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Global Nutrient Export from WaterSheds 2 (NEWS 2): Model Development and Implementation
Abstract
Global NEWS is a global, spatially explicit, multi-element and multi-form model of nutrient exports by rivers. Here we present NEWS 2, the new version of Global NEWS developed as part of a Millennium Ecosystem Assessment scenario implementation from hindcast (1970) to contemporary (2000) and future scenario trajectories (2030 & 2050). We provide a detailed model description and present an overview of enhancements to input datasets, emphasizing an integrated view of nutrient form sub-models and contrasts with previous NEWS models (NEWS 1). An important difference with NEWS 1 is our unified model framework (multi-element, multi-form) that facilitates detailed watershed comparisons regionally and by element or form. NEWS 2 performs approximately as well as NEWS 1 while incorporating previously uncharacterized factors. Although contemporary global river export estimates for dissolved inorganic nitrogen (DIN) and particulates show notable reductions, they are within the range of previous studies; global exports for other nutrient forms are comparable to NEWS 1. NEWS 2 can be used as an effective tool to examine the impact of polices to reduce coastal eutrophication at regional to global scales. Continued enhancements will focus on the incorporation of other forms and sub-basin spatial variability in drivers and retention processes.
... Other examples include the IMAGE-GNM (Integrated Model to Assess the Global Environment-Global Nutrient Model [11]), DynQual (dynamic surface water quality model [9]) and GLoWPa (Global Waterborne Pathogen [10]) with the grid scale resolution for the world. Global NEWS-2 (Nutrient Export from WaterSheds) is also for the world [14] while SPARROW is for America [15] but both take a basin scale resolution. MARINA (Model to Assess River Inputs of pollutaNts to seAs) is a global model with a sub-basin scale resolution [5,6,12,16,17]. ...
... This holds especially for the North American [33] and European waters [34] where eutrophication issues were observed [35]. Models were developed to understand the causes of eutrophication [14,36] and observed nutrient concentrations (https:// gemst at. org/) were used to validate the models. ...
... However, it has been limited to large-scale water quality models because of their diversities in scales such as daily [9] versus annual [12], and grid [11] versus (sub)basin [16]. Another reason is the diversity in modelled water quality parameters such as nutrients [11], pathogens [10], chemicals [23,25], plastics [56,57], and in modelling approaches suc has steady state [12,14] versus dynamic [9,11]). Models may need different input datasets including population [58], income [59], urbanization [60], and land use [49]. ...
Validating large-scale water quality models is challenging because of the variety of water quality constituents, and scales for which observations are limited. Here, in this perspective, we propose 13 alternative strategies to build trust in large-scale water quality models beyond validation and discuss their strengths and weaknesses regarding their validity, reliability, and applicability. Our alternative strategies aim to evaluate separately model inputs (Strategies 1–4), outputs (Strategies 5–6) and structures (Strategy 7) as well as these aspects together (Strategies 8–13). This is done via methods such as comparisons (Strategies 1–3, 6–8, 12–13), sensitivity analysis (Strategy 5), use of innovations (Strategy 9), expert knowledge (Strategy 11) and local models (Strategy 13). The proposed strategies vary in their validity, reliability, and applicability. Validation is an important starting point but should be used in combination with other strategies. Our proposed list opens the discussion to improve methods to evaluate global water quality models.
... During 1901- (Fig. 12b). For the most recent period 678 (2000s), the simulated riverine N export to oceans is converging, with 679 differences smaller than 10 % compared to other models such as GlobaNEWS2 680 (Mayorga et al., 2010), IMAGE-GNM, and DLEM (Fig. 12b). Although the 681 global riverine TN export to oceans simulated by ORCHIDEE-NLAT is close to 682 that simulated by GlobalNEWS2 (1970GlobalNEWS2 ( -2010, the TN export reported here 683 contains a slightly larger fraction of DIN and a slightly lower fraction of PON 684 compared to GlobalNEWS2 (Fig. 12b). ...
... The magnitude of TN exports simulated by ORCHIDEE-NLAT and 694 GlobalNEWS2 continue to diverge at basin scale (Fig. 13b). Balance Model (WBM plus ) (Fekete et al., 2010) , and NEWS models were used 724 to calculate nutrient retention in streams and reservoirs (Seitzinger et al., 2005, 725 2010; Mayorga et al., 2010). The hydrological model ued in IMAGE-GNM is GNM uses the nutrient spiraling approach (Newbold et al., 1981) to describe in-728 stream retention of both N and P with a yearly time step (following Wollheim et 729 al., 2008). ...
... We also evaluated the performance of 384 ORCHIDEE-NLAT in simulating annual lateral TN transfer against 385 observational data from the selected 189 sites around the world. The simulated 386 total amounts of PON, DON and DIN from land to river and from river to ocean 387 were further compared with previously published global N models, namely 388 IMAGE-GNM (Vilmin et al., 2018), FrAMES-N (Frame-work for Aquatic 389 Modeling in the Earth System) (Wollheim et al., 2008), MBM (Mass Balance 390Model)(Green et al., 2004), and Global NEWS2(Mayorga et al., 2010).391 ...
Lateral nitrogen (N) transport from land to oceans through rivers is an important component of the global N cycle. We developed a new model of this system, called ORCHIDEE-NLAT, which simulates the routing of water in rivers, and the pertaining transport of dissolved inorganic N (DIN), dissolved organic N (DON) and particulate organic N (PON) as well as the accompanying biogeochemical processes of decomposition for DON and PON, and denitrification for DIN during the transit from land to oceans through the river network. Evaluation against global observation-based datasets reveal that the model captures both the magnitude and seasonal variations of riverine water discharges and total nitrogen (TN) flows well. The ORCHIDEE-NLAT model was then applied to reconstruct the historical evolution of global TN flows from land to rivers, as well as the denitrification of DIN within the river network. Due to anthropogenic activities (e.g. mineral fertilisers and manure application, sewage water injection in rivers and land use change) and indirect climate and CO2 effects, the TN exports are modelled to increase from 27.1 Tg N yr-1 over 1901–1910 to 40.8 Tg N yr-1 over 2001–2014, with DIN (80 %) contributing most of this increase. The annual mean TN flow and DIN denitrification rates show substantial spatial heterogeneities. The seasonal amplitude of TN flow is of similar magnitude as the large-scale spatial variability. Compared to previously published global aquatic N transfer models (IMAGE-GNM, FrAMES-N, MBM, DLEM and Global NEWS2), our model produces similar global and continental-scale TN exports to the ocean, but shows distinct patterns at finer scale spatial scales (e.g. basin scale). ORCHIDEE-NLAT could also be coupled with other land surface models such as those used in the Nitrogen Model Intercomparison Project (NMIP). Our model provides a full simulation of N transport and reactivity from soils to oceans at an unprecedented spatio-temporal resolution (daily fluxes at 0.5° globally).
... The original empirical models of Ludwig et al. 24 and Galy et al. 3 quantified POC fluxes using single sediment flux distributions. Here we leverage four additional sediment flux products (WBMsed 2.0, Global NEWS, ISLSCPII and Peucker-Erhenbrink; see 'Revisiting Ludwig et al. (1998) and Galy et al. (2015) models-POC' in Methods) 25,[32][33][34] to rescale and harmonize these estimates, allowing us to better capture uncertainties arising from the variety of sediment load assessments. The initial estimates of global POC fluxes from Ludwig et al. and Galy et al. were 200 and 170 TgC yr −1 , respectively. ...
... Our updated estimates leveraging the four sediment products amount to 189 and 196 TgC yr −1 , respectively (Extended Data Fig. 5). When combined with the other empirical assessments 25,26,33 , the range in global POC exports among the five empirical estimates is 142-193 TgC yr −1 (circle symbols, Fig. 1), with a relative standard deviation (RSD) of 12%. The three spatially explicit, process-based estimates of global riverine POC export vary between 150 and 300 TgC yr −1 , exhibiting an RSD of 33%. ...
... Our study includes three spatially explicit, empirically derived global riverine DOC export estimates (range between 175 and 198 TgC yr −1 , RSD of 7%) 25,26,33 . The three process-based models simulating global DOC export (TRIPLEX-HYDRA, IMAGE-DGNM and DLEM-TAC) show larger differences than the empirical assessments and yield two-to threefold ranges in global DOC fluxes 6,27,29 , with an RSD value of 35% (Fig. 1). ...
Rivers are a key component of the global carbon cycle. They receive vast quantities of terrestrial carbon, of which a large fraction is ultimately exported to the coastal ocean. Our review of previously published assessments reveals that substantial uncertainties remain with regard to the spatial distribution and speciation of the carbon export. Accurate quantification of the relative contributions of dissolved, particulate, organic and inorganic carbon to the total amounts is, however, of crucial importance for the coupling between the terrestrial and marine carbon cycles. Breaking down existing spatially explicit assessments over large river basins, we find a disagreement in flux estimates that exceeds two orders of magnitude for more than half of the basins. Using machine-learning techniques in combination with a multi-model ensemble and an updated database of observations, we overcome the inconsistencies in existing assessments and narrow down uncertainties in riverine carbon exports. Our revised assessment yields a global riverine export of 1.02 ± 0.22 (2σ) PgC yr⁻¹. This carbon flux is partitioned into 0.52 ± 0.17, 0.30 ± 0.14, 0.18 ± 0.04 and 0.03 ± 0.02 PgC yr⁻¹ of dissolved inorganic, dissolved organic, particulate organic and particulate inorganic carbon, respectively. We estimate the carbon contribution through groundwater export to be minor (0.016 PgC yr⁻¹). Our assessment suggests an underestimation of the land-to-ocean carbon flux by 0.24 PgC yr⁻¹ by the Intergovernmental Panel on Climate Change (IPCC) and calls for a revision of the oceanic carbon budget.
... The underlying reason is that different forms of N came from different sources, which have evolved differently with increasing anthropogenic impacts rivers, initially, observational data in a limited number of representative rivers were extrapolated to estimate the global river export to oceans for different N forms [13][14][15][16][17][18] . Increasing data availability stimulated the development of global N budgets and exchanges among different compartments (land, ocean, atmosphere, rivers and so on) [19][20][21][22][23] , regression models ignoring temporal variability 5,7,[24][25][26][27][28][29][30][31][32][33] and, more recently, the use of machine-learning methods for global river biogeochemistry 34,35 . These approaches can spatially and quite accurately predict riverine N concentrations and thus identify world regions with large N exports. ...
... Our estimates of TN export for the 1970s are close to 35-37 TgN yr −1 reported by many studies for the same period using different appr oaches 7,19,23,27,55,56 (Fig. 2 and Supplementary Table 1). Our TN export estimates for the 1990s are similar to the 40-44 TgN yr −1 published by Seitzinger et al. 5 and Green et al. 28 . ...
... Our TN export estimates for the 1990s are similar to the 40-44 TgN yr −1 published by Seitzinger et al. 5 and Green et al. 28 . Moreover, our estimate for the 2000s is close to the 37-43 TgN yr −1 estimated by Mayorga et al. 27 , Seitzinger et al. 7 , Billen et al. 56 and Beusen et al. 6 (Fig. 2). However, though comparable in magnitude and showing similar increases during the twentieth century, studies based on much higher N inputs 4,44,54,58 or calculations considering small 25,26,33,59 or no inland-water N retention 21,22 yield much higher export estimates than ours. ...
Inland waters are an important component of the global nitrogen (N) cycle, functioning not only as land-to-sea transporters but also as active biogeochemical reactors. However, the latter role is not well understood regarding mechanisms, quantities or on a global scale. It remains unclear whether, when, how and why global inland-water biogeochemical N cycling has changed. Here we analyse the dynamic global inland-water N cycling processes in the Anthropocene by quantifying the long-term changes in different N forms, including their inputs to inland waters, transformation pathways, retention within inland waters, and river export to oceans. Using a spatially explicit, mechanistic, coupled hydrology and biogeochemistry model, we show that, during 1900–2010, the increase in total nitrogen (TN) river loading (from 27 to 68 Tg yr−1) resulted in an increase in TN export to oceans (from 20 to 42 Tg yr−1), despite an increase in inland-water retention (from 25% to 39%) primarily due to gaseous loss and burial. Moreover, the relative contributions of ammonium (NH4+), nitrate/nitrite (NOx−) and organic nitrogen (ON) changed because of threefold increases in global inland-water mineralization (transforming ON to NH4+) and N burial in sediments, a fourfold increase in nitrification (transforming NH4+ to NOx−) and a sixfold increase in denitrification (transforming NOx− to mainly N2).
... This flux is like that reported by Drake et al. (2021) in the Amazon River (0,82 Tg DIN yr −1 ). According to Mayorga et al. (2010), modeled N fluxes for South American rivers represent ~ 8.0 Tg N yr −1 . These authors also indicate DIN fluxes between 40-570 kg km −2 yr −1 for the study region, whereas our fluxes indicated a mean value of 196 kg km −2 yr −1 . ...
... This DIP load is approximately one third of the load reported by Drake et al. (2021) for the Amazon River (0.063 Tg P yr −1 ). Modeled yearly fluxes of DIP were reported by Mayorga et al. (2010), indicating a range of 2-5 kg km −2 yr −1 ; whereas our study indicated a mean value of 7.9 kg km −2 yr −1 . ...
... A temporal analysis of these fluxes between 2008-2018 showed a significant increasing trend for TOC (Fig. 4b). According to Mayorga et al. (2010), yearly DOC fluxes range between 250-4000 kg C km −2 yr −1 for the study region, whereas the observed range of DOC varied between 12-3300 kg C km −2 yr −1 . The slope of this series indicated an increase of 0.001 g C m −2 yr −1 for the TOC time series (Fig. 4b). ...
Public data from sixty tropical rivers were used to explore variations in C, N, and P and the potential impact of anthropogenic activities over a decade on freshwater bodies. The results showed the Total Organic Carbon (TOC) varying between 2.7–73.0 mg C l⁻¹; whereas Total Nitrogen observations showed concentrations between 0.9 and 32.0 mg N l⁻¹ and Total Phosphorus showed a higher frequency of data in the range of 0.02–0.4 mg P l⁻¹. The annual total C load (TC) was estimated at 32.7 Tg C yr⁻¹, where dissolved inorganic carbon (DIC) accounted for 69% of TC and dissolved organic carbon (DOC) contributed 22%. The stoichiometry showed P depletion relative to C and/or N in 39 of the sixty rivers (65%). This was further supported by the fact that most catchments had TOC values > 50% (C/N/P = 100%). A significant trend was found for yearly fluxes of TOC for the period 2008–2018 (Mann–Kendall test; p = 0.0006; α = 0.05). Land-use and cover at period 2008–2018 indicated a trend of increasing anthropized area of 4%, whereas the natural area decreased by 3.1%. The organic load trend analysis showed 85% of municipalities with a positive trend, this high rate in the remaining organic load is indicative of urban and agricultural in the region.
... The riverine fluxes of dissolved and particulate nutrients (nitrogen and phosphorus) are derived from the annual mean loads of inorganic and organic nitrogen and phosphorus from the Global Nutrient Export from WaterSheds2 (GlobalNEWS2), referenced to the year 2000 (Mayorga et al., 2010). We include riverine inputs of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP), and bio-available partic-210 ulate organic nitrogen (PON). ...
... anthropogenic increase in riverine nutrient supply (MOM6-COBALT-IND12 v1.0 includes nutrient inputs equivalent to year 2000 fromMayorga et al., 2010), which would likely introduce a long-term trend in coastal primary productivity and oxygen concentrations in the vicinity of large river systems.While this MOM6-COBALT-IND12 v1.0 configuration is remarkably successful at capturing many of the features and observed variability of the northern Indian Ocean, there are still some areas where there is potential for improvement. The 605 main model bias is the larger horizontal extent and volume of suboxia (oxygen concentrations < 5 µmol kg −1 ) simulated in the Bay of Bengal. ...
We introduce and evaluate the regional ocean model MOM6-COBALT-IND12 version 1 coupling the MOM6 ocean dynamics model to the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) biogeochemical model at a horizontal resolution of 1/12°. The model covers the northern Indian Ocean (north of 8° S), central to the livelihoods and economies of countries that comprise about one-third of the world’s population. We demonstrate that the model effectively captures the key physical and biogeochemical basin-scale features related to seasonal monsoon reversal, interannual Indian Ocean Dipole and multi-decadal variability, as well as intraseasonal and fine-scale variability (e.g., eddies and planetary waves), which are all essential for accurately simulating patterns of coastal upwelling, primary productivity, temperature, salinity, and oxygen levels. Well represented features include the timing and amplitude of the monsoonal blooms triggered by summer coastal upwelling and winter mixing, the strong contrast between the high evaporation / high salinity Arabian Sea and high precipitation / high runoff / low salinity Bay of Bengal, the seasonality of the Great Whirl gyre and coastal Kelvin upwelling/downwelling waves, as well as the physical and biogeochemical patterns associated with intraseasonal and interannual variability. A major model bias is the larger oxygen minimum zone simulated in the Bay of Bengal, a common challenge of ocean and Earth system models in this region. This bias was partly mitigated by improving the representation of the export and burial of organic detritus to the deep ocean (e.g., sinking speed, riverine lithogenic material inputs that protect organic material and burial fraction) and water-column denitrification (e.g., nitrate-based respiration at higher oxygen levels) using observational constraints. These results indicate that the regional MOM6-COBALT-IND12 v1.0 model is well suited for physical and biogeochemical studies on timescales ranging from weeks to decades, in addition to supporting marine resource applications and management in the northern Indian Ocean.
... River freshwater discharges in both simulations were sourced from the gridded daily Global Flood Awareness System (GloFAS) reanalysis version 4.0 (Harrigan et al., 2020). Riverine inputs of dissolved inorganic and organic nitrogen and phosphorus and particulate nitrogen were using the annual mean values (referenced to year 2000) from GLOBAL NEWS2 (Mayorga et al., 2010). The constant concentrations of DIC, alkalinity, and lithogenic minerals from river sources were allocated in both simulations, using the same values specified in Stock et al. (2020). ...
... For instance, N-budget for the northern Indian Ocean from [Srinivas and Sarin (2013); Sarma et al. (2022)] suggests that riverine supply is of comparable magnitude with atmospheric deposition supply with significant disparities between AS and BoB basins. Keeping the average external input of nitrogen from rivers constant to 2000 levels in our simulations captures an average input over the period, but lacks the increase in riverine nutrient inputs over the 1980-2020 period in the Bay of Bengal (Pedde et al., 2017), and might conceals potential impacts, such as eutrophication, which are not explicitly addressed in this study (Mayorga et al., 2010;Pedde et al., 2017;Suntharalingam et al., 2019;Sridevi and Sarma, 2022). ...
Since 1980, atmospheric pollutants in South Asia and India have dramatically increased in response to industrialization and agricultural development, enhancing the atmospheric deposition of anthropogenic nitrogen in the northern Indian Ocean and potentially promoting primary productivity. Concurrently, ocean warming has increased stratification and limited the supply of nutrients supporting primary productivity. Here, we examine the biogeochemical consequences of increasing anthropogenic atmospheric nitrogen deposition and contrast them with the counteracting effect of warming, using a regional ocean biogeochemical model of the northern Indian Ocean forced with atmospheric nitrogen deposition derived from an Earth System Model. Our results suggest that the 60% recent increase in anthropogenic nitrogen deposition over the northern Indian Ocean provided external reactive nitrogen that only weakly enhanced primary production (+10 mg C.m–2.d–1.yr–1 in regions of intense deposition) and secondary production (+4 mg C.m–2.d–1.yr–1). However, we find that locally this enhancement can significantly offset the declining trend in primary production over the last four decades in the central Arabian Sea and western Bay of Bengal, whose magnitude are up to -20 and -10 mg C.m–2.d–1.yr–1 respectively.
... Since nitrate is the only nitrogen nutrient explicitly resolved in the UVic version used, all bioavailable N has been included in the nitrate com-partment of the model. See Mayorga et al. (2010) for more details on the model configuration and Dumont et al. (2005) for more details on the validation of NEWS dissolved inorganic nitrogen (DIN). ...
... For the riverine nutrient input, we used the results of the NEWS 2 model. Mayorga et al. (2010) and Dumont et al. (2005) evaluated the individual models for the river export and found that despite uncertainties associated with model inputs, nutrient yield and export were close to observations and the global export estimates were similar to the results of previous analyses. Although the riverine nutrient export from NEWS 2 includes the anthropogenic component, based on the year 2000, we did not focus on the anthropogenic component in our experiment, and the millennial-scale simulations do not provide much insight into short-term variability. ...
Riverine nutrient export is an important process in marine coastal biogeochemistry and also impacts global marine biology. The nitrogen cycle is a key player here. Internal feedbacks are shown to regulate not only nitrogen distribution, but also primary production and thereby oxygen concentrations. Phosphorus is another essential nutrient and interacts with the nitrogen cycle via different feedback mechanisms. After a previous study of the marine nitrogen cycle response to riverine nitrogen supply, here we include phosphorus from river export with different phosphorus burial scenarios and study the impact of phosphorus alone and in combination with nitrogen in a global 3D ocean biogeochemistry model. Again, we analyse the effects on near-coastal and open-ocean biogeochemistry. We find that riverine export of bioavailable phosphorus alone or in conjunction with nitrogen affects marine biology on millennial timescales more than riverine nitrogen alone. Biogeochemical feedbacks in the marine nitrogen cycle are strongly influenced by additional phosphorus. Where bioavailable phosphorus increases with river input, nitrogen concentration increases as well, except for in regions with diminishing oxygen concentrations. High phosphorus burial rates decrease biological production significantly. Globally, the addition of riverine phosphorus in the modelled ocean leads to elevated primary production rates in the coastal and open oceans.
... b The sum of denitrification in "rivers" and "lakes and reservoirs" in Table 1 of Seitzinger et al. (2006). c Inputs of P to dam reservoirs in Table 1 of Maavara et al. (2015). d Retention of P by dam reservoirs in Table 1 of Maavara et al. (2015). ...
... c Inputs of P to dam reservoirs in Table 1 of Maavara et al. (2015). d Retention of P by dam reservoirs in Table 1 of Maavara et al. (2015). the Amazon, Ganges, and Yangtze shown in Fig. 2 of Harrison et al. (2019) imply that IMAGE-GNM likely overestimates global freshwater P retention. ...
Estimating global river solids, nitrogen (N), and phosphorus (P), in both quantity and composition, is necessary for understanding the development and persistence of many harmful algal blooms, hypoxic events, and other water quality issues in inland and coastal waters. This requires a comprehensive freshwater model that can resolve intertwined algae, solid, and nutrient dynamics, yet previous global watershed models have limited mechanistic resolution of instream biogeochemical processes. Here we develop the global, spatially explicit, and process-based Freshwater Algae, Nutrient, and Solid cycling and Yields (FANSY) model and incorporate it within the Land Model (LM3). The resulting model, LM3-FANSY v1.0, is intended as a baseline for eventual linking of global terrestrial and ocean biogeochemistry in next-generation Earth system models to project global changes that may challenge empirical approaches. LM3-FANSY explicitly resolves interactions between algae, N, P, and solid dynamics in rivers and lakes at 1° spatial and 30 min temporal resolution. Simulated suspended solids (SS), N, and P in multiple forms (particulate or dissolved, organic or inorganic) agree well with measurement-based yield (kg km-2 yr-1), load (kt yr-1), and concentration (mg L-1) estimates across a globally distributed set of large rivers, with an accuracy comparable to other global nutrient and SS models. Furthermore, simulated global river loads of SS, N, and P in different forms to the coastal ocean are consistent with published ranges, though regional biases are apparent. River N loads are estimated to contain approximately equal contributions by dissolved inorganic N (41 %) and dissolved organic N (39 %), with a lesser contribution by particulate organic N (20 %). For river P load estimates, particulate P, which includes both organic and sorbed inorganic forms, is the most abundant form (64 %), followed by dissolved inorganic and organic P (25 % and 11 %). Time series analysis of river solid and nutrient loads in large US rivers for the period ∼ 1963–2000 demonstrates that simulated SS and N loads in different N forms covary with variations of measurement-based loads. LM3-FANSY, however, has less capability to capture interannual variability of P loads, likely due to the lack of terrestrial P dynamics in LM3. Analyses of the model results and sensitivity to components, parameters, and inputs suggest that fluxes from terrestrial litter and soils, wastewater, and weathering are the most critical inputs to the fidelity of simulated river nutrient loads for observation-based estimates. Sensitivity analyses further demonstrate a critical role of algal dynamics in controlling the ratios of inorganic and organic nutrient forms in freshwaters. While the simulations are able to capture significant cross-watershed contrasts at a global scale, disagreement for individual rivers can be substantial. This limitation is shared by other global river models and could be ameliorated through further refinements in nutrient sources, freshwater model dynamics, and observations. Current targets for future LM3-FANSY development include the additions of terrestrial P dynamics, freshwater carbon, alkalinity, enhanced sediment dynamics, and anthropogenic hydraulic controls.
... The lateral transport of organic carbon by rivers (F ie , total fluvial load minus amount of carbon derived from solution of lithospheric carbonates) was estimated by scaling GlobalNEWS2 (Mayorga et al., 2010) adjusted for lithogenic carbon Hartmann et al., 2009) to match the latitudinal patterns of Resplandy et al. (2018) and to a global export constraint of 500 TgC yr 1 . Globally, the total fluvial carbon load in lateral transport sums to 728 TgC or 2,669 4 TgCO 2 yr 1 (Battin et al., 2023). ...
Accurate accounting of greenhouse‐gas (GHG) emissions and removals is central to tracking progress toward climate mitigation and for monitoring potential climate‐change feedbacks. GHG budgeting and reporting can follow either the Intergovernmental Panel on Climate Change methodologies for National Greenhouse Gas Inventory (NGHGI) reporting or use atmospheric‐based “top‐down” (TD) inversions or process‐based “bottom‐up” (BU) approaches. To help understand and reconcile these approaches, the Second REgional Carbon Cycle Assessment and Processes study (RECCAP2) was established to quantify GHG emissions and removals for carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), for ten‐land and five‐ocean regions for 2010–2019. Here, we present the results for the North American land region (Canada, the United States, Mexico, Central America and the Caribbean). For 2010–2019, the NGHGI reported total net‐GHG emissions of 7,270 TgCO2‐eq yr⁻¹ compared to TD estimates of 6,132 ± 1,846 TgCO2‐eq yr⁻¹ and BU estimates of 9,060 ± 898 TgCO2‐eq yr⁻¹. Reconciling differences between the NGHGI, TD and BU approaches depended on (a) accounting for lateral fluxes of CO2 along the land‐ocean‐aquatic continuum (LOAC) and trade, (b) correcting land‐use CO2 emissions for the loss‐of‐additional‐sink capacity (LASC), (c) avoiding double counting of inland water CH4 emissions, and (d) adjusting area estimates to match the NGHGI definition of the managed‐land proxy. Uncertainties remain from inland‐water CO2 evasion, the conversion of nitrogen fertilizers to N2O, and from less‐frequent NGHGI reporting from non‐Annex‐1 countries. The RECCAP2 framework plays a key role in reconciling independent GHG‐reporting methodologies to support policy commitments while providing insights into biogeochemical processes and responses to climate change.
... The model includes 34 tidal constituents (FES2014 tide model; Lyard et al., 2021), applied as a tidal potential and as a sea surface height and barotropic currents along the model's open lateral boundaries. River forcing was generated using the GlobalNEWS2 model (Beusen et al., 2009;Mayorga et al., 2010;Seitzinger et al., 2010) with constant land use and hydrology from the GlobalNEWS2 Realistic Hydrology 2000 (data set version 1.0. 12-11-2014), but varying precipitation from the HadGEM2 atmospheric forcing. ...
The Bay of Bengal is a dynamic region that experiences intense freshwater runoff, extreme meteorological events, and seasonally reversing surface currents. The region is particularly susceptible to anthropogenic climate change, driven in part by large air‐sea fluxes, persistent freshwater stratification, and low overturning rates. Predicting how this system is likely to change in the future is paramount for planning effective adaption and mitigation strategies. Using a relocatable, coupled physics‐ecosystem regional coastal ocean model (NEMO‐ERSEM), we investigate potential future changes in surface circulation and coastal nitrate pathways around the coast of the Bay of Bengal from 1980 to 2060, using a “business‐as‐usual” climate change scenario. We find that future surface currents are reduced in the northern Bay of Bengal(summer) and strengthened in the southern Bay of Bengal (fall). Coastal nitrate transports mirror this asymmetric change and decrease by as much as 14% in the northern Bay of Bengal, perpetuating a positive feedback loop whereby the northern Bay of Bengal becomes progressively fresher and more nutrient‐rich, strengthening surface stratification and increasing the risk of toxic algal blooms and eutrophication events. Conversely, in the southern Bay of Bengal, coastal nitrate transports increase by 52% that promotes localized diatom blooms despite reduced regional river runoff. This work highlights the need for more rigorous scenario testing in the region and presents new challenges for mitigating the impact of anthropogenic climate change across South Asia.
... We further built trust in our model by comparing our estimates with other existing modeling studies. Our global river exports of TDN and TDP were compared with the Global NEWS (Nutrient Export from WaterSheds, Mayorga et al., 2010;Seitzinger et al., 2010) and IMAGE-GNM models (Beusen et al., 2015(Beusen et al., , 2022. Our results are 35% higher than in Global NEWS and 10% lower than in IMAGE-GNM. ...
In the future, rivers may export more pollutants to coastal waters, driven by socio‐economic development, increased material consumption, and climate change. However, existing scenarios often ignore multi‐pollutant problems. Here, we aim to explore future trends in annual river exports of nutrients (nitrogen and phosphorus), plastics (macro and micro), and emerging contaminants (triclosan and diclofenac) at the sub‐basin scale worldwide. For this, we implement into the process‐based MARINA‐Multi model (Model to Assess River Inputs of pollutaNts to the seAs) two new multi‐pollutant scenarios: “Sustainability‐driven Future” (SD) and “Economy‐driven Future” (ED). In ED, river exports of nutrients and microplastics will double by 2100, globally. In SD, a decrease of up to 83% is projected for river export of all studied pollutants by 2100, globally. Diffuse sources such as fertilizers are largely responsible for increasing nutrient pollution in the two scenarios. Point sources, namely sewage systems, are largely responsible for increasing microplastic pollution in the ED scenario. In both scenarios, the coastal waters of the Indian Ocean will receive up to 400% more pollutants from rivers by 2100 because of growing population, urbanization, and poor waste management in the African and Asian sub‐basins. The situation differs for sub‐basins draining into the Mediterranean Sea and the Pacific Ocean (mainly less future pollution) and the Atlantic Ocean and Arctic Ocean (more or less future pollution depending on sub‐basins and scenarios). From 56% to 78% of the global population are expected to live in more polluted river basins in the future, challenging sustainable development goals for clean waters.
... Investigating riverine DOC contents and fluxes traditionally relied on field surveys/regular in situ measurements, quantification of organic carbon loss from terrestrial ecosystems, and models [13,[22][23][24][25][26][27]. However, field surveys/regular in situ measurements are costly and subject to significant spatial and temporal constraints. ...
Real-time monitoring of riverine-dissolved organic carbon (DOC) and its controlling factors is critical for formulating strategies regarding the river basin and marginal seas pollution prevention and control. In this study, we established a linear regression formulation that relates the permanganate index (CODMn) to the DOC concentration based on in situ measurements collected on five field surveys in 2023–2024. This regression formulation was used on a large number of data collected from automatic monitoring stations in the Qiantang River area to construct a daily quasi-in situ database of DOC concentration. By combining the quasi-in situ DOC data and Sentinel-2 measurements, an enhanced algorithm for empirical DOC estimation was developed (R² = 0.66) using the extreme gradient boosting (XGBoost) method and its spatial and temporal variations in the Qiantang River were analyzed from 2016 to 2023. Spatially, the main stream of the Qiantang River exhibited an overall decreasing and increasing trend influenced by population density, economic development, and pollutant discharge in the basin area, and the temporal distribution of DOC was controlled by meteorological conditions. The DOC contents had the highest in summer, primarily due to high rainfall and leaching. The inter-annual variation in DOC concentration was influenced by the total annual runoff volumes, with a minimum level of 2.24 mg L⁻¹ in 2023 and a maximum level of 2.45 mg L⁻¹ in 2019. The monthly DOC fluxes ranged from 6.3 to 13.8 × 10⁴ t, with the highest values coinciding with the maximum river discharge volumes in June and July. The DOC levels in the Qiantang River remained relatively high in recent years (2016–2023). This study enables the concerned stakeholders and researchers to better understand carbon transportation and its dynamics in the Qiantang River and its coastal areas.
... The model has a horizontal resolution of 1°with 60 vertical depth levels ranging in thickness from 10 m in the upper 150 m to 250 m in the deep ocean (Moore et al., 2013). Riverine nutrient fluxes (N, P, Si, Fe), dissolved inorganic carbon, alkalinity, and dissolved organic matter (DOM) fluxes were integrated into the CESM2 ocean model using estimates from Global NEWS (Mayorga et al., 2010) and the Integrated Model to Assess the Global Environment-Global Nutrient Model (IMAGE-GNM) (Beusen et al., 2015(Beusen et al., , 2016. Nutrient inputs from rivers encompass dissolved inorganic nitrogen (DIN), phosphorus (DIP), Si, and Fe, along with dissolved organic nitrogen and phosphorus, while carbon inputs included both inorganic and organic forms. ...
The Arctic Ocean experiences significant seasonal to interannual environmental changes, including in temperature, light, sea ice, and surface nutrient concentrations, that influence the dynamics of marine plankton populations. Here, we use a hindcast simulation (1948–2009) of size-structured Arctic Ocean plankton communities, ocean circulation, and biogeochemical cycles in order to better understand how seasonal to interannual changes in the environment influence phytoplankton physiology, plankton community structure, trophic dynamics, and fish production in the Arctic Ocean. The growth of model phytoplankton was primarily limited in winter, spring, and fall by light, but in summer, the growth of smaller and larger phytoplankton was mostly limited by temperature and nutrient availability, respectively. The dominant trophic pathway in summer was from phytoplankton to herbivorous zooplankton such that the average trophic position of model zooplankton was lower in the summer growing season compared to the rest of the year. On interannual timescales, changes in plankton community composition were strongly tied to interannual changes in bottom-up forcing by the environment. In the summer, in years with less ice and warmer temperatures, the biomass of phytoplankton and zooplankton was higher, the size–abundance relationship slopes were more negative (indicative of a phytoplankton community enriched in smaller phytoplankton), zooplankton had higher mean trophic position (indicative of greater carnivory), and potential fishery production was greater, fueled by increased mesozooplankton biomass and flux of organic matter to the benthos. The summertime shift toward greater carnivory in warmer and low-ice years was due primarily to changes in phenology, with phytoplankton and microzooplankton blooms occurring approximately 1 month earlier in these conditions and carnivorous zooplankton increasing in abundance during summer. The model provides a spatially and temporally complete overview of simulated changes in plankton communities in the Arctic Ocean occurring on seasonal to interannual timescales, and it provides insights into the mechanisms underlying these changes as well as their broader biogeochemical and ecosystem significance.
... We calculate each size class surface area by multiplying the estimated cumulative abundance by mean surface area. We estimate lake dissolved organic carbon (DOC) for each region by taking the DOC value at the river mouth with the largest discharge from the GLOBALNEWS data set (Mayorga et al., 2010). If the region does not discharge into the ocean directly, we use the DOC of the region it discharged into. ...
Inland waters emit significant amounts of carbon dioxide (CO2) to the atmosphere; however, the global magnitude and source distribution of inland water CO2 emissions remain uncertain. These fluxes have previously been “statistically upscaled” by independently estimating dissolved CO2 concentrations and gas exchange velocities to calculate fluxes. This scaling, while robust and defensible, has known limitations in representing carbon source limitations and spatial variability. Here, we develop and calibrate a CO2 transport model for the continental United States, simulating carbon transport and transformation in >22 million hydraulically connected rivers, lakes, and reservoirs. We estimate 25% lower CO2 fluxes compared to upscaling estimates forced by the same observational calibration data. While precise CO2 source distribution estimates are limited by the resolution of model parameterizations, our model suggests that stream corridor CO2 production dominates over groundwater inputs at the continental scale. Our results further suggest that the lack of observational networks for groundwater CO2 and scalable metabolic models of aquatic CO2 production remain the most salient barriers to further coupling of our model with other Earth system components.
... In all model simulations, river nutrient input was prescribed as a climatology based on the GLOBAL-NEWS2 dataset (Mayorga et al., 2010), while DIC and alkalinity river input are based on the output of the Global Erosion Model (Ludwig et al., 1996). We note here that this procedure is in contrast with the GCB protocol, which recommends river fluxes of nutrients and carbon to be switched off. ...
Given the role of the ocean in mitigating climate change through CO2 absorption, it is important to improve our ability to quantify the historical ocean CO2 uptake, including its natural variability, for carbon budgeting purposes. In this study we present an exhaustive intercomparison between two ocean modeling practices that can be used to reconstruct the historical ocean CO2 uptake. By comparing the simulations to a wide array of ocean physical and biogeochemical observational datasets, we show how constraining the ocean physics towards observed temperature and salinity results in a better representation of global biogeochemistry. We identify the main driver of this improvement to be a more vigorous large-scale meridional overturning circulation together with improvements in mixed-layer depth and sea surface temperature. Nevertheless, surface chlorophyll was rather insensitive to these changes, and in some regions its representation worsened. We identified the causes of this response to be a combination of a lack of robust parameter optimization and limited changes in environmental conditions for phytoplankton. We conclude that although the direct validation of CO2 fluxes is challenging, the pervasive improvement observed in most aspects of biogeochemistry when applying data assimilation of observed temperature and salinity is encouraging; therefore, data assimilation should be included in multi-method international efforts aimed at reconstructing the ocean CO2 uptake.
... As a result, the main sources of sedimentary DFe are along continental shelves and productive margins, with little contribution coming from the deep ocean. For the river source of DFe, discharge data for the year 2000 from the Global Nutrient Export from WaterSheds (GlobalNEWS, Mayorga et al., 2010) is combined with a constant DFe concentration of 10 nM. For hydrothermal vents, a constant flux of iron from the grid boxes containing vents is applied so that the total hydrothermal-vent iron flux is equal to approximately 5.0 Gmol yr −1 . ...
Although the northern Indian Ocean (IO) is globally one of the most productive regions and receives dissolved iron (DFe) from multiple sources, there is no comprehensive understanding of how these different sources of DFe can impact upper-ocean biogeochemical dynamics. Using an Earth system model with an ocean biogeochemistry component, this study shows that atmospheric deposition is the most important source of DFe to the upper 100 m of the northern IO, contributing more than 50 % of the annual DFe concentration. Sedimentary sources are locally important in the vicinity of the continental shelves and over the southern tropical IO, away from high atmospheric depositions. While atmospheric depositions contribute more than 10 % (35 %) to 0–100 m (surface-level) chlorophyll concentrations over large parts of the northern IO, sedimentary sources have a similar contribution to chlorophyll concentrations over the southern tropical IO. Such increases in chlorophyll are primarily driven by an increase in diatom population over most of the northern IO. The regions that are susceptible to chlorophyll enhancement following external DFe additions are where low levels of background DFe and high background nitrate-to-iron values are observed. Analysis of the DFe budget over selected biophysical regimes over the northern IO points to vertical mixing as the most important mechanism for DFe supply, while the importance of advection (horizontal and vertical) varies seasonally. Apart from removal of surface DFe by phytoplankton uptake, the subsurface balance between DFe scavenging and regeneration is crucial in replenishing the DFe pool to be made available to the surface layer by physical processes.
... eF is the watershed (land) export constant, which is 0.94 for DIN; this parameter is derived from the original Global Nutrient Export from WaterSheds 2 (NEWS 2) model by Mayorga et al. and Strokal et al. (7,59). Rnat j is the annual runoff from land to streams in subbasin j (m). ...
Excessive nitrogen (N) deposition affects aquatic ecosystems worldwide, but effectiveness of emissions controls and their impact on water pollution remains uncertain. In this modeling study, we assess historical and future N deposition trends in Chinese river basins and their contributions to water pollution via direct and indirect N deposition (the latter referring to transport of N to water from N deposited on land). The control of acid gas emissions (i.e., nitrogen oxides and sulfur dioxide) has had limited effectiveness in reducing total N deposition, with notable contributions from agricultural reduced N deposition. Despite increasing controls on acid gas emissions between 2011 and 2019, N inputs to rivers increased by 3%, primarily through indirect deposition. Simultaneously controlling acid gas and ammonia emissions could reduce N deposition and water inputs by 56 and 47%, respectively, by 2050 compared to 2019. Our findings underscore the importance of agricultural ammonia mitigation in protecting water bodies.
... Out of the stations that are used in the monthly river TN and TP concentration calibration, more than 75 % achieved an NSE>0.35 (Fig. 2), PBIAS ≤ ± 30 % and R 2 > 0.3, (Figs. 2 -4), showing a satisfactory performance (Moriasi et al., 2015) with good seasonality, particularly given the data scarcity in Africa. The validation efforts further show that the continental estimates of river sediment export to coastal waters (long-term averages of 3.1 Tg N/yr and 1.2 Tg P/yr for 1980 -2019) align with the ranges estimated by previous continental to global studies (4 -7.7 Tg N/yr and 1 -1.5 Tg P/yr) (Yasin et al., 2010;Mayorga et al., 2010;Seitzinger et al., 2005). Additionally, a comparative analysis for nutrient balance (Online Resource, Table A1) and nutrient export for four of Africa's major rivers (Online Resource, Table A2) was conducted, showing similar magnitudes of nutrient fluxes. ...
... Attempts to estimate carbon fluxes at the land-ocean interface initially focused on bicarbonate (Livingstone 1963;Kempe 1979). The focus then shifted to dissolved organic carbon (DOC) and particulate carbon (POC) (Schlesinger and Melack 1981;Galy et al. 2015) as well as carbon dioxide (CO 2 ) emissions to the atmosphere (Borges et al. 2005;Mayorga et al. 2010). Regnier et al. (2022) estimated that estuaries export $ 0.95 AE 0.15 PgC yr À1 to the ocean globally. ...
Previous estimates of dissolved carbon export from estuaries focused on larger systems in the Northern Hemisphere, with little data for smaller tropical estuaries often fringed by intertidal wetlands. We investigated lateral export (outwelling) and transformation rates of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total alkalinity (TA) as well as CO2 emissions from 18 diverse Australian estuaries. Most estuaries acted as net sources for DOC (72%), DIC (83%), and TA (50%). On average, estuaries exported 120 ± 55 and 344 ± 150 mmol m⁻² catchment yr⁻¹ DOC and DIC, respectively. Estuarine CO2 emissions (33 ± 20 mmol m⁻² estuary d⁻¹) equalled 13% ± 16% of the dissolved lateral carbon export. Carbon export positively correlated with runoff, rain, and intertidal wetland cover, and negatively correlated with estuary and catchment area. Mangroves and saltmarshes cover < 1% of all catchments but can contribute 46% ± 11% of the DOC and 67% ± 13% of the DIC exported to the ocean. Upscaling our observations, Australian estuaries export 2.8 ± 2.2 TgC yr⁻¹ DOC, 8.1 ± 6.2 TgC yr⁻¹ DIC, and 0.7 ± 0.6 Tmol yr⁻¹ TA. Small catchments (< 10 ha) making up 70% of all estuaries and accounting for 18% of the total freshwater flow provided 27% to the total dissolved carbon export. Overall, small tropical estuaries fringed by highly productive intertidal wetlands are hotspots of carbon exports and should be considered in marine carbon budgets.
... F river export is taken from spatially explicit estimates published by Zscheischler et al. (2017) after the predictive models of Hartmann et al. (2009) and Mayorga et al. (2010). In our accounting framework, F river export is fed by inputs from the lithosphere (F litho2river ) and the biosphere (F bio2river ) (Equation 13). ...
In the framework of the RECCAP2 initiative, we present the greenhouse gas (GHG) and carbon (C) budget of Europe. For the decade of the 2010s, we present a bottom‐up (BU) estimate of GHG net‐emissions of 3.9 Pg CO2‐eq. yr⁻¹ (using a global warming potential on a 100 years horizon), which are largely dominated by fossil fuel emissions. In this decade, terrestrial ecosystems acted as a net GHG sink of 0.9 Pg CO2‐eq. yr⁻¹, dominated by a CO2 sink that was partially counterbalanced by net emissions of CH4 and N2O. For CH4 and N2O, we find good agreement between BU and top‐down (TD) estimates from atmospheric inversions. However, our BU land CO2 sink is significantly higher than the TD estimates. We further show that decadal averages of GHG net‐emissions have declined by 1.2 Pg CO2‐eq. yr⁻¹ since the 1990s, mainly due to a reduction in fossil fuel emissions. In addition, based on both data driven BU and TD estimates, we also find that the land CO2 sink has weakened over the past two decades. A large part of the European CO2 and C sinks is located in Northern Europe. At the same time, we find a decreasing trend in sink strength in Scandinavia, which can be attributed to an increase in forest management intensity. These are partly offset by increasing CO2 sinks in parts of Eastern Europe and Northern Spain, attributed in part to land use change. Extensive regions of high CH4 and N2O emissions are mainly attributed to agricultural activities and are found in Belgium, the Netherlands and the southern UK. We further analyzed interannual variability in the GHG budgets. The drought year of 2003 shows the highest net‐emissions of CO2 and of all GHGs combined.
... Mortenson et al. (2020) incorporated a fixed concentration of riverine biogeochemical properties based on observations into their model and presented the effect of sea ice biogeochemical processes on OA. Using an Arctic sea ice-ocean model, Terhaar et al. (2019) conducted numerical experiments, where R-BGC increased by 1% per year until it doubled to quantify the effects of increased riverine inputs of carbon and nutrients on OA, based on annual mean values of R-BGC obtained from global erosion models (Ludwig & Probst, 1998;Mayorga et al., 2010). Consequently, Terhaar, Lauerwald, et al. (2021) first included observation-based monthly mean R-BGC in the same model and revealed the R-BGC effects on Arctic Mortenson et al. (2020) focused on the effect of sea ice biogeochemical processes (not R-BGC) on OA. f Terhaar, Lauerwald, et al. (2021) examined the R-BGC effects on NPP (not OA). ...
... Phytoplankton growth is influenced by nutrient availability, light penetration, and water temperature. In the version of the model we used, river supply of all elements except DIC and alkalinity remains constant across simulations and is obtained from the Global NEWS 2 datasets (Mayorga et al., 2010). For further insights into the model parameterizations, see Aumont et al. (2015). ...
The amount of radiative energy received at the Earth's surface depends on two factors: Earth–Sun distance and sunlight angle. Because of the former, high-eccentricity cycles can induce the appearance of seasons in the tropical ocean. In this paper, we use the Earth system model IPSL-CM5A2 to investigate the response of the low-latitude ocean to variations in Earth's orbit eccentricity. Sea surface temperature (SST) and primary production (PP) were simulated under six precession configurations at high eccentricity and two configurations at low eccentricity, representing extreme configurations observed over the past 1 million years. Results show that high eccentricity leads to increased seasonality in low-latitude mean SST, with an annual thermal amplitude of approximately 2.2 °C (vs. 0.5 °C at low eccentricity). Low-latitude mean PP, which already exhibits inherent seasonality under low-eccentricity conditions, sees its seasonality largely increased under high eccentricity. As a consequence, we show that on long timescales the intensity of SST seasonality exhibits only the eccentricity frequency, whereas that of PP additionally follows precession dynamics. Furthermore, the seasonal variations in both SST and PP at high eccentricities are influenced by the annual placement of the perihelion with its direct impact of radiative energy received in tropical regions. This leads to a gradual and consistent transition of seasons within the calendar. We introduce the concept of “eccentriseasons”, referring to distinct annual thermal differences observed in tropical oceans under high-eccentricity conditions, which shift gradually throughout the calendar year. These findings have implications for understanding low-latitude climate phenomena such as the El Niño–Southern Oscillation (ENSO) and monsoons in the past.
... Mortenson et al. (2020) incorporated a fixed concentration of riverine biogeochemical properties based on observations into their model and presented the effect of sea ice biogeochemical processes on OA. Using an Arctic sea ice-ocean model, Terhaar et al. (2019) conducted numerical experiments, where R-BGC increased by 1% per year until it doubled to quantify the effects of increased riverine inputs of carbon and nutrients on OA, based on annual mean values of R-BGC obtained from global erosion models (Ludwig & Probst, 1998;Mayorga et al., 2010). Consequently, Terhaar, Lauerwald, et al. (2021) first included observation-based monthly mean R-BGC in the same model and revealed the R-BGC effects on Arctic Mortenson et al. (2020) focused on the effect of sea ice biogeochemical processes (not R-BGC) on OA. f Terhaar, Lauerwald, et al. (2021) examined the R-BGC effects on NPP (not OA). ...
Arctic rivers carry not only large amounts of freshwater but also biogeochemical materials into the ocean and play important roles in Arctic ocean acidification (OA). This study quantitatively evaluated the effects of riverine biogeochemical fluxes (R‐BGC; carbon and nutrients) on the Arctic marine carbonate system and OA using multi‐decadal experiments (1979–2018) with a pan‐Arctic sea ice–ocean model. Improved initial and lateral boundary conditions of carbonate properties, observation‐based riverine biogeochemical data, and land model‐based interannually varying riverine freshwater discharge were adopted to enable more realistic experiments. The model simulated negative trends in aragonite saturation state (Ω) and pH in most regions of Arctic Ocean regardless of R‐BGC. The increased riverine freshwater promoted more OA through the higher dilution effect. Compared to the experiment with riverine discharge of only freshwater, the inclusion of R‐BGC caused positive anomalies in Ω and pH (by ∼0.14 and ∼0.03 in central basins, and by ∼0.15 and ∼0.06 in shelf seas, respectively). In the central basins, these anomalies were caused mostly by carbon (total alkalinity and dissolved inorganic carbon) of the R‐BGC. In the shelf seas, nutrient (nitrate and silicate) fluxes also contributed ∼14% and ∼32% of the anomalies owing to the enhanced primary production and a corresponding reduction in seawater pCO2. R‐BGC mitigated OA (ΔΩ = −1.53 × 10⁻³ year⁻¹ and ΔpH = −0.56 × 10⁻³ year⁻¹) in regions where riverine freshwater was accumulated (i.e., the Canada Basin, Chukchi Cap, Eurasian Basin, and East Siberian Sea). This study stressed the importance of including R‐BGC for OA model projection.
... The model has a horizontal resolution of 1 o with 60 vertical depth levels ranging in thickness from 10 m in the upper 150 m to 250 m in the deep ocean (Moore et al., 2013). Riverine nutrient fluxes (N, P, Si, Fe), dissolved inorganic carbon, alkalinity, and DOM fluxes were integrated into the CESM2 ocean model using estimates 165 from GlobalNEWS (Mayorga et al., 2010) and the Integrated Model to Assess the Global Environment-Global Nutrient Model (IMAGE-GNM) (Beusen et al., 2015(Beusen et al., , 2016. Nutrient inputs from rivers encompass dissolved inorganic nitrogen (DIN), phosphorus (DIP), Si, and Fe, along with dissolved organic nitrogen and phosphorus, while carbon inputs included both inorganic and organic forms. ...
The Arctic Ocean experiences significant seasonal to interannual environmental changes, including in temperature, light, sea ice, and surface nutrient concentrations, that influence the dynamics of marine plankton populations. Here, we use a hindcast simulation (1948–2009) of size-structured Arctic Ocean plankton communities, ocean circulation, and biogeochemical cycles in order to better understand how seasonal to interannual changes in the environment influence phytoplankton physiology, plankton community structure, trophic dynamics, and fish production in the Arctic Ocean. The growth of model phytoplankton was primarily limited in winter, spring, and fall by light, but in summer, the growth of smaller and larger phytoplankton was mostly limited by temperature and nutrient availability, respectively. The dominant trophic pathway in summer was from phytoplankton to herbivorous zooplankton, such that the average trophic position of model zooplankton was lower in the summer growing season compared with the rest of the year. On interannual timescales, changes in plankton community composition were strongly tied to interannual changes in bottom-up forcing by the environment. In the summer, in years with lower ice and warmer temperatures, the biomass of phytoplankton and zooplankton was higher, the size abundance relationship slopes were more negative (indicative of a phytoplankton community enriched in smaller phytoplankton), zooplankton had higher mean trophic position (indicative of greater carnivory), and potential fisheries production was greater, fueled by increased mesozooplankton biomass and flux of organic matter to the benthos. The summertime shift toward greater carnivory in warmer and low-ice years was due primarily to changes in phenology, with phytoplankton and microzoopankton blooms occurring approximately one month earlier in these conditions, and carnivorous zooplankton increasing in abundance during summer. The model provides a spatially and temporally complete overview of changes in plankton communities in the Arctic Ocean occurring on seasonal to interannual timescales, and provides insights on the mechanisms underlying these changes as well as their broader biogeochemical and ecosystems significance.
The northern Indian Ocean is a hotspot of nitrous oxide (N2 O) emission to the atmosphere. Yet, the direct link between production and emission of N2 O in this region is still poorly constrained, in particular the relative contributions of denitrification, nitrification and ocean transport to the N2 O efflux. Here, we implemented a mechanistically based N2 O cycling module into a regional ocean model of the Indian Ocean to examine how the biological production and transport of N2 O control the spatial variation of N2 O emissions in the basin. The model captures the upper ocean physical and biogeochemical dynamics of the northern Indian Ocean, including vertical and horizontal N2 O distribution observed in situ and regionally integrated N2 O emissions of 286 ± 152 Gg N yr−1 (annual mean ± seasonal range) in the lower range of the observation‐based reconstruction (391 ± 237 Gg N yr−1 ). N2 O emissions are primarily fueled by nitrification in or right below the surface mixed layer (∼ 57%, including 26% in the mixed layer and 31% right below), followed by denitrification in the oxygen minimum zones (∼ 30%) and N2 O produced elsewhere and transported into the region (∼ 13%). Overall, ∼ 74% of the emitted N2 O is produced in subsurface and transported to the surface in regions of coastal upwelling, winter convection or turbulent mixing. This spatial decoupling between N2 O production and emissions underscores the need to consider not only changes in environmental factors critical to N2 O production (oxygen, primary productivity etc.) but also shifts in ocean circulation that control emissions when evaluating future changes in global oceanic N2 O emissions.
This study presents a new physical-biogeochemical modelling framework for simulating lake methane (CH4) emissions at regional scales. The new model, FLaMe v1.0 (Fluxes of Lake Methane), rests on an innovative, computationally efficient lake clustering approach that enables the simulation of CH4 emissions across a large number of lakes. Building on the Canadian Small Lake Model (CSLM) that simulates the lake physics, we develop a suite of biogeochemical modules to simulate transient dynamics of organic Carbon (C), Oxygen (O2), and CH4 cycling. We first test the performance of FLaMe by analyzing physical and biogeochemical processes in two representative lakes (an oligotrophic, deep lake driven by cold climate versus a trophic, shallow lake driven by warm climate). Next, we evaluate the model by comparing simulated and observed timeseries of CH4 emissions in four well-surveyed lakes. We then apply FLaMe at the European scale to evaluate simulated diffusive and ebullitive lake CH4 fluxes against in-situ measurements in both boreal and central European regions. Finally, we provide a first assessment of the spatio-temporal variability in CH4 emissions from European lakes smaller than 1000 km2 (n=108407, total area = 1.33x105 km2), indicating a total emission of 0.97±0.23 Tg CH4 yr-1, with the uncertainty constrained by combining FLaMe and machine learning techniques. Moreover, 30 % and 70 % of these CH4 emissions are through diffusive and ebullitive pathways, respectively. Annually averaged CH4 emission rates per unit lake area during 2010–2016 have a South-to-North decreasing gradient, resulting in a mean over the European domain as 7.39 g CH4 m-2 yr-1. Our simulations reveal a strong seasonality in European lake CH4 emissions, with late summer emissions nearly ten times higher than winter values. This pronounced seasonal variation highlights the importance of accounting for the sub-annual variability in CH4 emissions to accurately constrain regional CH4 budgets. In the future, FLaMe could be embedded into Earth System Models to investigate the feedback between climate warming and global lake CH4 emissions.
The flux of dissolved organic carbon (DOC) from land to sea is an important transfer within the global carbon cycle. The biogeochemical fate of this terrestrial DOC (tDOC) remains poorly understood and is usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from tropical peatland‐draining rivers discharging onto the Sunda Shelf. We developed a new light‐driven parameterization of tDOC remineralization that accounts for photochemical, microbial, and interactive photochemical–microbial degradation, and simulated the transport and remineralization of tDOC through the Sunda Shelf seas using the regional 3D hydrodynamical HAMSOM and biogeochemical ECOHAM models (only for the carbonate system). Our realistic hindcast simulations for 1958–2022 show that about 50% of riverine tDOC is remineralized before leaving the shelf. This lowers seawater pH across the entire inner Sunda Shelf by an average of 0.005 (by up to 0.05 in the Malacca Strait). Correspondingly, seawater pCO2 is raised, increasing yearly CO2 outgassing from the shelf by 19% (3.1 Tg C yr−1 , 0.14 mol m−2 yr−1 ) during 2013–2022. Even regional ocean acidification trends increase, because river discharge and tDOC flux increase. Our model reveals large spatial variability with greatest inputs and remineralization of tDOC close to major peatlands, especially off Sumatra and Borneo. The interannual variability in tDOC input and the monsoonal current reversal lead to strong temporal variability in carbonate system parameters in these areas. Our results highlight the importance of representing tDOC in ocean models, and reveal the fate of tropical peatland tDOC.
Quantifying the global extent of anthropogenic impacts on freshwater quality remains challenging due to limited monitoring data, especially in low and middle-income regions. To address this gap and improve our understanding of surface water quality, we introduce CoSWAT-WQ, a large-scale water quality model developed to simulate river water quality constituents of Total Nitrogen (TN) and Total Phosphorus (TP), across global and regional freshwater systems. CoSWAT-WQ, an adaptation of the Soil and Water Assessment Tool (SWAT), is run at a global scale, providing high-resolution simulations that capture spatial and daily temporal dynamics of riverine nutrient loads. Here, we describe the model's inputs, setup structure, processes, and evaluate its performance by comparing model outputs to in-situ water quality observations and other global nutrient models. CoSWAT-WQ achieves comparable ranges of river nutrient loads in comparison to other global nutrient models. Additionally, a normalized root mean square error (nRMSE) < 1 was achieved with in-situ observations at more than 80 % of the gauging stations for TN and TP concentrations. However, there was a general weak underestimation of observed concentrations and variability as seen with low Kling–Gupta efficiency (KGE) values for selected stations. Despite its limitations, the model enables the simulation of river TN and TP constituents at a global scale while keeping local relevance. CoSWAT-WQ’s modular setup allows coupling with sectoral models addressing lake systems, agricultural runoff, and aquatic biodiversity, thereby broadening its applicability for cross-sectoral assessments. The model outputs offer valuable data that can inform ecological risk assessments, human health evaluations, and policy decisions on global freshwater quality management.
The Arabian Sea oxygen minimum zone (OMZ) has the smallest horizontal area of all open ocean OMZs but is the world's third most intense OMZ with the largest vertical extent. This study used a regional physical-biogeochemical coupled model, HYbrid Coordinate Ocean Model- ECOSystem (HYCOM-ECOSMO), to investigate the variations in Arabian Sea OMZ and deoxygenation for the period 2000–2020. The model was evaluated against BGC-Argo data and World Ocean Atlas 2018 (WOA18) to check the consistency of eddy-permitting simulation. It accurately simulated dissolved oxygen (DO) profiles, with RMSE of 16.5 µmol kg−1 for WOA18 and 21 µmol kg−1 for BGC-Argo. Model efficiency was estimated at 0.81 with percentage bias of 31% indicating that model performs well with observations. Interannual and seasonal variabilities showed good agreement with BGC-Argo profiling floats, but with slight DO overestimation. Despite a slight decreasing trend in the model's OMZ DO, Argo data indicated a minor increase. Sensitivity experiments identified detritus remineralization and sinking rates as key factors influencing DO levels. Surface DO, temperature, and Brunt-Väisälä frequency showed spatial warming impacts. OMZ exhibited seasonal variation, with higher DO concentrations during the winter monsoon due to convective mixing. ENSO and Indian Ocean Dipole (IOD) phases minimally influence surface DO levels, with lagged impacts (1–10 months) on temperature, productivity, organic matter sinking, and microbial respiration. The system responds more promptly to IOD than to ENSO. These findings establish a baseline for future research on marine ecosystems, fishery and regional climate projections in the Arabian Sea.
A Bay of Biscay model configuration is used as a test case to assess the data-based consistency of ensemble-based ocean model uncertainties of several types: [A] built-in stochastic parameterizations at regional ocean scales, [B] ocean model response to a global atmospheric model ensemble and [C] both A and B simultaneously. Ensembles of varying length were generated. In addition to a seasonal-range ensemble, three medium-range ensembles were carried out over successive overlapping segments permitting to compare consistency metrics for different lead times. The largest spread was obtained for the C case, although most of the model uncertainties were attributable to the stochastic ocean parameterizations in A. We addressed the question of which ensemble type and lead time was able to provide the most realistic model uncertainties given observations of SST, sea level, and Chlorophyll a, using a theoretical and diagnostic consistency analysis framework expanded from Vervatis et al. (2021a). In our results, consistency was satisfactory for the stochastic ensembles of types A and C, for the “aged” error cases (but only marginally with respect to the “young” error cases), and whenever physical and biogeochemical uncertainty processes were active in the region and could be detected by the observational networks, such as the onset of the spring shoaling of the thermocline and the phytoplankton abundance primary bloom. Sea level empirical consistency was improved when a wide range of low- to high-frequency errors were included in the signal of dynamic atmospheric process in the data and in the model inverse barometer. These findings provide additional insight that can help configure ensemble-based methods in academic studies and in operational ocean forecasting systems.
At the interface between the continental and oceanic domains, estuaries are essential components of the land–ocean aquatic continuum. These coastal ecosystems play a significant role in biogeochemical cycles, as they transform and export large amounts of terrigenous carbon and nutrients from rivers to marine waters. Because of this intense biogeochemical processing, they are significant ecosystems in terms of greenhouse gas exchange with the atmosphere. However, in spite of recent advances in remote sensing and the need for accurate estimates to calculate regional and global estuarine budgets, the global quantification of the estuarine spatial extent available for gas exchange has not been updated in over a decade and remains poorly constrained. This is due to the lack of a global extensive database, the diversity of estuaries, and the controversial definition of their boundaries. To address these challenges, a hybrid approach was developed that combines the surface areas of over 700 estuaries worldwide (extracted from the literature or calculated using geographical information systems) with a novel extrapolation method to provide type-specific regional estimates for 45 regions. The three estuarine types considered are ‘tidal systems and deltas’, ‘lagoons’, and ‘fjords’. The upscaling formula applied is determined and calibrated using data from several regions where an extensive survey of total estuarine surface areas was available. The new global estimate of 733,801 ± 39,892 km2 (mean ± 2 σ) is 31% lower than the previous global assessment. It also provides quantitative uncertainty estimates for regional and global estuarine surface areas as well as a breakdown between tidal systems and deltas (294,956 ± 30,780 km2), lagoons (179,946 ± 12,056 km2), and fjords (259,899 ± 22,328 km2). This decrease of the global estuarine surface area is related to the novel method used in this study and does not reflect a temporal trend.
We simulated the input, physical transport, and biogeochemical remineralization of terrestrial dissolved organic carbon in the Sunda Shelf of Southeast Asia, using a 3-D physical and biogeochemical model system (HAMSOM and ECOHAM)
Land and ocean ecosystems are strongly connected and mutually interactive. As climate changes and other anthropogenic stressors intensify, the complex pathways that link these systems will strengthen or weaken in ways that are currently beyond reliable prediction. In this review we offer a framework of land–ocean couplings and their role in shaping marine ecosystems in coastal temperate rainforest (CTR) ecoregions, where high freshwater and materials flux result in particularly strong land–ocean connections. Using the largest contiguous expanse of CTR on Earth—the Northeast Pacific CTR (NPCTR)—as a case study, we integrate current understanding of the spatial and temporal scales of interacting processes across the land–ocean continuum, and examine how these processes structure and are defining features of marine ecosystems from nearshore to offshore domains. We look ahead to the potential effects of climate and other anthropogenic changes on the coupled land–ocean meta‐ecosystem. Finally, we review key data gaps and provide research recommendations for an integrated, transdisciplinary approach with the intent to guide future evaluations of and management recommendations for ongoing impacts to marine ecosystems of the NPCTR and other CTRs globally. In the light of extreme events including heatwaves, fire, and flooding, which are occurring almost annually, this integrative agenda is not only necessary but urgent.
Since their advent over 2 decades ago, autonomous Argo floats have revolutionized the field of oceanography, and, more recently, the addition of biogeochemical and biological sensors to these floats has greatly improved our understanding of carbon, nutrient, and oxygen cycling in the ocean. While Argo floats offer unprecedented horizontal, vertical, and temporal coverage of the global ocean, uncertainties remain about whether Argo sampling frequency and density capture the true spatiotemporal variability in physical, biogeochemical, and biological properties. As the true distributions of, e.g., temperature or oxygen are unknown, these uncertainties remain difficult to address with Argo floats alone. Numerical models with synthetic observing systems offer one potential avenue to address these uncertainties. Here, we implement synthetic biogeochemical Argo floats into the Energy Exascale Earth System Model version 2 (E3SMv2), which build on the Lagrangian In Situ Global High-Performance Particle Tracking (LIGHT) module in E3SMv2 (E3SMv2-LIGHT-bgcArgo-1.0). Since the synthetic floats sample the model fields at model run time, the end user defines the sampling protocol ahead of any model simulation, including the number and distribution of synthetic floats to be deployed, their sampling frequency, and the prognostic or diagnostic model fields to be sampled. Using a 6-year proof-of-concept simulation, we illustrate the utility of the synthetic floats in different case studies. In particular, we quantify the impact of (i) sampling density on the float-derived detection of deep-ocean change in temperature or oxygen and on float-derived estimates of phytoplankton phenology, (ii) sampling frequency and sea-ice cover on float trajectory lengths and hence float-derived estimates of current velocities, and (iii) short-term variability in ecosystem stressors on estimates of their seasonal variability.
Dissolved organic carbon (DOC) is a key component of coastal biogeochemical cycles, but its composition and reactivity depend on the relative contribution of autochthonous aquatic versus allochthonous terrigenous DOC (tDOC). In complex coastal waters, tDOC is commonly quantified using the bulk DOC stable carbon isotope ratio (δ13CDOC). However, several limitations can hamper the use of δ13CDOC in marine ecosystems, such as (1) the narrow and often overlapping separation of the autochthonous and allochthonous endmembers, and (2) mineralization of tDOC to dissolved inorganic carbon creates a reservoir effect such that autochthonous DOC can acquire a terrigenous-like δ13CDOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ2Hn) has emerged as a new tool that can potentially overcome these limitations: (1) δ2Hn has a large separation between aquatic and terrigenous endmembers (>50‰) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of δ2Hn obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ13CDOC and chromophoric DOM (CDOM) optical properties. We collected samples at a site in Southeast Asia’s Sunda Shelf that experiences substantial seasonal variation in tDOC input, driven primarily by the monsoon-induced physical advection of peatland-derived tDOC. Over a 1-year monthly time series, the terrigenous fraction of DOC (fterr) determined using δ2Hn of SPE-DOM and δ13CDOC of bulk DOC was well correlated (r2 = 0.42), and there was no significant difference in fterr between the two isotope systems. In fact, δ2Hn displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ13CDOC. Our results indicate that δ2Hn of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ13CDOC, and is a viable alternative in settings where δ13CDOC is inadequate.
We developed a simple model that related NO3 export to point-source N loading and nonpoint source N loads from chemical fertilizers and NO(y) deposition and tested it at the global scale using data from 35 large rivers with a global distribution. The model explained well (r2 > 0.8) the nearly 1000-fold variation in NO3 export from different regions of the world. The model suggests that human activity is the dominant control of NO3 export even though less than 20 of the 100 Tg N yr-1 added to land in fertilizer and NO(y) deposition is at present exported from rivers as NO3. Watershed export to rivers may increase in the future due to either increased loads to the watershed or decreased watershed retention. Simple models, coupled with continued measurements of NO3 in rivers, will be of use in interpreting these regional changes.
Global nitrogen (N) budgets for intensive agricultural systems were compiled for a 0.5 by 0.5 degree resolution. These budgets include N inputs (N fertilizer, animal manure, biological N fixation and atmospheric N deposition) and outputs (N removal from the field in harvested crops and grass and grass consumption by grazing animals, ammonia volatilization, denitrification and leaching). Data for the historical years 1970 and 1995 and a projection for 2030 were used to study changes in the recovery of N and the different loss terms for intensive agricultural systems. The results indicate that the overall system N recovery and fertilizer use efficiency slowly increased in the industrialized countries between 1970 and 1995, the values for developing countries have decreased in the same period. For the coming three decades our results indicate a rapid increase in both the industrialized and developing countries. High values of > 80% for fertilizer use efficiency may be related to surface N balance deficits, implying a depletion of soil N and loss of soil fertility. The projected intensification in most developing countries will cause a gradual shift from deficits to surpluses in the coming decades. The projected fast growth of crop and livestock production, and intensification and associated increase in fertilizer inputs will cause a major increase in the surface N balance surplus in the coming three decades. This implies increasing losses of N compounds to air (ammonia, nitrous oxide and nitric oxide), and groundwater and surface water (nitrate).
Renewable fresh water comprises a tiny fraction of the global water pool but is the foundation for life in terrestrial and freshwater ecosystems. The benefits to humans of renewable fresh water include water for drinking, irrigation, and industrial uses, for production of fish and waterfowl, and for such instream uses as recreation, transportation, and waste disposal. In the coming century, climate change and a growing imbalance among freshwater supply, consumption, and population will alter the water cycle dramatically. Many regions of the world are already limited by the amount and quality of available water. In the next 30 yr alone, accessible runoff is unlikely to increase more than 10%, but the earth's population is projected to rise by approximately one-third. Unless the efficiency of water use rises, this imbalance will reduce freshwater ecosystem services, increase the number of aquatic species facing extinction, and further fragment wetlands, rivers, deltas, and estuaries. Based on the scientific evidence currently available, we conclude that: (1) over half of accessible freshwater runoff globally is already appropriated for human use; (2) more than 1 x 10(9) people currently lack access to clean drinking water and almost 3 x 10(9) people lack basic sanitation services; (3) because the human population will grow faster than increases in the amount of accessible fresh water, per capita availability of fresh water will decrease in the coming century; (4) climate change will cause a general intensification of the earth's hydrological cycle in the next 100 yr, with generally increased precipitation, evapotranspiration, and occurrence of storms, and significant changes in biogeochemical processes influencing water quality; (5) at least 90% of total water discharge from U.S. rivers is strongly affected by channel fragmentation from dams, reservoirs, interbasin diversions, and irrigation; and (6) globally, 20% of freshwater fish species are threatened or extinct, and freshwater species make up 47% of all animals federally endangered in the United States. The growing demands on freshwater resources create an urgent need to link research with improved water management. Better monitoring, assessment, and forecasting of water resources will help to allocate water more efficiently among competing needs, Currently in the United States, at least six federal departments and 20 agencies share responsibilities for various aspects of the hydrologic cycle. Coordination by a single panel with members drawn from each department, or by a central agency, would acknowledge the diverse pressures on freshwater systems and could lead to the development of a well-coordinated national plan.
Yields of total fixed nitrogen and nitrogen fractions are summarized for thirty-one watersheds in which anthropogenic disturbance of the nitrogen cycle, either through land use or atmospheric deposition, is negligible or slight. These yields are taken as representative of background conditions over a broad range of watershed areas, elevations, and vegetation types. The data set focuses on watersheds of the American tropics, but also includes information on the Gambia River (Africa) and some small watersheds in the Sierra Nevada of California. For the tropical watersheds, total nitrogen yield averages 5.1 kg ha ^−1 y^−1. On average, 30% of the total is particulate and 70% is dissolved. Of the dissolved fraction, an average of 50% is organic and 50% is inorganic, of which 20% is ammonium and 80% is nitrate. Yields are substantially lower than previously estimated for background conditions. Yields of all nitrogen fractions are strongly related to runoff, which also explains a large percentage of variance in yield of total nitrogen (r^2 = 0.85). For total nitrogen and nitrogen fractions, yield increases at about two-thirds the rate of runoff; concentration decreases as runoff increases. There is a secondary but significant positive relationship between elevation and yield of DIN. Ratios DON/TDN and PN/TN both are related to watershed area rather than runoff; DON/TDN decreases and PN/TN increases toward higher stream orders. The analysis suggests for tropical watersheds the existence of mechanisms promoting strong homeostasis in the yield of N and its fractions for a given moisture regime, as well as predictable downstream change in proportionate representation N fractions. Yields and concentrations for small tropical watersheds are much larger than for the few temperate ones with which comparisons are possible.
The uneven distribution of biological nitrogen fixation in terrestrial ecosystems has yet to be explained. Latitudinal gradients in temperature and phosphorus may hold the answer.
We present a global model that describes the fate of nitrogen (N) from point and nonpoint sources in the hydrological system up to the river mouths at the 0.5° by 0.5° spatial and annual temporal resolution. Estimates for point sources are based on population densities, per capita human N emissions, and data on sanitation coverage and wastewater treatment. For nonpoint sources, we use spatial information on land use, climate, hydrology, geology, and soils, combined with data on N inputs (fertilizers and animal manure, biological N fixation, and atmospheric deposition), and outputs (N removal in harvested agricultural products, ammonia emissions). Denitrification in the root zone and nitrate leaching to groundwater are calculated with a model that combines the effect of temperature, crop type, soil properties, and hydrological conditions. The nitrate concentration of the outflow for shallow and deep groundwater layers is based on historical inputs of fertilizer N and the effects of residence time and denitrification. In-stream N retention is based on a global estimate of 30% of the N discharged to surface water. Calculated and reported total N concentrations of discharge near the river outlet agree fairly well. However, our model systematically overestimates total N concentrations for river basins with mean annual temperature >0°C.
This paper demonstrates the potential of combining observed river discharge information with climate-driven water balance model (WBM) outputs to develop composite runoff fields. Such combined runoff fields simultaneously reflect the numerical accuracy of the discharge measurements and preserve the spatial and temporal distribution of simulated runoff. Selected gauging stations from the World Meteorological Organization Global Runoff Data Centre (GRDC) data archive were geographically coregistered to a gridded simulated topological network at 30' (longitude × latitude) spatial resolution (STN-30p). Interstation regions between gauging stations along the STN-30p network were identified, and annual interstation runoff was calculated. The annual interstation runoff was compared with outputs from WBM calculations, which were performed using long-term mean monthly climate forcings (air temperature and precipitation). The simulated runoff for each cell was multiplied by the ratio of observed to simulated runoff of the corresponding interstation region from the GRDC data set to create spatially distributed runoff fields at 30' resolution. The resulting composite runoff fields (UNH/GRDC Composite Runoff Fields V1.0) are released to the scientific community along with intermediate data sets, such as station attributes and long-term monthly regimes of the selected gauging stations, the simulated topological network (STN-30p), STN-30p derived attributes for the selected stations, and gridded fields of the interstation regions along STN-30p. These data sets represent high-resolution fields that are of value to a broad range of water-related research, including terrestrial modeling, climate-atmosphere interactions, and global water resource assessments.
The Amazon Basin is the world's largest tropical forest region and one where rapid human changes to land cover have the potential to cause significant changes to hydrological and biogeochemical processes. The Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multidisciplinary, multinational research program led by Brazil. The goal of LBA is to understand how the Amazon Basin functions as a regional entity in the earth system and how these functions are changing as a result of ongoing human activity. This compilation of nine papers focuses on a central LBA question in the area of nutrient dynamics and surface water chemistry - how do changes in land use alter fluxes of dissolved and particulate materials from uplands across riparian zones and down the channels of river corridors? These papers cover work conducted in small watersheds on a wide range of topics within the spirit and geographical focus area of LBA: water balance and runoff generation, nutrient transformations in riparian zones and stream channels, carbon fluxes in water moving from land to water and the influence of soils on flowpath structure and stream chemistry. Important new insights can be gained from these and other studies. Forest clearing for pastures results in a decrease in soil hydraulic conductivity that forces water into surficial flowpaths throughout most of the rainy season across wide regions of the Amazon. Riparian zones along small forest streams appear to be very effective in removing nitrate arriving from the uplands, while forest streams take up nitrate at very low rates, allowing them to travel downstream for long distances. Although substantial, the contribution of dissolved organic C (DOC) to the carbon flux from forests to streams appears to be lower than the flux of dissolved inorganic C that is subsequently outgassed as CO2. Remaining key challenges within LBA will be to synthesize existing data sets on river networks, soils, climate, land use and planned infrastructure for the Amazon to develop models capable of predicting hydrologic and biogeochemical fluxes at a variety of scales relevant to the development of strategies for sustainable management of the Amazon's remarkable forest, soil and freshwater resources.
This study was carried out to support and enhance a series of global studies assessing contemporary and future changes in nutrient export from watersheds (Global Nutrient Export from Watersheds (NEWS)). Because hydrography is one of the most important drivers in nutrient transport, it was essential to establish how climatic changes and direct human activities (primarily irrigation and reservoir operations) affect the hydrological cycle. Contemporary and future hydrography was established by applying a modified version of a global water balance and transport model (WBMplus) driven by present and future climate forcing, as described in the Millennium Ecosystem Assessment scenarios (1970–2050). WBMplus represents a major upgrade to previous WBM implementations by incorporating irrigational water uptake and reservoir operations in a single modeling framework. Contemporary simulations were carried out by using both observed climate forcings from the Climate Research Unit of East Anglia (CRU) data sets and from Global Circulation Model (GCM) simulations that are comparable to the future simulations from the same GCM forcings. Future trends in three key human activities (land use, irrigation, and reservoirs operation for hydropower) were taken from the Integrated Model to Assess the Global Environment (IMAGE). The reservoir operation required establishing a realistic distribution of future reservoirs since the IMAGE model provided only the hydropower potentials for the different future scenarios.
The Millennium Ecosystem Assessment scenarios for 2000 to 2050 describe contrasting future developments in agricultural land use under changing climate. Differences are related to the total crop and livestock production and the efficiency of nutrient use in agriculture. The scenarios with a reactive approach to environmental problems show increases in agricultural N and P soil balances in all developing countries. In the scenarios with a proactive attitude, N balances decrease and P balances show no change or a slight increase. In Europe and North America, the N balance will decline in all scenarios, most strongly in the environment-oriented scenarios; the P balance declines (proactive) or increases slowly (reactive approach). Even with rapidly increasing agricultural efficiency, the global N balance, ammonia, leaching and denitrification loss will not decrease from their current levels even in the most optimistic scenario. Soil P depletion seems to be a major problem in large parts of the global grassland area.
Here we describe, test, and apply a spatially explicit, global model of river-borne dissolved inorganic phosphorus (DIP) export called NEWS-DIP. Among the innovations in NEWS-DIP are increased spatial resolution (0.5 × 0.5°), explicit treatment of sewage, fertilizer, manure, and weathering P sources, and inclusion of reservoir retention and consumptive water use terms. The NEWS-DIP model performed better than pre-existing global models in predicting DIP yield for both calibration and validation basins (r2 = 0.72 and 0.56, respectively). NEWS-DIP predicts that of the 34 Tg of P yr−1 loaded on watersheds by human activity globally, approximately 3% reaches river mouths as DIP; anthropogenic sources account for 65% (0.71 Tg yr−1) of the DIP exported to the coastal zone, with the remainder (0.38 Tg yr−1) attributable to natural weathering processes; DIP yields range over 5 orders of magnitude, from less than 0.01 to 1153 kg P km−2 yr−1 with highest predicted DIP yields clustering in East Asia, Europe, and Indonesia; human sewage is the largest anthropogenic source of DIP to the coastal zone on all continents and to all ocean basins. NEWS-DIP also suggests that despite regional variability, at the global scale, non-point sources of DIP such as inorganic P fertilizer and manure are much less important in determining coastal export of DIP than point sources and natural weathering processes.
Climate change is one of a number of human forcings that affect river export of nutrients from watersheds. Past trends (1970-2000) and four future (2030 and 2050) scenarios of N, P, C and Si sources and controlling factors in watersheds globally were analyzed using the NEWS model. Hydrology is an important factor controlling river export of nutrients. Changes in river nutrient export between 2000 and 2030 due to future changes in hydrology related to climate change were compared to changes due to a scenario that integrated changes in a range of social-economic factors. These included changes in food and energy production, nutrient management practices, and GDP, among other factors. The four future scenarios were developed using the Millennium Ecosystem Assessment (MA) scenarios as a basis. The Global Orchestration scenario from the MA generally exhibited the largest human impacts on climate as well as social-economic development. At the global scale by 2030, climate change alone was projected to slightly decrease annual dissolved inorganic N export (0.2 Tg N), compared to a global increase of 3.5 Tg N using the integrated assessment approach. However, there was considerable regional variability. For example, climate accounted for much of the decrease in DIN export projected in parts of Europe and the eastern USA. In other regions such as China, climate accounted for only a small portion of the large increase in projected DIN export. Climate had a larger effect on dissolved organic N, P and C export than dissolved inorganic N and P. The effects of climate forcing on land-ocean interactions should be studied within the context of multiple human forcings.
The various forms (dissolved and particulate, organic and inorganic) of carbon, nitrogen, and phosphorus in world rivers are reviewed from literature data. Natural levels are based mainly on major rivers for the subarctic and tropical zones which are still unpolluted and on smaller streams for the temperate zone. Atmospheric fallout is also reviewed. Natural contents of dissolved organic carbon (DOC) are mainly dependent on environmental conditions: DOC varies from 1 mg 1â»Â¹ in the mountainous alpine environments to 20 mg 1â»Â¹ in some taiga rivers. The world DOC average is 5.75 mg lâ»Â¹. Nitrogen forms include dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN = N - NHâ/sup +/ + N - NOââ» + N - NOââ»), and particulate organic nitrogen (PON). Natural levels are very low: DIN = 120 ..mu..g 1â»Â¹ of which only 15 percent is present as ammonia, and 1 percent as nitrite. Phosphorus is naturally present in very low amounts: around 10 ..mu..g 1â»Â¹ for P-POâ³ and 25 ..mu..g 1â»Â¹ for total dissolved phosphorus (TDP which includes the organic form). The average nutrient content of rains has been estimated with a set of unpolluted stations: P - POâ = 5 ..mu..g 1â»Â¹, TDP = 10, N - NOâ = 5, N - NHâ = 225, DON = 225, and N - NOâ = 175 ..mu..g 1â»Â¹. TOC levels are probably around several mg 1â»Â¹. These contents are very similar to those found in unpolluted rivers. Man's influence on surface waters has now greatly increased natural nutrient levels. Total dissolved P and N have globally increased by a factor of two and locally (Western Europe, North America) by factors of 10 to 50. These increases were found to be directly proportional to the watershed population and to its energy consumption.
The evolution of river systems and their related fluxes is considered at various time scales: (i) over the last 18 000 years, under climatic variability control, (ii) over the last 50 to 200 years (Anthropocene) due to direct human impacts. Natural Holocene variations in time and space depend on (i) land-to-ocean connections (endorheism, glacial cover, exposure of continental shelf); (ii) types of natural fluvial filters (e.g., wetlands, lakes, floodplains, estuaries). Anthropocene changes concern (i) land–ocean connection (e.g., partial to total runoff reduction resulting from water management), (ii) modification and removal of natural filters, (iii) creation of new filters, particularly irrigated fields and reservoirs, (iv) acceleration and/or development of material sources from human activities. The total river basin area directly affected by human activities is of the same order of magnitude (>40 Mkm2) as the total area affected over the last 18 000 years. A tentative analysis of 38 major river systems totaling 55 Mkm2 is proposed for several criteria: (i) trajectories of Holocene evolution, (ii) occurrence of natural fluvial filters, (iii) present-day fluvial filters: most river basins are unique. Riverine fluxes per unit area are characterized by hot spots that exceed the world average by one order of magnitude. At the Anthropocene (i.e., since 1950), many riverine fluxes have globally increased (sodium, chloride, sulfate, nitrogen, phosphorous, heavy metals), others are stable (calcium, bicarbonate, sediments) or likely to decrease (dissolved silica). Future trajectories of river fluxes will depend on the balance between increased sources of material (e.g., soil erosion, pollution, fertilization), water abstraction for irrigation and the modification of fluvial filters, particularly the occurrence of reservoirs that already intercept half of the water and store at least 30% of river sediment fluxes. In some river systems, retention actually exceeds material production and river fluxes are actually decreasing. These trajectories are specific to each river and to each type of river material. Megacities, mining and industrial districts can be considered as hot spots of contaminants fluxes, while major reservoirs are global-scale sinks for all particulates. Global picture should therefore be determined at a fine resolution, since regional differences in Anthropocene evolution of river fluxes may reach one order of magnitude, as illustrated for total nitrogen. To cite this article: M. Meybeck, C. Vörösmarty, C. R. Geoscience 337 (2005).
Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome- and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our ``best estimate'' of potential nitrogen fixation by natural ecosystem is ~195 TgNyr-1, with a range of 100-290 TgNyr-1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.
This paper presents a multiple linear regression model developed for describing global river export of sediments (suspended solids, TSS) to coastal seas, and approaches for estimating organic carbon, nitrogen, and phosphorous transported as particulate matter (POC, PN, and PP) associated with sediments. The model, with river-basin spatial scale and a 1-year temporal scale, is based on five factors with a significant influence on TSS yields (the extent of marginal grassland and wetland rice, Fournier precipitation, Fournier slope, and lithology), and accounts for sediment trapping in reservoirs. The model generates predictions within a factor of 4 for 80% of the 124 rivers in the data set. It is a robust model which was cross-validated by using training and validation sets of data, and validated against independent data. In addition, Monte Carlo simulations were used to deal with uncertainties in the model coefficients for the five model factors. The global river export of TSS calculated thus is 19 Pg yr−1 with a 95% confidence interval of 11–27 Pg yr−1 when accounting for sediment trapping in regulated rivers. Associated POC, PN, and PP export is 197 Tg yr−1 (as C), 30 Tg yr−1 (N), and 9 Tg yr−1 (P), respectively. The global sediment trapping included in these estimates is 13%. Most particulate nutrients are transported by rivers to the Pacific (∼37% of global particulate nutrient export), Atlantic (28–29%), and Indian (∼20%) oceans, and the major source regions are Asia (∼50% of global particulate nutrient export), South America (∼20%), and Africa (12%).
Here we present the COSCATs global database of 151 catchments in exorheic areas. The catchments connect to oceans through coastal segments according to three sets of criteria: natural limits (continents, oceans, regional seas, major capes, and bays), continental shelf topography (sills, basins, island chains), and geophysical dynamics (climate, ocean currents and tectonics). The COSCATs segmentation scheme is designed to improve Earth System analysis and to harmonize reporting of global riverine transfers from land to oceans. Each COSCAT is characterized by its coastal segment limits and length (median 2 400 km), by its catchment characteristics, including area (median 0.45 M km2), width, latitudinal range, runoff average value and direction, including its related physiographic units (n = 500). We apply the COSCAT segmentation to all 151 basins to estimate water discharge and total nitrogen impacts to oceans and find that the average runoff (mm/yr) and N yields (YN in kg km-2 yr-1) range over more than 3 orders of magnitude at this coarse resolution, and that their average population density ranges over 2 orders of magnitude. Hyperactive regions, defined as segments with 5 to 10 times the world average yield (river transfers per unit area of land), are differentially placed for water runoff and total contemporary nitrogen. COSCATs have been designed to facilitate the budget reporting and the analysis of global scale heterogeneity for riverine fluxes and can be applied to other material, such as suspended solids, carbon species or other nutrients, particularly for areas draining into regional seas.
Over the past three decades, tropical forest clearing and burning have greatly altered the Amazonian landscape by increasing the cover of pastures and secondary forests. The alteration of biogeochemical processes on these lands is of particular interest on highly weathered Oxisols that cover large areas in the region because of concerns regarding possible nutrient limitation in agricultural land uses and during forest regrowth. The objectives of this study were to quantify (1) the reaccumulation of nutrients in biomass of secondary land uses, (2) changes in soil nutrient contents, (3) internal nutrient cycles, and (4) input–output budgets for the landscape mosaic.
Nutrient stocks and fluxes were quantified from 1996 to 1998 in mature forest, 19‐yr‐old secondary forest, degraded pastureland, and managed pastureland in the Brazilian state of Pará. Mature forests contain 130 Mg C/ha in aboveground biomass while secondary forest, degraded pasture, and managed pasture contain 34, 4, and 3 Mg C/ha, respectively. Reaccumulation of N, P, K, Ca, and Mg in aboveground biomass of secondary forest was 20%, 21%, 42%, 50%, and 27% of that present in mature forest, while degraded pasture contained 2%, 4%, 15%, 11%, and 6%. Managed pasture had similar accumulations as degraded pasture except for Ca (3%).
Changes in soil stocks of C, N, and P were not detected among land uses, except in fertilized managed pastures, where total soil P (0–10 cm) was elevated. Conversely, Mehlich‐III‐extractable P of all secondary lands were very low (<1 μg/g) and were 1 kg/ha less than contents (0–10 cm) in mature forest. NaOH‐extractable P was present in 100‐fold higher concentrations and may gradually contribute to meeting plant demands over decadal time scales. Soil cation contents (0–20 cm) were elevated in secondary lands with increases of ∼85, 500, and 75 kg/ha for K, Ca, and Mg, respectively. These increases could account for a substantial portion of cation contents originally in the aboveground biomass of mature forest.
The recycling of nutrients through ∼9.0 Mg·ha ⁻¹ ·yr ⁻¹ of litterfall in secondary forest of 132, 2.8, 32, 106, and 23 kg·ha ⁻¹ ·yr ⁻¹ for N, P, K, Ca, and Mg, respectively, is similar to mature forest. Nutrient returns in both pasturelands were smaller for all elements except K, which was similar to the forested sites. In these pasture ecosystems, grass turnover has replaced litterfall return as the predominate mechanism of nutrient recycling.
Soil solution fluxes of total N were higher in mature forest (12 kg·ha ⁻¹ ·yr ⁻¹ at 25 cm depth) compared to secondary lands (<4 kg·ha ⁻¹ ·yr ⁻¹ ), indicating that cycling of available forms of N has diminished. Conversely, fluxes of cationic elements appear elevated in secondary lands and are charge balanced in solution by HCO 3 ⁻ derived from biological activity in the soil surface. Despite detectable increases in soil cation fluxes, rainwater inputs and stream water outputs of these elements across the watershed were not significantly different.
The aggregate picture for this landscape is one in which the secondary forest, although still of smaller stature and lower in species diversity compared to mature forest, is recuperating important nutrient cycling functions. Conversely, pasturelands, which dominate the landscape, are not only of smaller stature, but are also accumulating and cycling a smaller total mass of nutrients. This ecosystem conversion has released C and N from biomass mostly to the atmosphere and has redistributed K, Ca, and Mg from biomass mostly to the soil. Presently, base cation enriched soils are slowly re‐equilibrating to an acidic condition through decadal‐scale processes of plant uptake and biogenically driven soil leaching. Our mass balance approach has revealed low soil available N and P, diminished rates of cycling of these elements in secondary lands, and low precipitation inputs of P, which may constrain long‐term recuperation of ecosystem carbon.
Enormous meandering rivers are the most remarkable fluvial feature of the Amazon landscape, but these rivers are only the largest component of a much denser network of streams which finely dissects and drains the basin. In terms of combined length and total amount of lotic habitat, streams dominate over their more visible downstream counterparts; this dominance is especially dramatic for first- and second- order streams which alone may account for greater than 80% of total channel length in meso-scale Amazon drainag basins (Table 12.1). The flow of Amazon streams emerges directly from the extensive forests and savannas that compose the basin. Bio- geochemical cycles in streams are thus intricately associated with processes operat- ing in adjoining riparian and upland ecosys- tems. Terrestrial processes regulate the input of organic and inorganic species to stream systems, and the chemistry of inflowing
A regression model (RivR-N) was developed that predicts the proportion of N removed from streams and reservoirs as an inverse function of the water displacement time of the water body (ratio of water body depth to water time of travel). When applied to 16 drainage networks in the eastern U.S., the RivR-N model predicted that 37% to 76% of N input to these rivers is removed during transport through the river networks. Approximately half of that is removed in 1st through 4th order streams which account for 90% of the total stream length. The other half is removed in 5th order and higher rivers which account for only about 10% of the total stream length. Most N removed in these higher orders is predicted to originate from watershed loading to small and intermediate sized streams. The proportion of N removed from all streams in the watersheds (37-76%) is considerably higher than the proportion of N input to an individual reach that is removed in that reach (generally 20%) because of the cumulative effect of continued nitrogen removal along the entire flow path in downstream reaches. This generally has not been recognized in previous studies, but is critical to an evaluation of the total amount of N removed within a river network. At the river network scale, reservoirs were predicted to have a minimal effect on N removal. A fairly modest decrease (10 percentage points) in the N removed at the river network scale was predicted when a third of the direct watershed loading was to the two highest orders compared to a uniform loading.
Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSI) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSI. The resulting changes in the nutrient composition (DSI:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSI concentrations and the DSI:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction of dams for flow regulation.
Most modern estimates of dissolved nitrogen and phosphorus delivery to the ocean use Meybeck's estimates from approximately 30 large rivers. We have derived an extended database of approximately 165 sites with nutrient loads. For both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the logarithmic yields (log [load/area]) can be parameterized as functions of log (population density) and log (runoff/area) (R2 for DIN and DIP approximately 0.6). Landscape production of DIN and DIP is largely assimilated. Even though DIN and DIP follow substantially different biogeochemical cycles, loading for DIN and DIP is tightly coupled (R2 for log DIN versus log DIP approximately 0.8), with a constant loading ratio of about 18:1. Estimates of DIN and DIP fluxes are distributed globally around the world coastlines by using basin population density and runoff at 0.5° increments of latitude and longitude. We estimate that total loads for the 1990s are about three times Meybeck's estimates for the 1970s.
1] Here we describe, test, and apply a system of spatially explicit, global models for predicting river export of three dissolved organic matter (DOM) components: dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). The DON and DOP models represent the first attempt to model DON and DOP export in a spatially explicit, global manner. DOC, DON, and DOP models explain 88%, 77%, and 91% of the variability in DOC, DON, and DOP yield (kg C, N, or P km À2 yr À1) from validation basins, respectively, and all models are relatively bias free. When applied globally, these models predict that 170 Tg C yr À1 , 10 Tg N yr À1 , and 0.6 Tg P yr À1 are exported by rivers to the coastal zone as DOC, DON, and DOP, respectively. Because predicted spatial patterns of export for DOC, DON, and DOP are all largely driven by water runoff, geographic distributions of high and low fluxes are fairly consistent across elements, with high fluxes of DOC, DON, and DOP generally predicted for high runoff systems and low fluxes predicted for arid systems. However, there are important regional differences in predicted rates of DOC, DON, and DOP export due to anthropogenic inputs of DON and DOP and wetland influence on DOC. Citation: Harrison, J. A., N. Caraco, and S. P. Seitzinger (2005), Global patterns and sources of dissolved organic matter export to the coastal zone: Results from a spatially explicit, global model, Global Biogeochem. Cycles, 19, GB4S04, doi:10.1029/2005GB002480.
2006. [1] We present an overview of riverine nitrogen flux calculations that were prepared for the International Nitrogen Initiative's current global assessment of nitrogen cycles: past, present, and future (Galloway et al., 2004). We quantified anthropogenic and natural inputs of reactive nitrogen (N) to terrestrial landscapes and the associated riverine N fluxes. Anthropogenic inputs include fossil-fuel derived atmospheric deposition, fixation in cultivated croplands, fertilizer use, and the net import in human food and animal feedstuffs. Natural inputs include natural biological N fixation in forests and other noncultivated vegetated lands, and fixation by lightning. We use an empirical model relating total N inputs per landscape area to the total flux of N discharged in rivers based on watershed data from contrasting ecosystems spanning multiple spatial scales. With this approach, we simulate riverine N loads to the coastal zone and to inland waters from the continents. Globally, rivers exported about 59 Tg N yr À1 , with 11 Tg N yr À1 transported to dry lands and inland receiving waters, and 48 Tg N yr À1 transported to the coastal zone. Rates of riverine N loss vary greatly among the continents, reflecting the regional differences in population and the associated anthropogenic N inputs. We compare our estimates to other approaches that have been reported in the literature. Our work provides an understanding of the sources of N to landscapes and the associated N fluxes in rivers, and highlights how anthropogenic activities impact N cycling around the world.
This paper reviews knowledge of nitrogen and phosphorus generation from land use and export to waterways, including studies relevant to Australia. It provides a link between current and future modelling requirements, and the context for incorporation of this knowledge into catchment models for use by catchment managers. Selected catchment models used by catchment managers are reviewed, and factors limiting their application are addressed. The review highlights the importance of dissolved N and P for overland flow and groundwater pathways, for sheep, beef and dairy grazing land use. Consequently, the effectiveness of riparian buffers to remove N and P may not be adequate. Consideration of the effects of rainfall and hydrology, dissolved P and N losses from pastures and event-based catchment-scale loads are therefore important factors that should be incorporated into catchment models. The review shows that it is likely that nutrient losses under Australian dairying conditions have many similarities to worldwide studies. Catchment models need to represent the importance of event-based loads, intensively farmed land use, management and forms of nutrients. Otherwise there is a likelihood of either underestimating nutrient losses, or potentially overestimating the effectiveness of riparian buffers.
Published data and analyses from temperate and tropical aquatic systems are used to summarize knowledge about the potential impact of land-use alteration on the nitrogen biogeochemistry of tropical aquatic ecosystems, identify important patterns and recommend key needs for research. The tropical N-cycle is traced from pre-disturbance conditions through the phases of disturbance, highlighting major differences between tropical and temperate systems that might influence development strategies in the tropics. Analyses suggest that tropical freshwaters are more frequently N-limited than temperate zones, while tropical marine systems may show more frequent P limitation. These analyses indicate that disturbances to pristine tropical lands will lead to greatly increased primary production in freshwaters and large changes in tropical freshwater communities. Increased freshwater nutrient flux will also lead to an expansion of the high production, N- and light-limited zones around river deltas, a switch from P- to N-limitation in calcareous marine systems, with large changes in the community composition of fragile mangrove and reef systems. Key information gaps are highlighted, including data on mechanisms of nutrient transport and atmospheric deposition in the tropics, nutrient and material retention capacities of tropical impoundments, and N/P coupling and stoichiometric impacts of nutrient supplies on tropical aquatic communities. The current base of biogeochemical data suggests that alterations in the N-cycle will have greater impacts on tropical aquatic ecosystems than those already observed in the temperate zone.
We evaluated the accuracy of six watershed models of nitrogen export in streams (kg km2 yr–1) developed for use in large watersheds and representing various empirical and quasi-empirical approaches described in the literature. These models differ in their methods of calibration and have varying levels of spatial resolution and process complexity, which potentially affect the accuracy (bias and precision) of the model predictions of nitrogen export and source contributions to export. Using stream monitoring data and detailed estimates of the natural and cultural sources of nitrogen for 16 watersheds in the northeastern United States (drainage sizes = 475 to 70,000 km2), we assessed the accuracy of the model predictions of total nitrogen and nitrate-nitrogen export. The model validation included the use of an error modeling technique to identify biases caused by model deficiencies in quantifying nitrogen sources and biogeochemical processes affecting the transport of nitrogen in watersheds. Most models predicted stream nitrogen export to within 50% of the measured export in a majority of the watersheds. Prediction errors were negatively correlated with cultivated land area, indicating that the watershed models tended to over predict export in less agricultural and more forested watersheds and under predict in more agricultural basins. The magnitude of these biases differed appreciably among the models. Those models having more detailed descriptions of nitrogen sources, land and water attenuation of nitrogen, and water flow paths were found to have considerably lower bias and higher precision in their predictions of nitrogen export.
This paper provides a global synthesis of reactive nitrogen (Nr) loading to the continental landmass and subsequent riverine nitrogen fluxes under a gradient of anthropogenic disturbance, from pre-industrial to contemporary. A mass balance model of nitrogen loading to the landmass is employed to account for transfers of Nr between atmospheric input sources (as food and feed products) and subsequent consumer output loads. This calculation produces a gridded surface of nitrogen loading ultimately mobilizable to aquatic systems (Nmob). Compared to the pre-industrial condition, nitrogen loading to the landmass has doubled from 111 to 223Tg/year due to anthropogenic activities. This is particularly evident in the industrialized areas of the globe where contemporary levels of nitrogen loading have increased up to 6-fold in many areas. The quantity of nitrogen loaded to the landscape has shifted from a chiefly fixation-based system (89% of total loads) in the pre-industrial state to a heterogeneous mix in contemporary times where fertilizer (15%), livestock (24%) and atmospheric deposition (15%) dominate in many parts of the industrialized and developing world. A nitrogen transport model is developed from a global database of drainage basin characteristics and a comprehensive compendium of river chemistry observations. The model utilizes constituent delivery coefficients based on basin temperature and hydraulic residence times in soils, rivers, lakes and reservoirs to transport nitrogen loads to river mouths. Fluxes are estimated for total nitrogen, dissolved inorganic nitrogen, and total organic nitrogen. Model results show that total nitrogen fluxes from river basins have doubled from 21Tg/year in the pre-industrial to 40Tg/year in the contemporary period, with many industrialized areas of the globe showing an increase up to 5-fold. DIN fluxes from river basins have increased 6-fold from 2.4Tg/year in the pre-industrial to 14.5Tg/year in the contemporary period. The amount of nitrogen loading delivered to river mouth as flux is greatly influenced by both basin temperatures and hydraulic residence times suggesting a regional sensitivity to loading. The global, aggregate nitrogen retention on the continental land mass is 82%, with a range of 0–100% for individual basins. We also present the first seasonal estimates of riverine nitrogen fluxes at the global scale based on monthly discharge as the primary driver.
We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr-1 out of a total for the entire North Atlantic region of 13.1 Tg yr-1. On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km-2 yr-1.
Non-point sources of nitrogen dominate riverine fluxes to the coast in all regions. River fluxes of total nitrogen from the temperate regions of the North Atlantic basin are correlated with population density, as has been observed previously for fluxes of nitrate in the world’s major rivers. However, more striking is a strong linear correlation between river fluxes of total nitrogen and the sum of anthropogenically-derived nitrogen inputs to the temperate regions (fertilizer application, human-induced increases in atmospheric deposition of oxidized forms of nitrogen, fixation by leguminous crops, and the import/export of nitrogen in agricultural products). On average, regional nitrogen fluxes in rivers are only 25% of these anthropogenically derived nitrogen inputs. Denitrification in wetlands and aquatic ecosystems is probably the dominant sink, with storage in forests perhaps also of importance. Storage of nitrogen in groundwater, although of importance in some localities, is a very small sink for nitrogen inputs in all regions. Agricultural sources of nitrogen dominate inputs in many regions, particularly the Mississippi basin and the North Sea drainages. Deposition of oxidized nitrogen, primarily of industrial origin, is the major control over river nitrogen export in some regions such as the northeastern U.S.
Using data from relatively pristine areas as an index of change, we estimate that riverine nitrogen fluxes in many of the temperate regions have increased from pre-industrial times by 2 to 20 fold, although some regions such as northern Canada are relatively unchanged. Fluxes from the most disturbed region, the North Sea drainages, have increased by 6 to 20 fold. Fluxes from the Amazon basin are also at least 2 to 5 fold greater than estimated fluxes from undisturbed temperate-zone regions, despite low population density and low inputs of anthropogenic nitrogen to the region. This suggests that natural riverine nitrogen fluxes in the tropics may be significantly greater than in the temperate zone. However, deforestation may be contributing to the tropical fluxes. In either case, projected increases in fertilizer use and atmospheric deposition in the coming decades are likely to cause dramatic increases in nitrogen loading to many tropical river systems.
We examine the global distribution of dissolved inorganic nitrogen (DIN) and particulate nitrogen (PN) export to coastal systems
and the effect of human activities and natural processes on that export. The analysis is based on DIN and PN models that were
combined with spatially explicit global databases. The model results indicate the widely uneven geographic distribution of
human activities and rates of nitrogen input to coastal systems at the watershed, latitudinal, and regional-continental scales.
Future projections in a business-as-usual scenario indicate that DIN export rates increase from approximately 21 Tg N yr−1 in 1990 to 47 Tg N yr−1 by 2050. Increased DIN inputs to coastal systems in most world regions are predicted by 2050. The largest increases are predicted
for Southern and Eastern Asia, associated with predicted large increases in population, increased fertilizer use to grow food
to meet the dietary demands of that population, and increased industrialization. Results of an alternative scenario for North
America and Europe in 2050 indicate that reductions in the human consumption of animal protein could reduce fertilizer use
and result in substantial decreases in DIN export rates by rivers. In another scenario for 2050, future air pollution control
in Europe that would reduce atmospheric deposition of nitrogen oxides in watersheds is predicted to decrease DIN export by
rivers, particularly from Baltic and North Atlantic watersheds. Results of a newly developed global PN river export model
indicate that total global PN and DIN export by rivers in 1990 are similar, even though the global distribution of the two
differ considerably.
Drawing upon a variety of existing maps, data and information, a new Global Lakes and Wetlands Database (GLWD) has been created. The combination of best available sources for lakes and wetlands on a global scale (1:1 to 1:3 million resolution), and the application of Geographic Information System (GIS) functionality enabled the generation of a database which focuses in three coordinated levels on (1) large lakes and reservoirs, (2) smaller water bodies, and (3) wetlands. Level 1 comprises the shoreline polygons of the 3067 largest lakes (surface area ≥50 km2) and 654 largest reservoirs (storage capacity ≥0.5 km3) worldwide, and offers extensive attribute data. Level 2 contains the shoreline polygons of approx. 250,000 smaller lakes, reservoirs and rivers (surface area ≥0.1 km2), excluding all water bodies of level 1. Finally, level 3 represents lakes, reservoirs, rivers, and different wetland types in the form of a global raster map at 30-second resolution, including all water bodies of levels 1 and 2.In a validation against documented data, GLWD proved to represent a comprehensive database of global lakes ≥1 km2 and to provide a good representation of the maximum global wetland extent. GLWD-1 and GLWD-2 establish two global polygon maps to which existing lake registers, compilations or remote sensing data can be linked in order to allow for further analyses in a GIS environment. GLWD-3 may serve as an estimate of wetland extents for global hydrology and climatology models, or to identify large-scale wetland distributions and important wetland complexes.
In this paper, we develop and apply a framework for estimating the potential global-scale impact of reservoir construction on riverine sediment transport to the ocean. Using this framework, we discern a large, global-scale, and growing impact from anthropogenic impoundment. Our study links information on 633 of the world's largest reservoirs (LRs) (≥0.5 km3 maximum storage capacity) to the geography of continental discharge and uses statistical inferences to assess the potential impact of the remaining >44,000 smaller reservoirs (SRs). Information on the LRs was linked to a digitized river network at 30′ (latitude×longitude) spatial resolution. A residence time change (ΔτR) for otherwise free-flowing river water is determined locally for each reservoir and used with a sediment retention function to predict the proportion of incident sediment flux trapped within each impoundment. The discharge-weighted mean ΔτR for individual impoundments distributed across the globe is 0.21 years for LRs and 0.011 years for SRs. More than 40% of global river discharge is intercepted locally by the LRs analyzed here, and a significant proportion (≈70%) of this discharge maintains a theoretical sediment trapping efficiency in excess of 50%. Half of all discharge entering LRs shows a local sediment trapping efficiency of 80% or more. Analysis of the recent history of river impoundment reveals that between 1950 and 1968, there was tripling from 5% to 15% in global LR sediment trapping, another doubling to 30% by 1985, and stabilization thereafter. Several large basins such as the Colorado and Nile show nearly complete trapping due to large reservoir construction and flow diversion. From the standpoint of sediment retention rates, the most heavily regulated drainage basins reside in Europe. North America, Africa, and Australia/Oceania are also strongly affected by LRs. Globally, greater than 50% of basin-scale sediment flux in regulated basins is potentially trapped in artificial impoundments, with a discharge-weighted sediment trapping due to LRs of 30%, and an additional contribution of 23% from SRs. If we consider both regulated and unregulated basins, the interception of global sediment flux by all registered reservoirs (n≈45,000) is conservatively placed at 4–5 Gt year−1 or 25–30% of the total. There is an additional but unknown impact due to still smaller unregistered impoundments (n≈800,000). Our results demonstrate that river impoundment should now be considered explicitly in global elemental flux studies, such as for water, sediment, carbon, and nutrients. From a global change perspective, the long-term impact of such hydraulic engineering works on the world's coastal zone appears to be significant but has yet to be fully elucidated.
One of the most significant challenges for the science of global change is determining how hydrological and biogeochemical cycles function at the land surface on regional to continental scales. At these scales, river basins are natural integrators of surficial processes. As the main pathway for the ultimate preservation of terrigenous production in modern environments, the transfer of organic matter from the land to the oceans is a key link in the global carbon cycle (Hedges, in press). Carbon and nutrient fluxes in large rivers are reflections of their watersheds and floodplains, and some extension of the properties of the smaller rivers that form them (Richey et al.,1990). Organic and inorganic materials measured in the main channel are a mixture of materials originating from sources thousands of kilometers away in upland regions, as well as material introduced continuously from the adjacent floodplain. Organic matter of both sources has been subjected to within channel transport and reactive processes before it is discharged into the coastal zone.
Computing the surface area of land parcels, administrative districts, or more extensive geographic features is an area of concern to surveyors, geographers, and a host of planners in various levels of government. A computer compatible method is described for calculating the area of any irregular surface feature on Earth directly from geodetic coordinates along its boundary. The method is evaluated in three respects: computation accuracy vs. feature size, computation accuracy at various locations on the spheroid, and computation accuracy vs. precision of geodetic coordinates.
This paper demonstrates the potential of combining observed river discharge information with climate-driven water balance model (WBM) outputs to develop composite runoff fields. Such combined runoff fields simultaneously reflect the numerical accuracy of the discharge measurements and preserve the spatial and temporal distribution of simulated runoff. Selected gauging stations from the World Meteorological Organization Global Runoff Data Centre (GRDC) data archive were geographically coregistered to a gridded simulated topological network at 30′ (longitude × latitude) spatial resolution (STN-30p). Interstation regions between gauging stations along the STN-30p network were identified, and annual interstation runoff was calculated. The annual interstation runoff was compared with outputs from WBM calculations, which were performed using long-term mean monthly climate forcings (air temperature and precipitation). The simulated runoff for each cell was multiplied by the ratio of observed to simulated runoff of the corresponding interstation region from the GRDC data set to create spatially distributed runoff fields at 30′ resolution. The resulting composite runoff fields (UNH/GRDC Composite Runoff Fields V1.0) are released to the scientific community along with intermediate data sets, such as station attributes and long-term monthly regimes of the selected gauging stations, the simulated topological network (STN-30p), STN-30p derived attributes for the selected stations, and gridded fields of the interstation regions along STN-30p. These data sets represent high-resolution fields that are of value to a broad range of water-related research, including terrestrial modeling, climate-atmosphere interactions, and global water resource assessments.
The spatial organization of the Earth's land mass is analyzed using a
simulated topological network (STN-30p) representing potential flow
pathways across the entire nonglacierized surface of the globe at 30-min
(longitude × latitude) spatial resolution. We discuss a
semiautomated procedure to develop this topology combining digital
elevation models and manual network editing. STN-30p was verified
against several independent sources including map products and drainage
basin statistics, although we found substantial inconsistency within the
extant literature itself. A broad suite of diagnostics is offered that
quantitatively describes individual grid cells, river segments, and
complete drainage systems spanning orders 1 through 6 based on the
Strahler classification scheme. Continental and global-scale summaries
of key STN-30p attributes are given. Summaries are also presented which
distinguish basins that potentially deliver discharge to an ocean
(exorheic) from those that potentially empty into an internal receiving
body (endorheic). A total of 59,122 individual grid cells constitutes
the global nonglacierized land mass. At 30-min spatial resolution, the
cells are organized into 33,251 distinct river segments which define
6152 drainage basins. A global total of 133.1 × 106
km2 bear STN-SOp flow paths with a total length of 3.24
× 106 km. The organization of river networks has an
important role in linking land mass to ocean. From a continental
perspective, low-order river segments (orders 1-3) drain the largest
fraction of land (90%) and thus constitute a primary source area for
runoff and constituents. From an oceanic perspective, however, the small
number (n=101) of large drainage systems (orders 4-6) predominates;
draining 65% of global land area and subsuming a large fraction of the
otherwise spatially remote low-order rivers. Along river corridors, only
10% of land mass is within 100 km of a coastline, 25% is within 250 km,
and 50% is within 750 km. The global mean distance to river mouth is
1050 km with individual continental values from 460 to 1340 km. The
Mediterranean/Black Sea and Arctic Ocean are the most land-dominated of
all oceans with land:ocean area ratios of 4.4 and 1.2, respectively;
remaining oceans show ratios from 0.55 to 0.13. We discuss limitations
of the STN-30p together with its potential role in future global change
studies. STN-30p is geographically linked to several hundred river
discharge and chemistry monitoring stations to provide a framework for
calibrating and validating macroscale hydrology and biogeochemical flux
models.
Samples from several major world rivers were analyzed for their
particulate nitrogen (PN) contents, C/N ratios, and total particulate
amino acid (TPAA) contents. The results show a global PN transport of 33
× 1012 g N yr-1, more than 80% of which
occurs in rivers carrying high suspended matter concentrations such as
the Ganges, Brahmaputra, Mekong, and Huanghe. Quantitatively, nitrogen
transported in the particulate fraction exceeds that reported for
dissolved inorganic nitrogen. The bulk of the PN transport is associated
with material having relatively low C/N ratios and low TPAA contents.
TPAA accounted for about 20% of the total PN transport. Although natural
processes leading to high rates of erosion in Asian rivers might have
partly contributed to the observed PN transport pattern, our results
suggest that human activities such as deforestation and increased use of
nitrogen fertilizer in their drainage basins are also an important
factor. The biogeochemical nature of the remaining 80% is only poorly
known, but should be of relevance in studying the response of estuaries
and coastal seas to human activities in the terrestrial environment.
Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSi) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSi. The resulting changes in the nutrient composition (DSi:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction of dams for flow regulation.
The construction of a 0.5° lat x 0.5 ° long surface climatology of global land areas, excluding Antarctica, is described. The climatology represents the period 1961--90 and comprises a suite of nine variables: precipitation, wet-day frequency, mean temperature, diurnal temperature range, vapor pressure, sunshine, cloud cover, ground frost frequency, and wind speed. The climate surfaces have been constructed from a new dataset of station 1961--90 climatological normals, numbering between 19,800 (precipitation) and 3,615 (wind speed). The station data were interpolated as a function of latitude, longitude, and elevation using thin-plate splines. The accuracy of the interpolations are assessed using cross validation and by comparison with other climatologies. This new climatology represents an advance over earlier published global terrestrial climatologies in that it is strictly constrained to the period 1961--90, describes an extended suite of surface climate variables, explicitly incorporates elevation as a predictor variable, and contains an evaluation of regional errors associated with this and other commonly used climatologies. The climatology is already being used by researchers in the areas of ecosystem modelling, climate model evaluation, and climate change impact assessment. The data are available from the Climatic Research Unit and images of all the monthly fields can be accessed via the World Wide Web.
During the last half of the 20th century, cumulative annual discharge from 137 representative rivers (watershed areas ranging from 0.3 to 6300 × 103 km2) to the global ocean remained constant, although annual discharge from about one-third of these rivers changed by more than 30%. Discharge trends for many rivers reflected mostly changes in precipitation, primarily in response to short- and longer-term atmospheric–oceanic signals; with the notable exception of the Parana, Mississippi, Niger and Cunene rivers, few of these “normal" rivers experienced significant changes in either discharge or precipitation. Cumulative discharge from many mid-latitude rivers, in contrast, decreased by 60%, reflecting in large part impacts due to damming, irrigation and interbasin water transfers. A number of high-latitude and high-altitude rivers experienced increased discharge despite generally declining precipitation. Poorly constrained meteorological and hydrological data do not seem to explain fully these “excess” rivers; changed seasonality in discharge, decreased storage and/or decreased evapotranspiration also may play important roles.
As a limiting nutrient in aquatic systems, phosphorus (P) plays an important role in controlling freshwater and coastal primary productivity and ecosystem dynamics, increasing frequency and severity of harmful and nuisance algae blooms and hypoxia, as well as contributing to loss of biodiversity. Although dissolved inorganic P (DIP) often constitutes a relatively small fraction of the total P pool in aquatic systems, its bioavailability makes it an important determinant of ecosystem function. Here we describe, apply, evaluate, and interpret an enhanced version of the Global Nutrient Export from Watersheds (NEWS)-DIP model: NEWS-DIP-Half Degree (NEWS-DIP-HD). Improvements to NEWS-DIP-HD over the original NEWS DIP model include (1) the preservation of spatial resolution of input data sets at the 0.5 degree level and (2) explicit downstream routing of water and DIP from half-degree cell to half-degree cell using a global flow-direction representation. NEWS-DIP explains 78% and 62% of the variability in per-basin DIP export (DIP load) for U.S. Geological Survey (USGS) and global stations, respectively, similar to the original NEWS-DIP model and somewhat more than other global models of DIP loading and export. NEWS-DIP-HD output suggests that hot spots for DIP loading tend to occur in urban centers, with the highest per-area rate of DIP loading predicted for the half-degree grid cell containing Tokyo (6366 kg P km-2 yr-1). Furthermore, cities with populations >100,000 accounted for 35% of global surface water DIP loading while covering less than 2% of global land surface area. NEWS-DIP-HD also indicates that humans supply more DIP to surface waters than natural weathering over the majority (53%) of the Earth's land surface, with a much larger area dominated by DIP point sources than nonpoint sources (52% versus 1% of the global land surface, respectively). NEWS-DIP-HD also suggests that while humans had increased DIP input to surface waters more than fourfold globally by the year 2000, human activities such as dam construction and consumptive water use have somewhat moderated the effect of humans on P transport by preventing (conservatively) 0.35 Tg P yr-1 (˜20% of P inputs to surface waters) from reaching coastal zones globally.
The replacement of undisturbed tropical forest with cattle pasture has the potential to greatly modify the hydrology of small watersheds and the fluxes of solutes. We examined the fluxes of water, Cl−, , Na+, K+, Mg2+ and Ca2+ in different flow paths in ∼1 ha catchments of undisturbed open tropical rainforest and a 20 year-old pasture established from forest in the southwestern Brazilian Amazon state of Rondônia. Storm flow discharge was 18% of incident rainfall in pasture, but only 1% in forest. Quickflow predominated over baseflow in both catchments and in both wet and dry seasons. In the pasture, groundwater and quickflow were important flow paths for the export of all solutes. In the forest, quickflow was important for export, but all other solutes were exported primarily by groundwater outflow. Both catchments were sinks for and Ca2+, and sources of Na+. The pasture catchment also lost K+ and Mg2+ because of higher overland flow frequency and volume and to cattle excrement. These results show that forest clearing dramatically influences small watershed hydrology by increasing quickflow and water export to streams. They also indicate that tropical forest watersheds are highly conservative for most solutes but that pastures continue to lose important cations even decades after deforestation and pasture establishment.
A manageable, relatively inexpensive model was constructed to predict the loss of nitrogen and phosphorus from a complex catchment to its drainage system. The model used an export coefficient approach, calculating the total nitrogen (N) and total phosphorus (P) load delivered annually to a water body as the sum of the individual loads exported from each nutrient source in its catchment. The export coefficient modelling approach permits scaling up from plot-scale experiments to the catchment scale, allowing application of findings from field experimental studies at a suitable scale for catchment management. The catchment of the River Windrush, a tributary of the River Thames, UK, was selected as the initial study site. The Windrush model predicted nitrogen and phosphorus loading within 2% of observed total nitrogen load and 0.5% of observed total phosphorus load in 1989. The export coefficient modelling approach was then validated by application in a second research basin, the catchment of Slapton Ley, south Devon, which has markedly different catchment hydrology and land use. The Slapton model was calibrated within 2% of observed total nitrogen load and 2.5% of observed total phosphorus load in 1986. Both models proved sensitive to the impact of temporal changes in land use and management on water quality in both catchments, and were therefore used to evaluate the potential impact of proposed pollution control strategies on the nutrient loading delivered to the River Windrush and Slapton Ley.
During the last half of the 20th century, cumulative annual discharge from 137 representative rivers (watershed areas ranging from 0.3 to 6300 × 103 km2) to the global ocean remained constant, although annual discharge from about one-third of these rivers changed by more than 30%. Discharge trends for many rivers reflected mostly changes in precipitation, primarily in response to short- and longer-term atmospheric–oceanic signals; with the notable exception of the Parana, Mississippi, Niger and Cunene rivers, few of these “normal" rivers experienced significant changes in either discharge or precipitation. Cumulative discharge from many mid-latitude rivers, in contrast, decreased by 60%, reflecting in large part impacts due to damming, irrigation and interbasin water transfers. A number of high-latitude and high-altitude rivers experienced increased discharge despite generally declining precipitation. Poorly constrained meteorological and hydrological data do not seem to explain fully these “excess” rivers; changed seasonality in discharge, decreased storage and/or decreased evapotranspiration also may play important roles.
The consequences of a decline in soil nitrogen following ploughing of permanent pasture and land-use change are explored in terms of nitrate export at the catchment scale. The release of reserves of organic nitrogen built up in grassland soils is modelled as a first-order kinetic decay. The build up of reserves of nitrogen upon reversion to pasture is modelled both as a first-order process and under the assumption that new grassland can absorb all the nitrogen applied to it. Results show that the release and sequestration of nitrogen in these reservoirs shows supply-limited hysteresis, and consequently the ploughing-up of permanent pasture has the dominant effect. Allowing for the present land-use and the effect of rainfall, the model is compared to streamwater nitrate concentrations measured in the Slapton Wood catchment, south west England. Significant overestimates are observed that suggest that nitrogen released from ploughing up of grassland is either in an organic form or that significant denitrification capacity is available. Optimising the model against the data from Slapton Wood catchment confirms there is an elastic capacity for denitirification within the catchment. At the catchment scale the grassland reservoirs acts as a constant source of nitrogen whilst the effect of the reversion of land to permanent pasture, at the catchment scale, attains rapid equilibrium and does not continue to remove significant levels of nitrogen after the first year.