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Geochemical isotopic composition in the Loess Plateau and corresponding source analyses: A case study of China's Yangjuangou catchment

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... Meanwhile, dual isotope analyses (i.e., simultaneous monitoring of two different isotopes) have been used to identify different nitrogen sources in aquatic environments Fenech et al., 2012;Jiang et al., 2016;Xue et al., 2009) because single-isotope analyses often fail to distinguish potential source materials with similar isotopic compositions (Fenech et al., 2012;Minet et al., 2012;Xue et al., 2009). Many studies have reported the successful application of dual isotope techniques in the identification of NO 3 − -N sources using dual nitrate nitrogen (δ 15 N NO3 ) and oxygen (δ 18 O NO3 ) isotope ratio axes (Fenech et al., 2012;Hu et al., 2019;Mingzhu et al., 2014;Nestler et al., 2011;Ohte et al., 2010;Wang et al., 2017;Xue et al., 2009;Yu et al., 2018;Zeng and Wu, 2015). Meanwhile, to date, ammonium nitrogen (δ 15 N NH4 ) and δ 15 N NO3 isotope ratios have only been applied independently to identify nitrogen sources in aquatic environments including groundwater (Gooddy et al., 2016;Hood et al., 2014;Lee et al., 2016;Li et al., 2007;Sugimoto et al., 2011;Wells et al., 2016). ...
... Prior to the assessment of nitrogen sources using δ 15 N NH4 and δ 15 N NO3 axes, the dual isotope technique using δ 18 O NO3 and δ 15 N NO3 was employed, which has been successfully applied in identifying NO 3 − -N sources in aquatic environments (Fenech et al., 2012;Hu et al., 2019;Mingzhu et al., 2014;Nestler et al., 2011;Ohte et al., 2010;Wang et al., 2017;Xue et al., 2009;Yu et al., 2018;Zeng and Wu, 2015). In Fig. 2a, the ranges in the δ 18 O NO3 and δ 15 N NO3 values for soil and mineral fertilizers, sewage, and manure were obtained from Kendall et al. (2007), which are considered typical ranges used in this field. ...
Article
We proposed a novel approach based on dual ammonium and nitrate nitrogen isotope ratios (δ¹⁵NNH4 and δ¹⁵NNO3, respectively) axes to identify nitrogen sources in intensive livestock farming watersheds, especially those with swine excreta treatment facilities. The δ¹⁵NNH4 and δ¹⁵NNO3 values in water samples were measured monthly in 2016–2017. Soil and mineral fertilizers, sewage, sewage effluent, manure, and swine effluents were the five sources considered to identify nitrogen sources. The results showed that nitrogen pollution from agricultural activities was well reflected by the seasonal δ¹⁵NNH4 and δ¹⁵NNO3 patterns in the river, and microbial nitrification was suggested as the dominant nitrogen transformation process in the river. This study revealed that δ¹⁵NNH4 and δ¹⁵NNO3 axes provided better results than the traditionally used nitrate oxygen (δ¹⁸ONO3) and δ¹⁵NNO3 axes for identifying nitrogen sources in agricultural watersheds with swine excreta treatment facilities. The mixing model results showed that stream water was severely contaminated with swine effluents (e.g., a mean minimum contribution of 31%), thus affecting the quality of the mainstream (p = 0.068 < 0.10). This study was the first successful application of dual δ¹⁵NNH4 and δ¹⁵NNO3 axes to better understand nitrogen sources in intensive livestock farming watersheds with swine excreta treatment facilities.
... Given the wide planting of non-native trees, it is important to elucidate how the ecosystem components and ecosystem functioning respond to non-native forest plantations (NNFPs) so that the knowledge can be used to assess the ecosystem health of restored systems, and guide the selection of plant species in ecological restoration. To date, the majority of research on this topic focused on soil moisture (Yang et al., 2015;Feng et al., 2016), soil physicochemical properties (Wang et al., 2017), and plant diversity (Kou et al., 2016), less is known the effects of NNFPs on soil fauna and ecosystem processes they mediate (Zhu et al., 2020). ...
... The region has a semiarid continental climate with a mean annual precipitation of 535 mm and a mean annual temperature of 9.9 ℃. The geomorphology is composed of loess hills and gullies that have maximum altitudinal differences from hill top to gully bottom of 225 m and a gully density of 2.74 km 2 (Wang et al., 2017). The soil is mainly derived from loess, with a texture that ranges from fine silt to silt and is vulnerable to erosion (Fu et al., 2000). ...
... Many studies have presented mathematical and statistical indices to quantify landscape patterns [27][28][29], such as edge density, the landscape diversity index, the landscape shape index and patch density [30][31][32][33]. These indices are calculated using the popular Fragstats software, based on the geometry of patches and their spatial relationships [34][35][36]. ...
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Understanding the influence of landscape patterns on the water quality of agricultural wetlands is critically important for their management and related decision-making. However, the question of how to quantify this objectively remains a challenge in the relevant scientific fields. In this study, the location-weighted landscape index (LWLI), a process-oriented indicator that integrates ecological processes with landscape patterns based on the source and sink theory, was modified into the SLWLI by assigning nutrient-based weights in the Honghe Hani Rice Terraces World Heritage Site (HHRT). The results indicate that the five watersheds are dominated by sink landscapes, representing 64 percent of the total area. Rice terraced fields were a composite “source–sink” landscape, and their areas in the five watersheds ranged from 4.82% to 20.40%. The nutrient retention function of the sink landscapes of total nitrogen (TN) ranged from 0.64 to 0.86, whereas the total phosphorus (TP) ranged from 0.72 to 0.82, showing good retention function in regard to both nutrients. The contribution rates of forest land and rice terraces to TN and TP retention were greater than 47.07% and 17.07%, respectively, which indicates their key regulation of the nutrient retention function, reducing the risk of water eutrophication and leading to optimized conservation. The vertical pattern of the HHRT plays an important role in nutrient retention function. The SLWLI is an effective index that can be used to assess nutrient retention function and to identify sink landscapes for regulating water pollution in agricultural wetlands.
... Land use changes in erosion-prone areas had long-term effects on non-point source pollution Wang et al. 2017). USEPA (2003) survey results indicate that agricultural land use was the largest source of pollution for rivers and lakes in the USA and that agricultural land use was the main reason for deteriorating water qualities in 40% of the rivers and lakes in the USA. ...
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Non-point source pollution in rivers is an important factor affecting water quality. Quantifying the load of non-point source pollutants in the water and implementing improvement measures are critical for guaranteeing drinking water quality. In this study, the Dan River watershed, which is an important water source for Beijing, was investigated. Through a combination of water sampling and numerical simulations, the temporal and spatial distributions of nitrate nitrogen (NO3⁻-N) and ammoniacal nitrogen (NH4⁺-N) loads in the watershed were determined, and the effects of vegetation restoration and agricultural management on reducing nitrogen pollution in the river were predicted. The NO3⁻-N and NH4⁺-N loads in the watershed were higher during the wet season (July–September), accounting for more than 50% of the annual nitrogen output. The Soil and Water Assessment Tool (SWAT) was used to simulate the nitrogen load in the watershed. Pollution from nitrogen loading was serious in the lower reaches of the river; however, vegetation restoration can reduce the nitrogen output. Through scenario simulations, we found that an increase in forestland in the watershed would reduce the NO3⁻-N and NH4⁺-N loads. The nitrate and NH4⁺-N loads in the watershed also decreased with reduced fertilizer use and reduced irrigation application in the watershed. Thus, reasonable land planning and agricultural management measures can effectively reduce nitrogen loss, which is an effective way to control non-point source pollution in watersheds and ensure river water quality.
... Geographic location of the Yangjuangou catchment in the Loess Plateau region, China; the red and yellow dots denote the weather station and runoff sampling site (source:Wang et al., 2017). ...
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Dissolved organic carbon (DOC) transported by runoff has been identified as an important role in the global carbon cycle. Despite there being many studies on DOC concentration and flux, little information is available for the semi-arid catchments of the Loess Plateau region (LPR). The primary goal of this study was to quantify DOC exported and driven by a sequence of rainfall events during the concentrated rainfall season. In addition, factors that affect DOC export from a small headwater catchment will be investigated accordingly. Runoff discharge and DOC concentration were monitored at the outlet of the Yangjuangou catchment in Yanan, Shaanxi Province, China. The results showed that DOC concentration was highly variable, with event-based DOC concentrations ranging from 5.14 to 13.14 mg L−1. Hysteresis analysis showed a nonlinear relationship between DOC concentration and flow rate in the hydrological process. The monthly DOC flux loading from the catchment was varied from 94.73 to 110.17 kg km−2, while the event-based DOC flux ranged from 0.18 to 2.84 kg km−2 in the period of June to September. Variations of event-driven DOC concentration contributed slightly to a difference in DOC flux, whereas intra-events of rainfall amount and runoff discharge led to evident differences in DOC export. In conclusion, our case results highlighted the advantages of high-frequency monitoring for DOC export and indicated that event-driven DOC export is largely influenced by the interaction of catchment hydrology and antecedent condition within a catchment. Engineers and scientists can take advantage of the derived results to better develop advanced field monitoring work. In addition, more studies are needed to investigate the magnitude of terrestrial DOC export in response to projected climate change at larger spatio-temporal scales, which may have implications for the carbon balance and carbon cycle model from an ecologically restored catchment in the LPR.
... The visible deposition couplet with double-layer and annual freeze-thaw layer of 'bean curd' structure is easy to identify from the sediment profile in the field. Therefore, the sediment sequences within the check dams and landslide-dammed reservoirs are the ideal material to reconstruct inter-annual changes of soil erosion, hydrologic events, eco-environment, land use and human activities on the CLP during the past hundreds of years Jin et al., 2017;Li et al., 2016;Wang et al., 2017;Wei et al., 2017;Zhao et al., 2015Zhao et al., , 2017. For example, Zhang et al. (2009) traced eco-environmental changes in the Huangtuwa landslide-dammed reservoir using the distributions of pollen and annual freeze-thaw layers, and further discussed sediment discharge changes in response to different land uses during the 31 years after 1569. ...
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The deposition couplets within the check dams and landslide-dammed reservoirs on the Chinese Loess Plateau (CLP) document the processes and histories of watershed soil erosion, transportation and deposition, related to floods, eco-environment and land use changes, and human activities. Previously, dating the couplets was dominantly dependent on multiple intercomparisons among specific sediment yields of visible couplets, ¹³⁷ Cs activities distributions and erosive rainfall events by meteorological records. However, inter-annual division of the deposition couplets and dating historical sequence beyond meteorological records in the landslide-dammed reservoir are little known. Based on high-resolution XRF core scanning on a 22.75 m sediment sequence in Jingbian (JB) landslide-dammed reservoir on the central hilly-gully area of the CLP, and cross checking of multiple dating methods, this study tried to propose a new method to build the accurate chronology sequence based on inter-annual division by annual freeze-thaw layer and to further date the sediment without rainfall records. The results showed that a total of 126 deposition couplets and 78 annual freeze-thaw layers were identified in the JB sequence. Multimethod cross-dating, including ¹³⁷ Cs activities, annual freeze-thaw layers, couplet specific sediment yield and modern rainfall records, was the most accurate method of dating the JB sediment sequences since 1960s with detail meteorological records. Furthermore, the correspondence between annual freeze-thaw layers and the historical grades of flood index from literature was the valid method to date the JB historical sequences without rainfall records. Consequently, the JB sequence was deposited during the period between 1855 and 2014. High-resolution dating of the JB sequence provides the chronology for recovering natural and anthropogenic information on the central hilly-gully area of CLP since 1850s. The proposed methods will also shed new light on the accurate dating of the sediment sequences within other check dams and landslide-dammed reservoirs on the central and northern CLP.
... Land-use changes in vulnerable erosion-prone areas have long-term effects on hydrological processes (Liang et al., 2015;Wang et al., 2017;Zuo et al., 2016). An increase in the vegetative cover on hillslopes reduces runoff generation and sediment detachment. ...
... For example, Mladenov et al. (2012) used a long-term dataset of weekly DOC deposition and demonstrated that atmospheric wet deposition of dissolved carbon represented a significant source to an alpine catchment in the Rocky Mountains of Colorado, USA. Wang et al. (2017) reported that atmospheric wet deposition might be a large source of DOC in stream water, based on isotopic characteristics of carbon in a semi-arid catchment in the LPR. Consequently, the large magnitude of dissolved carbon flux via rainfall played an important role in ecological processes and acted as one of the key driving forces of the global carbon biogeochemical cycle. ...
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TS2Wet dissolved carbon deposition is a critical node of the global carbon cycle, but little is known about dissolved organic and inorganic carbon (DOC and DIC) concentrations and fluxes in the semi-arid areas of the Loess 5 Plateau Region (LPR). In this study, we measured variations in DOC and DIC concentrations in rainfalls at Yangjuangou Ecological Restoration and Soil and Water Observatory. Rainwater samples were collected in 16 rainfall events from July to September and the event-based, monthly concentrations and fluxes of DOC and DIC were quantified. The results showed that the event-based concentrations and fluxes of DOC and DIC were highly variable, ranging from 0.56 to 28.71 mg CL^-1 and from 3.47 to 17.49 mg CL^-1, respectively. The corresponding event-based fluxes ranged from 0.21–258.36mgC m^-2 and from 4.12 to 42.32mg C m^-2. The monthly concentrations of DOC and DIC were 24.62 and 4.30 (July), 3.58 and 10.52 (August), and 1.01 and 5.89 (September) mg C L^-1, respectively. Thus, the monthly deposition fluxes of DOC and DIC were 541.64/94.60, 131.03/385.03, and 44.44/259.16mg C m^-2 for July, August, and September, respectively. In addition, the concentrations of DOC and DIC for the concentrated rainfall season (July–September) in the studied catchment were 7.06 and 7.00 mg C L^-1, respectively. The estimated annual wet dissolved carbon depositions were 1.91 and 1.89 g C m^-2 yr^-1 for DOC and DIC, respectively. The results of this study suggest the variation in concentrations and fluxes of DOC and DIC and explore that these variation may be related to the dissolved carbon source and the rainfall characteristics during the concentrated rainfall season in the semi-arid catchment of the LPR. Furthermore, these results also suggest that dissolved carbon may be an important external input of carbon into terrestrial ecosystems.
... The Yellow River contributed 16.8 Tg of PIC, which accounted for 60.9% of total PIC transported from the Yellow River to the China Seas. This was due to the severe soil erosion taking place in northwestern China (Miao et al. 2010, Gao et al. 2015a, b, Wang et al. 2017 and that the Yellow River has the largest sediment transport pathway between the three rivers , b, Wang et al. 2016. Song et al. (2016) reported that riverine DOC and POC concentrations in China decreased along with mean annual precipitation and mean annual temperatures, but riverine DOC and POC transport loads increased chiefly in conjunction with mean annual precipitation and mean annual temperatures. ...
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Rivers play an important role in carbon (C) exchange between terrestrial and oceanic water bodies and the atmosphere. The aim of this study was to systematically quantify fluxes in riverine C export and C exchange in the air–sea interface of marine ecosystems in China. Results show that annual C transport from rivers to coastal ecosystems in China can reach up to 64.35 TgC, which accounts for approximately 4.8%–8.1% of global C transport from river systems. In the Bohai Sea, particulate inorganic carbon is the main form of C influx, and it can reach up to 20.79 TgC/yr. Conversely, dissolved inorganic carbon is the main form of C influx into the East China Sea, and it can reach up to 10.52 TgC/yr, which is 42.6% of the total annual C imported into the East China Sea. China's marine ecosystems including the Yellow Sea, the Bohai Sea, the East China Sea, and the South China Sea can absorb 65.06 TgC/yr from the atmosphere.
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The sediment-reducing effect of check dams and the safety issues following dam breaks are long-standing concerns. This study analyzed the runoff change and sediment source during rainstorms in a small watershed using a multivariate mixed model and a comparative analysis of watersheds. The problem of sediment loss from dammed farmland following check dam break during rainstorms was evaluated. The results showed that the flood peak lag time (PLT) was significantly influenced by pre-soil moisture in cases of small amounts of rainfall but not during rainstorms. Ecological construction significantly reduced the linear correlation between rainfall and runoff modulus (RM). The reduction in sediment delivery modulus (SDM) due to the check dam was more significant than that in RM. The reduction in RM and SDM under rainstorm conditions were 16%–74% and 53%–93%, respectively. The contributions of inter-gully and gully lands to the sediment deposited in dammed farmland during a large rainstorm on July 26, 2017 were 38.07% and 61.93%, respectively. Soil erosion remained significant during large rainstorms. The increase in vegetation coverage on the hill slope increased the amount of sediment from gully lands. Check dam breaches have accounted for a loss of only 1.2% of the total area of the dammed farmland, and thus have not caused a large loss of sediment. However, breaches in them clearly increased the coefficient of variation of RM and SDM. Therefore, check dams have a critical effect on controlling sediment delivery at the watershed scale. Dam breaks do not result in a large percentage of sediment loss in the dammed farmland.
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The increasingly fragile ecological environment and associative nitrogen (N) biogeochemical cycle have become critical environmental and ecological issues in China's Loess Plateau. However, N flow and N source for typical catchments remains poorly understood in the Loess Plateau. In this study, we measured concentrations and isotopic signatures of N, hydrogen (H), and oxygen (O) in both rainfall and river water. Results showed that baseflow variation in total nitrogen (TN) concentrations ranged from 0.16 to 32.70 mg L‐1. The monthly TN deposition flux and monthly TN wet deposition concentration to river water were shown significant variations between rainy and dry seasons. The range of variation in δ2H values for rainfall and baseflow were from ‐90.0‰ to 19.8‰ and from ‐67.2‰ to ‐38.4‰, respectively, while δ18O‐H2O values ranged from ‐12.1‰ to +2.7‰ and from ‐9.3‰ to ‐3.6‰, respectively. The Local Meteoric Water Line (LMWL) in the check‐dam catchment was δ2H= 7.35 δ18O + 2.29 (R2=0.93). Furthermore, NO3‐ δ15N and δ18O values in baseflow ranged from ‐2.0‰ to +20.5‰ and from +8.0‰ to +15.6‰, respectively. The results indicated that rainfall was affected by below‐cloud secondary evaporation and caused strong isotopic kinetic fractionation to occur during the falling process. The NO3‐ in runoff mainly derived from the nitrification of soil organic matter (SOM), manure or sewage, for which the proportion of manure or sewage was from 50.5% to 83%.
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a b s t r a c t a r t i c l e i n f o Research on rivers has traditionally involved concentration and flux measurements to better understand weathering, transport and cycling of materials from land to ocean. As a relatively new tool, stable isotope measurements complement this type of research by providing an extra label to characterize origin of the transported material, its transfer mechanisms, and natural versus anthropogenic influences. These new stable isotope techniques are scalable across a wide range of geographic and temporal scales. This review focuses on three aspects of hydrological and geochemical river research that are of prime importance to the policy issues of climate change and include utilization of stable water and carbon isotopes: (i) silicate and carbonate weathering in river basins, (ii) the riverine carbon and oxygen cycles, and (iii) water balances at the catchment scale. Most studies at watershed scales currently focus on water and carbon balances but future applications hold promise to integrate sediment fluxes and turnover, ground and surface water interactions, as well as the understanding of contaminant sources and their effects in river systems.
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Flooding associated with tropical storms can cause extreme perturbations in riverine and coastal ecosystems. Measuring isotope variability of tropical storm events can help investigate the impacts of flooding. We measured the water isotope composition (δD and δ18O) of rain and associated floodwater collected during two storms and subsequent major and minor flooding events in the subtropical coast of eastern Australia. Compared with baseline regional rainfall isotope values of -15.0±1.9‰ for δD and -3.3±0.2‰ for δ18O, floodwater had lower values with -33.8±2.5‰ δD and -5.1±0.4‰ δ18O for the major flood and -29.4±1.0‰ δD and -4.6±0.1‰ δ18O for the minor flood. The low isotope composition of the floodwater was associated with the transport of large quantities of suspended sediments, with sediment loads 30 to 70 times larger than during base flow conditions. Floods carried up to 35% of the annual phosphorus and up to 208% of the currently calculated average annual nitrogen load of the Brisbane River. The dramatic changes caused by a rapid increase in discharge from 2 to 2015m3s-1 over 2days in the major flood would have major consequences in riverine and coastal ecosystems of the region. These changes could potentially be traced using the isotope composition of the floodwaters.
Chapter
Publisher Summary This chapter focuses on the uses of isotopes to understand water chemistry.I Isotopic compositions generally cannot be interpreted successfully in the absence of other chemical and hydrologic data. The chapter focusses on uses of isotopes in tracing sources and cycling of nitrogen in the water-component of forested catchment, and on dissolved nitrate in shallow waters, nutrient uptake studies in agricultural areas, large-scale tracer experiments, groundwater contamination studies, food-web investigations, and uses of compound-specific stable isotope techniques. Shallow waters moving along a flowpath through a relatively uniform material and reacting with minerals probably do not achieve equilibrium but gradually approach some steady-state composition. The chapter also discusses the use of isotopic techniques to assess impacts of changes in land-management practices and land use on water quality. The analysis of individual molecular components for isotopic composition has much potential as a method for tracing the source, biogeochemistry, and degradation of organic liquids and gases because different materials have characteristic isotope spectrums or biomarkers.
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Alpine shrubs and soils in catchments of the headwaters of the Yangtze River in west China are very important for reducing flooding and sustaining base flow during the summer. However, the contributions of precipitation, throughfall, and interflow to soil water and their effective contribution times in alpine shrub soil are not fully understood. In this study, we investigated a time series of stable isotopes in soil water, precipitation, canopy throughfall, interflow, litter, and humus water in a hillslope area. In addition, the spatial variation of water isotopes in soil profile in the Wolong Valley, located in the upper watershed of the Yangtze River, Sichuan, China, was considered. We found that (1) precipitation and throughfall significantly affected water isotopes in litter, humus, and shallow soil (0 cm to 50 cm deep) by affecting preferential flows. (2) Rainwater from a small precipitation event (about 4.0 mm d− 1) also penetrated soil to depths of 40 cm to 50 cm. (3) Interflow could comprise as much as 96% of water in soil columns during non-rainy days, but the proportion would decrease quickly after the precipitation amount reached more than 3 mm d− 1. (4) Mean effective contribution times of recharge in soil (0 cm to 50 cm deep) occurred 3 to 5 days despite the occurrence of large precipitation events (15.0 mm and 18.9 mm). Therefore, preferential flows composed of precipitation/throughfall and interflow were dominant in hillslope hydrology in the southeast edge of Tibet, which caused runoff to increase during the rainy season.
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This study demonstrates the application of an improved Evolutionary optimization Algorithm (EA), titled Multi-Objective Complex Evolution Global Optimization Method with Principal Component Analysis and Crowding Distance Operator (MOSPD), for the hydropower reservoir operation of the Oroville–Thermalito Complex (OTC) – a crucial head-water resource for the California State Water Project (SWP). In the OTC's water-hydropower joint management study, the nonlinearity of hydropower generation and the reservoir's water elevation–storage relationship are explicitly formulated by polynomial function in order to closely match realistic situations and reduce linearization approximation errors. Comparison among different curve-fitting methods is conducted to understand the impact of the simplification of reservoir topography. In the optimization algorithm development, techniques of crowding distance and principal component analysis are implemented to improve the diversity and convergence of the optimal solutions towards and along the Pareto optimal set in the objective space. A comparative evaluation among the new algorithm MOSPD, the original Multi-Objective Complex Evolution Global Optimization Method (MOCOM), the Multi-Objective Differential Evolution method (MODE), the Multi-Objective Genetic Algorithm (MOGA), the Multi-Objective Simulated Annealing approach (MOSA), and the Multi-Objective Particle Swarm Optimization scheme (MOPSO) is conducted using the benchmark functions. The results show that best the MOSPD algorithm demonstrated the best and most consistent performance when compared with other algorithms on the test problems. The newly developed algorithm (MOSPD) is further applied to the OTC reservoir releasing problem during the snow melting season in 1998 (wet year), 2000 (normal year) and 2001 (dry year), in which the more spreading and converged non-dominated solutions of MOSPD provide decision makers with better operational alternatives for effectively and efficiently managing the OTC reservoirs in response to the different climates, especially drought, which has become more and more severe and frequent in California.
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Forests play a vital role in global carbon (C) cycling. Accordingly, afforestation engineering programs that promote increased terrestrial C stocks are an important means to help gradually decrease atmospheric CO2 emissions. China, however, had increased its afforested area bordering hydroclimatic zones to 275.71 million hm2 between 1949 and 2010. Ecosystem water use efficiency (EWUE) and plant water use efficiency (PWUE) provide data on ecosystem sensitivity to water availability across rainfall regimes. The water consumption cost of C sequestration (WCCC) is also an important parameter that gauges the cost of C sequestration under afforestation. However, abrupt changes in EWUE and PWUE (threshold values of 1.5 and 3.6 gC kg−1 H2O, respectively) have been measured within the 400–500 mm precipitation climatic isoline boundary situated between semi-humid and arid zones. The threshold value of the corresponding WCCC was 1.0 kg H2O gC−1. Forest ecosystems in China typically generate high EWUE and PWUE values (2.80 ± 0.77 and 4.25 ± 1.02 gC kg−1 H2O, respectively) but low WCCC values (0.52 ± 0.42 kg H2O g−1 C), providing proof that afforestation is the best choice in increasing terrestrial C stocks. However, China's major afforestation engineering programs have concentrated efforts toward low EWUE and PWUE and high WCCC in the western region of the 400–500 mm precipitation isoline boundary, belonging to the arid and semiarid zones, which introduced potential environmental risks. Therefore, policies related to large-scale C sequestration initiatives under afforestation must first fully consider the statuses of WCCC and WUE.
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Urban watersheds are often sources of nitrogen (N) to downstream systems, contributing to poor water quality. However, it is unknown which components (e.g., land cover and stormwater infrastructure type) of urban watersheds contribute to N export and which may be sites of retention. In this study we investigated which watershed characteristics control N sourcing, biogeochemical processing of nitrate (NO3-) during storms, and the amount of rainfall N that is retained within urban watersheds. We used triple isotopes of NO3- (δ15N, δ18O, and Δ17O) to identify sources and transformations of NO3- during storms from 10 nested arid urban watersheds that varied in stormwater infrastructure type and drainage area. Stormwater infrastructure and land cover-retention basins, pipes, and grass cover-dictated the sourcing of NO3- in runoff. Urban watersheds were strong sinks or sources of N to stormwater depending on runoff, which in turn was inversely related to retention basin density and positively related to imperviousness and precipitation. Our results suggest that watershed characteristics control the sources and transport of inorganic N in urban stormwater but that retention of inorganic N at the timescale of individual runoff events is controlled by hydrologic, rather than biogeochemical, mechanisms.
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Groundwater is the primary water resource for human use on the Loess Tableland of China; however, the depletion of groundwater has been very severe in recent years. To understand the reasons for groundwater depletion, it is necessary to understand its recharge mechanisms. The objective of this study is to reveal the mechanisms of groundwater recharge on the Loess Tableland using deuterium and oxygen-18 in precipitation, in soil water under three land use types (natural grassland, wheat field, and apple orchard), and in groundwater. Results indicate that piston flow and preferential flow coexist in rainfall infiltration; however, their occurrences are greatly influenced by land use type. They both occurred in the natural grassland and wheat field; however, only piston flow was detected in the apple orchard. Groundwater is likely to be recharged by both piston flow and preferential flow through the unsaturated zone, while preferential flow should be the main recharge mechanism, as the isotopic composition of the groundwater is very close to that of precipitation while quite different from that of deep soil water. However, groundwater recharge by preferential flow only occurred in the natural grassland or wheat field, but not in apple orchard due to its thick dry soil layer. Regarding groundwater recharge by piston flow, it possibly occurs in most years for the natural grassland and in wet years for the wheat field; however, it is possibly difficult for the apple orchard to recharge groundwater by piston flow. The above results suggest that the conversion of farmland to apple orchard can affect the natural mode of the water cycle and reduce groundwater recharge on the Loess Tableland; therefore, attention should be paid to adjustments of land use.© 2014 American Society of Agricultural and Biological Engineers.
Article
Anthropogenic S emissions in the Athabasca oil sands region (AOSR) in Alberta, Canada, affect SO4 deposition in close vicinity of industrial emitters. Between May 2008 and May 2009, SO4-S deposition was monitored using open field bulk collectors at 15 sites and throughfall collectors at 14 sites at distances between 3 and 113 km from one of the major emission stacks in the AOSR. At forested plots >90 km from the operations, SO4 deposition was ∼1.4 kg SO4-S ha−1 yr−1 for bulk deposition and ∼3.3 kg SO4-S ha−1 yr−1 for throughfall deposition. Throughfall SO4 deposition rates in the AOSR exceeded bulk deposition rates at all sites by a factor of 2–3, indicating significant inputs of dry deposition especially under forest canopies. Both bulk and throughfall SO4 deposition rates were elevated within 29 km distance of the industrial operations with deposition rates as high as 11.7 kg SO4-S ha−1 yr−1 for bulk deposition and 39.2 kg SO4-S ha−1 yr−1 for throughfall at industrial sites. Sulfur isotope ratio measurements of atmospheric SO4 deposited in the AOSR revealed that at a few selected locations 34S-depleted SO4, likely derived from H2S emissions from tailing ponds contributes to local atmospheric SO4 deposition. In general, however, δ34S values of SO4 deposition at distant forested plots (>74 km) with low deposition rates were not isotopically different from δ34S values at sites with high deposition rates in the AOSR and are, therefore, not suitable to determine industrial S contributions. However, O isotope ratios of atmospheric SO4 in bulk and throughfall deposition in the AOSR showed a distinct trend of decreasing δ18O-SO4 values with increasing SO4 deposition rates allowing quantification of industrial contributions to atmospheric SO4 deposition. Two-end-member mixing calculations revealed that open field bulk SO4 deposition especially at industrial sites in close proximity (<29 km) to the operations is significantly (17–59%) affected by industrial S emissions and that throughfall generally contained 49–100% SO4 of industrial origin. Hence, it is suggested that δ18O values of SO4 may constitute a suitable tracer for quantifying industrial contributions to atmospheric SO4 deposition in the AOSR.
Article
Major ion composition of waters, δ13C of its DIC (dissolved inorganic carbon), and the clay mineral composition of bank sediments in the Brahmaputra River System (draining India and Bangladesh) have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputra mainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index of alteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in the Ganga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputra basin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicate erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rate of ∼300 t km−2 y−1, one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes much slower chemical erosion (∼40 t km−2 y−1). The abundance of total dissolved solids (TDS, 102–203 mg/L) in the time-series samples collected over a period of one year shows variations in accordance with the annual discharge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cyclic component) shows a strong positive correlation with Si, indicating their common source: silicate weathering. Estimates of silicate cations (Nasil+Ksil+Casil+Mgsil) suggest that about half of the dissolved cations in the Brahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO2 consumption rate due to silicate weathering in the Brahmaputra watershed is ∼6 × 105 moles km−2 y−1; whereas that in the Eastern Syntaxis subbasin is ∼19 × 105 moles km−2 y−1, similar to the estimates for some of the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of most intensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern Himalaya.
Article
Isotopic composition of dissolved inorganic carbon () in the Ottawa River basin is about −8 and −16‰ for lowland carbonate and upland silicate tributaries, respectively. This suggests that (1) the source of DIC to the Ottawa River is soil respiration and carbonate weathering, (2) exchange with the atmosphere is unidirectional or volumetrically unimportant, and (3) in-river respiration and photosynthesis are not significant influences on the river carbon budget. Accepting these constraints, chemical and isotopic data are used to reconstitute soil pCO2 for tributary catchments. Averages for upland silicate, mixed, and lowland carbonate basins are calculated to be roughly 2000, 5000, and 30,000 ppm, respectively. These values are used as input to model the pathway of carbon through the watershed—rain water to soil water to river water. The flux of carbon from the Ottawa River as DIC is calculated to be 4.3×1010 mol C/a. Utilizing carbon isotopes, 75% and 25% of the Ca2++Mg2+ flux is calculated to originate from carbonate and silicate weathering, respectively, and 61% of the DIC is calculated to originate from organic respiration. The latter represents some 6% of respired carbon in the basin, assuming an average respiration rate of 0.5 mmol C m−2 h−1. Based on a diffusion model, CO2 evasion to the atmosphere from the Ottawa River and its tributaries is estimated to be 1.3×1010 mol C/a or 30% of the DIC flux.
Article
Highlights ► Relationships between water resources of the Okanagan Valley remain poorly defined. ► Stable isotopes were used to characterize rainfall, rivers and lakes, and ground water resources. ► Precipitation isotopes resulted in a local meteoric water line of δD = 6.6 (δ18O) − 22.7 for the Okanagan. ► Isotopic mass-balance modeling suggested ∼35% of precipitation falling upon the watershed is lost to evaporation. ► Ground water in the valley aquifers is anthropogenically recharged by irrigation water from the Okanagan River.
Article
▪ Abstract Changes of the isotopic composition of water within the water cycle provide a recognizable signature, relating such water to the different phases of the cycle. The isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes. As a result, meteoric waters are depleted in the heavy isotopic species of H and O relative to ocean waters, whereas waters in evaporative systems such as lakes, plants, and soilwaters are relatively enriched. During the passage through the aquifers, the isotope composition of water is essentially a conservative property at ambient temperatures, but at elevated temperatures, interaction with the rock matrix may perturb the isotope composition. These changes of the isotope composition in atmospheric waters, surface water, soil, and groundwaters, as well as in the biosphere, are applied in the characterization of hydrological system as well as indicators of paleo-c...
Article
The shallowly buried marginal part of the Cambrian–Vendian confined aquifer system of the Baltic Basin is characterised by fresh and low δ18O composition water, whereas the deeply settled parts of the aquifer are characterized by typical Na–Ca–Cl basinal brines. Spatial variation in water geochemistry and stable isotope composition suggests mixing origin of the diluted water of three end-members—glacial melt water of the Weichselian Ice Age (115 000–10 000 BP), Na–Ca–Cl composition basin brine and modern meteoric water. The mixing has occurred in two stages. First, the intrusion and mixing of isotopically depleted glacial waters with basinal brines occurred during the Pleistocene glacial periods when the subglacial melt-water with high hydraulic gradient penetrated into the aquifer. The second stage of mixing takes place nowadays by intrusion of meteoric waters. The freshened water at the northern margin of the basin has acquired a partial equilibrium with the weakly cemented rock matrix of the aquifer.
Article
The main Patagonian rivers (Colorado, Negro, Chubut, Deseado, Coyle, Chico, Santa Cruz and Gallegos) were sampled between September 1995 and November 1998 to determine their chemical and isotopic compositions, the origins of the suspended and dissolved river loads and their inputs to the South Atlantic Ocean. This paper focuses on the dissolved inorganic carbon (DIC) transport and its δ13C isotopic signature. The δ13CDIC values vary between −12·8 and −1·8‰ and allow one to distinguish two river groups: (i) the Colorado, Negro, Chubut and Santa Cruz, which display the highest values and the lowest seasonal variations; (ii) the Deseado, Coyle, Chico and Gallegos, which show the lowest values and the highest seasonal variations. For the first group, δ13CDIC is mainly controlled by important exchanges between the river waters and atmospheric CO2, due to the presence of lakes and dams. For the second group, δ13CDIC also appears to be controlled by the oxidation of organic carbon, showing a negative relationship between δ13CDIC and the dissolved organic carbon. These biogeochemical processes interfere with the contribution of carbonate and silicate weathering to the riverine DIC and do not allow use of δ13CDIC alone to distinguish these contributions. The annual DIC flux exported by Patagonian Rivers to the South Atlantic Ocean averages 621 × 109 g. of C, i.e. a specific yield of 2·7 g m−2 year−1. The mean δ13CDIC can be estimated to − 4·9‰, which is high compared with other rivers of the world. Copyright
Article
Arctic amplification, the observation that surface air temperature changes in the Arctic exceed those of the Northern Hemisphere as a whole, is a pervasive feature of climate models, and has recently emerged in observational data relative to the warming trend of the past century. The magnitude of Arctic amplification is an important, but poorly constrained variable necessary to estimate global average temperature change over the next century. Here we evaluate the mechanisms responsible for Arctic amplification on Quaternary timescales, and review evidence from four intervals in the past 3 Ma for which sufficient paleoclimate data and model simulations are available to estimate the magnitude of Arctic amplification under climate states both warmer and colder than present. Despite differences in forcings and feedbacks for these reconstructions compared to today, the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4, suggesting that Arctic warming will continue to greatly exceed the global average over the coming century, with concomitant reductions in terrestrial ice masses and, consequently, an increasing rate of sea level rise.
Article
This study presents new δ18O and δD data from 191 streams across the Himalaya and Tibetan Plateau to better constrain the spatial variability of stable isotopes in modern precipitation over this region. Moisture penetrating into the southeastern Tibetan Plateau is predominantly derived from monsoonal airmasses originating from the Bay of Bengal and transported into the eastern Himalayan syntaxis along the Brahmaputra River. Progressive rainout during orographic lifting and cooling results in clear relationships between δ18O and δD and catchment hypsometric elevation on the plateau margin. However, monsoonal-derived moisture is progressively mixed with central Asian airmasses in more western and northern parts of the Tibetan Plateau. As a result, predicted isotope–elevation relationships that are based on empirical lapse rates or thermodynamic models of the isotopic evolution of an airmass produce large (1–3 km) misfits between measured and predicted catchment elevations for much of the Tibetan Plateau, including some areas directly north of the central Himalayan crest. This suggests that changes in the δ18O or δD of paleoprecipitation on the central and southwestern Tibetan Plateau may reflect surface uplift along moisture transport pathways or changes in the penetration of monsoonally-derived moisture rather than regional surface uplift histories.
Article
Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock–soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1–5 × 10−17 mol m−2 s−1) and illite (2–9 × 10−17 mol m−2 s−1) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases.
Article
Soil organic carbon (SOC) is one of the key components for assessing soil quality. Meanwhile, the changes in the stocks SOC may have large potential impact on global climate. It is increasingly important to estimate the SOC stock precisely and to investigate its variability. In this study, Yangjuangou watershed was selected to investigate the SOC distribution under different land uses. We found that SOC concentration decreased with increasing soil depth under all land uses and was significantly different across the vertical soil profile (P < 0.01). However, considering effect of land use on SOC, it is only significant (P < 0.01) in the topsoil (0–5 cm) layer. This indicated that land use has a large effect on the stocks of SOC in the surface soil. The stratification ratio of SOC > 1.2 may mean that soil quality is improving. The order of the SOC density (0–30 cm) under different land uses is forestland > orchard land > grassland > immature forestland > terraced cropland. The SOC stock is found to be as large as 2.67 × 103 t (0–30 cm) in this watershed. Considering time effect of restoration, the slope cropland just abandoned is more efficient for SOC accumulation than trees planted in the semi-arid hilly loess area.
Article
Evidence is accumulating that land use changes and other human activity during the past 100 to 200 years have contributed to decreased CH4 oxidation in the soil. Recent studies have documented the effect of land use change on CH4 oxidation in a variety of ecosystems. Increased N additions to temperate forest soils in the northeastern United States decreased CH4 uptake by 30 to 60%, and increased N fertilization and conversion to cropland in temperate grasslands decreased CH4 uptake by 30 to 75%.Using these data, we made a series of calculations to estimate the impact of land use and management changes which have altered soil the CH4 sink in temperate forest and grassland ecosystems. Our study indicates that as the atmospheric mixing ratio of CH4 has increased during the past 150 y, the temperate CH4 sink has risen from approximately 8 Tg y−1 to 27 Tg y−1, assuming no loss of land cover to cropland conversion. The net effect of intensive land cover changes and extensive chronic disturbance (i.e., increased atmospheric N deposition) to these ecosystems have resulted in about 30% reduction in the CH4 sink relative to the soil sink assuming no disturbance to any of the temperate ecosystems. This will impact the global CH4 budget even more as atmospheric CH4 concentrations increase and as a result of further disturbance to other biomes. Determining the reasons for the decreased CH4 uptake due to land disturbance is necessary to understand the role of CH4 uptake in conjunction with the increasing atmospheric CH4 concentrations. Without accounting for this approximately 20 Tg y−1 temperate soil sink, the atmospheric CH4 concentration would be increasing about 1.5 times the current rate.
Article
The stable carbon isotopic composition of dissolved inorganic carbon (δ<sup>13</sup>C<sub>DIC</sub>) is traditionally determined using either direct precipitation or gas evolution methods in conjunction with offline gas preparation and measurement in a dual-inlet isotope ratio mass spectrometer. A gas evolution method based on continuous-flow technology is described here, which is easy to use and robust. Water samples (100–1500 μl depending on the carbonate alkalinity) are injected into He-filled autosampler vials in the field and analysed on an automated continuous-flow gas preparation system interfaced to an isotope ratio mass spectrometer. Sample analysis time including online preparation is 10 min and overall precision is 0.1 ‰. This method is thus fast and can easily be automated for handling large sample batches.
Article
The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.
Article
Elevated concentrations of sodium (Na+) and chloride (Cl-) in surface and ground water are common in the United States and other countries, and can serve as indicators of, or may constitute, a water quality problem. We have characterized the most prevalent natural and anthropogenic sources of Na+ and Cl- in ground water, primarily in Illinois, and explored techniques that could be used to identify their source. We considered seven potential sources that included agricultural chemicals, septic effluent, animal waste, municipal landfill leachate, sea water, basin brines, and road deicers. The halides Cl-, bromide (Br), and iodide (I) were useful indicators of the sources of Na+-Cl- contamination. Iodide enrichment (relative to Cl-) was greatest in precipitation, followed by uncontaminated soil water and ground water, and landfill leachate. The mass ratios of the halides among themselves, with total nitrogen (N), and with Na+ provided diagnostic methods for graphically distinguishing among sources of Na+ and Cl- in contaminated water. Cl/Br ratios relative to Cl- revealed a clear, although overlapping, separation of sample groups. Samples of landfill leachate and ground water known to be contaminated by leachate were enriched in I and Br; this provided an excellent fingerprint for identifying leachate contamination. In addition, total N, when plotted against Cl/Br ratios, successfully separated water contaminated by road salt from water contaminated by other sources.