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Modeling Nitrate-Nitrogen Load Reduction Strategies for the Des Moines River, Iowa Using SWAT
Abstract
The Des Moines River that drains a watershed of 16,175 km(2) in portions of Iowa and Minnesota is impaired for nitrate-nitrogen (nitrate) due to concentrations that exceed regulatory limits for public water supplies. The Soil Water Assessment Tool (SWAT) model was used to model streamflow and nitrate loads and evaluate a suite of basin-wide changes and targeting configurations to potentially reduce nitrate loads in the river. The SWAT model comprised 173 subbasins and 2,516 hydrologic response units and included point and nonpoint nitrogen sources. The model was calibrated for an 11-year period and three basin-wide and four targeting strategies were evaluated. Results indicated that nonpoint sources accounted for 95% of the total nitrate export. Reduction in fertilizer applications from 170 to 50 kg/ha achieved the 38% reduction in nitrate loads, exceeding the 34% reduction required. In terms of targeting, the most efficient load reductions occurred when fertilizer applications were reduced in subbasins nearest the watershed outlet. The greatest load reduction for the area of land treated was associated with reducing loads from 55 subbasins with the highest nitrate loads, achieving a 14% reduction in nitrate loads achieved by reducing applications on 30% of the land area. SWAT model results provide much needed guidance on how to begin implementing load reduction strategies most efficiently in the Des Moines River watershed.
... It is widely recognized that there is no silver bullet [8] for resolving the 'wicked' problem of nonpoint source water pollution [9] in the Mississippi watershed. To achieve the 45% nutrient reduction goal, in-field nutrient management must be combined with edge-of-field measures as well as downstream nutrient removal practices [8,[10][11][12]. While agronomic and environmental management techniques to control and remove N loss have advanced, there is limited evidence that existing policies are effective in facilitating the adoption of these techniques [8,13,14]. ...
... At the aggregated level, post-application treatments like controlled drainage and wetland construction yield much larger N loss reductions (31 and 27 kg of N/ha) than split N application and the N loss tax (7 and 5 kg of N/ha) 10 . Removing one kg of N loss costs $1.8 by wetlands and $0.8 by controlled drainage 11 . ...
... The 28.9% simply represents the aggregated N loss rate at the national level. 10 These results are generally comparable to those recorded in the CEAP regional reports, although a straight comparison between the two may not be reasonable given the difference of the actions considered in each study. 11 This number accounts only the cost for the control system but not the installation of the subsurface-drains itself due to lack of information. ...
Reducing nutrient loss from agriculture to improve water quality requires a combination of management practices. However, it has been unclear what pattern of mitigation is likely to emerge from different policies, individually and combined, and what the consequences would be for local and national land use and farm returns. We address this research gap by constructing an integrated multi-scale framework for evaluating alternative nitrogen loss management policies for corn production in the US. This approach combines site- and practice-specific agro-ecosystem processes with a grid-resolving economic model to identify locations that can be prioritized to increase the economic efficiency of the policies. We find that regional measures, albeit effective in reducing local nitrogen loss, can displace corn production to the area where nitrogen fertilizer productivity is low and nutrient loss rate is high, thereby offsetting the overall effectiveness of the nutrient management strategy. This spatial spillover effect can be suppressed by implementing the partial measures in tandem with nationwide policies. Wetland restoration combined with split fertilizer application, along with a nitrogen loss tax could reduce nitrate nitrogen loading in the Mississippi River by 30% while only increasing corn prices by less than 2%.
... Land management practices that can prevent high N load are constantly being sought for mitigation and conservation purposes. For instance, researchers found that the total N load in rivers could be reduced by decreasing the N application in the area near watershed outlets (Schilling and Wolter, 2009). ...
... Although these models are able to generally predict the impacts of agricultural activities on N, some limitations hinder their effectiveness in assessing field scale spatio-temporal variation in N. SWAT and APEX, for example, both perform simulations on sub-basin or watershed levels (Schilling and Wolter, 2009;Tuppad et al., 2010). Since land management practices are typically implemented at the local scale, the ability of these numerical models to assess changes in N at this scale is crucial. ...
... Since land management practices are typically implemented at the local scale, the ability of these numerical models to assess changes in N at this scale is crucial. Furthermore, most model applications mainly focus on the growing seasons under average rainfall events without considering the uncertainties brought about by future climate (Schilling and Wolter, 2009). Since rainfall is the main driver of surface runoff that facilitates N losses to water bodies, assessing N load during rainfall events in a probabilistic manner is necessary. ...
The use of Nitrogen (N) fertilizer boosted crop production to accommodate 7 billion people on Earth in the 20th century but with the consequence of exacerbating N losses from agricultural landscapes. Land management practices that can prevent high N load are constantly being sought for mitigation and conservation purposes. This study was aimed at evaluating the impacts of different land management practices under projected climate scenarios on surface runoff linked N load at the field scale level. A framework to analyze changes in N load at a high spatiotemporal resolution under high greenhouse emission climate projections was developed using the Pesticide Root Zone Model (PRZM) for the Willow Creek Watershed in the Fort Cobb Experimental Watershed in Oklahoma. Specifically, 12 combinations of land management and climate scenarios were evaluated based on their N load via surface runoff from 2020 to 2070. Results showed that crop rotation practices lowered both the N load and the probability of high N load events. Spring application reduced the negative effects in summer and fall from other land management practices but at the risk of increased probability of generating high N load in April and May. The fertilizer application rate was found to be the most critical factor that affected the amount and the probability of high N load events. By adopting a target application management approach, the monthly maximum N can be decreased by 13% while the annual mean N load by 6%. The model framework and analysis method developed in this research can be used to analyze tradeoffs between environmental welfare and economic benefits of N fertilizer at the field scale level.
... Alternative tile drainage algorithms that were developed on the basis of the physically-based Hooghoudt and Kirkham tile drain equations have also been grafted into more recent SWAT version 2012 (SWAT2012) codes (Moriasi et al. 2012;. At present, over 50 studies have been documented that incorporate tile drain representation in SWAT (CARD, 2018), many of which have been performed in the U.S. Corn Belt region (e.g., Du et al., 2005;Green et al., 2006;Jha et al., 2007;Schilling et al. 2008;Schilling and Wolter, 2009;Moriasi et al. 2012;Yen et al., 2015;Gassman et al., 2017a;Ikenberry, 2017;Panagopoulos et al., 2015;Valcu-Lisman et al., 2017). ...
... The BRW was subdivided into 30 subwatersheds, which were further delineated by a total of 2,212 HRUs, to perform the SWAT simulations. The tile drain depth was set at 1,200 mm, which is consistent with other several other previous SWAT studies performed in the region (Jha et al., 2007;Schilling and Wolter, 2009) and it was assumed that all cropland less than 2% in slope was managed with tile drains. 2005), which have been used extensively to assess SWAT statistical accuracy (e.g., Bressiani et al., 2015). ...
... River was the subject of Total Maximum Daily Load (TMDL) assessment for the nitrate impairment of the drinking water source for the City of Des Moines (Schilling and Wolter, 2009). Using a SWAT modeling framework similar to the Boone River SWAT model, results from a 13-year simulation period (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) indicated that tile drainage accounted for 32% of the annual discharge in the Des Moines watershed. ...
The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales within the 2370 km² Boone River watershed (BRW), a subbasin within the Des Moines River watershed. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km², with downstream effects diminishing with increasing watershed size. Developing a better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export.
... Nonetheless, particularly for the purpose of investigating basin management strategies, most previous process-based watershed models were applied only for studies of fairly small basins (mostly much less than 20,000 km 2 ) that do not cover different climate regimes (e.g., Chaplot et al., 2004;Hu et al., 2007;Jha et al., 2007;Krause et al., 2008;Schilling & Wolter, 2009;Wendland et al., 2005). This was because of their focus on the local impacts of different farming practices or land use changes on river N exports. ...
... That is, a small reduction in N inputs have a greater improvement on water quality, perhaps because terrestrial and/or aquatic ecosystems in the Han River Basin, including the densely populated, large urban areas, are saturated with respect to N and do not have much capacity to assimilate marginal N inputs under current N loadings. This finding is consistent with a previous empirical study, showing a 33% decrease in Mississippi River nitrate-N exports in response to a 14.2% reduction in net anthropogenic N inputs (defined as the sum of fertilizer, fixation, and atmospheric deposition N inputs minus N exported in food and feed; McIsaac et al., 2001), yet the results differ from a modeling study of the Des Moines River Basin, which reported a 25.2% decrease in nitrate-N exports as a result of a 29.5% reduction in fertilizer (Schilling & Wolter, 2009). However, this discrepancy between our study and McIsaac et al. (2001) versus Schilling and Wolter (2009) is probably because the later only reduced fertilizer N inputs, and other inputs (e.g., atmospheric deposition) were not accounted for when calculating the input reduction percentage. ...
... This finding is consistent with a previous empirical study, showing a 33% decrease in Mississippi River nitrate-N exports in response to a 14.2% reduction in net anthropogenic N inputs (defined as the sum of fertilizer, fixation, and atmospheric deposition N inputs minus N exported in food and feed; McIsaac et al., 2001), yet the results differ from a modeling study of the Des Moines River Basin, which reported a 25.2% decrease in nitrate-N exports as a result of a 29.5% reduction in fertilizer (Schilling & Wolter, 2009). However, this discrepancy between our study and McIsaac et al. (2001) versus Schilling and Wolter (2009) is probably because the later only reduced fertilizer N inputs, and other inputs (e.g., atmospheric deposition) were not accounted for when calculating the input reduction percentage. ...
The spread of coastal hypoxia is a pressing global problem, largely caused by substantial nitrogen (N) exports from river basins to the coastal ocean. Most previous process-based modeling studies for investigating basin management strategies to reduce river N exports focused on the impacts of different farming practices or land use, used watershed models that simplified many mechanisms that critically affect the state of N storage in land, were limited mainly to fairly small basins, and did not span multiple climate regimes. Here we use a process-based land-river model to simulate historical (1999–2010) river flows and nitrate-N exports throughout the entire drainage network of South Korea (100,210 km²), which encompasses varying climate, land use, and hydrogeological characteristics. Based on projections by using multiple scenarios of N input reductions and climates, we explore the impacts of various ecosystem factors (i.e., N storage in basins, climate and its variability, anthropogenic N inputs, and basin location) on river nitrate-N exports. Our findings have fundamental implications for reducing coastal hypoxia: (1) a small reduction of N inputs in basins, including intensively utilized human land use, can have a greater improvement on water quality; (2) heightening climate variability may not increase long-term mean river N exports yet can significantly mask N input reduction effects by producing N export extremes associated with recurring coastal hypoxia; and (3) N exports to the coastal ocean can be most efficiently reduced by decreasing N inputs in subbasins, which are receiving high anthropogenic N inputs and are close to the coast.
... Water quality models are increasingly used to evaluate the effectiveness of BPMs for reducing pollutant loads or sediment losses from a watershed (Kirsch, Kirsch, & Arnold, 2002;Lamba, Thompson, Karthikeyan, Panuska, & Good, 2016;Schilling & Wolter, 2009). Models can simulate the effects of BMP implementation on a watershed scale and allow for comparisons among different load reduction strategies (Schilling & Wolter, 2009). ...
... Water quality models are increasingly used to evaluate the effectiveness of BPMs for reducing pollutant loads or sediment losses from a watershed (Kirsch, Kirsch, & Arnold, 2002;Lamba, Thompson, Karthikeyan, Panuska, & Good, 2016;Schilling & Wolter, 2009). Models can simulate the effects of BMP implementation on a watershed scale and allow for comparisons among different load reduction strategies (Schilling & Wolter, 2009). A number of models have been applied to address BMP optimization, such as the Soil Water and Assessment Tool (SWAT) (Ghebremichael, Veith, & Hamlett, 2013), the Annualized Agricultural Nonpoint Source model (AGNPS) (Bhuyan, Marzen, Koelliker, Harrington, & Barnes, 2002;Mostaghimi, Park, Cooke, & Wang, 1997), the Hydrological Simulation Program-FORTRAN (HSPF) (Chichakly, Bowden, & Eppstein, 2013;Shenk, Wu, & Linker, 2012) and the Storm Water Management Model (SWMM) (Lee et al., 2012). ...
... A number of models have been applied to address BMP optimization, such as the Soil Water and Assessment Tool (SWAT) (Ghebremichael, Veith, & Hamlett, 2013), the Annualized Agricultural Nonpoint Source model (AGNPS) (Bhuyan, Marzen, Koelliker, Harrington, & Barnes, 2002;Mostaghimi, Park, Cooke, & Wang, 1997), the Hydrological Simulation Program-FORTRAN (HSPF) (Chichakly, Bowden, & Eppstein, 2013;Shenk, Wu, & Linker, 2012) and the Storm Water Management Model (SWMM) (Lee et al., 2012). The SWAT model has emerged as one of the best available water quality models on the watershed and river basin scale for simulating the effectiveness of BMPs (Liu et al., 2013) and has been extensively used for a broad range of hydrological and environmental problems (Arabi, Frankenberger, Engel, & Arnold, 2008;Dixon & Earls, 2012;Gassman, Sadeghi, & Srinivasan, 2014;Schilling & Wolter, 2009;Ullrich & Volk, 2009). Especially in agricultural fields, numerous SWAT studies on agricultural BMPs (such as filter strips, contour farming, parallel terraces, grassed waterways and nutrient management plans) have been proposed to reduce the losses of sediment and nutrient loads at different spatial levels and temporal scales (Behera & Panda, 2006;Bracmort, Arabi, Frankenberger, Engel, & Arnold, 2006;Kaini, Artita, & Nicklow, 2012;Lam, Schmalz, & Fohrer, 2011;Tuppad, Kannan, Srinivasan, Rossi, & Arnold, 2010;Vach e, Eilers, & Santelmann, 2002). ...
Increasing fertilizer inputs in tea (Camellia sinensis L.) fields, especially in hilly areas, poses serious potential risks of surface water contamination. Best management practices (BMPs) have been extensively implemented to control nutrient loads from non-point sources. However, little research has been conducted to evaluate the effects of dynamic BMPs on nitrogen (N) load reductions in tea fields with different slope gradients. This study is conducted in subtropical tea fields in the Machuan River watershed, which adjoins Huangshan Mountain of Anhui Province in China. Three BMP scenarios, viz., avoiding fertilizing before rain, contour planting and applying slow-release fertilizers, are simulated to model the N load reduction using the Soil and Water Assessment Tool (SWAT). The objectives of this study are as follows: (1) to build a calibrated SWAT model for the Machuan River watershed, (2) to evaluate and compare the effects of three BMPs on N load reduction in tea fields with different slope gradients, and (3) to identify critical slope gradients and BMPs for effective N load control. The results indicate that three BMP scenarios significantly impact N losses reduction in tea fields and that their effects vary among different slopes and months. Compared with the baseline condition, the N losses rate could be reduced by approximately 24%, 28%, and 66% for three scenarios, respectively. Generally, tea planted on steep-sloping land leads to great losses of N, and tea fields on slopes exceeding 15° should be prioritized for BMP implementation as tea fields planted here tend to have the highest N losses. This will offer sound information for the development of better pollution-control strategies for tea fields in the Machuan River watershed.
... For the entire watershed, annual nitrate loading during the simulation period was 24.6 kg ha −1 yr −1 ranging from 14.0 to 50.7 kg ha −1 yr −1 . These values are in the range reported in other studies (Rode et al., 2009;Schilling and Wolter, 2009). Rode et al. (2009) reported the nitrate leaching under agricultural land of three catchments in Germany to range from 18.5 to 41.2 kg N ha −1 yr −1 . ...
... Rode et al. (2009) reported the nitrate leaching under agricultural land of three catchments in Germany to range from 18.5 to 41.2 kg N ha −1 yr −1 . Measured and SWAT simulated average annual nitrate load were 17.5 kg N ha −1 yr −1 and 18.0 kg N ha −1 yr −1 in an agricultural watershed in Minnesota (Schilling and Wolter, 2009). In our study, nitrate load experienced a substantial increase from growing season to non-growing season. ...
... Although N input to water bodies from agricultural non-point sources of pollutants are difficult to control, it is of prime importance to continually search for ways to reduce inputs of contaminants into surface waters (Larose et al., 2011). The reduction of fertilizer application is one of the most effective BMPs to minimize the nitrate pollution in agriculture areas (Lam et al., 2011;Schilling and Wolter, 2009;Yevenes and Mannaerts, 2011). Schilling and Wolter (2009), employing the SWAT model, showed that the reduction of fertilizer applications from 170 to 50 kg ha -1 achieved a 34.4% reduction in NO3-N load in the Des Moines River watershed in USA. ...
... The reduction of fertilizer application is one of the most effective BMPs to minimize the nitrate pollution in agriculture areas (Lam et al., 2011;Schilling and Wolter, 2009;Yevenes and Mannaerts, 2011). Schilling and Wolter (2009), employing the SWAT model, showed that the reduction of fertilizer applications from 170 to 50 kg ha -1 achieved a 34.4% reduction in NO3-N load in the Des Moines River watershed in USA. Yevenes and Mannaerts (2011), simulating land-use alternatives on NO3-N load with the SWAT model in Portugal, stated that a fertilizer reduction scenario was effectively implemented to evaluate remedial NO3-N control policies. ...
Water pollution by nitrogen originates at diffuse and point sources. In surface aquatic systems, nitrate is one of the most problematic forms of nitrogen, causing phytoplankton and macrophyte growth and consequently water eutrophication. This study evaluated whether the Soil and Water Assessment Tool (SWAT) model can simulate nitrate load in a rural watershed in daily and monthly time increments. The study investigated 462 km² of the upper part of the Stör catchment, a typical rural lowland catchment located in Northern Germany. The results showed that simulations of nitrate load at monthly increments are better predictors of observed data than daily simulations. The most effective practices to minimize the NO3-N load were the reduction of nitrogen fertilizer application and the increasing of conservation areas, such as field filter strips.
... It is well understood that tile drainage is the primary NO 3 -N loss pathway from agricultural fields. Thus far, studies estimating the tile NO 3 -N contributions at the watershed scale have often relied on models to delineate loads from groundwater, tile water, and surface water runoff (Schilling and Wolter 2009;Jha et al. 2010). Studies separating tile NO 3 -N from other pathways using field measurements are sparse. ...
... After adjusting the values reported in Fig. 8 to include just agricultural row crop areas in Otter Creek, the highest tile drain losses of nitrate are approximately 11 kg/ha. In tile drained areas on the Des Moines Lobe in central Iowa, nitrate yields commonly exceed 20 kg/ha (Ikenberry et al. 2014;Jha et al. 2010;Schilling and Wolter 2009). We suspect that the low yields measured in our study are due to the low flow conditions that persisted during 2015. ...
Nitrogen losses from artificially drained watersheds degrade water quality at local and regional scales. In this study, we used an end-member mixing analysis (EMMA) together with high temporal resolution water quality and streamflow data collected in the 122 km2 Otter Creek watershed located in northeast Iowa. We estimated the contribution of three end-members (groundwater, tile drainage, and quick flow) to streamflow and nitrogen loads and tested several combinations of possible nitrate concentrations for the end-members. Results indicated that subsurface tile drainage is responsible for at least 50% of the watershed nitrogen load between April 15 and November 1, 2015. Tiles delivered up to 80% of the stream N load while providing only 15–43% of the streamflow, whereas quick flows only marginally contributed to N loading. Data collected offer guidance about areas of the watershed that should be targeted for nitrogen export mitigation strategies.
... Other instances of the impact of conservation practices on agricultural operations and ecosystem restorations include a study which reported on non-point source pollution that resulted in a 95% increase in total nitrate content in the Des Moines River (Schilling and Wolter, 2009). The study found that applying nutrient management plans resulted in a 38% reduction in nitrate load. ...
Conservation practices such as crop rotation, filter strips, and constructed wetlands are nature-based approaches intended to safeguard natural resources in agricultural landscapes. In this study, we reviewed the literature on how conservation practices, both at watershed and field scales, have been proven to subdue flood peaks, surface runoff, soil erosion, sediment transport, and nutrient loss. We classified different conservation practices based on the mode of their application (i.e., in-field, edge-of-field, and structural practices) and described what prior research efforts have concluded about the efficacy of different practices. At the field scale, practices such as reduced or no-till farming, grassed waterways, and creation of wetlands significantly reduced the peak flow. Similarly, water quality was improved with implementation of conservation practices such as using cover crops, filter strips, and managing residue and tillage. The assessment of conservation practices across the literature was found to be challenging as different conservation practices showed a similar response, thus making it complex to assess the individual effect. A wide range of challenges related to the data, modeling/analysis, and management aspects of conservation practices were identified, and recommendations were provided to overcome these challenges.
... For example, the SWAT model was applied to evaluate the response of runoff to land cover and rainfall spatial variability in the Walnut Gulch Experimental Watershed in the early twenty-first century (Hernandez et al. 2000). Furthermore, the model was also applied to simulate streamflow and nitrate loads and evaluate a suite of basin-wide changes and targeting configurations to potentially reduce nitrate loads in the Des Moines River watershed (Schilling and Wolter 2009). The number of SWAT applications regarding LULC change assessment and nitrate loading significantly increased during the period 2010-2020 (Guse et al. 2015;Rajib et al. 2016;Liang et al. 2020;CARD 2022). ...
Tri An Reservoir is a vital source of water for agriculture, industry, hydropower, and public usage in Southern Vietnam. Due to human activities, water eutrophication has become a serious problem in recent decades. This study investigated for the first time the impact of land use and land cover (LULC) change on streamflow and nitrate load from the upstream Dong Nai River basin, which is the largest watershed of the reservoir. The study utilized several LULC scenarios, including LULC 2000, 2010, and 2020. The SWAT model was applied to model the watershed during the period 1997–2009. Results showed that the hydrological model performed satisfactorily based on the Nash–Sutcliffe efficiency (NSE) coefficient, the root mean square error observations standard deviation ratio (RSR), and the percent bias (PBIAS). The average simulated values of monthly streamflow and nitrate load were 453.7, 450.0, 446.7 m3/s and 17,699.43, 17,869.13, 17,590.81 tonnes for the LULC 2000, 2010, and 2020 scenarios, respectively. There were no significant differences in streamflow and nitrate load at the basin level under the different LULC scenarios. However, when looking at the subbasin level, there were differences in nitrate load among the scenarios. This suggests that the impacts of LULC on nitrate load may be more pronounced at smaller scales. Overall, our finding underscores the importance of modeling techniques in predicting the impacts of LULC change on streamflow and water quality, which can ultimately aid in the sustainable management of water resources.
... Detailed information is provided in previous studies. 48 To characterize the microbial nitrogen transformation level in the river of the three groups, the riverine microbial-related parameters were made identical for each reach to ensure that the SWAT-simulated riverine microbial nitrogen transformation was identical in all rivers. The relative deviations (i.e., the deviation between the measured and simulated concentrations divided by the measured concentrations) of nitrogen species varied among the different regions (Figure 5a−c). ...
The characterization of variations in riverine microbiota that stem from contaminant sources and transport modes is important for understanding biogeochemical processes. However, the association between complex anthropogenic nitrogen pollution and bacteria has not been extensively investigated owing to the difficulties faced while determining the distribution of nitrogen contaminants in watersheds. Here, we employed the Soil and Water Assessment Tool alongside microbiological analysis to explore microbial characteristics and their responses to complex nitrogen pollution patterns. Significant variations in microbial communities were observed in sub-basins with distinct land-water pollution transport modes. Point source-dominated areas (PSDAs) exhibited reduced microbial diversity, high number of denitrification groups, and increased nitrogen cycling compared with others. The negative relative deviations (-3.38) between the measured and simulated nitrate concentrations in PSDAs indicated that nitrate removal was more effective in PSDAs. Pollution sources were also closely associated with microbiota. Effluents from concentrated animal feeding operations were the primary factors relating to the microbiota compositions in PSDAs and balanced areas. In nonpoint source-dominated areas, contaminants from septic tanks become the most relevant sources to microbial community structures. Overall, this study expands our knowledge regarding microbial biogeochemistry in catchments and beyond by linking specific nitrogen pollution scenarios to microorganisms.
... Since the process of pollution sources entering into the water body is very complex, there are two major commonly used methods available for estimating pollution load: the ECM and the mechanism model (Zhou et al., 2014). Although SWAT, MIKE, HSPF, EDFDC, and other models have the functions of spatial information analysis, database construction, mathematical calculation, and visual expression and are widely used in basin pollution load and water quality prediction, they require more basic data (Wool et al., 2003;Schilling Keith and Wolter Calvin, 2009;Lee et al., 2018;Borah et al., 2019;Yonce et al., 2019;Hively et al., 2020). Restricted by the data constraints of the study areas, we adopted an export coefficient method for the estimation of the exogenous pollution load of the Zhangze Reservoir and the flux model method for endogenous sediment release. ...
Raising the water pollution control countermeasures on the basis of rational pollution load estimation is significant for improving water quality. Zhangze Reservoir, the largest water body in Changzhi city, China, was selected for this study. Considering the information constraints of the reservoir basin, the pollution load estimation method system covering point sources, non-point sources, and internal sources is systematically constructed using an export coefficient model, an sediment pore water diffusion model, and other methods, with the aim of estimating the discharge of pollutants into the Zhangze Reservoir from domestic sources, industrial sources, agricultural sources, sediment release, and urban runoff. The findings indicate the following: 1) the pollution loads of COD, NH3-N, TN, and TP are 3,157.82t, 177.62t, 760.72t, and 42.29t, respectively; 2) in terms of the distribution of pollution sources, urban domestic sources top the rest, accounting for 65.47% of COD, 82.83% of NH3-N, 79.00% of TN, and 60.65% of TP, followed by the agricultural source; 3) the total discharge of the main water pollutants exceeds the water environmental capacity, characterized as the dominating existence of domestic point source, the coexistence of point sources and non-point sources, and the coexistence of exogenous sources and endogenous sources. Finally, control countermeasures are raised to minimize the total discharge of pollutants for improving the water quality.
... It is widely recognized that there is no silver bullet for resolving the wicked problem of nonpoint source water pollution in the Mississippi watershed [8]. To achieve the 45% nutrient reduction goal, in-field nutrient management must be combined with edge of field measures as well as downstream nutrient removal practices [8,9,10,11]. While agronomic and environmental management techniques to control and remove lost N have advanced, there is limited evidence that existing policies are effective in facilitating the adoption of these techniques [12,13,14]. ...
Reducing the size of the hypoxic zone in the Gulf of Mexico has proven to be a challenging task. A variety of mitigation options have been proposed, each likely to produce markedly different patterns of mitigation with widely varying consequences for the economy. The general consensus is that no single measure alone is sufficient to achieve the EPA Task Force goal for reducing the Gulf hypoxic zone and it appears that a combination of management practices must be employed. However, absent a highly resolved, multi-scale framework for assessing these policy combinations, it has been unclear what pattern of mitigation is likely to emerge from different policies and what the consequences would be for local, regional and national land use, food prices and farm returns. We address this research gap by utilizing a novel multi-scale framework for evaluating alternative N loss management policies in the Mississippi River basin. This combines fine-scale agro-ecosystem responses with an economic model capturing domestic and international market and price linkages. We find that wetland restoration combined with improved N use efficiency, along with a leaching tax could reduce the Mississippi River N load by 30-53\% while only modestly increasing corn prices. This study underscores the value of fine-resolution analysis and the potential of combined economic and ecological instruments in tackling nonpoint source nitrate pollution.
... Therefore, detailed geographical information and accurate model parameters are crucial to produce reliable tile drain effects within ecohydrological model simulation (Moriasi et al., 2013;Valayamkunnath et al., 2020). Accurate accounting of nitrogen (N) fertilizer application rates is equally important for model simulations because N is one of the most important nutrients used to enhance corn productivity (Gassman et al., 2017a;Yang et al., 2016) and contributes to serious water quality impairment across the study area (Schilling and Wolter, 2009;Gassman et al., 2017a). Corn growth and yield could be negatively affected by inadequate N fertilizer amounts in model development, causing misleading water balance simulations. ...
Improving food systems to address food insecurity and minimize environmental impacts is still a challenge in the 21st century. Ecohydrological models are a key tool for accurate system representation and impact measurement. We used a multi-phase testing approach to represent baseline hydrologic conditions across three agricultural basins that drain parts of north central and central Iowa, U.S.: the Des Moines River Basin (DMRB), the South Skunk River Basin (SSRB), and the North Skunk River Basin (NSRB). The Soil and Water Assessment Tool (SWAT) ecohydrological model was applied using a framework consisting of the Hydrologic and Water Quality System (HAWQS) online platform, 40 streamflow gauges, the alternative runoff curve number method, additional tile drainage and fertilizer application. In addition, ten SWAT baselines were created to analyze both the HAWQS parameters (baseline 1) and nine alternative baseline configurations (considering the framework). Most of the models achieved acceptable statistical replication of measured (close to the outlet) streamflows, with Nash-Sutcliffe (NS) values ranging up to 0.80 for baseline 9 in the DMRB and SSRB, and 0.78 for baseline 7 in the NSRB. However, water balance and other hydrologic indicators revealed that careful selection of management data and other inputs are essential for obtaining the most accurate representation of baseline conditions for the simulated stream systems. Using cumulative distribution curves as a criterion, baselines 7 to 10 showed the best fit for the SSRB and NSRB, but none of the baselines accurately represented 20% of low flows for the DMRB. Analysis of snowmelt and growing season periods showed that baselines 3 and 4 resulted in poor simulations across all three basins using four common statistical measures (NS, KGE, Pbias, and R²), and that baseline 9 was characterized by the most satisfactory statistical results, followed by baselines 5, 7 and 1.
... Among a plethora of hydrological models, United States Department of Agriculture's (USDA) SWAT has gained popularity for its wide range of applications in hydrological studies. SWAT is a GIS-based, semi-distributed, and continuous time step hydrological model that allows faster implementation of input datasets over large river basin and provides an efficient and affordable insight into the effects of land use change on water quantity and quality , Schilling and Wolter, 2009, Volk et al., 2009 ). The SWAT model has been implemented across several watersheds, climatic zones, environmental conditions, and management systems worldwide. ...
Rapid economic development in India has intensified the changes in natural land cover and thus affects the availability of water resources. The present study aims to analyze past land use changes for three time periods and assess their impacts on water resources in the Upper Bhima basin. An object-based image analysis (OBIA) technique was used to identify the progressive land use changes for the year 1992, 2000, and 2009. Soil and Water Assessment Tool (SWAT), an open source hydrological model was used to simulate hydrological processes and assess the impacts of land use change on surface runoff, groundwater flow, lateral flow, percolation, and evapotranspiration in the basin. The model was calibrated and validated for a baseline period (1985-1994) using 1992 land use/land cover (LULC) map. The model calibration and validation is found to be satisfactory. The optimized SWAT model parameters along with 1992 and 2009 LULC maps were used to quantify the impacts in the Upper Bhima basin. The major land use changes were identified as conversion from waste land and agricultural land to built up area (2.6 % in 1992 to 7.3% in 2009). The modeling results showed that urbanization has a significant (p<0.001) positive relation with surface runoff (R = 0.96), water yield (R = 0.57), and a negative relationship with baseflow (R = -0.95), and percolation (R = -0.99). The study also revealed that the nature of LULC change has differential impacts at both basin and sub-basin scale. At basin level, the overall impacts of LULC change on hydrological parameters are small however at sub-basin level, the surface runoff and water yield has increased up to 13.6%, 8%, respectively. Similarly, the lateral flow changed from -9% in sub-basin 42 to 10% in sub-basin 20 and evapotranspiration changed from -3% in sub-basin 43 to 59% in sub-basin 6.
... Floodplains in the region are typically composed of fine-textured alluvium sourced from silty upland glacial sediments. While the landscape is generally defined as relic or well-developed, extensive soil erosion and deposition has been enhanced by post-settlement agricultural practices (Cruse et al., 2006) and postsettlement deposition of floodplain soils (Schilling & Wolter, 2009 15% Phleum pratense (timothy). Buffer vegetation was generally healthy and well managed by the landowner. ...
Agricultural drainage tiles are primary contributors to NO3-N export from Iowa croplands. Saturated buffers are a relatively new conservation practice that diverts tile water into a distribution tile installed in a riparian buffer parallel to a stream with the intent of enhancing NO3-N processing within the buffer. In this study, tile NO3-N concentration reductions were characterized through two different saturated buffers at a working farm site in eastern Iowa. Study objectives were to (1) evaluate the hydrogeology and water quality patterns in the saturated buffer and (2) quantify the reduction in tile NO3-N concentration from the saturated buffer installation. Results showed that the two saturated buffers are reducing NO3-N concentrations in tile drainage water from input concentrations of approximately 15 mg/l to levels < 1.5 mg/l at the streamside well locations. The reduction occurs rapidly in the fine-textured and organic-rich alluvial soils with most of the reduction occurring within 1.5 m of the distribution line. Denitrification is hypothesized as being primarily responsible for the concentration reductions based on soil and water chemistry conditions, completion of a geophysical survey (quantifying low potential for N loss to deeper aquifers), and comparisons to other similar Iowa sites. The study provides more assurance to new adopters that this practice can be installed in many areas throughout the Midwestern Cornbelt region.
... Numerous methods have been proposed to identify best management practices (BMPs) for nutrient management. BMPs include efficient use of irrigation water(Abdelraouf and Ragab, 2018;Li et al., 2018;Zhang et al., 2018), decreasing fertilizer loadingSchilling and Wolter, 2009), enhancing soil covers(Gaynor and Findlay, 1995;Bosch et al., 2013;, and associated combinations(Novotny, 2002;Rong and Xuefeng, 2011;Chukalla et al., 2018;. Often, the effect of proposed BMPs on N and P concentrations and loadings is evaluated using computational modeling tools. ...
Nitrogen (N) and Phosphorus (P) are essential elements for animal nutrition and plant growth. However, over the previous decades, excessive loading of fertilizers in agricultural activities has led to elevated concentrations of N and P contaminations in surface waters and groundwater worldwide and associated eutrophication. Therefore, precisely understanding and representation of water movement and fate and transport of N and P within a complex dynamic groundwater-surface water system affected by agricultural practices is of essential importance for sustaining ecological health of the stream-aquifer environment while maintaining high agricultural productivity. Modeling tools often are used to assess N and P contamination and evaluate the impact of management practices. Such models include land surface-based watershed models such SWAT, and aquifer-based models that simulate spatially-distributed groundwater flow. However, SWAT simulates groundwater flow in a simplistic fashion and therefore is not suited for watersheds with complex groundwater flow patterns and groundwater-surface interactions, whereas groundwater models do not simulate land surface processes.
This dissertation establishes the modeling capacity for assessing the movement, transformation, and storage of nitrate (NO3) and soluble P in intensively managed irrigated stream-aquifer systems. This is accomplished by (1) developing a method to apply the SWAT model to such a system, and includes: designating each cultivated field as an individual hydrologic response unit (HRU), crop rotations to simulate the impact of changing crop types for each cultivated field, including N and P mass in irrigation water, and seepage from earthen irrigation canals into the aquifer; (2) simulating land surface hydrology, groundwater flow, and groundwater-surface water interactions in the system using the coupled flow model SWAT-MODFLOW, with the enhanced capability of linkage between SWAT groundwater irrigation HRUs and MODFLOW pumping cells, and the use of MODFLOW’s EVT package to simulate groundwater evapotranspiration; and (3) linking RT3D, a widely used groundwater reactive solute transport model, to SWAT-MODFLOW to credibly represent of NO3-N and soluble P fate and transport processes in irrigated agroecosystems to evaluate best management practices for nutrient contamination. This last phase will also address the uncertainty in system output (in-stream nutrient loads and concentrations, groundwater nutrient concentrations model predictions).
Each modeling phase is applied to a 734 km2 study region in the Lower Arkansas River Valley (LARV), an alluvial valley in Colorado, USA, which has been intensively irrigated for over 100 130 years and is threatened by shallow water tables and nutrient contamination. Multiple best management practices (BMPs) are investigated to analyze the effectiveness in reducing NO3-N and soluble P contamination in the LARV. These strategies are related to irrigation management, nutrient management, water conveyance efficiency, and tillage operations. The most effective individual BMP in most areas is to decrease fertilizer by 30%, resulting in average NO3-N and soluble P concentrations within the region could be reduced by 14% and 9%, respectively. This individual BMP could lower the average NO3-N concentrations by 19% and soluble P concentrations by 2%. Combinations of using 30% irrigation reduction, 30% fertilization reduction, 60% canal seepage, and conservation tillage are predicted to have the greatest overall impact that can not only provide a decrease of groundwater concentration in NO3-N up to 41% and soluble P concentration up to 8%, but also reduce the median of the in-stream NO3-N and soluble P to meet the Colorado interim standard. As nutrient conditions within the Lower Arkansas River Valley are typical of those in many other intensively irrigated regions, the results of this dissertation and the developed modeling tools can be applied to other watersheds worldwide.
... Surface waters located within the Upper Mississippi River basin contain some of the highest concentrations of nonpoint source NO 3 − in the USA (David et al., 2010;Schilling et al., 2012). Nitrate as nitrogen concentrations in surface waters regularly exceed the US Environmental Protection Agency (USEPA) maximum contaminant level for drinking water of 10 mg/L (Jha et al., 2010;Schilling & Wolter, 2009). Excess NO 3 − in surface waters leads to eutrophication, the enrichment of an aquatic ecosystem with excess nutrients (Boesch, 2002;Nixon, 1995;Ryther & Dunstan, 1971). ...
n agricultural land-use regions, excess nitrate in the soil contributes to eutrophication and pollution of both surface and subsurface waters. This study examines the role of plant uptake within the vadose zone of a saturated riparian buffer (SRB) to reduce nitrate from the soil porewater. Two hypotheses were explored:1) During the growing season, nitrate removal will be greater in the presence of plants than where plants are absent (barren) and 2) Following the growing season, nitrate concentration in the soils underlying a barren plot will be less than in the soils underlying a plot with plants. Within the SRB, three experimental blocks each composed of two plots were established. One plot allowed the growth of plants, primarily switchgrass (Panicum virgatum L.); the other plot was barren. Statistical comparison of among the treatments, pre-growing season, plot with plants, and barren plot, and among the different depths, 30 cm, 60 cm, and 90 cm identified significantly different soil NO3--N concentrations. Plots with plants experienced a reduction in nitrate from the soil and vadose waters. Nitrate concentrations in the soils underlying the barren plot were higher than in the soils with plants, a result of no plant uptake and of the decomposition of the residual plants material returning nitrogen to the vadose. Sustained nitrate concentrations imply plants withdraw nitrate from the vadose zone, generating organic matter that is recycled year-to-year, suggesting that the plants serve as a short-term nitrate sink.
... Numerous methods have been proposed to identify best management practices (BMPs) for nutrient management. BMPs include efficient use of irrigation water (Abdelraouf and Ragab, 2018;Li et al., 2018;Zhang et al., 2018), decreasing fertilizer loading (Almasri and Kaluarachchi, 2007;Schilling and Wolter, 2009), enhancing soil covers (Gaynor and Findlay, 1995;Bosch et al., 2013;Almendinger and Ulrich, 2017), and associated combinations (Novotny, 2002;Rong and Xuefeng, 2011;Zhang et al., 2013;Dozier et al., 2017;Chukalla et al., 2018;Robertson et al., 2018;Deng and Bailey, 2020). Often, the effect of proposed BMPs on N and P concentrations and loadings is evaluated using computational modelling tools. ...
In irrigated semi-arid watersheds, over-fertilization often leads to nitrogen (N) and phosphorus (P) contamination in aquifers and river systems. Modelling tools often are used to evaluate the effect of management practices on nutrient contamination levels. In this study, we assess a suite of best management practices (BMPs) for N and P in a regional irrigated stream-aquifer system using a new numerical model. As it is necessary to consider the interaction of flow in the stream and aquifer, SWAT-MODFLOW model that couples surface and groundwater flow was used. The reactive processes were modelled using RT3D that simulate the reactive groundwater transport. The model is based on the coupled flow model SWAT-MODFLOW, with the groundwater reactive transport model RT3D included to simulate the reactive groundwater transport of NO3 and soluble P and their interactions within the soil-aquifer-stream system. The assessment is performed for a highly managed 732 km² region in the Lower Arkansas River Valley (south-eastern Colorado), with model results compared to observed groundwater nutrient concentration, in-stream nutrient concentration and loading, and crop yield. A total of 28 BMPs scenarios are evaluated regarding their impact on NO3 and soluble P contamination in the aquifer and river system. The most effective individual BMP in most areas is to decrease fertilizer by 30%, resulting in NO3 and soluble P reduced by 20% and 2% for groundwater concentrations, 25% and 10% for river concentrations, and 27% and 6% for mass loadings into surface water, respectively. Combinations of using 30% irrigation reduction, 30% fertilization reduction, 60% canal seepage reduction, and conservation tillage yield the greatest overall impact to lower NO3 and soluble P concentrations up to 41% and 8% in groundwater, 52% and 40% in river, and 63% and 49% for mass loadings. Targeting BMPs on localized problem areas shows great promise in reducing contamination while maintaining region-wide crop yield. The study demonstrates the SWAT-MODFLOW-RT3D modelling code is a useful tool to examine NO3 and P transport and quantify BMP effects in groundwater-driven watersheds.
... The physical-based model, with a physicochemical background, has been used to simulate chemical compounds in aquatic systems (Mostaghimi, 2003;Nasr et al., 2007). Among the models, the Soil and Water Assessment Tool (SWAT; Arnold et al. (1998)) is popular in simulating numerous compounds (e.g., phosphorous, nitrogen, and organic carbon) (Nasr et al., 2007;Oeurng et al., 2011;Schilling and Wolter, 2009). This is because the physical-based model can consider the hydrological process and the farm plan that can influence the transport and source of the pollutants (Douglas-Mankin et al., 2010). ...
In recent years, as agricultural activities and types of crops have become diverse, the occurrence of micro-pollutants has been reported more frequently in rural areas. These pollutants have detrimental effects on human health and ecological systems; thus, it is important to manage and monitor their presence in the environment. The modeling approach could be an effective way to understand and manage these pollutants. This study predicts the concentrations of micro-pollutants (MPs) using deep learning (DL) models, and the results are then compared with simulation results obtained from the soil water assessment tool (SWAT) model. The SWAT model showed an unacceptable performance owing to the resulting negative Nash–Sutcliffe efficiency (NSE) values for the simulations. This may be caused by the limitations of SWAT, which pertains to adopting simplified equations to simulate micro-pollutants. In addition, the ambiguous plan of pesticide application increased the model uncertainty, thereby deteriorating the model result. Here, we developed two different DL models: long short-term memory (LSTM) and convolutional neural network (CNN). LSTM exhibited the highest model performance, with NSE values of 0.99 and 0.75 for the training and validation steps, respectively. In the multi-target MP model, the error decreased as the number of simulated pollutants increased. The simulation of the four pollutants had the highest error, while the six-target simulation had the lowest error. In conclusion, this study demonstrated that the LSTM model has the potential to improve the prediction of MPs in aquatic systems.
... Several studies have been performed on nitrate-N load estimation using various approaches. Soil and water assessment tool (SWAT) and hydrological simulation program-Fortran, which are process-based models, were used to calculate nitrate-N loads using the product of discharge and nitrate-N concentration [8][9][10]. A multiple log-linear regression equation known as LOADEST was used for solving uncertainties in estimating nitrate-N load [11][12][13]. ...
The long short-term memory (LSTM) model has been widely used for a broad range of applications entailing the estimation of variables in different fields to improve water quality management in rivers. The main objectives of this study are (1) to develop a novel LSTM-based model for the estimation of nitrate-N loads, which adversely affect water resources, and (2) to evaluate the performance of the model by comparing it with that of Monte Carlo sub-sampling and the weighted regressions on time discharge and season (WRTDS) model. We evaluated the model performance using various numbers of hidden layers, ranging from one to four, in the LSTM model to determine the appropriate number of hidden layers; furthermore, we applied the sampling frequencies of 6, 12, and 24 to assess their impact. Seven polluted river basins in the United States were used for analysis, and the relative root mean squared error (rRMSE) and the mean percentage error (MPE) metrics were applied for the validation of the model estimates. The proposed model achieved accurate nitrate-N load estimates using three to four hidden layers, and improved model performance was observed when the sampling frequency was increased. The differences among the results obtained using the LSTM model were examined based on a binning technique via a log-log plot of nitrate-N concentration against discharge. The binning analysis showed that the slope obtained from the average rates of discharge and low discharge values apparently influenced the estimates. Furthermore, box plot analyses of the statistical indices such as rRMSE and MPE demonstrate that the LSTM model seems to exhibit better performance than the WRTDS model. The results of the examination demonstrate that the LSTM model may be a good alternative with regard to estimating nitrate-N loads for the control of water quality constituents.
... Intensely drained row crop areas have a major impact on stream nitrate loading in Iowa. For example, Jha et al. (2010) reported that 92 % of the NO 3 -N load in the Raccoon River was derived from tiled row crop sources and in the Des Moines River, the percentage was even higher (95 %, Schilling and Wolter, 2009). Hence, we consider the nutrient loads exported by tiles and groundwater from our monitored farm field to be consistent with typical DML conditions. ...
It is understood that the major transport pathways for the soluble nutrients nitrate (NO3-N) and orthophosphorus (OP) from cropped fields to streams in the U.S. Cornbelt are tile drainage and groundwater seepage. The relative contribution of each, however, has not been well quantified and can vary between fields and watersheds. In this study, we used intermittent grab sample water quality monitoring and tile flow measurements, and the groundwater model MODFLOW to source dissolved nutrients from a cropped field to a low-order stream in the intensively-drained and cropped Des Moines Lobe landform of north central Iowa. Based on monitoring of eight tile outlets, nine groundwater wells and the receiving stream over a two-year period, nutrient loads from tiles were found to contribute approximately 98 % of the nitrate load to Hardin Creek. The loading pattern for OP was also dominated by discharge from constructed drainage, with 99.7 % sourced back to field tiles. Results from the farm field fit within the scaling pattern observed within the Des Moines Lobe region of Iowa showing that water yields and NO3-N loads are dominated by tile drainage at the field and local watershed scale. Loads dominated by tile flows suggest edge-of-field interception and treatment of tile water for mitigation of stream nutrients. Study results are consistent with local and regional assessments showing tile drainage to be an important pathway for both water and nutrients in the stream network of the US Cornbelt.
... In addition, at least 80 SWAT-related studies report some combination of evaluating pollution emanating from both diffuse and point sources (CARD, 2019). Over 50% of these combined pollutant source studies report results for a single constiuent, e.g., nitrate (Jha et al., 2010;Lam et al., 2010;Martinkova et al., 2018;Schilling and Wolter, 2009), total nitrogen (Motallebi et al., 2017;Yang et al., 2017), phosphorus (Kirsch et al., 2002;Makarewicz et al., 2015;White et al., 2014) and pathogens (Bai et al., 2016;Coffey et al., 2010). Other SWAT-based studies report the impacts of accounting for diffuse and point sources on multiple constituents including Grizzetti et al. (2003), Panagopoulos et al. (2015, and Santhi et al. (2001). ...
Although multiple pollution sources could discharge significant nutrient loads within a river basin, few studies have attempted to analyze the spatiotemporal distribution patterns of nutrient pollution source compositions across a basin. Using the Xiaohong River Basin as a case example, this study aims to establish a feasible framework for characterizing the spatiotemporal patterns of regional pollution source compositions that could be implemented in watersheds worldwide. Firstly, SWAT (Soil and Water Assessment Tool) was used to simulate nutrient loads from all main human activities in the basin. Driven by hourly rainfall, the SWAT model was able to simulate both monthly total nitrogen (TN) and total phosphorous (TP) loads in the basin satisfactorily. Source attribution of TN and TP loads through SWAT scenario analysis combined with the k-means clustering analysis was then used to characterize the spatiotemporal distribution patterns in the nutrient pollution source compositions. A total of six types of TN pollution source compositions and five types of TP pollution source compositions have been identified across the region. In general, crop production dominated TN load contributions in 24 sub-basins along the main reach and upstream tributaries, concentrated animal feeding operations (CAFOs) joined crop production as the leading source in eight sub-basins, and wastewater treatment plants (WWTPs) and industry contributed significant TN loads in the remaining three sub-basins. For TP, crop production dominated load contributions in six sub-basins along the upstream tributaries, CAFOs and crop production dominated in 28 sub-basins, while WWTPs were the leading source in the remaining sub-basin. Therefore, implementing effective programs to promote the utilization of organic fertilizers from animal manure for crop production is essential to alleviate the serious nutrient pollution situation in the basin.
... Many studies revealed the important contribution of nonpoint load from agriculture land to nitrate export and highlighted the importance of agriculture strategy on stream water quality (Flipo et al. 2007;Hesse et al. 2008;Huang et al. 2009;Schilling and Wolter 2009;Tong and Chen 2002). In the case of the Joumine basin, point and diffuse loads are the main pollution sources of water enrichment. ...
The protection of the aquatic environment while managing the risk of water scarcity in the Mediterranean region is
challenging. Ensuring future sustainability of water resources needs improved monitoring networks and early warning
system of future trends of water quality. A specific concern is given to nonpoint source pollution from agricultu re, which is
often the main source of water quality degradation in rivers. In this work, we focused on the Joumine river basin, a rural-
catchment situated north Tunisia dominated by agricultural activities and exposed to e utrophication problems. Aiming to
present an assessment framework of the spatial–temporal water quality variability and quantify “press ure-impact”
relationships, we used a physically based modeling approach involving the river/basin integrated model PEGASE
(Planification Et Gestion de l’ASsa inissement des Eaux). PEGASE simulates watercourses physicochemical quality
depending on the morphology of the drainage network, hydrometeorological conditions and natural and anthropogenic
influences. Simulation results showed a better description of Joumine river water quality and helped in identifying exposed
areas to nutrients export. Results have also emphasized the contribution of different pollution sources. We were able to
examine the potential impact of agriculture diffuse pollution and we found that Nitrate is the element mostly threatening
water quality. The nutrients patterns sugges t that climate and farming pract ices are important factors controlling their
transfer. These findings demonstrate that the adopted assessment approach in investigating the behavior of the studied
hydrosystem can be a u seful support to develop an appropriate surface water quality management program in a semiarid
context.
... Prior research has demonstrated that states have historically lagged in attaining water quality goals specifically, because BMP application to date has neither been spatially targeted to critical sources and/or pathways of contamination nor applied in accordance with watershed-scale hydrologic considerations (Tomer and Locke 2011). Fortunately, the technical capacity for land management agencies and/or watershed-level entities and allied stakeholders to spatially target BMPs based on high-resolution geospatial analysis is steadily increasing (e.g., Walter et al. 2007;Schilling and Wolter 2009;White et al. 2014;Tomer et al. 2015). Yet comprehensively tracking the cost of BMP application has been a challenge largely because up-to-date data regarding the direct and potential opportunity costs of BMP use is lacking (Tyndall and Roesch 2014). ...
The US Cornbelt leads North American production of intensively managed, row-crop corn and soybeans. While highly productive, agricultural management in the region is often linked with nonpoint source nutrient pollution that negatively impacts water quality. Presently, conservation programs designed to install best management practices (BMPs) to mitigate agricultural nonpoint source pollution have not been targeted to those areas of the landscape that contribute disproportionately to surface water quality concerns. We used an innovative spatially targeted conservation protocol coupled with a GIS-based landscape planning tool to evaluate the cost and effect on water quality from nitrate-nitrogen loss under alternative landscape scenarios in an Iowa watershed. Outputs indicate large reductions in watershed-level nitrate-nitrogen loss could be achieved through coordinated placement of BMPs on high-contributing parcels with limited reduction of cultivated land, resulting in improved surface water quality at relatively low economic costs. For example, one scenario, which added wetlands, cover crops, and saturated buffers in the watershed, required the removal of <5% of cultivated area to reduce nitrate-nitrogen loss by an estimated 49%, exceeding the Iowa Nutrient Reduction Strategy goal for enhancing water quality. Annualized establishment and management costs of landscape scenarios that met the nonpoint source nitrogen reduction goal varied from $3.16 to $3.19 million (2017 US dollars). These results support our hypothesis that water quality can be improved by targeting high-contributing parcels, and highlights the potential to minimize tradeoffs by coupling targeted conservation and planning tools to help stakeholders achieve water quality outcomes within agricultural landscapes.
... Elevated NO 3 -N concentrations in agricultural regions threaten water resources at local (Kaushal et al. 2011) and regional scales (McLellan et al. 2015). Drinking water supplies that use surface water (Schilling and Wolter 2009;Jha et al. 2010) or groundwater (Nolan et al. 1997;Burow et al. 2010) are impacted when NO 3 -N concentrations exceed the US maximum contaminant level (MCL) of 10 mg/L or, in the EU, a maximum admissible concentration (MAC) of 50 mg/L as NO 3 . High NO 3 -N concentrations discharging to streams via baseflow and tile drainage also contribute to nutrient enrichment that impairs many local (Dodds and Welch 2000;Chambers et al. 2012) and regional river systems, including the Mississippi River and the Gulf of Mexico (David et al. 2010). ...
Evaluating the patterns of NO3-N concentrations at karst springs can be used to infer hydrologic processes and nutrient dynamics in karst aquifers. In this study, NO3-N concentrations observed at two karst springs in northeast Iowa (USA) were evaluated for a 2-year period using high-frequency sensors. Despite similar watershed land use dominated by intense row cropping of corn and soybean production (>70%), NO3-N concentrations and temporal patterns were very different between the two springs. At the Manchester spring, NO3-N stored in overburden materials above the karst-enhanced Silurian-age bedrock provides a continuing source of NO3-N to the spring. Rainfall events mobilize the stored NO3-N and concentrations increase. At Big Spring, the karst system is overlain by a thin layer of sediments and the bedrock is dominated by sinkholes and losing streams. Rainfall events dilute the spring NO3-N concentrations which rapidly decreased during events before rebounding to previous levels. Spectral analyses revealed that concentrations at both springs were a fractal process, with the scaling exponent at Manchester (2.0) considerably larger than that measured at Big Spring (1.4), indicating a higher degree of autocorrelation in NO3-N concentrations at Manchester, consistent with the conceptual model. Overall, results argue for greater use of high-frequency NO3-N monitoring at karst springs to better assess short- and long-term variations in NO3-N concentrations and to unravel karst processes.
... Goolsby and Battaglin (2001) estimated NO 3 -N flux from 42 interior Mississippi River subbasins and found five major Iowa watersheds (Raccoon, Cedar, Iowa, Skunk, and Des Moines) among the eight highest for NO 3 -N yield (load per unit of watershed area). Several other studies have characterized NO 3 -N concentrations and loads within Iowa's largest interior rivers, including the Des Moines River (McIsaac and Libra 2003;Schilling and Wolter 2009), the Cedar River ( Yang and Jin 2010), the Iowa River ( Sprague et al. 2011), and the Raccoon River ( Jones et al. 2016). ...
We evaluated Iowa Department of Natural Resources nitrate (NO3–N) and US Geological Survey hydrological data from 1987 to 2016 in nine agricultural watersheds to assess how transport of this pollutant has changed in the US state of Iowa. When the first 15 years of the 30-year water-quality record is compared to the second 15 years (1987–2001 and 2002–2016), three different metrics used to quantify NO3–N transport all indicate levels of this pollutant are increasing. Yield of NO3–N (kg ha⁻¹) averaged 18% higher in the second 15 years, while flow-weighted average concentrations (mg L⁻¹) were 12% higher. We also introduced the new metric of NO3–N yield (g ha⁻¹) per mm precipitation to assess differences between years and watersheds, which averaged 21 g NO3–N ha⁻¹ per 1 mm of precipitation across all watersheds and was 13% higher during the second half of the record. These increases of NO3–N occurred within a backdrop of increasing wetness across Iowa, with precipitation and discharge levels 8 and 16% higher in the last half of the record, indicating how NO3–N transport is amplified by increasing precipitation levels. The implications of this are that in future climate scenarios where rainfall is more abundant, detaining water and increasing evapotranspiration within the cropping system will be necessary to control NO3–N losses. Land use changes that include use of cover crops, living mulches, and perennial plants should be expanded to improve water quality and affect the water balance within agricultural basins.
... Spatial heterogeneity of NPS should be recognized as an important consideration for BMP placement at the watershed scale. It's reported that some sub-watersheds contribute significantly more nutrient loads than others (Schilling and Wolter, 2009;Strauss et al., 2007;White et al., 2009). Confining BMPs to high polluted areas is usually more cost-effective than implementing universal controls or random placement (Giri et al., 2012;Strauss et al., 2007). ...
Targeting nonpoint source (NPS) pollution hot spots is of vital importance for placement of best management practices (BMPs). Although physically-based watershed models have been widely used to estimate nutrient emissions, connections between nutrient abatement and compliance of water quality standards have been rarely considered in NPS hotspot ranking, which may lead to ineffective decision-making. It’s critical to develop a strategy to identify priority management areas (PMAs) based on water quality response to nutrient load mitigation. A water quality constrained PMA identification framework was thereby proposed in this study, based on the simulation-optimization approach with ideal load reduction (ILR-SO). It integrates the physically-based Soil and Water Assessment Tool (SWAT) model and an optimization model under constraints of site-specific water quality standards. To our knowledge, it was the first effort to identify PMAs with simulation-based optimization. The SWAT model was established to simulate temporal and spatial nutrient loading and evaluate effectiveness of pollution mitigation. A metamodel was trained to establish a quantitative relationship between sources and water quality. Ranking of priority areas is based on required nutrient load reduction in each sub-watershed targeting to satisfy water quality standards in waterbodies, which was calculated with genetic algorithm (GA). The proposed approach was used for identification of PMAs on the basis of diffuse total phosphorus (TP) in Lake Dianchi Watershed, one of the three most eutrophic large lakes in China. The modeling results demonstrated that 85% of diffuse TP came from 30% of the watershed area. Compared with the two conventional targeting strategies based on overland nutrient loss and instream nutrient loading, the ILR-SO model identified distinct PMAs and narrowed down the coverage of management areas. This study addressed the urgent need to incorporate water quality response into PMA identification and showed that the ILR-SO approach is effective to guide watershed management for aquatic ecosystem restoration.
... In this study, a more restrictive application scenario of nutrient management was simulated by reducing fertilizer inputs by 50% of the pre-CP scenario. Several SWAT model studies have also reported a 50% fertilizer reduction scenario [48][49][50]; therefore, this study was designed to further the results reported by past studies. ...
Assessing the performance of appropriate agricultural conservation practices (CPs) frequently relies on the use of simulation models as a cost-effective tool instead of depending solely on the monitoring of water quality at individual field and watershed levels. This study evaluates the predicted impacts of several CPs on nutrient and sediment loss at the hydrological response unit scale in the L'Anguille River Watershed, which is a watershed identified as a "focus watershed" under the Mississippi River Basin healthy watershed Initiative (MRBI) program. The Soil and Water Assessment Tool model was calibrated and validated between 1998-2005 and 2006-2012, respectively for flow, sediment, total phosphorus, and nitrate nitrogen. Out of the seven MRBI CPs modeled in this study, the highest reduction in sediment (80%) and nutrient (58% for total phosphorus and 16% for total nitrogen) was predicted for the critical area planting practice, followed by filter strip, irrigation land leveling, grade stabilization structure, irrigation pipeline, nutrient management, and irrigation water management. Some of the predicted impacts conflicted with expected CP performance. The study underscores the importance of the proper formulation of CP algorithms in using simulation models for predicting impacts on water quality.
... There is a growing consensus that in-field conservation practices, such as improved nutrient management, conservation tillage, and cover crops, will not, singly or together, achieve the 45% nutrient reduction goals (Schilling and Wolter 2009;Iowa Nutrient Reduction Strategy 2013). Meeting these water quality goals will require combining in-field nutrient management practices with downstream nutrient removal practices, such as biore- actors and filter strips at the edges of fields and wetland creation and floodplain restoration below fields . ...
... Variations in NO 3 -N mass loading rates were due to variations in both groundwater recharge and NO 3 -N concentrations. The range and average NO 3 -N mass loading rate are similar to annual yields reported for subsurface drainage at a golf course in Texas (<3 kg/ ha; King et al., 2006) and North Carolina (4.8 kg/ha; Line et al., 2002) and are much lower than reported for Iowa agricultural areas which regularly exceed 20 kg/ha (Schilling and Wolter, 2009;Jha et al., 2010;Ikenberry et al., 2014). For comparison, we compared annual NO 3 -N mass loading rates measured at the golf courses to NO 3 -N yields measured in their respective watersheds and found that the golf course NO 3 -N mass loading rates were approximately 10% of the annual watershed yield for 2015 and 2016 (Table 6). ...
Golf courses are often considered by the public to be significant nitrogen (N) and phosphorus (P) nonpoint sources but only limited information exists on nutrient concentrations and loads in golf course groundwater. In this study, we measured N and P concentrations in groundwater and available surface water at six randomly selected Iowa golf courses to assess the loading risk posed by these facilities to groundwater and local rivers. At each course, three shallow monitoring wells were installed, one each on representative tee, fairway, and rough locations. Wells and available surface water were sampled on eight occasions during 2015 and 2016. NO3-N concentrations were not detected above 1 mg/L at three of the six courses monitored in this study and the overall mean NO3-N concentration in Iowa golf courses was 2.2 mg/L. The mass of NO3-N recharged to groundwater averaged 3.3 kg/ha at the six courses, which represents approximately one-tenth of the NO3-N load exported by the watershed that contains the course and represented approximately 0.1 to 8% of the fertilizer N applied. Groundwater orthophosphorus concentrations averaged 0.13 mg/L and were similar to those measured in a variety of settings across Iowa. Study results should prove useful in evaluating nutrient contributions from golf courses in Midwestern states where nutrient reduction strategies are being pursued.
... A tile drain depth (DDRAIN) of 1,200 mm was simulated for the BRW study, which is consistent with several other previous SWAT studies performed in the region (Jha et al., 2007(Jha et al., , 2010Schilling and Wolter, 2009) indicates that about 70% of the cropland is tile drained based on surveys of landowners who live in the six counties that the BRW is located in. Thus the present configuration of tile-drained cropland in the BRW SWAT model may be somewhat overestimated. ...
Several biofuel cropping scenarios were evaluated with an improved version of Soil and Water Assessment Tool (SWAT) as part of the CenUSA Bioenergy consortium for the Boone River Watershed (BRW), which drains about 2,370 km² in north central Iowa. The adoption of corn stover removal, switchgrass, and/or Miscanthus biofuel cropping systems was simulated to assess the impact of cellulosic biofuel production on pollutant losses. The stover removal results indicate removal of 20 or 50% of corn stover in the BRW would have negligible effects on streamflow and relatively minor or negligible effects on sediment and nutrient losses, even on higher sloped cropland. Complete cropland conversion into switchgrass or Miscanthus, resulted in reductions of streamflow, sediment, nitrate, and other pollutants ranging between 23-99%. The predicted nitrate reductions due to Miscanthus adoption were over two times greater compared to switchgrass, with the largest impacts occurring for tile-drained cropland. Targeting of switchgrass or Miscanthus on cropland ≥2% slope or ≥7% slope revealed a disproportionate amount of sediment and sediment-bound nutrient reductions could be obtained by protecting these relatively small areas of higher sloped cropland. Overall, the results indicate that all biofuel cropping systems could be effectively implemented in the BRW, with the most robust approach being corn stover removal adopted on tile-drained cropland in combination with a perennial biofuel crop on higher sloped landscapes. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.
... SWAT has a number of modules for BMP simulation with guidance for their use (Arabi et al., 2008;Waidler et al., 2009), yet there remains much latitude in BMP parameterization left to the discretion of the modeler, leading to substantial variability to the reported effectiveness of BMPs (Arabi et al., 2007;Karamouz et al., 2015). In addition, BMP effectiveness varies with spatial scale, i.e., with location within a watershed (Schilling and Wolter, 2009;Bosch et al., 2013). Consequently there remain many improvements to BMP simulations that deserve further research (Xie et al., 2015), and reporting of modeled BMP parameterization and effectiveness at specifically stated scales is needed to help ascertain the ranges of possible model and ecosystem response. ...
Phosphorus export coefficients (kg/ha/yr) from selected land covers, also called phosphorus yields, tend to get smaller as contributing areas get larger because some of the phosphorus mobilized on local fields gets trapped during transport to regional watershed outlets. Phosphorus traps include floodplains, wetlands, and lakes, which can then become impaired by eutrophication. The Sunrise River watershed in east central Minnesota, United States, has numerous lakes impaired by excess phosphorus. The Sunrise is tributary to the St. Croix River, whose much larger watershed is terminated by Lake St. Croix, also impaired by excess phosphorus. To support management of these impairments at both local and regional scales, a Soil and Water Assessment Tool (SWAT) model of the Sunrise watershed was constructed to estimate load reductions due to selected best management practices (BMPs) and to determine how phosphorus export coefficients scaled with contributing area. In this study, agricultural BMPs, including vegetated filter strips, grassed waterways, and reduction of soil-phosphorus concentrations reduced phosphorus loads by 4-20%, with similar percentage reductions at field and watershed spatial scales. Phosphorus export coefficients from cropland in rotation with corn, soybeans, and alfalfa decreased as a negative power function of contributing area, from an average of 2.12 kg/ha/yr at the upland field scale (~0.6 km²) to 0.63 kg/ha/yr at the major river basin scale (20,000 km²). Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.
... Similar SWAT studies have previously been performed, for example, Santhi et al. [45] evaluated the long-term effects of Water Quality Management Plans on non-point source pollution in a Texas catchment. Schilling and Wolter [46] examined a suite of measures, among these fertilizer application reductions, to meet regulatory limits for public water supplies. White et al. [47] modelled nutrient loads from six Oklahoma catchments, also identifying critical source areas for sediment and phosphorous. ...
The main environmental stressor of the Baltic Sea is elevated riverine nutrient loads, mainly originating from diffuse agricultural sources. Agricultural practices, intensities, and nutrient losses vary across the Baltic Sea drainage basin (1.75 × 10⁶ km², 14 countries and 85 million inhabitants). Six “Soil and Water Assessment Tool” (SWAT) models were set up for catchments representing the major agricultural systems, and covering the different climate gradients in the Baltic Sea drainage basin. Four fertilizer application scenarios were run for each catchment to evaluate the sensitivity of changed fertilizer applications. Increasing sensitivity was found for catchments with an increasing proportion of agricultural land use and increased amounts of applied fertilizers. A change in chemical fertilizer use of ±20% was found to affect watershed NO3-N loads between zero effect and ±13%, while a change in manure application of ±20% affected watershed NO3-N loads between zero effect and −6% to +7%.
... Benefits that accrue from conservation practices are affected by numerous variables such as soil type, distance to a water body, topography, and crop (Qiu, 2003;Gitau et al., 2005). To address this complexity numerous decision tools have been developed to help technical assistance providers identify the most cost-effective locations for implementing conservation prac- tices (Hession and Shanholtz, 1988;Richardson and Gatti, 1999;Veith et al., 2003;Mishra et al., 2007;Schilling and Wolter, 2009;Tuppad et al., 2010). However, these systems are often difficult to use and focused on individual outcomes (e.g., sediment or nutrients) ( Legge et al., 2013), which are barriers that can limit their use for targeting. ...
There is growing evidence that addressing nonpoint source pollution within intensely agricultural regions of the Great Lakes will require innovative solutions to achieve meaningful ecological outcomes. Recognizing this, a broad coalition of partners is collaborating across Michigan's Saginaw Bay watershed to develop and test innovative approaches to achieve the vision of Strategic Agricultural Conservation. The strategy focuses on using science, technology, and new ways of incentivizing practices and delivering services to producers to address challenges and barriers to Strategic Agricultural Conservation. It uses science to model relations between conservation actions, water quality and fish community health, allowing the coalition to establish realistic ecological outcomes and both short and long-term implementation goals at a variety of scales. It uses a decision tool and pay-for-performance methods to strategically target conservation practices and increase their efficiency. It uses nontraditional partners to help increase the ability to engage landowners and streamlined the application process to help increase landowner participation. Finally, it uses secure, privacy respecting, methods to track practices and progress towards short and long-term goals. Herein we present three case studies that demonstrate the practical application of this strategy including developing and testing new innovative conservation programs across the Saginaw Bay watershed. The success of this work will ultimately be determined by a variety of factors that affect conservation at landscape scales. However, what is clear is that without the science and complementary decision tool, this collaborative adaptive management approach would be impossible to implement across such a large geography.
... Contrary to the simulations of land models limited to the terrestrial component, most watershed models do estimate stream N concentrations and loads, but they simplify or neglect many key mechanisms describing terrestrial N dynamics (e.g., vegetation and land-use dynamics, interactive C-N feedbacks on vegetation and soil microbial processes; phenological leaf drop and its contribution to soil organic matter pools). INCA-N and SWAT are widely used geographic information system (GIS)-based watershed models (Wade et al., 2002;Schilling and Wolter, 2009). However, when it comes to large-scale applications, because these models are semi-distributed, they are less capable of representing spatial variability, requiring users to define the number and sizes of sub-basins, in which land use and all of the processes for each land use are assumed to be homogeneous and needed to be defined individually. ...
We developed a process model LM3-TAN to assess the combined effects of direct
human influences and climate change on terrestrial and aquatic nitrogen (TAN)
cycling. The model was developed by expanding NOAA's Geophysical Fluid
Dynamics Laboratory land model LM3V-N of coupled terrestrial carbon and
nitrogen (C-N) cycling and including new N cycling processes and inputs such
as a soil denitrification, point N sources to streams (i.e., sewage), and
stream transport and microbial processes. Because the model integrates
ecological, hydrological, and biogeochemical processes, it captures key
controls of the transport and fate of N in the vegetation–soil–river system in a
comprehensive and consistent framework which is responsive to climatic
variations and land-use changes. We applied the model at 1/8°
resolution for a study of the Susquehanna River Basin. We simulated with
LM3-TAN stream dissolved organic-N, ammonium-N, and nitrate-N loads
throughout the river network, and we evaluated the modeled loads for
1986–2005 using data from 16 monitoring stations as well as a reported budget
for the entire basin. By accounting for interannual hydrologic variability,
the model was able to capture interannual variations of stream N loadings.
While the model was calibrated with the stream N loads only at the last
downstream Susquehanna River Basin Commission station Marietta (40°02' N,
76°32' W), it captured the N loads well at
multiple locations within the basin with different climate regimes, land-use
types, and associated N sources and transformations in the sub-basins.
Furthermore, the calculated and previously reported N budgets agreed well at
the level of the whole Susquehanna watershed. Here we illustrate how point
and non-point N sources contributing to the various ecosystems are stored,
lost, and exported via the river. Local analysis of six sub-basins showed
combined effects of land use and climate on soil denitrification rates, with
the highest rates in the Lower Susquehanna Sub-Basin (extensive agriculture;
Atlantic coastal climate) and the lowest rates in the West Branch Susquehanna
Sub-Basin (mostly forest; Great Lakes and Midwest climate). In the re-growing
secondary forests, most of the N from non-point sources was stored in the
vegetation and soil, but in the agricultural lands most N inputs were removed
by soil denitrification, indicating that anthropogenic N applications could
drive substantial increase of N2O emission,
an intermediate of the denitrification process.
... Researchers used SWAT to model nitrogen in small, agricultural watersheds in Canada (Ahmad et al. 2011), Italy (Salvetti et al. 2008), and India (Mishra et al. 2010). Additional studies developed SWAT models for a large agricultural watershed in the UK (Grizzetti et al. 2005) and Iowa and Minnesota, USA (Schilling and Wolter 2009). ...
Identifying sources of nitrogen input and nitrogen yield to waterways are important aspects of watershed management. A Soil and Water Assessment Tool (SWAT) model was developed for the Cibolo and Dry Comal Creek watersheds in south central Texas, USA to assess how individual nitrogen source categories contribute to nitrogen yield. The model includes nitrogen inputs from atmospheric deposition, fertilizer, manure, a wastewater treatment plant (WWTP), and on-site sewage facilities (OSSFs). Model calibration was successful with all model performance parameters rated satisfactory or better except for percent bias (PBIAS) during the streamflow calibration period in Cibolo Creek. The high PBIAS value likely resulted from the model underestimating streamflow during two flood events coupled with potential measurement errors for the observed streamflow values during the floods. The largest contributors to nitrogen input were livestock (42.0 %), atmospheric deposition (26.8 %), deer (9.9 %), and farm fertilizer use (9.7 %). The largest contributors to nitrogen yield were atmospheric deposition (56.3 % of nitrogen yield), livestock (25.9 %), and the WWTP (7.1 %). The difference in ranking between nitrogen inputs and nitrogen yields occurred because the percentage of nitrogen input that becomes nitrogen yield varied with source. The WWTP (100 % of the WWTP nitrogen input became nitrogen yield) was highest because it directly discharged to the waterway. The WWTP was followed by feral hogs (8.0 %), atmospheric deposition (5.8 %), and waterfowl (2.1 %). These results provide a source-by-source assessment of nitrogen inputs and their corresponding yields allowing watershed managers to better assess nitrogen management strategies.
... Understanding N-retention processes in Saylorville Reservoir is important because the downstream municipal water supply, Des Moines Water Works, uses surface water from the Des Moines River as part of its water supply for 500,000 people in central Iowa. Recent assessment of the Des Moines River indicates that the river is impaired for use as a drinking water supply due to high levels of NO 3 -N that exceed the Maximum Contaminant Level (MCL) of 10 mg/l (Schilling and Wolter 2009). ...
Nitrogen (N) removal within reservoirs can be substantial, but few studies have reported the relative importance of various N-retention pathways. Assessing N-removal processes in reservoirs is important for quantifying the impacts of reservoirs on downstream water quality. In this study, we used a time-series approach to quantify the relative importance of various N-removal processes in the Saylorville Reservoir in Iowa. Dynamic regression modeling of upstream–downstream changes in key water-quality surrogates (pH, hardness, alkalinity, and suspended solids) and their relation to N concentration changes were used to estimate the relative importances of denitrification, N assimilation by algal uptake, and sedimentation of N on N retention in the reservoir. Assuming that decreasing N concentrations in the reservoir are the sum of these three processes, we estimate that denitrification is the dominant N removal process (60.9 %) followed by algal assimilation (37.9 %) and sedimentation (1.2 %). Our approach represents a new method of establishing the relative importance of N-removal processes in reservoirs and quantifying the impacts of reservoirs on downstream water quality.
... The Clear Creek SWAT model was executed for a total simulation period of 13 years (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012). Calibration was achieved by adjusting several hydrologic parameters, including the curve number, soil available water capacity, evaporation compensation coefficient and groundwater delay within their acceptable ranges (e.g., (Elhakeem and Papanicolaou, 2009;Schilling and Wolter, 2009). Model calibration was evaluated using the coefficient of determination (r 2 ) and the Nash-Sutcliffe coefficient (ENS), which are described by Krause et al. (2005) and are the most common statistics used to evaluate SWAT simulations (Douglas-Mankin et al., 2010;Gassman et al., 2007;Tuppad et al., 2011;Gassman et al., 2014). ...
Agricultural expansion and urbanization, coupled with climate change represent major threats to the sustainability of river ecosystems and infrastructure. In this study, we evaluated how subbasins with different dominant land covers within the 27.5 km2 Clear Creek, IA watershed affect key hydrologic indicators. Hydrologic output from two stream gages and a calibrated Soil Water Assessment Tool (SWAT) model were used as input to the Indicators of Hydrologic Alteration (IHA). Study results indicated that land cover plays a dominant role in controlling hydrologic variability at the subbasin level within a watershed. Subbasins dominated by urban development had nearly 30 more reversals than row crop or grassdominated subbasins and the duration of small and large flood events were half as long. Row crop dominated subbasins had greater water yield and maximum flows and higher peak flows, whereas grass-dominated subbasins had lower rise and fall rates, fewer zero days and fewer reversals. Hydrologic variations from land cover differences were more prominently expressed at the subbasin level than at the watershed level, as the dominant land cover represented a greater percentage of the total land area. Study results suggest that future changes in LU/LC and climate will have significant effects on the hydrology of Clear Creek Watershed. © 2015 Keith Schilling, Matthew Streeter, Kasey Hutchinson, Christopher Wilson, Ben Abban, Kenneth Wacha and Athanasios Papanicolaou.
Nutrient export from the agricultural Midwest threatens the Gulf of Mexico and new conservation practices are needed to reduce the loss of nutrient from subsurface tile drainage systems. Oxbows are natural waterbodies formed when a river cuts off a meander loop and water quality benefits of reconstructed oxbows are being increasingly recognized. In this study, we monitored four reconstructed oxbow sites (two tile‐fed, two non‐tile) over a 2‐year period in north‐central Iowa and assessed their capacity for NO 3 ‐N and dissolved reactive phosphorus (DRP) reductions. Water flow and quality monitoring of tiles, shallow groundwater, oxbow and receiving streams documented that the oxbows were dominated by tile drainage inputs. NO 3 ‐N concentrations were highest in the drainage tiles flowing into the tile‐fed oxbows (mean 8–10 mg/L) and much lower in floodplain groundwater (<1–2 mg/L). Annual NO 3 ‐N loads into the tile‐fed oxbows were substantially larger than input loads into the non‐tiled oxbows. For the two tile‐fed oxbows, the 2‐year NO 3 ‐N retention efficiencies were very similar (0.76–0.77) and on a monthly basis, greater retention efficiencies were measured in summer and fall. DRP concentrations and loads into the tile‐fed oxbows were too low to allow for meaningful estimates of retention. Reconstructing oxbows to receive tile drainage water should be considered a sustainable conservation practice for tile drainage treatment in agricultural areas.
This chapter presents a case study on the impact of land use land cover (LULC) change on water balances in an increasingly urbanized Palar and other sub-basins that form the upper basin area of the east flowing rivers between Pennar and Cauvery basin. The study area covers 35,392 km² in hydrologically important peninsular regions of Andhra Pradesh, Karnataka, and Tamil Nadu. The LULC changes were quantified between 1995 and 2020, which showed that the study area experienced a significant increase in built-up area and decrease in wetland and forest coverage. The impacts of these LULC changes on the water balances were analyzed using Soil and Water Assessment Tool (SWAT). A SUFI-2 algorithm was used to calibrate and validate the model for streamflow. The results of the SWAT model were used to assess the LULC impact on water balances, which can be used to develop water resources planning and management strategies in the urbanized peninsular region of India.
Prediction of flow rate in rivers is essential for the planning and management of water resources. This study shows that, based on a Machine Learning approach, accurate models for streamflow prediction can be developed. The Des Moines watershed, which includes both Des Moines River and Raccoon River, was chosen as a case study. Only the daily river discharge was considered for the modeling of 10 stations located on different tributaries of the two rivers. Four machine learning algorithms were applied for the streamflow prediction: Random Subspace, M5P, Random Forest and Bagging. The performance of applied algorithms was assessed using statistical performance indicators and graphical representations. Three stations were selected for the training and testing of the different Machine Learning models. Then, the best model was validated on the other seven stations. Prediction accuracy was also assessed as the forecast horizon increased. Overall, M5P algorithms led to the best predictions, with R 2 equal to 0.970 and 0.960 for the stations of East Fork Des Moines River at Dakota City and Des Moines River near Tracy, respectively. Accurate predictions were obtained also on the Raccoon River, with R 2 equal to 0.938 and 0.887 for the stations of North Raccoon River near Jefferson and Raccoon River at Van Meter, respectively.
Determining the groundwater contribution of nonpoint source pollution at a watershed scale is a challenging issue. In this study, we utilized a top-down approach to characterize representative groundwater response units (GRUs) based on land use, and landscape position (e.g., upland, sideslope, floodplain) in the 275-km² Clear Creek, Iowa watershed. Groundwater monitoring wells were then established along downslope transects in representative GRUs. This unique combination of top-down/ bottom-up approaches allowed us to estimate groundwater pollutant loads at the watershed scale with minimal monitoring. For the 2015 study period, results indicated that more groundwater recharge occurred in the floodplain (404 mm) compared to the uplands or sideslopes (281 and 165 mm, respectively), irrespective of land use. Recharge in the floodplains consisted of 37% of the annual precipitation, while upland wells averaged 26% and sideslopes averaged 15% of the annual precipitation. Less recharge was found to occur beneath perennial grass compared to row crop and urbanized areas. Baseflow discharge accounted for 69% of the total NO3-N exported from the Clear Creek watershed, with row crop areas contributing approximately 95% of the annual load. Orthophosphorus (OP) yields were approximately 0.72 kg/ha beneath urban and suburban areas, three times higher than row crop or perennial areas. Urban and suburban areas accounted for 21.4% of groundwater OP and chloride loads in the watershed compared to only 8.5% of the land area. Overall, the groundwater load allocation model for baseflow nutrient discharge to Clear Creek can be used to target future NPS load reduction strategies at the watershed scale. The use of GRUs can pinpoint better areas of concern for controlling nutrient loads.
About 50% of U.S. water pollution problems are caused by non-point source (NPS) pollution, primarily sediment and nutrients from agricultural areas, despite the widespread implementation of agricultural Best Management Practices (BMPs). However, the effectiveness of implementation strategies and type of BMPs at watershed scale are still not well understood. In this study, the Soil and Water Assessment Tool (SWAT) ecohydrological model was used to assess the effectiveness of pollutant mitigation strategies in the Raccoon River watershed (RRW) in west-central Iowa, USA. We analyzed fourteen management scenarios based on systematic combinations of five strategies: fertilizer/manure management, changing row-crop land to perennial grass, vegetative filter strips, cover crops and shallower tile drainage systems, specifically aimed at reducing nitrate and total suspended sediment yields from hotspot areas in the RRW. Moreover, we assessed implications of climate change on management practices, and the impacts of management practices on water availability, row crop yield, and total agricultural production. Our results indicate that sufficient reduction of nitrate load may require either implementation of multiple management practices (38.5% with current setup) or conversion of extensive areas into perennial grass (up to 49.7%) to meet and maintain the drinking water standard. However, climate change may undermine the effectiveness of management practices, especially late in the 21st century, cutting the reduction by up to 65% for nitrate and more for sediment loads. Further, though our approach is targeted, it resulted in a slight decrease (~ 5%) in watershed average crop yield and hence an overall reduction in total crop production, mainly due to the conversion of row-crop lands to perennial grass. Such yield reductions could be quite spatially heterogeneously distributed (0 to 40%).
This study represents an application of the Soil and Water Assessment Tool
(SWAT) to an agricultural catchment, called Akarsu irrigation district, in the Lower
Seyhan Plain located at the Eastern Mediterranean region of Turkey. The catchment
covers a drainage area of 9 495 ha. The main objective of this study was to introduce
SWAT model and calibrate the model against daily drainage discharge and nitrate
(NO3) concentrations derived from the field stations. The modelling period was
covering the hydrological years of 2009 and 2010. Major input variables to run the
model were climate data, soil type, land use, management practices and topography
at watershed scale. Sequential Uncertainty Fitting (SUFI-2) linked to SWAT was
utilized for calibration of the model. SUFI-2 is associated with SWAT in the
Calibration Uncertainty Program called SWAT-CUP. Calibrations were conducted
using the SWAT-CUP program and SUFI-2 algorithm. Calibration based on the
Nash-Sutcliffe coefficient of efficiency (E) and R2 was done through using daily
data. Interpretation of model calibration results led us to conclude that the simulated
SWAT drainage flows and their corresponding nitrate loads were reasonably
accurate for the Akarsu irrigation district.
Key Words: SWAT, SWAT-CUP, SUFI-2, Nitrate
A fully integrated, physically-based model of a drained and farmed wetland complex in the Prairie Pothole Region of Iowa (termed Ellsworth #2) was developed to investigate hydrologic connectivity of surface and groundwater sources. The model is based on the code HydroGeoSphere, which solves surface and subsurface flow and the interaction between these domains. Physical processes governing the hydrologic response of a wetland, precipitation, plant transpiration, and surface and subsurface evaporation, were included in the model. The model was run for 6 years and predicted water table location was compared against measurements. Results indicated that intermittent ponding was observed in pothole depressions but hydrologic connectivity among three pothole depressions via surface water ponding was rarely established. Extensive subsurface tile drainage system simulated in the model removed a substantial volume of water and reduced the amount and duration of ponding. Groundwater discharge contributed to some degree during nearly all ponding events in the pothole depressions as exfiltration ranged from 1.7 to 23.6 % of annual precipitation. Despite simplifications, the model captures the interactions among hydrologic processes and provides important information for scientists and decision makers to effectively plan for current and future management of these drained and farmed wetland complexes.
Groundwater discharge to a lake can be an important component to water and nutrient budgets. In this study, we evaluated groundwater loading of nitrate-nitrogen (NO3-N) and phosphorus (P) to West Lake Okoboji, Iowa, using a watershed-based approach based on groundwater recharge and land cover class. Our objectives were to assess groundwater level fluctuations and nutrient concentrations under representative land use classes and develop an allocation model for groundwater nutrient loads based on land cover class. Monitoring wells were installed at 21 locations around the lake and sampled during a three-year study period. Groundwater quality varied among the land cover types with average NO3-N concentrations the highest beneath cropped fields (8.8 mg l− 1) and residential areas (2 mg l− 1), and P concentrations ranging between 0.05 and 0.1 mg l− 1 throughout the region. NO3-N loads were the highest under cropped fields and this source accounted for approximately 90% of the NO3-N, whereas P loads were more evenly distributed among source areas. Groundwater recharge averaged approximately 76 mm year− 1 for vegetated areas and substantially less for urban areas. Based on mass balance, groundwater discharge may account for 80% of the NO3-N in the lake compared to 10% of the P. Results are instructive to more effectively target implementation of conservation practices to major nutrient loading areas for reduction of NO3-N and P delivered to the lake.
Strategies to reduce nitrate-nitrogen (nitrate) pollution delivered to streams often seek to increase groundwater residence time to achieve measureable results, yet the effects of tile drainage on residence time have not been well documented. In this study, we used a geographic information system groundwater travel time model to quantify the effects of artificial subsurface drainage on groundwater travel times in the 7443-ha Bear Creek watershed in north-central Iowa. Our objectives were to evaluate how mean groundwater travel times changed with increasing drainage intensity and to assess how tile drainage density reduces groundwater contributions to riparian buffers. Results indicate that mean groundwater travel times are reduced with increasing degrees of tile drainage. Mean groundwater travel times decreased from 5.6 to 1.1 yr, with drainage densities ranging from 0.005 m (7.6 mi) to 0.04 m (62 mi), respectively. Model simulations indicate that mean travel times with tile drainage are more than 150 times faster than those that existed before settlement. With intensive drainage, less than 2% of the groundwater in the basin appears to flow through a perennial stream buffer, thereby reducing the effectiveness of this practice to reduce stream nitrate loads. Hence, strategies, such as reconnecting tile drainage to buffers, are promising because they increase groundwater residence times in tile-drained watersheds.
We investigated how projected changes in land cover and climate affected simulated nitrate (NO3−) and organic nitrogen (ORGN) discharge for two watersheds within the Neuse River Basin North Carolina, USA for years 2010 to 2070. We applied the Soil and Water Assessment Tool (SWAT) watershed model to predict nitrogen discharge using (1) atmospheric carbon dioxide (CO2) concentrations predicted by the Intergovernmental Panel on Climate Change (IPCC), (2) land cover change predicted by the Integrated Climate and Land Use Change (ICLUS) project and (3) precipitation and temperature simulated by two statistically downscaled and bias-corrected Global Circulation Models (GCMs). We determined the sensitivity of simulated nitrogen discharge to separate changes in each treatment ([1] CO2, [2] land cover, and [3] precipitation and temperature (PT)) by comparing each treatment to a reference condition. Results showed nitrogen discharges were most sensitive to changes in PT over the 60-year simulation. Nitrogen discharges had similar sensitivities to the CO2 and land cover treatments which were only one-tenth the influence of the PT treatment. Under the CO2 treatment, nitrogen discharges increased with increasing ambient CO2. NO3− discharge decreased with increased urbanization; however, ORGN had a varied response. Under the PT treatment, there was high spatial variability in nitrogen discharges. In a single year, certain sub-basins showed an 80% increase in nitrogen discharge relative to reference, while others showed a 400% decrease. With nitrogen discharge showing high sensitivity to PT change, we suggest more emphasis should be placed on investigating impacts of PT on nutrient transport in the Neuse River Basin. This article is protected by copyright. All rights reserved.
Over the last century, land use and land cover (LULC) in the United
States Corn Belt region shifted from mixed perennial and annual cropping
systems to primarily annual crops. Historical LULC change impacted the
annual water balance in many Midwestern basins by decreasing annual
evapotranspiration (ET) and increasing streamflow and base flow. Recent
expansion of the biofuel industry may lead to future LULC changes from
increasing corn acreage and potential conversion of the industry to
cellulosic bioenergy crops of warm or cool season grasses. In this
paper, the Soil and Water Assessment Tool (SWAT) model was used to
evaluate potential impacts from future LULC change on the annual and
seasonal water balance of the Raccoon River watershed in west-central
Iowa. Three primary scenarios for LULC change and three scenario
variants were evaluated, including an expansion of corn acreage in the
watershed and two scenarios involving expansion of land using warm
season and cool season grasses for ethanol biofuel. Modeling results
were consistent with historical observations. Increased corn production
will decrease annual ET and increase water yield and losses of nitrate,
phosphorus, and sediment, whereas increasing perennialization will
increase ET and decrease water yield and loss of nonpoint source
pollutants. However, widespread tile drainage that exists today may
limit the extent to which a mixed perennial-annual land cover would ever
resemble pre-1940s hydrologic conditions. Study results indicate that
future LULC change will affect the water balance of the watershed, with
consequences largely dependent on the future LULC trajectory.
Changes in land-use or management practices may affect water outflow, sediment, nutrients and pesticides loads. Thus, there is an increasing demand for quantitative information at the watershed scale that would help decision makers or planners to take appropriate decisions. This paper evaluates by a modeling approach the impact of farming practices and land-use changes on water discharge, sediment and NO3-N loads at the outlet of a 51.29 km2 watershed of central Iowa (Walnut Creek watershed). This intensively farmed (corn-soybean rotation) watershed is characterized by a flat topography with tiles and potholes. Nine scenarios of management practices (nitrogen application rates: increase of current rate by 20, 40%, decrease of current rate by 20, 40 and 60%; no tillage) and land-use changes (from corn-soybean rotation to winter wheat and pasture) were tested over a 30 yr simulated period. The selected model (Soil and Water Assessment Tool, SWAT) was first validated using observed flow, sediment and nutrient loads from 1991 to 1998. Scenarios of N application rates did not affect water and sediment annual budgets but did so for NO3-N loads. Lessening the N rate by 20, 40 and 60% in corn-soybean fields decreased mean NO3-N annual loads by 22, 50 and 95%, respectively, with greatest differences during late spring. On the other hand, increasing input N by 20 and 40% enhanced NO3-N loads by 25 and 49%, respectively. When replacing corn-soybean rotation by winter wheat, NO3-N loads increased in early fall, immediately after harvest. Pasture installation with or without fertilization lessened flow discharge, NO3-N and sediment delivery by 58, 97 and 50%, respectively. No-tillage practices did not significantly affect the water resource and sediment loads. Finally, such realistic predictions of the impact of farming systems scenarios over a long period are discussed regarding environmental processes involved.
At present, there are over 34,000 impaired waters and over 58,000 associated impairments officially listed in the U.S. Nutrients and sediment are two of the most common pollutants included in the list. States are required to identify and list those waters within their boundaries that are not meeting standards, to prioritize them, and to develop Total Maximum Daily Loads (TMDLs) for the pollutants of concern. Models are used to support development of TMDLs, typically to estimate source loading inputs, evaluate receiving water quality, and determine source load allocations so that receiving water quality standards are met. Numerous models are available today, and selection of the most suitable model for a specific TMDL project can be daunting. This article presents a critical review of models simulating sediment and nutrients in watersheds and receiving waters that have potential for use with TMDL development and implementation. The water quality models discussed, especially those with sediment and/or nutrient components, include loading models (GWLF and PLOAD), receiving water models (AQUATOX, BATHTUB, CE-QUAL-W2, QUAL2E, and QUAL2K), and watershed models having both loading and receiving components (AGNPS, AnnAGNPS, CASC2D/GSSHA, DWSM, HSPF, KINEROS2, LSPC, MIKE SHE, and SWAT). Additional models mentioned include another receiving water quality model (WASP), watershed models (ANSWERS storm event, ANSWERS continuous, PRMS storm event, SWMM, and WEPP), and BMP models (APEX, REMM, and VFSMOD). Model sources, structures, and procedures for simulating hydrology, sediment, and nutrients are briefly described for the reviewed models along with an assessment of their strengths, limitations, robustness, and potentials for using in sediment and/or nutrient TMDLs. Applications of AGNPS, APEX, BATHTUB, CE-QUAL-W2, GWLF, and SWAT in TMDL developments are presented. Applications of some of the other models (DWSM, GSSHA, and KINEROS2) relevant to TMDL studies are also presented. The models proved to be useful; however, they require a learning process. Simple models are easy to use but have limitations; comprehensive models are labor and data intensive but offer extensive analysis tools. Finally, recommendations are offered for advancing the sediment and nutrient modeling technologies as applied to TMDL development and implementation. Advances could be made towards: making the best use of existing models, enhancing the existing models, combining strengths of existing models, developing new models or supplemental components with physically based robust routines, numerous field applications, sensitivity analyses, full documentation, and rigorous education and training.
Watershed models are powerful tools for simulating the effect of watershed processes and management on soil and water resources. However, no comprehensive guidance is available to facilitate model evaluation in terms of the accuracy of simulated data compared to measured flow and constituent values. Thus, the objectives of this research were to: (1) determine recommended model evaluation techniques (statistical and graphical), (2) review reported ranges of values and corresponding performance ratings for the recommended statistics, and (3) establish guidelines for model evaluation based on the review results and project-specific considerations; all of these objectives focus on simulation of streamflow and transport of sediment and nutrients. These objectives were achieved with a thorough review of relevant literature on model application and recommended model evaluation methods. Based on this analysis, we recommend that three quantitative statistics, Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR), in addition to the graphical techniques, be used in model evaluation. The following model evaluation performance ratings were established for each recommended statistic. In general, model simulation can be judged as satisfactory if NSE > 0.50 and RSR < 0.70, and if PBIAS + 25% for streamflow, PBIAS + 55% for sediment, and PBIAS + 70% for N and P. For PBIAS, constituent-specific performance ratings were determined based on uncertainty of measured data. Additional considerations related to model evaluation guidelines are also discussed. These considerations include: single-event simulation, quality and quantity of measured data, model calibration procedure, evaluation time step, and project scope and magnitude. A case study illustrating the application of the model evaluation guidelines is also provided.
An investigation was conducted to evaluate strengths and limitations of manual calibration and the existing autocalibration tool in the watershed-scale model referred to as the Soil and Water Assessment Tool (SWAT). Performance of the model was tested on the Little River Experimental Watershed in Georgia and the Little Washita River Experimental Watershed in Oklahoma, both USDA-ARS watersheds. A long record of multi-gauge streamflow data on each of the watersheds was used for model calibration and validation. Model performance of the streamflow response in SWAT was assessed using a six-parameter manual calibration based on daily mass balance and visual inspection of hydrographs and duration of daily flow curves, a six-parameter autocalibration method based on the daily sum of squares of the residuals after ranking objective function (referred to as SSQRauto6), a six-parameter method based on the daily sum of squares of residuals (SSQauto6), and an eleven-parameter method based on the daily sum of square of residuals (SSQauto11). Results show that for both watersheds, manual calibration generally outperformed the autocalibration methods based on percent bias (PBIAS) and simulation of the range in magnitude of daily flows. For the calibration period on Little River subwatershed F, PBIAS was 0.0%, -24.0%, -21.5%, and +29.0% for the manual, SSQRauto6, SSQauto6, and SSQauto11 methods, respectively. Based on the coefficient of efficiency (NSE), the SSQauto6 and SSQauto11 methods gave substantially better results than manual calibration on the Little River watershed. On the Little Washita watershed, however, the manual approach generally outperformed the automated methods, based on the NSE error statistic. Results of this study suggest that the autocalibration option in SWAT provides a powerful, labor-saving tool that can be used to substantially reduce the frustration and uncertainty that often characterize manual calibrations. If used in combination with a manual approach, the autocalibration tool shows promising results in providing initial estimates for model parameters. To maintain mass balance and adequately represent the range in magnitude of output variables, manual adjustments may be necessary following autocalibration. Caution must also be exercised in utilizing the autocalibration tool so that the selection of initial lower and upper ranges in the parameters results in calibrated values that are representative of watershed conditions.
Fecal contamination of surface waters is a critical water-quality issue, leading to human illnesses and deaths. Total Maximum Daily Loads (TMDLs), which set pollutant limits, are being developed to address fecal bacteria impairments. Watershed models are widely used to support TMDLs, although their use for simulating in-stream fecal bacteria concentrations is somewhat rudimentary. This article provides an overview of fecal microorganism fate and transport within watersheds, describes current watershed models used to simulate microbial transport, and presents case studies demonstrating model use. Bacterial modeling capabilities and limitations for setting TMDL limits are described for two widely used watershed models (HSPF and SWAT) and for the load-duration method. Both HSPF and SWAT permit the user to discretize a watershed spatially and bacteria loads temporally. However, the options and flexibilities are limited. The models are also limited in their ability to describe bacterial life cycles and in their ability to adequately simulate bacteria concentrations during extreme climatic conditions. The load-duration method for developing TMDLs provides a good representation of overall water quality and needed water quality improvement, but intra-watershed contributions must be determined through supplemental sampling or through subsequent modeling that relates land use and hydrologic response to bacterial concentrations. Identified research needs include improved bacteria source characterization procedures, data to support such procedures, and modeling advances including better representation of bacteria life cycles, inclusion of more appropriate fate and transport processes, improved simulation of catastrophic conditions, and creation of a decision support tool to aid users in selecting an appropriate model or method for TMDL development.
Reliable water quality models are needed to forecast the water quality consequences of different agricultural nutrient management scenarios. In this study, the Soil and Water Assessment Tool (SWAT), version 2000, was applied to simulate streamflow, riverine nitrate (NO(3)) export, crop yield, and watershed nitrogen (N) budgets in the upper Embarras River (UER) watershed in east-central Illinois, which has extensive maize-soybean cultivation, large N fertilizer input, and extensive tile drainage. During the calibration (1994-2002) and validation (1985-1993) periods, SWAT simulated monthly and annual stream flows with Nash-Sutcliffe coefficients (E) ranging from 0.67 to 0.94 and R(2) from 0.75 to 0.95. For monthly and annual NO(3) loads, E ranged from -0.16 to 0.45 and R(2) from 0.36 to 0.74. Annual maize and soybean yields were simulated with relative errors ranging from -10 to 6%. The model was then used to predict the changes in NO(3) output with N fertilizer application rates 10 to 50% lower than original application rates in UER. The calibrated SWAT predicted a 10 to 43% decrease in NO(3) export from UER and a 6 to 38% reduction in maize yield in response to the reduction in N fertilizer. The SWAT model markedly overestimated NO(3) export during major wet periods. Moreover, SWAT estimated soybean N fixation rates considerably greater than literature values, and some simulated changes in the N cycle in response to fertilizer reduction seemed to be unrealistic. Improving these aspects of SWAT could lead to more reliable predictions in the water quality outcomes of nutrient management practices in tile-drained watersheds.
The Soil and Water Assessment Tool (SWAT) model is a continuation of nearly 30 years of modeling efforts conducted by the U.S. Department of Agriculture (USDA), Agricultural Research Service. SWAT has gained international acceptance as a robust interdisciplinary watershed modeling tool, as evidenced by international SWAT conferences, hundreds of SWAT-related papers presented at numerous scientific meetings, and dozens of articles published in peer-reviewed journals. The model has also been adopted as part of the U.S. Environmental Protection Agency's BASINS (Better Assessment Science Integrating Point & Nonpoint Sources) software package and is being used by many U.S. federal and state agencies, including the USDA within the Conservation Effects Assessment Project. At present, over 250 peer-reviewed, published articles have been identified that report SWAT applications, reviews of SWAT components, or other research that includes SWAT. Many of these peer-reviewed articles are summarized here according to relevant application categories such as streamflow calibration and related hydrologic analyses, climate change impacts on hydrology, pollutant load assessments, comparisons with other models, and sensitivity analyses and calibration techniques. Strengths and weaknesses of the model are presented, and recommended research needs for SWAT are provided.
The Raccoon River Watershed (RRW) in West-Central Iowa has been recognized as exporting some of the highest nitrate-nitrogen loadings in the United States and is a major source of sediment and other nutrient loadings. An integrated modeling framework has been constructed for the RRW that consists of the Soil and Water Assessment Tool (SWAT) model, the interactive SWAT (i_SWAT) software package, Load Estimator (LOADEST) computer program, and other supporting software and databases. The simulation framework includes detailed land use and management data such as different crop rotations and an array of nutrient and tillage management schemes, derived from the U.S. Department of Agriculture's National Resources Inventory databases and other sources. This paper presents the calibration and validation of SWAT for the streamflow, sediment losses, and nutrient loadings in the watershed and an assessment of land use and management practice shifts in controlling pollution. Streamflow, sediment yield, and nitrate loadings were calibrated for the 1981-1992 period and validated for the 1993-2003 period. Limited field data on organic nitrogen, organic phosphorus, and mineral phosphorus allowed model validation for the 2001-2003 period. Model predictions generally performed very well on both an annual and monthly basis during the calibration and validation periods, as indicated by coefficient of determination (R2) and Nash-Sutcliffe simulation efficiency (E) values that exceeded 0.7 in most cases. A set of land use change scenarios based on taking cropland out of production indicated a significant benefit in reducing sediment yield at the watershed outlet. A second scenario set found that relatively small reductions in nutrient applications resulted in significant reductions in nitrate loadings at the watershed outlet, without affecting crop yields significantly.
The Raccoon River watershed (RRW) in west-central Iowa has been recognized as exporting some of the highest nitrate-nitrogen loadings in the U.S. and is a major source of sediment and other nutrient loadings. An integrated modeling framework has been constructed for the 9,400 km 2 RRW that consists of the SWAT (Soil and Water Assessment Tool) model, the interactive SWAT (i_SWAT) software package, the Load Estimator (LOADEST) computer program, and other supporting software and databases. The simulation framework includes detailed land use and management data, such as different crop rotations, and an array of nutrient and tillage management schemes, derived from the USDA National Resources Inventory (NRI) databases and other sources. This article presents the calibration and validation of SWAT for the streamflow, sediment losses, and nutrient loadings in the watershed, and an assessment of land use and management practice shifts in controlling pollution. Streamflow, sediment yield, and nitrate loadings were calibrated for the period 1981-1992 and validated for the period 1993-2003. Limited field data on organic nitrogen, organic phosphorus, and mineral phosphorus allowed model validation for the period 2001-2003. Model predictions generally performed very well on both an annual and monthly basis during the calibration and validation periods, as indicated by R 2 and Nash-Sutcliffe efficiency (E) values that exceeded 0.7 in most cases. A set of land use change scenarios depicting conversion of cropland into land set-aside resulted in large reductions of sediment yield at the watershed outlet. A second scenario set found that reductions in nutrient applications of 10% to 20% resulted in similar predicted percentage reductions in nitrate loadings at the watershed outlet and in corresponding corn yield reductions of 3% to 6%. © 2007 American Society of Agricultural and Biological Engineers.
This paper presents a critical review of models simulating sediment and nutrients in
watersheds and receiving waters that have potential for use with TMDL development and
implementation. The water quality models discussed, especially those with sediment and/or nutrient
components include two loading models, five receiving water models, and nine watershed models
having both loading and receiving simulation capabilities. Other potential TMDL models are also
mentioned. Applications of some of the models, including in TMDL studies, are presented. The
models proved to be useful, however, still a learning process. Recommendations are offered for
advancing the sediment and nutrient, as well as their basic hydrologic, modeling technologies as
applied to TMDL development and implementation. An extended abstract of the paper is presented
here while the full-length paper has been accepted for publication in the Transactions of the ASABE.
A Total Maximum Daily Load (TMDL) program has been initiated in the North Bosque River Watershed in Texas, USA, where point and nonpoint sources of pollution are of a concern. The Soil and Water Assessment Tool (SWAT), which had been validated for flow and sediment and nutrient transport, was applied to quantify the effects of Best Management Practices (BMPs) related to dairy manure management and municipal wastewater treatment plant effluent. Results are presented for the period from 1960 through 1998 for three sites along the North Bosque River. Results are presented as annual time-weighted concentrations (average of the daily load divided by daily flow over a year) and annual flow-weighted concentrations (total cumulative load divided by total cumulative flow over a year). The wastewater treatment plant BMPs resulted in greater improvement in time-weighted instream soluble phosphorus concentrations than dairy BMPs. On the other hand, dairy BMPs made greater differences in flow-weighted concentrations. This study showed that SWAT could be a useful tool for studying the effects of alternative management scenarios for pollution control from point and nonpoint sources in large watersheds.
A conceptual, continuous time model called SWAT (Soil and Water Assessment Tool) was developed to assist water resource managers in assessing the impact of management on water supplies and nonpoint source pollution in watersheds and large river basins. The model is currently being utilized in several large area projects by EPA, NOAA, NRCS and others to estimate the off-site impacts of climate and management on water use, non-point source loadings, and pesticide contamination. Model development, operation, limitations, and assumptions are discussed and components of the model are described. In Part II, a GIS input/output interface is presented along with model validation on three basins within the Upper Trinity basin in Texas.
SWAT (Soil and Water Assessment Tool) is a conceptual, continuous time model that was developed in the early 1990s to assist water resource managers in assessing the impact of management and climate on water supplies and non-point source pollution in watersheds and large river basins. SWAT is the continuation of over 30 years of model development within the US Department of Agriculture's Agricultural Research Service and was developed to ‘scale up’ past field-scale models to large river basins. Model components include weather, hydrology, erosion/sedimentation, plant growth, nutrients, pesticides, agricultural management, stream routing and pond/reservoir routing. The latest version, SWAT2000, has several significant enhancements that include: bacteria transport routines; urban routines; Green and Ampt infiltration equation; improved weather generator; ability to read in daily solar radiation, relative humidity, wind speed and potential ET; Muskingum channel routing; and modified dormancy calculations for tropical areas. A complete set of model documentation for equations and algorithms, a user manual describing model inputs and outputs, and an ArcView interface manual are now complete for SWAT2000. The model has been recoded into Fortran 90 with a complete data dictionary, dynamic allocation of arrays and modular subroutines. Current research is focusing on bacteria, riparian zones, pothole topography, forest growth, channel downcutting and widening, and input uncertainty analysis.
Human activities may cause enhanced loads of total N (TN) and total P (TP) in running waters, which may have a serious impact on their ecological quality. Likely sources for N and P are industries, dwellings and agriculture. A GIS based model is proposed to analyse which of these sources cause the main problems. Simulated concentrations were compared with standards for TN and TP representing natural conditions in running water. The model was applied on the strongly disturbed catchment of the river Dommel (Belgium/The Netherlands). It was taken into account that river sediments may act as a sink for nutrients.Flows from point sources and dwellings were taken from databases. Diffuse sources were assumed to be a percentage of the anthropogenic input, defined as the nutrient input on the soil minus crop yield. A regression analysis on ten headwaters without point sources suggests that 5.5% of anthropogenic N and 0.02% of anthropogenic P has leached to the river network. Applying the model for all tributaries of the Dommel yielded a good fit between observed and simulated TN and TP loads (R2 of 0.88 and 0.95, respectively).All analysed tributaries exceed the standards for TN and TP. Analysis suggests that point sources are -overall- the main contributor to nutrient fluxes, although they affect few tributaries only. Most intermediate and small tributaries are affected by diffuse flows from agricultural pastures (24.2 kg TN ha−1 yr−1; 0.82 kg TP ha−1 yr−1). It can be argued that reduction of point sources may help to minimise the magnitude of the nutrient fluxes on the catchment scale, reduction of diffuse sources may help to restore a good water quality on the scale of individual tributaries.
Subsurface drainage is a common practice in many agricultural watersheds in the Mid-Western region of the United States. A typical drainage system in east central Illinois is not spaced in a parallel manner, but the subsurface drain lines are laid out in a random and irregular fashion. These subsurface drain lines most often discharge into numerous man-made drainage channels, which ultimately drain to the rivers and the reservoirs. The Little Vermilion River (LVR) watershed in east central Illinois, USA is an example of a watershed with altered hydrology from subsurface drainage systems. A continuous monitoring study has been conducted from 1991 to 2003 on this watershed to quantify the effects of cropping management practices and random subsurface drainage systems on fertilizer and pesticide transport in subsurface flow. This study investigates the losses and concentrations of nitrate-N and atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) from the LVR watershed that is flat (1% or less slope) and has intensive crop productions. Two of the drainage areas used in this study within this watershed have corn (Zea Mays L.) and soybeans (Glycine max L.) in rotations and the other two have seed corn and soybean in rotations. Long-term data collected from the LVR suggests that surface runoff rarely occurs in this watershed, and most of the soil-water is removed by the subsurface drainage systems. The concentrations of nitrate-N in subsurface drains varied depending on fertilizer application methods; pre-planting application contributed to nitrate-N concentrations higher than 10 mg L−1 (Drinking Water Standard of the US Environmental Protection Agency for nitrate-N) in subsurface drainage water. The pre-planting N application method resulted long-term average nitrate-N concentrations of 15, 17, 19, and 20 mg L−1 at the four monitoring sites, and the long-term average losses were 33, 23, 26, and 25 kg ha−1 of nitrate-N from the respective sites. Atrazine concentrations were lower than 3 μg L−1 (Drinking Water Standard of the US Environmental Protection Agency for Atrazine) in most water samples. The long-term average annual atrazine concentrations were 0.87, 1.0, 1.22, and 0.94 μg L−1 for the four sites. The mean annual atrazine losses varied from 0 to 7.12 g ha−1 during the monitoring period, with major losses of atrazine in drainage water occurring within 3 months of atrazine application.
The principles governing the application of the conceptual model
technique to river flow forecasting are discussed. The necessity for a
systematic approach to the development and testing of the model is
explained and some preliminary ideas suggested.
The water quality of 13 rivers in the lowland, agricultural county of Suffolk is investigated using routine monitoring data for the period 1981 to 2006 collected by the Environment Agency of England and Wales (EA), and its predecessors, with particular emphasis on phosphorus (as total reactive phosphorus, TRP) and total (dissolved and particulate) oxidised nitrogen (TOxN--predominantly nitrate NO3). Major ion and flow data are used to outline fundamental hydrochemical characteristics related to the groundwater provenance of base-flow waters. Relative load contributions from point and diffuse sources are approximated using Load Apportionment Modelling for both TRP and TOxN where concurrent flow and concentration data are available. Analyses indicate a mixture of point and diffuse sources of TRP, with the former being dominant during low flow periods, while for TOxN diffuse sources dominate. Out of 59 sites considered, 53 (90%) were found to have annual average TRP concentrations greater than 0.05 mg P l(-1), and 36 (61%) had average concentrations over 0.120 mg P l(-1), the upper thresholds for 'High' and 'Good' ecological status, respectively. Correspondingly, for TOxN, most of the rivers are already within 70% of the 11.3 mg N l(-1) threshold, with two rivers (Wang and Ore) being consistently greater than this. It is suggested that the major challenge is to characterise and control point-source TRP inputs which, being predominant during the late spring and summer low-flow period, coincide with the peak of primary biological production, thus presenting the major challenge to achieving 'good' ecological status under the Water Framework Directive. Results show that considerable effort is still required to ensure appropriate management and develop tools for decision-support.
Nitrate N fluxes from tile-drained watersheds have been implicated in water quality studies of the Mississippi River basin, but actual NO3-N loads from small watersheds during long periods are poorly documented. We evaluated discharge and NO3-N fluxes passing the outlet of an Iowa watershed (5134 ha) and two of its tile-drained subbasins (493 and 863 ha) from mid-1992 through 2000. The cumulative NO3-N load from the catchment was 168 kg ha(-1), and 176 and 229 kg ha(-1) from the subbasins. The outlet had greater total discharge (1831 mm) and smaller flow-weighted mean NO3-N concentration (9.2 mg L(-1)) than the subbasins, while the larger subbasin had greater discharge (1712 vs. 1559 mm) and mean NO3-N concentration (13.4 vs. 11.3 mg L(-1)) than the smaller subbasin. Concentrations exceeding 10 mg L(-1) were common, but least frequent at the outlet. Nitrate N was generally not diluted by large flows, except during 1993 flooding. The outlet showed smaller NO3-N concentrations at low flows. Relationships between discharge and NO3-N flux showed log-log slopes near 1.0 for the subbasins, and 1.2 for the outlet, considering autocorrelation and measurement-error effects. We estimated denitrification of subbasin NO3-N fluxes in a hypothetical wetland using published data. Assuming that temperature and NO3-N supply could limit denitrification, then about 20% of the NO3-N would have been denitrified by a wetland constructed to meet USDA-approved criteria. The low efficiency results from the seasonal timing and NO3-N content of large flows. Therefore, agricultural and wetland best management practices (BMPs) are needed to achieve water quality goals in tile-drained watersheds.
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