Soil Water Assessment Tool Model Research Articles

Application of SWAT Model to Simulate Nitrate and Phosphate Leaching from Agricultural Lands to Rivers

In the present study, the amount of nitrate and phosphate leaching from agricultural lands into the Zanjanrood River in Iran was simulated using the Soil & Water Assessment Tool (SWAT) model. The measured average monthly discharges at the Sarcham station were used to calibrate and validate the SWAT model, and the SWAT Calibration and Uncertainty Program (SWAT-CUP) model was applied to perform the uncertainty and sensitivity analyses. Three scenarios for the irrigation methods and five for the fertilizer rates were defined. The p-factor and r-factor were used for the uncertainty analysis, and two statistical indices of the coefficient of determination (R2) and Nash-Sutcliffe efficiency (NS) were utilized in the validation model. For the calibration of the monthly runoff at the basin’s outlet, the coefficients of r-factor, p-factor, R2, and NS were obtained as 0.27, 0.11, 0.83, and 0.53, respectively. The results showed that by increasing the pressurized irrigation areas, the nitrate and phosphate pollutions in the river basin were not significantly affected. With regard to fertilizer rates, by reducing the consumption of urea and phosphate fertilizers up to 50%, the amount of nitrate and phosphate leaching into the Zanjanrood River was reduced up to 16.7% and 19.2%, respectively. On the other hand, an increase of 50% in fertilizer application increased nitrate and phosphate leaching into the river by 17.2% and 17.7%, respectively. In addition, by reducing the fertilization rate and preventing unnecessary fertilization by farmers, the pollution of water resources can be largely prevented.

Modeling Hydrological Responses to Land Use Dynamics, Choke, Ethiopia

The main objective of this study was to assess the hydrological response of the Choke mountain range to land use dynamics. Two watersheds, Muga and Suha watersheds, were selected for detailed evaluation and analysis. The study was conducted using integrated applications of remote sensing and the Soil Water Assessment Tool (SWAT). The SWAT model was calibrated using Sequential Uncertainty Fitting (SUFI-2) algorithm in SWAT-CUP. Decadal land use maps (1985, 1995, and 2005) were used to simulate the hydrological responses. Simulated model results showed that over the past two decades (1985–2004), the total annual surface and lateral streamflows in the watershed increased at a rate of 1.2 mm/year and 0.57 mm/year, whereas the annual total groundwater flow and percolation in the basin decreased at a rate of 1.6 mm/year and 1.77 mm/year respectively. The decrease in the streamflow was more pronounced during the dry season (October to May), for which statistically significant declines of the base flow or the low flow at a rate of 0.37 m3/year and 0.73 m3/year in the Muga and Suha watersheds, respectively, were found. In the wet season (June to September), the peak flow has increased by 50% in Muga and 94% in Suha watersheds. Results of this study showed that the SWAT model can be an effective and useful tool for the assessment of response of watersheds to land use alterations.

Application of hydrological model for assessment of water security using multi-model ensemble of CORDEX-South Asia experiments in a semi-arid river basin of India

This study analyses the impacts of climate change on water resources in the Banas River Basin, which is located in a semi-arid part of the state of Rajasthan in India. A bias-corrected ensemble mean of three CORDEX-SA driving GCM experiments (CNRM-CM5, CCSM4, and MPI-ESM-LR) was used with the Soil Water Assessment Tool (SWAT) to predict water yield and evapotranspiration for three future periods (2011–2040, 2041–2070, and 2071–2099) under two representative concentration pathway scenarios (RCP4.5 and RCP8.5), and the results were compared with the data of a historical period (1979–2008). The SUFI-2 method was used for the calibration and validation of the SWAT model. The model was calibrated for the period of five years (1982–1986) and validated for the next five years (1987–1991). The values of R2, NSE, bR2, PBIAS, and RSR were, respectively, 0.78, 0.77, 0.61, 47.4 and 0.48 during the calibration period, and 0.71, 0.65, 0.60, 72.1 and 0.59 during the validation period. Trend analysis was also performed for annual future predicted flows using the Sen Slope method. From the results, it can be predicted that precipitation, evapotranspiration, and water yield will increase in all the three future periods under both RCP4.5 and RCP8.5. Water availability in the future in the basin (zone wise) was identified using appropriate indicators. Per-capita water availability and meteorological variation were used to calculate future water availability and, considering these indicators, it can be concluded that zone 3 will better than the other zones. Zone 3 exhibits high values of per capita water availability and meteorological variation. This study will be useful in understanding the impacts of climate change on the water availability of the river basin and may help in overall water management in the present and the future.

Assessment of Water Balance for Russian Subcatchment of Western Dvina River Using SWAT Model

The study provides a new assessment of the water balance components of the catchment (evapotranspiration, surface and lateral flow etc. and its spatial distribution and temporal variability) for the transboundary catchment of Western Dvina river within the poorly gauged Russian part of the catchment.The study focuses on modeling the inland flow generation processes using open source data and the SWAT (Soil Water Assessment Tool) hydrological model. The high interannual variability of river flow and impact of snowmelt processes were especially taken into account when setting up the model and processing the calibration. The database of daily meteorological data for the period 1981 – 2016 was prepared using global atmospheric reanalysis ERA-Interim data and observed station data from the GSOD NCDC/NOAA and ECA&D datasets. The considered datasets were tested on plausibility and regionalized. The catchment model was built on the basis of open land use / land cover (LULC) data sets, topography and soil, so that the entire transboundary catchment area could be easily implemented in the next step. For the daily model calibration, 19 sensitive parameters were chosen manually. The most sensitive are the parameters which consider snow melting processes and flow recession curve number. The highest impact on water balance components has the area and distribution of wetlands. Lakes strongly affect the evapotranspiration rate. The study provides further research with uncertainty analysis and recommendations for model improvement and model limitations. The developed modelling approach can be used to assess water resources, climate change impacts, and water quality issues in comparable regions.

Multiple modeling to estimate sediment loss and transport capacity employing hourly rainfall and In-Situ data: A prioritization of highland watershed in Awash River basin, Ethiopia

In present study, the sediment yield/loss and Transport Capacity (TC) has been estimated in a highland watershed of Adama Woreda, part of Awash River Basin in Ethiopia. For the study, average hourly 14 years rainfall data obtained from different gauging stations in the study area during 2001–2014 was utilized to calculate rain erosivity (R factor). Subsequently, soil erodibility K, length-slope LS factors of Revised Universal Soil Loss Equation (RUSLE) were adopted for the calculation of WATEM/SEDEM sediment delivery algorithm. The average R factor value for the watershed was evaluated as 2075.27 MJ/ha mm/year. The grid based KTC (Transport Capacity Coefficient) was determined for the WATEM/SEDEM sediment transport estimation through the results of Soil Water Assessment Tool (SWAT) and Revised/Modified USLE. The SWAT simulated suspended solids (SS) versus two years (2013 and 2014) rainfall measured records at single observatory was used for the SWAT calibration and validation. The SWAT sediment yields and Transport Capacity (TC) were used to determine the KTC at each sub-watershed. The mean value of KTC was estimated 8.033 and minimum and maximum values were found 0.080 and 51.49 respectively. Moreover, the Sediment Deliver Ratio (SDR) model equation was calculated and resulted values were found >0.7 in the main streams. For the prioritization, sediment yield and loss estimated values using Sediment Yield Index (SYI) and RUSLE along with SWAT models were employed. The results generated through these models were averaged and accordingly the ranks and severity classes were determined.

Revisiting SWAT as a Saturation-Excess Runoff Model

The Soil Water Assessment Tool (SWAT) is employed throughout the world to simulate watershed processes. A limitation of this model is that locations of saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth cannot be simulated realistically. The objective of this research is to overcome this limitation with minor changes in the original SWAT code. The new approach is called SWAT-with-impervious-layers (SWAT-wil). Adaptations consisted of redefining the hillslope length, restricting downward percolation from the root zone, and redefining hydrologic response units (HRUs) such that they are associated with the landscape position. Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service (SCS) curve number runoff equation. We tested the model for the Town Brook watershed in the Catskill Mountains. The results showed that the discharge calculated with SWAT-wil agreed with observed outflow and results simulated with the original SWAT and SWAT-hillslope (SWAT-HS) models that had a surface aquifer that transferred water between groups of HRUs. The locations of the periodically saturated runoff areas were predicted by SWAT-wil at the right locations. Current users can utilize the SWAT-wil approach for catchments where VSA hydrology predominates.

Incorporating a non-reactive heavy metal simulation module into SWAT model and its application in the Athabasca oil sands region.

Heavy metal contaminations in an aquatic environment is a serious issue since the exposure to toxic metals can cause a variety of public health problems. A watershed-scale model is a useful tool for predicting and assessing heavy metal fate and transport in both terrestrial and aquatic environments. In this study, we developed a simulation module for non-reactive heavy metals and incorporated it into the widely used Soil Water Assessment Tool (SWAT) model. The simulated processes in the developed model include heavy metaldeposition, partitioning in soil and water, and transport by different pathways in both terrestrial and aquatic environments. Three-phase partitioning processes were considered in the module by simulating heavy metals portioning to dissolved organic carbon in the soil and stream. This developed module was used for watershed-scale simulation of heavy metal processes in the Muskeg River watershed (MRW) of the Athabasca oil sands region in western Canadafor the first time. The daily streamflow and sediment load from 2015 to 2017 were first calibrated and validated. Subsequently, the daily Leadand Copperloads at the outlet station were used for heavy metal calibration and validation. The performances for the daily heavy metal loads simulation during the wholesimulation period can be considered as "satisfactory" based on the recommended model performance criteria with the Nash-Sutcliffe efficiency as 0.41 and 0.71 for Pb and Cu loads, respectively. The simulation results indicate that the spring and summer are hot moments for heavy metal transport and the snowmelt in spring and rainfall runoff events in summer are the main driving forces for the metal transport in the MRW. We believe the developed model can be a useful tool for simulating the fate and transport of non-reativeheavy metals at watershed scale and further used to assess management scenarios for mitigating heavy metal pollution in the Athabasca oil sands region.

A Race Against Time: Modeling Time Lags in Watershed Response

AbstractLand use change and agricultural intensification have increased food production but at the cost of polluting surface and groundwater. Best management practices implemented to improve water quality have met with limited success. Such lack of success is increasingly attributed to legacy nutrient stores in the subsurface that may act as sources after reduction of external inputs. However, current water‐quality models lack a framework to capture these legacy effects. Here we have modified the SWAT (Soil Water Assessment Tool) model to capture the effects of nitrogen (N) legacies on water quality under multiple land‐management scenarios. Our new SWAT‐LAG model includes (1) a modified carbon‐nitrogen cycling module to capture the dynamics of soil N accumulation, and (2) a groundwater travel time distribution module to capture a range of subsurface travel times. Using a 502‐km2 Iowa watershed as a case study, we found that between 1950 and 2016, 25% of the total watershed N surplus (N Deposition + Fertilizer + Manure + N Fixation − Crop N uptake) had accumulated within the root zone, 14% had accumulated in groundwater, while 27% was lost as riverine output, and 34% was denitrified. In future scenarios, a 100% reduction in fertilizer application led to a 79% reduction in stream N load, but the SWAT‐LAG results suggest that it would take 84 years to achieve this reduction, in contrast to the 2 years predicted in the original SWAT model. The framework proposed here constitutes a first step toward modifying a widely used modeling approach to assess the effects of legacy N on the time required to achieve water‐quality goals.

An Integrated Modelling Approach to Study Future Water Demand Vulnerability in the Montargil Reservoir Basin, Portugal

This paper describes an integrated modelling approach to study water use vulnerability in a typical Mediterranean basin under different climate change projections. The soil water assessment tool (SWAT) and the MOHID (from modelo hidrodinâmico) Water model were used to evaluate the impacts of two climate scenarios (GFDL-ESM2M and IPSL-CM5A-LR) on water availability in Montargil’s basin and reservoir (Portugal) during two decadal timelines (2030 and 2060). Reservoir performance metrics were estimated considering also two water demand scenarios: an average of the water demand in the last 10 years; and the largest annual demand of the last 10 years. The SWAT model results showed a future decrease of inflows to the reservoir, with its volumetric reliability decreasing from 100% in the historical simulation to about 60–70% in the IPSL-CM5A-LR climate scenario and 40–50% in the GFDL-ESM2M climate scenario. The time reliability also decreased to less than 30%, while the resiliency for the water demand decreased to an average 20–35% for both climate scenarios. These impacts indicate the importance of the managing systems in an integrative mode to prevent water resources reduction in the region.

A SWAT-Copula based approach for monitoring and assessment of drought propagation in an irrigation command

Since drought is a natural disaster that causes significant loss to society and the agrarian economy, detection of drought period is very important for different drought management strategies involving irrigation scheduling, canal scheduling, and reservoir operation. For estimation of drought, although there are a number of drought indices, viz., univariate Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), Palmer Drought Severity Index (PDSI), and Soil Moisture Stress Index (SSI) existing in the literature, they are not fully effective for early warning and onset detection of drought events due to the scarce-knowledge of interdependency among the drought-causing factors. As an indirect method, the hydrological Soil Water Assessment Tool (SWAT) alone cannot be used for drought assessment. Hence, in this study, the SWAT is coupled with the multivariate copulas to effectively predict the drought years. The developed approach is tested in the Kangsabati River basin (12,014 km2) having about 48% of the area under paddy land use. The SWAT model is set up for the study area at daily time-scale with the Nash-Sutcliffe Criterion (ENS) of 0.64 and R2 of 0.64 during the calibration period. The SPI and Standardized Runoff Index (SRI) are calculated for all the selected areas at four time-scales of 3-months, 6-months, 9-months and 12-months using the time series data from 1982–2010. The correlation analysis between SPI and SRI showed that the onset time for hydrological drought after the occurence of meteorological drought is three months. For identification of drought years, the multivariate distribution of SPI and SSI was constructed using the members of Archimedean copula family. Out of all the members of the copula family, Frank copula gave the best joint probability distribution function. The newly developed Multivariate Standardized Drought Index (MSDI) is capable of determining drought years with very good accuracy. Hence, the results reveal that the developed SWAT-Copula based approach has the potential to be implemented in data-scarce regions for effective drought monitoring with the minimum observed inputs.

Impact of model structure on the accuracy of hydrological modeling of a Canadian Prairie watershed

Study regionPrairie Pothole Region of Canada. Study focusThe Prairie region spans across approximately 870,000 km2 of the Great Plains region of Canada (80%) and the United States (20%). The presence of a large number of depressional wetlands (potholes) results in dynamic surface-water and stream connectivity during wet and dry year necessitating an improved understanding of watershed-scale interactions of the Prairie Potholes. The Soil Water Assessment Tool (SWAT) hydrological model with three structural variants is utilized to assess the degree of accuracy associated with increasing model complexity and its impact on the model calibration of the Upper Assiniboine River Basin at Kamsack. New hydrologic insight for the regionThe SWAT model was calibrated and verified with three different structural arrangements in 1) lumped pothole, 2) semi discretized pothole, 3) and fully discretized pothole representation. The fully discretized pothole version of the SWAT reflected streamflow best (KGE of 0.78) but with greater uncertainty, larger data and computational resource requirements. The fully discretized (modified) model, however, was able to capture the high flow and the fill-and-spill processes, which is a defining characteristic of the Prairie Pothole Region (PPR). Significant improvements to the predictive ability of SWAT in the case of the modified model was observed, thus allowing an enhanced understanding of the aggregate effect of potholes in this watershed.

Calculation of Water Demand for Multiple Uses in a Specified Region Using a SWAT Model

The calculation of water demand for multiple uses including the irrigation of crops and municipal uses in a specified region is an essential and urgent task for sustainable water management in such regions. To achieve the aim of this study, the Soil Water Assessment Tool (SWAT) was used with the aid of a Geographic Information System (GIS). The region of Jilawia and Muharam, which lies North of Babylon in Iraq, was selected as a case study so that the water available and demand for that water could be calculated. This region is irrigated from the Great Al–Mussiab Project out to a 12 km radius, covering a 20000 dunum (50 kM2) area, as well as receiving rainfall water. Four types of seasonal crops, Rice, Corn, Wheat, and Cucumbers, are grown, and several residential units have been suggested as planned investment projects in the region of study which will also need sufficient available water. The results of the simulation model in the SWAT covered the period from 2010 to 2020. The Curve Number (CN) and the surface runoff of the region were both calculated, and the SWAT model calibrated against the measured reference evapotranspiration. The results of this calibration were somewhat convergent, demonstrating an error of 12.5%. The results of the calculation of water available and demand showed that sufficient water is available for only municipal uses for 2,300 persons, fulfilling the full capacity of the investment projects, with a deficit of water more generally appearing where both irrigation and municipal use are taken into account. These deficits are 154.70%, 68.97%, and 11.53% with Rice, Corn, and Cucumber crops respectively; however, there is a surplus of water of 34.13% with a Wheat crop. The results also showed that the area of the study region could support 39.25%, 59.17%, and 89.65% of the total available area being turned to Rice, Corn, and Cucumber crops, respectively, to achieve the planned investment projects. SWAT is shown to be an appropriate time saving tool for calculating water demand for multiple uses in a specified region. It can also be concluded that Jilawia and Muharam is a suitable region for the planned investment projects, including managed agriculture and municipal projects, based on the available water in this region.

Integrating organic chemical simulation module into SWAT model with application for PAHs simulation in Athabasca oil sands region, Western Canada

An integrated watershed model is a key tool for assessing the fate and transport of organic chemicals. We developed and integrated an organic chemical simulation module into Soil Water Assessment Tool (SWAT) model and used it for Polycyclic Aromatic Hydrocarbons (PAHs) simulation in Muskeg River Watershed in Athabasca oil sands region, Western Canada. The model uses a three-phase partition process to simulate the chemicals partitioning to Dissolved Organic Carbon (DOC) in both soil water and streams. The simulation performances for Phenanthrene and Pyrene loads for a period of 2015–2017 were assessed to be of ‘satisfactory’ quality. The model simulation results indicated that the summer season is the hot moment and the rainfall-runoff event is the driving force for the PAHs transport in the watershed. The developed simulation module could be a useful modeling tool to simulate and asess the pathway, fate and transport of organic chemicals at the watershed scale.

O Modelo SWAT como Ferramenta para a Gestão de Recursos Hídricos: Um exemplo aplicado no rio Apeú, Castanhal/PA

The management of water resources has driven research, which seeks alternatives to know the processes occurring in river basins. Therefore, the objective of this study was to apply the SWAT (Soil & Water Assessment Tool) model as a complementary tool for the management of water resources, evaluating the Apeu river basinwith the intention of predicting through the model how different actions related to land use and management practices may influence the useful life of the river basin. The results were satisfactory, with Nash-Sutcliffe (NSE) values of 0.89% and R² of 0.99, demonstrating the significance of the SWAT model for the planning and maintenance of water resources.

The Impacts of Climate Variability and Land Use Change on Streamflow in the Hailiutu River Basin

The Hailiutu River basin is a typical semi-arid wind sandy grass shoal watershed in northwest China. Climate and land use have changed significantly during the period 1970–2014. These changes are expected to impact hydrological processes in the basin. The Mann–Kendall (MK) test and sequential t-test analysis of the regime shift method were used to detect the trend and shifts of the hydrometeorological time series. Based on the analyzed results, seven scenarios were developed by combining different land use and/or climate situations. The Soil Water Assessment Tool (SWAT) model was applied to analyze the impacts of climate variability and land use change on the values of the hydrological components. The China Meteorological Assimilation Driving Datasets for the SWAT model (CMADS) was applied to enhance the spatial expressiveness of precipitation data in the study area during the period 2008–2014. Rather than solely using observed precipitation or CMADS precipitation, the precipitation values of CMADS and the observed precipitation values were combined to drive the SWAT model for better simulation results. From the trend analysis, the annual streamflow and wind speed showed a significant downward trend. No significant trend was found for the annual precipitation series; however, the temperature series showed upward trends. With the change point analysis, the whole study period was divided into three sub-periods (1970–1985, 1986–2000, and 2001–2014). The annual precipitation, mean wind speed, and average temperature values were 316 mm, 2.62 m/s, and 7.9 °C, respectively, for the sub-period 1970–1985, 272 mm, 2.58 m/s, and 8.4 °C, respectively, for the sub-period 1986–2000, and 391 mm, 2.2 m/s, and 9.35 °C, respectively, for the sub-period 2001–2014. The simulated mean annual streamflow was 35.09 mm/year during the period 1970–1985. Considering the impact of the climate variability, the simulated mean annual streamflow values were 32.94 mm/year (1986–2000) and 36.78 mm/year (2001–2014). Compared to the period 1970–1985, the simulated mean annual streamflow reduced by 2.15 mm/year for the period 1986–2000 and increased by 1.69 mm/year for the period 2001–2014. The main variations of land use from 1970 to 2014 were the increased area of shrub and grass land and decreased area of sandy land. In the simulation it was shown that these changes caused the mean annual streamflow to decrease by 0.23 mm/year and 0.68 mm/year during the periods 1986–2000 and 2001–2014, respectively. Thus, the impact of climate variability on the streamflow was more profound than that of land use change. Under the impact of coupled climate variability and land use change, the mean annual streamflow decreased by 2.45 mm/year during the period 1986–2000, and the contribution of this variation to the decrease in observed streamflow was 27.8%. For the period 2001–2014, the combined climate variability and land use change resulted in an increase of 0.84 mm/year in annual streamflow. The results obtained in this study could provide guidance for water resource management and planning in the Erdos plateau.

Coupling hydrological modeling and support vector regression to model hydropeaking in alpine catchments

Water management in the alpine region has an important impact on streamflow. In particular, hydropower production is known to cause hydropeaking i.e., sudden fluctuations in river stage caused by the release or storage of water in artificial reservoirs. Modeling hydropeaking with hydrological models, such as the Soil Water Assessment Tool (SWAT), requires knowledge of reservoir management rules. These data are often not available since they are sensitive information belonging to hydropower production companies. In this short communication, we propose to couple the results of a calibrated hydrological model with a machine learning method to reproduce hydropeaking without requiring the knowledge of the actual reservoir management operation. We trained a support vector machine (SVM) with SWAT model outputs, the day of the week and the energy price. We tested the model for the Upper Adige river basin in North-East Italy. A wavelet analysis showed that energy price has a significant influence on river discharge, and a wavelet coherence analysis demonstrated the improved performance of the SVM model in comparison to the SWAT model alone. The SVM model was also able to capture the fluctuations in streamflow caused by hydropeaking when both energy price and river discharge displayed a complex temporal dynamic.

Accessing the capability of TRMM 3B42 V7 to simulate streamflow during extreme rain events: Case study for a Himalayan River Basin

The paper examines the quality of Tropical Rainfall Monitoring Mission (TRMM) 3B42 V7 precipitation product to simulate the streamflow using Soil Water Assessment Tool (SWAT) model for various rainfall intensities over the Himalayan region. The SWAT model has been set up for Gandak River Basin with 41 sub-basins and 420 HRUs. Five stream gauge locations are used to simulate the streamflow for a time span of 10 years (2000–2010). Daily streamflow for the simulation period is collected from Central Water Commission (CWC), India and Department of Hydrology and Meteorology (DHM), Nepal. The simulation results are found good in terms of Nash–Sutcliffe efficiency $$(\hbox {NSE}) {>}0.65$$ , coefficient of determination $$(R^{2}) {>}0.67$$ and Percentage Bias $$\hbox {(PBIAS)}{<}15\%$$ , at each stream gauge sites. Thereafter, we have calculated the PBIAS and RMSE-observations standard deviation ratio (RSR) statistics between TRMM simulated and observed streamflow for various rainfall intensity classes, viz., light ( $${<}7.5 \, \hbox {mm}/\hbox {d}$$ ), moderate (7.5 to 35.4 mm/d), heavy (35.5 to 124.4 mm/d) and extremely heavy ( $${>}124.4 \, \hbox {mm}/\hbox {d}$$ ). The PBIAS and RSR show that TRMM simulated streamflow is suitable for moderate to heavy rainfall intensities. However, it does not perform well for light- and extremely-heavy rainfall intensities. The finding of the present work is useful for the problems related to water resources management, irrigation planning and hazard analysis over the Himalayan regions.

Evaluation of Watershed-Scale Simulations of In-Stream Pesticide Concentrations from Off-Target Spray Drift.

The estimation of pesticide concentrations in surface water bodies is a critical component of the environmental risk assessment process required by regulatory agencies in North America, the European Union, and elsewhere. Pesticide transport to surface waters via deposition from off-field spray drift can be an important route of potential contamination. The spatial orientation of treated fields relative to receiving water bodies make prediction of off-target pesticide spray drift deposition and resulting aquatic estimated environmental concentrations (EECs) challenging at the watershed scale. The variability in wind conditions further complicates the simulation of the environmental processes leading to pesticide spray drift contributions to surface water. This study investigates the use of the Soil Water Assessment Tool (SWAT) for predicting concentrations of malathion (O,O-deimethyl thiophosphate of diethyl mercaptosuccinate) in a flowing water body when exposure is a result of off-target spray drift, and assesses the model's performance using a parameterization typical of a screening-level regulatory assessment. Six SWAT parameterizations, each including incrementally more site-specific data, are then evaluated to quantify changes in model performance. Results indicate that the SWAT model is an appropriate tool for simulating watershed scale concentrations of pesticides resulting from off-target spray drift deposition. The model predictions are significantly more accurate when the inputs and assumptions accurately reflect application practices and environmental conditions. Inclusion of detailed wind data had the most significant impact on improving model-predicted EECs in comparison to observed concentrations.

High resolution water body mapping for SWAT evaporative modelling in the Upper Oconee watershed of Georgia, USA

AbstractTechnological improvements in remote sensing and geographic information systems have demonstrated the abundance of artificially constructed water bodies across the landscape. Although research has shown the ubiquity of small ponds globally, and in the southeastern United States in particular, their cumulative impact in terms of evaporative alteration is less well quantified. The objectives of this study are to examine the hydrologic and evaporative importance of small artificial water bodies in the Upper Oconee watershed in the northern Georgia Piedmont, USA, by mapping their locations and modelling these small reservoirs using the Soil Water Assessment Tool. Comparative Soil Water Assessment Tool models were run with and without the inclusion of small reservoir surface area and volume. The models used meteorological inputs from 1990–2013 to represent years with drought, high precipitation, and moderate precipitation for both the calibration and evaluation periods. Statistical comparison of streamflow indicated that the calibration methodology produced results where the default model simulation without reservoirs fit observed flows more closely than the modified model with small reservoirs included (e.g., Nash–Sutcliffe efficiency of 0.72 vs. 0.64, r2 of 0.73 vs. 0.66, and percent bias of 11.4 vs. 21.6). In addition, Penman–Monteith, Hargreaves, and Priestley–Taylor evapotranspiration equations were used to estimate actual evaporation from 2,219 small water bodies identified throughout the 1,936.8 km2 watershed. Depending on the evaporation equation used, water bodies evaporated an average of 0.03–0.036 km3/year for the period 2003–2013. Using Penman–Monteith further, if the reservoirs were not considered and average actual evapotranspiration rates from the rest of the basin were applied, only 0.016 km3 of water would have left the basin as a result of evapotranspiration. This finding suggests construction of small reservoirs increased evaporation by an average of 0.017 km3 per year (approximately 46,500 m3/day). As the construction of small reservoirs continues and high resolution image data used to map these water bodies becomes increasingly available, watershed models that evolve to address the cumulative impacts of small water bodies on evaporation and other hydrologic processes will have greater potential to benefit the water resource management community.

Hydrological modeling of the pipestone creek watershed using the Soil Water Assessment Tool (SWAT): Assessing impacts of wetland drainage on hydrology

Study regionPrairie Pothole Region of North America. Study focusThe Prairie Pothole Region of North America has experienced extensive wetland drainage, potentially impacting peak flows and annual flow volumes. Some of this drainage has occurred in closed basins, possibly impacting lake water levels of these systems. In this study we investigated the potential impact of wetland drainage on peak flows and annual volumes in a 2242km2 watershed located in southeastern Saskatchewan (Canada) using the Soil Water Assessment Tool (SWAT) model. New hydrological insightsThe SWAT model, which had been calibrated and validated at daily and monthly time steps for the 1997–2009 period, was used to assess the impact of wetland drainage using three hypothetical scenarios that drained 15, 30, and 50% of the non-contributing drainage area. Results of these simulations suggested that drainage increased spring peak flows by about 50, 79 and 113%, respectively while annual flow volumes increased by about 43, 68, and 98% in each scenario. Years that were wetter than normal presented increased peak flows and annual flow volumes below the average of the simulated period. Alternatively, summer peak flows presented smaller increases in terms of percentages during the simulated period.