Soil And Water Assessment Tool Calibration Research Articles

Staged SWAT calibration with bias-corrected precipitation product for enhancing flow data continuity in tributaries of the Mekong River

Accurate and continuous flow data are crucial for effective water management in large river basins worldwide. However, these catchments often face challenges regarding data continuity in the mainstream and their tributaries. This study proposes a methodological framework for enhancing flow data continuity that uses a staged calibration scheme of Soil and Water Assessment Tool (SWAT) modeling with an appropriate satellite precipitation product (SPP) for each watershed. This framework is successfully applied in the Mun-Chi River Basin, the largest tributary of the Lower Mekong River, over a 20-year period at 34 stations. The staged calibration involves partitioning the catchment into smaller sub-catchments, guided by the flow monitoring stations and flow discontinuity structures such as dams. The sequential calibration from upstream to downstream partitions the overall model calibration challenge into smaller problems and contributes to a more efficient and accurate calibration and validation. Alternative SPPs were considered to overcome monitoring gaps. Their bias was corrected using quantile mapping, and their performance was evaluated with flow simulations using the SWAT model. The assessment indicates that the CMORPH-CRT product, with a spatial resolution of 0.25°, demonstrates good suitability for hydrological modeling of the Mun-Chi River Basin. The proposed methodological framework provides a continuous time series of flow discharge at multiple stations within the watershed, offering valuable insights for sustainable water resource management strategies in river systems under changing climate and land use conditions, and supporting future studies on environmental issues.

Groundwater Recharge and Surface Runoff Modeling Response to Land Use and Land Cover Dynamics in a Mae Wong Watershed of Thailand

This study aims to develop efficient management strategies and assess the spatiotemporal dynamics of land use and land cover (LULC) on surface runoff modeling response dynamics for the long-term sustainability of watersheds. The soil and water assessment tool (SWAT) model was used to evaluate the LULC dynamics on GRSR in the Mae Wong Watershed (MWW) of Thailand. Using Landsat images, three different LULC maps (2011, 2021, and 2031) were created using the cellular automata markov chain (CA-Markov) model, and TerrSet 2020 geospatial monitoring and modeling software. In the overall MWW, the forestland has undergone deforestation and decreased by 2.10% of the total area and 2.72% of the total area has been transformed into agricultural lands due to human activity and population growth. The soil, LULC, weather, and the digital elevation model (DEM) were all used in the SWAT simulation procedure. To understand the groundwater recharge and surface runoff (GRSR) responses of each hydrologic response units (HRUs), the SWAT model was calibrated and verified using streamflow and the sequential uncertainty fitting (SUFI-2) technique from the SWAT calibration and uncertainty program (SWAT-CUP). The results indicate that there is a good agreement for both the calibration and validation phases of all LULC simulations. The study indicated that groundwater recharge has decreased over the last two decades while surface runoff has increased due to the forest area being converted to agricultural land. Thus, the study can support maximizing water management and strategies for systematically attaining sustainability.

Efficiency analysis of best management practices under climate change conditions in the So-okcheon watershed, South Korea

Best management practices (BMPs) are widely applied to address water quality degradation issues attributed to non-point source pollutants. The objective of this study was to assess the efficiency of two types of BMPs, vegetation filter strips (VFS) and wetlands, in reducing total nitrogen (TN) and total phosphorus (TP) in a watershed, and to investigate whether the efficiency of the two BMPs is maintained under future climate scenarios by employing the Soil and Water Assessment Tool (SWAT). A set of parameters was calibrated using the SWAT Calibration and Uncertainty Program (SWAT-CUP) to ensure acceptable simulation results. Three BMP scenarios were developed by applying VFS and wetlands individually and in combination. Projected climate data from 18 global circulation models under Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) were used to depict climate change conditions. VFS demonstrated 18.2% and 22.9% greater reduction efficiency for TN and TP, respectively, relative to wetlands. When the two BMPs were simultaneously applied, the reduction efficiency was even greater than that of single-BMP implementation (by 7.4% for TN and 6.8% for TP compared to VFS alone and by 25.5% for TN and 29.7% for TP compared to wetlands alone). To assess the effect of climate change, the model simulated results for the period of 2021–2,100. The differences in efficiency between the combined BMP scenario and the individual BMPs increased with greater intensity of climate change, especially in the distant future. Therefore, this study supports the effectiveness of nutrient pollution control by applying multiple BMPs rather than by applying individual BMPs. Furthermore, this research underscores the adaptability and reliability of natural-based solutions in mitigating non-point source pollution in a changing climate, which is essential for effective ecological restoration in complex urban-agricultural landscapes. The study provides valuable insights for watershed managers and policymakers seeking effective strategies to combat nutrient pollution in the face of a changing climate within the unique landscape of South Korea.

Assessing Hydropower Potential in Nepal's Sunkoshi River Basin: An Integrated GIS and SWAT Hydrological Modeling Approach.

This study assessed the hydropower potential of a mountain watershed within the Sunkoshi River basin in Sindhupalchok, Nepal, utilizing geographic information systems (GIS) and the soil and water assessment tool (SWAT) hydrological model. Topographical, soil, land use, meteorological, and discharge data were employed to assess the study area for the appropriateness of hydropower generation. SWAT was utilized to delineate the Sunkoshi basin into 23 distinct subbasins and involved the creation of a detailed river network, incorporating various hydrological attributes including stream links, stream order, stream length, and slope gradient. After that, it was employed to simulate river discharges within these subbasins. The Sequential Uncertainty Fitting Version 2 (SUFI-2) algorithm, integrated within the SWAT Calibration and Uncertainty Program (SWAT-CUP), was employed to calibrate and validate the model. This step involved the adjustment of 25 selected parameters to enhance the model's accuracy and reliability in representing the hydrological processes of the Sunkoshi basin. Model performance was assessed utilizing three well-established efficiency criteria: coefficient of determination (R2 = 0.79), Nash-Sutcliffe efficiency (NSE = 0.73), and percent bias (PBIAS = 17.59). The study identified 36 sites across streams of order 3, 4, and 5 as having potential for hydropower generation. The hydropower potential at each identified site was evaluated using estimated stream flow and topographical head at various probability of exceedance (PoE) levels (40%, 45%, 50%, and 60%). The aggregate hydropower potential of the basin was quantified, yielding a potential of 371.30 MW at a 40% PoE. The findings suggest that an integrated approach combining SWAT-based hydrological modeling within a GIS can accurately assess a river basin's hydropower potential and provide insights into further evaluation of the comprehensive environmental assessment of the fragile Himalayan watersheds.

Modeling the Potential Influence of Subsurface Tile Drainage Systems on Downstream Flooding in a Midwestern Agricultural Watershed

Highlights Tile flow was simulated to study the impacts of tile drainage on watershed hydrology and downstream flood days. Three tile drainage scenarios with varied extents of drained land were compared with a baseline scenario. The depth of tile flow contributing to streamflow was found to increase as the area of drained land increased. Results suggest that drainage systems decrease flood days when compared to the baseline scenario. Abstract. Subsurface tile drainage systems are common in agricultural regions of the Midwestern United States. Drainage systems remove excess water from the surface and soil profile of agricultural fields, allowing crop production in previously unsuitable locations. However, these systems impact watershed hydrology and could influence flooding events. Therefore, this study aimed to determine whether tile drainage systems influence downstream flood days in a Midwestern agricultural watershed, specifically the Skunk Creek watershed. The Soil and Water Assessment Tool (SWAT) model was used to simulate the hydrologic processes of the Skunk Creek Watershed for an 18-year period (2004–2021). The model was calibrated and validated using observed daily streamflow data with the SWAT Calibration and Uncertainty Program (SWAT-CUP) software and through the manual modification of missed precipitation data. Three tile drainage scenarios—ranging from 15% to 75% tile-drained agricultural land—were individually incorporated into the model. Results indicated that with increased tile drainage, surface runoff, groundwater flow, deep aquifer recharge, and percolation decreased, whereas lateral soil flow, tile flow, evapotranspiration, and water yield increased. A comparison of tile drainage scenarios suggests that increasing the area of tile-drained agricultural land decreases total flood days. However, the impact of tile drainage systems on downstream peak flood flows varied by event, with large precipitation events initiating flood days across all scenarios. While tile flow decreased flood days on a daily time step, flash floods might have occurred on a sub-daily time step but were not captured in this study. Future studies can replicate the approach with a sub-daily time step for simulating hourly flood events with various tile drainage scenarios. Keywords: Midwestern United States, Streamflow, SWAT, Tile drainage, Watershed hydrology.

Water Quality Assessment and Nutrient Management by SWAT from Sanitation and Agricultural Sources in Warana River Basin, MH, India

The Warana river watershed in Western Maharashtra, India has been recognized for exporting some of the highest nitrate-nitrogen loadings in western Maharashtra and is also a major source of sediment and other nutrient loadings. An integrated modeling framework has been constructed with Soil and Water Assessment Tool (SWAT) model and the interactive SWAT CUP. The simulation framework includes a detailed land use and management data such as different crop rotations and an array of nutrient and tillage management schemes, derived from the various source including the state department of Agriculture. This paper presents the calibration and validation of SWAT for the streamflow, sediment losses, and nutrient loadings in the watershed. Streamflow, sediment yield, and nitrate loadings were calibrated for the 1979-1984 period and validated for the 1984-2014 period. Secondary field data on organic nitrogen, organic phosphorus, and mineral phosphorus is used in model to validate for the 2007-2014 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. The first scenario set with application of fertilizer and no sanitation practices and 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.

Impacts of hydrometeorological factors on discharge simulation in the North West Himalayas: a SUFI-2 algorithm-driven investigation using the SWAT model.

The Soil and Water Assessment Tool (SWAT) is a computational hydrological model extensively utilised for developing sustainable strategies and viable approaches for prudent management of water resources. The central emphasis of this study is on the utilisation of SWAT model along with SWAT-CUP (SWAT calibration toolbox) to simulate streamflow in the upper Jhelum basin, the North West Himalayas, for a period of 20years from 2000 to 2019. The global sensitivity analysis algorithm, Sequential Uncertainty Fitting 2 (SUFI-2) of SWAT-CUP, is used for sensitivity and uncertainty analysis. The optimised parameter set estimated by SUFI-2 constitutes 11 parameters that are found to be sensitive with soil conservation service (SCS) curve number (CN) being the most influential parameter followed by snowmelt base temperature. Autocorrelation analysis using the autocorrelation function was conducted on the temperature and precipitation time series data, followed by a pre-whitening procedure to remove any autocorrelation effects. Subsequently, the modified Mann-Kendall (MMK) test was applied to examine trends in the annual temperature and precipitation data. The results indicated statistically significant positive trends in both datasets on an annual scale. The results for the calibration period (2003-2014) for monthly simulation displayed good model performance at three gauging stations, Rambiara, Sangam and Ram Munshi Bagh with R2 values of 0.83, 0.847, 0.829, P factor values of 0.73, 0.76, 0.75 and R factor values of 0.61, 0.58, 0.63, respectively. The validation results for monthly simulation for the 2015-2019 period showed good model agreement with R2 values of 0.817, 0.853, and 0.836, P factor values of 0.76, 0.8, and 0.75 and R factor values of 0.62, 0.53, and 0.65, respectively. The study concludes that the SWAT hydrological model can perform satisfactorily in high mountainous catchments and can be employed to analyse the impact of land use-land cover changes and the effect of climate variation on streamflow dynamics.

Spatial-Temporal Response of Sediment Loads to Climate Change and Soil Conservation Practices in the Northern Aegean Watershed, Türkiye

Climate change and agricultural activities are significant sources of stress to the natural environment and water resources. These also affect erosion and the associated estimation of sediment yields, which is also a crucial task in the hydrological models. The presented study is significant for the development of sustainable watershed management practices. It also aims to determine the effects of climate change and different agricultural best management practices (BMPs) on the sediment loads of the North Aegean Basin in Türkiye by using the Soil and Water Assessment Tool (SWAT) model. While sediment calibration was performed for 2014, streamflow calibration and verification were performed using the SWAT Calibration and Uncertainty Program (SWAT-CUP) for the period 2012–2013 and 2014–2015, respectively. The obtained results showed that the climate change scenarios reduce the surface waters of the basin and sediment yield in accordance with the hydrological transport processes. During the 2012–2030 time period, runoff in the basin for the RCP4.5 and RCP8.5 climate change scenarios decreased by 38.5% and 31.8%, respectively, and the basin sediment yield decreased by 55.7% and 50.7%, respectively. The sediment yields to water resources had distinctive reductions due to BMPs such as zero tillage, vertical tillage, cover crop, and terracing. Considering the RCP4.5 and RCP8.5 scenarios, BMPs reduced the sediment yield in the range of 0.93–4.03% and 0.89–3.85%, respectively. Determining the sediment transport by using hydrological modeling and the effects of climate change for different agricultural practices on erosion will be useful for decision-makers.

Assimilation versus optimization for SWAT calibration: accuracy, uncertainty, and computational burden analysis

Abstract The accurate estimation of runoff by hydrological models depends on proper model calibration. Sequential Data Assimilation (DA), as an online method, is used to estimate complex hydrological models' states and parameters simultaneously. Although DA was applied for estimating the Soil and Water Assessment Tool (SWAT) model's state and/or parameter, the previous research did not pay attention to the model calibration by DA or the comparison between DA and popular SWAT calibration methods. This paper compares Ensemble Kalman Filter (EnKF), a well-known DA method, with Sequential Uncertainty Fitting (SUFI2), a popular SWAT calibration method, to calibrate the model. We test the impact of the selected objective function in the SUFI2 application. We evaluate the results based on the multiple deterministic and uncertainty-based Goodness of Fit (GOF) measures and compare all scenarios based on the simulation accuracy, computational burden, uncertainty assessment, and parameter ranges. Results show that under the SUFI2 application, some GOFs might be located in unsatisfactory ranges while the algorithm obtains (very) good results concerning the objective functions. On the other hand, EnKF simultaneously locates most GOFs in (very) good ratings. Moreover, we found that the selection of SUFI2's objective function and the specification of uncertainty's error in EnKF have significant effects on the results.

Comparison of Flood Frequency at Different Climatic Scenarios in Forested Coastal Watersheds

Climate change-induced extreme precipitation causes coastal flooding. A streamflow simulation in coastal watersheds, Wolf River Watershed (WRW) and Jourdan River Watershed (JRW), was conducted using the Soil and Water Assessment Tool (SWAT) to compare variation in flow at different climates and to analyze the flood frequency. Baseline models were auto-calibrated with SWAT calibration and uncertainty programs (SWAT-CUP). Kling–Gupta efficiency (KGE), defined as the objective function in SWAT-CUP, ranged from 0.8 to 0.7 in WRW and from 0.55 to 0.68 in JRW during the calibration–validation process. Results indicated reliability of the model performances. Monthly averaged baseline flow was 1% greater than historical and 8.9% lower than future climate in WRW. In JRW, monthly averaged baseline flow was 11% greater than historical and 5.7% lower than future climate. Flood frequency analysis showed the highest 1% exceedance probability in annual maximum series (AMS) of baseline model in WRW, whereas AMS of projected model was estimated the highest in JRW. This study aids in preparing for future flood management.

Addressing data challenges in riverine nutrient load modeling of an intensively managed agro‐industrial watershed

AbstractData limitations often challenge the reliability of water quality models, especially in intensively managed watersheds. While numerous studies report successful hydrological model setup and calibration, few have addressed in detail the data challenges for multisite and multivariable model calibration to an intensively managed watershed. In this study, we address some of these challenges based on our reflective experience calibrating the Soil and Water Assessment Tool (SWAT) to the Upper Sangamon River Watershed in central Illinois based on daily flow, annual crop yield, and monthly sediment, nitrate, and total phosphorus loads. We highlight some challenges in SWAT calibration processes due to data errors and inconsistencies, and insufficient precipitation and water quality observations. Following, we demonstrate the merits of additional weather and water quality observations that could help reduce input uncertainties, and we provide suggestions for selecting appropriate observations for the model calibration. After dealing with the data issues, we show that the SWAT model could be calibrated with acceptable results for the case study watershed.

SWAT_DA: Sequential Multivariate Data Assimilation‐Oriented Modification of SWAT

AbstractMultivariate data assimilation (DA), a novel way to couple big data with land surface models, was extensively employed in forecasting‐reanalyzing systems (FRSs), for example, ECMWF and GLDAS. Meanwhile, most (distributed) hydrological models, like soil and water assessment tool (SWAT), have not been equipped with straightforward ways to link to DA algorithms. Therefore, it is one of the main barriers to utilizing such hydrological models in FRSs. This paper deals with multivariate DA into SWAT (DA‐SWAT), which is complicated since the original model does not provide full access to the models' initial conditions (ICs) at the hydrologic response unit (HRU) scale. The preceding DA‐SWAT works commonly used an integrated approach in which the DA and SWAT codes were implemented in the same programming environment. We discuss how this approach complicates and prevents the application of DA‐SWAT in multivariate, multimodel, and multisensor systems. Accordingly, we proposed a new approach for DA‐SWAT by which SWAT can be perfectly linked with any DA algorithm of interest coded in any desired programming environment. Our framework utilizes input/output text files to access ICs and to link DA with SWAT. Moreover, we designed some univariate and multivariate scenarios for assimilating in situ streamflow measurement and MODIS's snow cover fraction (SCF) data, which has not yet been focused on in the SWAT calibration context. Results show that compared to the univariate assimilation of streamflow (SCF), the multivariate assimilation mitigates the equifinality problem and more accurately estimates SCF (streamflow) by improving NS and PBIAS measures with the differences of 0.4 (0.86), 12% (64%), respectively.

Ecohydrologic modeling using nitrate, ammonium, phosphorus, and macroinvertebrates as aquatic ecosystem health indicators of Albaida Valley (Spain)

Study regionAlbaida Valley, Spain. Study focusRivers are experiencing a rapid biodiversity loss largely due to water quality degradation imposed by anthropogenic activities. To show the capability of modeling to inform water management at watershed scales, this research develops an eco-hydrological model of the Albaida Valley (Spain). SWAT (Soil and Water Assessment Tool) is used for modeling of discharge and nutrients after calibration with SWAT-CUP (SWAT Calibration and Uncertainty Program). Results from SWAT are coupled to regressions between nutrients concentrations and macroinvertebrate-based metrics obtained from field monitoring. The spatio-temporal assessment of ecological status of streams is then carried out using simulated chemical and biological quality indicators (nitrate, ammonium, phosphorus, and macroinvertebrates). New hydrological insights for the regionManagement measures (e.g., improving treatment of wastewater and/or adopting policies for reducing fertilizer use) are needed as the ecological status of Albaida Valley rivers is mostly classified as poor because of nutrients pollution. The reasonably low uncertainty in the model prediction (expressed by R-factor: discharge (0.2–0.61), nitrate (0.79–1.27) and total phosphorus (0.8–1.68)) demonstrates the potential of the presented model for future applications (i.e., for investigating possible responses of the Albaida Valley ecosystem to changes in climate, land-use, and local management policies). The modeling approach provided in this study could be generally used as a complementary technique to field monitoring in assessing and managing ecological conditions of rivers.

Response of runoff and nitrogen loadings to climate and land use changes in the middle Fenhe River basin in Northern China

Abstract Investigating and understanding the responses of runoff and nitrogen loading to climate and land use change is particularly important for future water resources management. In this article, the Soil and Water Assessment Tool (SWAT) was used to simulate runoff and nitrogen loading in the middle reaches of the Fenhe River. The model was calibrated by the SWAT calibration and uncertainty procedure (SWAT-CUP) to achieve the accuracy of simulating runoff and nitrogen loadings. Furthermore, 20 climate change scenarios and seven extreme land use change scenarios were set up and run on the calibrated model. The results showed that runoff and nitrogen loading decreased when temperature increased and increased with increasing precipitation. Runoff was more sensitive to changes in precipitation (±10%) than temperature (±2 °C), while nitrogen loading showed the opposite pattern. When the two climatic factors changed in the same direction, the combined effect was larger than either factor alone, whereas the change in the opposite direction produced a weaker effect. The changes produced by different extreme land use scenarios on runoff and nitrogen loading were significantly different and were more obvious during the flood season than in the non-flood season. The results of this study provide a useful guide for water resource managers.

Identification of soil erosion hot-spot areas for prioritization of conservation measures using the SWAT model in Ribb watershed, Ethiopia

Soil erosion is one of the pressing environmental problem in Ethiopia Thus, identifying soil erosion hot-spot areas of a watershed/basin and isolating best management practices (BMPs) for soil erosion-prone sub-watersheds is imperative for effective soil loss reduction and lessening its on-site and off-site effects. The present study aimed to identify erosion hot-spot areas for prioritization of conservation measures and evaluate BMPs for reducing soil erosion using the Soil and Water Assessment Tool (SWAT) model in Ribb watershed, Ethiopia. The SWAT model was calibrated (1995–2002) and validated (2003–2008) using flow and sediment data in the SWAT Calibration and Uncertainty Program (SWAT-CUP). The NSE (Nash–Sutcliffe efficiency), R 2 (Coefficient of determination), PBIAS (Percent bias) and RSR (Root Mean Square Error — observations standard deviation ratio) values during flow and sediment calibration and validation periods ranged from 0.7 to 0.9, 0.71 to 0.92, -0.4% to 15.2% and 0.32 to 0.55, respectively, indicating that SWAT is able to simulate streamflow and sediment with sufficient accuracy. The calibrated and validated SWAT model was used to evaluate the effectiveness of three BMPs against the baseline conditions for reducing soil erosion, such as filter strips Scenario (Scenario F), stone/soil bunds Scenario (Scenario S) and reforestation Scenario (Scenario R). The result indicated that about 95% of the watershed are indentified as soil erosion hot-spot areas, which experienced sediment yield > 11 t ha − 1 yr − 1 . The mean annual sediment yield (1995–2008) of the watershed at the baseline conditions was nearly 29 t ha − 1 yr − 1 . The implementation of Scenario F, Scenario S and Scenario R reduced the baseline sediment yield by 28%, 76% and 62% at the watershed scale, respectively. At the sub-watershed levels, Scenario S and Scenario R reduced the mean annual sediment yield from 45%–90% and 0%–90%, respectively. Therefore, the study suggests implementations of Scenario S and Scenario R for effective soil erosion reduction in the study watershed in particular and to other watersheds in Ethiopia in general which have similar environmental settings. • SWAT was applied to identify soil erosion hot-spot areas and evaluate BMPs. • Filter strips, stone/soil bunds and reforestation Scenarios were evaluated. • About 95% of the watershed are identified as soil erosion hot-spot areas. • Stone/soil bunds (76%) and reforestation (62%) Scenarios reduced maximum sediment. • Implementations of stone/soil bunds and reforestation Scenarios are suggested.

Hydrologic response of arid and semi-arid river basins in Iraq under a changing climate

Abstract An assessment of the total hydrologic response of arid and semi-arid river basins to various scenarios of climate change by considering evapotranspiration, streamflow, and snowmelt is essential for sustainable management of water resources. The Diyala River Basin in Iraq has been chosen as a typical case study of dozens of river basins in arid and semi-arid regions. Here, the Long Ashton Research Station-Weather Generator (LARS-WG), the Soil and Water Assessment Tool (SWAT), and the SWAT Calibration and Uncertainty Program (CUP) were used to evaluate the total response by considering three Representative Concentration Pathways (RCPs); RCPs 2.6, 4.5, and 8.5 over three periods, 2021–2040, 2041–2061, and 2061–2080. The results indicate that by the year 2080, the basin will experience a temperature increase by 6.6, 10.1, and 16.6% for RCP 2.6, RCP 4.5, and RCP 8.5, respectively. The corresponding reduction in precipitation will be 3.2, 6.4, and 8.7%, resulting in 38.8, 47.9, and 52.8% fall in streamflow for RCPs 2.6, 4.5, and 8.5, respectively. Due to the increase in temperature, an earlier and less contribution of snowmelt is expected in the projected streamflow. Our findings provide a useful reference and a guide to decision makers for developing adaption plans to sustainably manage water resources in the Diyala River Basin and other similar basins in arid and semi-arid regions.

Spatial and temporal variability evaluation of sediment yield and sub-basins/hydrologic response units prioritization on Genale Basin, Ethiopia

Erosion of the topsoil is a serious environmental problem worldwide that critically alarms agricultural and upland areas. Most wetlands, rivers, and reservoirs are losing their capacity because of sediment deposition into the water body from the upstream of watersheds, as they were not managed. Soil and Water Assessment Tool (SWAT) model is used to simulate streamflow, sediment load, & identify spatiotemporal variability of the sediment yield and sediment delivery ratio (SDR), point out erosion-prone area, and prioritize sub-basins/hydrologic response units (HRUs) for management. The model is implemented by utilizing a digital elevation model, land use, soil type, and slope of the Genale basin; 464 HRUs were created, spreading over 25 sub-basins within the drainage area 54,942Km2. SUFI 2 algorithm of SWAT calibration & uncertainty programs (SWAT_CUP) is used to calibrate (1990 – 2005) and validated (2006 – 2013) streamflow & sediment on monthly scale, and the resulted statistics are; R2 = 0.87, 0.85 and NSE = 0.81, 0.78 for flow calibration and validation respectively whereas R2 = 0.84, 0.82 and NSE = 0.79, 0.75 for sediment calibration and validation respectively. Significant spatiotemporal variability of sediment yield was observed in the watershed, as the characteristics of the terrain, land use, & soil type in each sub-basin varies. Among the total 25 sub-basins, three sub-basins produced a very high sediment rate (21–31 ton/ha/year), one produced high (16–20 ton/ha/year), one is moderate (11–15 ton/ha/year), three were low (6–10 ton/ha/year), and the rest 16 sub-basins were under very low categories. In the case of SDR, three sub-basins have very high SDR (>0.452), two sub-basins resulted in high SDR (0.326–0.451), which are located at the upland basin. Based on the model results, sub-basins 6, 8, 12, 10, and 7 were identified as sediment-prone areas. Further investigation at the HRUs scale is taken to understand critical erosion areas and minimize the cost of management practice, time & human resources. HRU analysis has revealed the immense scope of minimizing cost by concentrating management measures only in the critical HRUs. For example, Sub-basin-6 has 31 HRUs, of which only seven are assessed to have the high rates of sediment yield and thus prioritized as; HRU-159, HRU-160, HRU-161, HRU-168, HRU-170, and HRU-171, which are located on agricultural and arable lands of steep slopes.

Spatio-temporal modeling of surface runoff in ungauged sub-catchments of Subarnarekha river basin using SWAT

In this present study, Soil and Water Assessment Tool (SWAT) embedded with ArcGIS interface has been used to simulate the surface runoff from the un-gauged sub-catchments in the upper catchment of Subarnarekha basin. Model calibration and validation were performed with the help of Sequential Uncertainty Fitting (SUFI-2) in-built in the SWAT-CUP package (SWAT Calibration Uncertainty Programs). The model was calibrated for a period from 1996 to 2008 with 3 years warm up period (1996-1998) and validated for a period of 5 years from 2009 to 2013. The model evaluation was performed by Nash - Sutcliffe coefficient (NSE), Coefficient of determination (R2) and Percentage Bias (PBIAS). The degree of uncertainty was evaluated by P and R factors. Basing upon the R2, NSE and PBIAS values respectively, of the order of 0.90, 0.90 and -12%, during calibration and 0.85, 0.83 and -15% during validation, substantiate performance of the model. All uncertainties of model parameters have been well taken by the P and R factors respectively, of the order of 0.95 and 0.77 during calibration and 0.82 and 0.87 during validation. The runoff generation from 19 sub-catchments of Adityapur catchment varies from 29.2-44.1% of the annual rainfall and average surface runoff simulated for the entire catchment is 545 mm. As the surface runoff generated in most of the sub-catchments amounts to above 30% of rainfall, it is recommended for adequate number of structural interventions at appropriate locations in the catchment to store the rainfall excess for providing irrigation, recharging groundwater and restricting the sediment and nutrient loss.

Evaluating the added value of multi-variable calibration of SWAT with remotely sensed evapotranspiration data for improving hydrological modeling

Hydrological processes in a watershed consist of multiple sub-processes (such as plant growth, evapotranspiration, water yield, and soil–water balance) that have complex interactions. The common practice of calibrating hydrological models against only a single variable (e.g., streamflow) can lead to parameter uncertainty (also known as equifinality), resulting in significant uncertainties in the representation and simulation of sub-processes. As multi-variable calibration can be a potential solution to this issue, we tested the integration of spatially and temporally gridded remotely sensed evapotranspiration (RS-ET) data with the Soil and Water Assessment Tool (SWAT) hydrological model. This approach was intended to reduce equifinality by enhancing related hydrological sub-processes in both space and time rather than improving the evaluation metrics at the streamflow outlet. We further introduced the principle of repeated measure design in the calibration process, where the SWAT was calibrated under two different schemes: Scheme1 (using only streamflow data) and Scheme2 (using both RS-ET and streamflow data). The model's performance was evaluated using the concept of stability at multiple spatial scales (basin outlet, sub-basins, and hydrological response units) and aspects (different model outputs and most sensitive calibrated parameters). The significance of the difference between the stabilities produced by the two schemes was estimated using the Mann–Whitney U test. Testing this approach in Meichuan Basin (China) showed that Scheme2 substantially reduced equifinality for calibrated parameters and model outputs compared to Scheme1. In addition, the model solutions and outputs for Scheme2 were significantly different from Scheme1. Our results demonstrate the added value of using increasingly available open-access RS-ET data for improving hydrological model calibration.

Impacts of climate variability and changing land use/land cover on River Mpanga flows in Uganda, East Africa

We analyzed River Mpanga Catchment (RMC) land use/land cover (LULC) types based on Landsat images for 2000, 2008 and 2014. Soil and Water Assessment Tool (SWAT) was driven by daily meteorological data from 2000 to 2011 to investigate impacts of LULC changes on river flow variation. In 2000, 2008, and 2014, cropland covered 33.0%, 69.1%, and 72.2% of RMC area, respectively. However, the fractions of the RMC area covered by grassland in 2000, 2008, and 2014 were 39.4%, 12.5%, and 10.4%, respectively. The portion of RMC area covered by human settlement increased from 0.2% in 2000 to 0.5% by 2014. RMC was characterized by increasing trends in annual rainfall and river flows. SWAT calibration and validation at daily scale over the periods 2000–2005 and 2006–2011 yielded Nash Sutcliffe Efficiency of 0.77 and 0.75, respectively. Contribution from transitions in LULC types to river flow changes over the period 2000–2008 was 7.65%. Generally, 70.46% of the total river flow variation was contributed by climate variability in terms of changes in climatic conditions. However, 21.89% of the total river flow variance remained unexplained and this could be attributed to other factors not considered in this study including extra impacts of human activities such water abstractions for agricultural, industrial and domestic needs. These findings are important for planning predictive land and water resources management amidst impacts of climate variability and human activities on water resources.