Modeling of Non-Point Source Contaminants in a River Basin: Implications for Watershed Management in Semi-Arid Regions

Abstract Rivers are vital components of freshwater resources but are increasingly affected by nonpoint-source pollution and eutrophication. This deterioration is primarily driven by nitrogen and phosphorus inputs from agricultural runoff, wastewater, and industrial discharges. Monitoring these pollutants is essential for identifying their sources and implementing effective water quality management strategies. This study applies the Soil and Water Assessment Tool (SWAT) to simulate hydrological and biogeochemical processes in a semi-arid river basin, with an emphasis on streamflow and nutrient loads at the subbasin level. Model calibration and validation were performed using SWAT Calibration and Uncertainty Procedures (SWAT-CUP), and model performance was evaluated using the coefficient of determination ( R 2 ) and the Nash–Sutcliffe efficiency (NSE). A combined spatial and temporal analysis was conducted to assess nitrogen and phosphorus dynamics as nonpoint-source contaminants. Spatial variability was observed, with total nitrogen ranging from 9.09 to 27.89 kg/ha/year and total phosphorus from 2.10 to 6.75 kg/ha/year across 31 subbasins. Subbasins with disproportionately high nutrient loads were identified as critical source areas (CSAs). Subbasins 3, 8, 9, 19, and 22 collectively contributed approximately 35 percent of the total nutrient loads, representing only 16 percent of the basin area. Temporal analysis indicated an upward trend in nitrogen and phosphorus loads. Five high-risk subbasins were identified as dominant contributors to total nutrient loads. This study demonstrates the effectiveness of the SWAT-based framework in identifying critical source areas and supporting targeted water quality management in semi-arid river basins.