Modeling sediment yield and assessing conservation measure effectiveness with SWAT+

Soil erosion is a fundamental physical process driving land degradation across various spatial and temporal scales. The Soil and Water Assessment Tool (SWAT) model is a robust tool for predicting soil erosion and evaluating water and soil quality within watersheds. The latest version, SWAT+, introduces advanced encoding capabilities and improved performance, making it better suited for addressing complex watershed modeling challenges. This study implemented the SWAT+ model to quantify soil erosion rates within the Chehelchay watershed in northern Iran. The foundational dataset comprises a 30-meter resolution Digital Elevation Model (DEM), land use classification, soil, and weather data. Model performance was evaluated using Nash-Sutcliffe Efficiency (NSE), coefficient of determination (R²), root mean square error (RMSE), and percent bias (PBIAS). The SWAT+ simulation revealed substantial spatial variation in erosion patterns across the watershed, with annual sediment yields in critical HRUs, reflecting diverse erosion intensities driven by variations in land use, soil characteristics, and slope. Among the Hydrological Response Units (HRUs), 50 critical units, representing approximately 9% of the total watershed area, generate sediment yields exceeding 5 tons per hectare per year. The most severe erosion occurs predominantly in the central zone of the watershed. Downstream regions exhibit minimal soil loss due to gentle topography while upstream areas maintain soil stability through protective forest cover, resulting in negligible erosion rates. Best Management Practices (BMPs) were designed to safeguard water and soil resources at a watershed level. The study evaluated three strategic conservation interventions: alfalfa cultivation, agroforestry implementation, and garden development. When applied in combination, these measures achieved approximately 30% reduction in sediment yield at the HRU level. This integrated approach demonstrates the potential of combining multiple land management strategies to combat erosion effectively.