Model-based evaluation of structural best management practices in reducing agricultural nonpoint source pollution in the Sand River catchment

The intensified use of chemical fertilisers, coupled with unmanaged agricultural land, results in nonpoint source (NPS) pollution of surface water resources through runoff. This study aimed to evaluate the effectiveness of Best Management Practices (BMPs), including grassed waterways (GWW) and vegetative filter strips (VFS), and their combination in reducing sediment, total inorganic phosphorus (TIP), and total inorganic nitrogen (TIN) loads in a catchment under semi-arid conditions. Furthermore, the study sought to compare the performance of these BMPs when implemented at the catchment level versus the sub-catchment level. The Soil and Water Assessment Tool (SWAT+) was used to simulate annual TIN, TIP, and sediment loads under various scenarios (base, GWW, VFS, and GWW + VFS) in the Sand River Catchment (SRC). A percentage reduction equation was applied to determine the load reductions achieved in each scenario. Calibration and validation of the SWAT+ model showed good performance, with R² and NSE values above 0.72 indicating strong correlation, and PBIAS values (− 17.30 to 14.74) within acceptable limits, confirming model reliability. The simulated results have revealed a lower reduction in sediment, TIP, and TIN loads using VFS at sub-catchment and catchment levels (27/23%, 23/20%, and 30/25%, respectively), compared to GWW (47/38%, 35/31%, and 43/31%, respectively). VFS + GWW exceedingly reduced sediment, TIP, and TIN loads at the sub-catchment level as compared to the catchment level (70/58%, 66/56%, and 71/62%, respectively). The study concludes that implementing BMPs at the sub-catchment level can be one of the most effective management strategies for NPS pollution in the SRC.