Applicability evaluation of agricultural Best Management Practices to estimate reduction efficiency of suspended solids

Nonpoint source (NPS) pollution affects water quality, and the path of leakage and discharge of NPS pollutants is not clear. NPS pollution is heavily influenced by weather conditions such as precipitation, making NPS pollution difficult to manage. The Soyang River watershed, in the upper reaches of the Han River, is dominated by mountain farmland, which causes the water to become turbid due to soil erosion during rainfall. Therefore, South Korea's Ministry of Environment has designated Mandae District as an NPS pollution management area to install and manage various NPS pollution reduction facilities (NPRF). However, research on the NPS reduction efficiency is insufficient. Therefore, a Soil and Water Assessment Tool (SWAT) model was used to evaluate soil loss and suspended solids (SS) reduction efficiency by NPRF installed in the Mandae District. In this study, hydrologic response units (HRUs) were modified to spatially consider agricultural fields in steep-sloping areas in South Korea, and soil loss, as well as SS reduction efficiency, were calculated reflecting temporal/spatial characteristics of best management practices (BMPs). In order to accurately evaluate the NPRF, this study digitized all fields in the Mandae District and applied them to land use maps where slope and slope length were entered for each agricultural field. To this end, the digitized boundaries of all agricultural fields have been updated into land use maps. In addition, the estimated soil loss was corrected by calibrating the exponent and coefficient of the MUSLE (Modified Universal Soil Loss Equation) formula. Reduction efficiency was assessed by comparing the amount of soil loss and SS before and after the installation of the NPRF with a scenario-based analysis. As a result, the soil loss reduction efficiency of the NPRF in Mandae District was 7.8% and the SS reduction efficiency was 5.8%. Both soil loss and SS were sensitive to slope length in evaluating the effectiveness of reduction. The results showed that the longer the slope length, the greater the soil loss or SS.