Phosphorus critical source areas identification in an agricultural watershed of Three Gorges Reservoir Area, China

Agricultural non-point sources (ANPS) pollution are considered to pose risks to water quality in Three Gorges Reservoir Areas (TGRA), especially when those pollution source comes from the hydrologically sensitive areas (HSAs) that generate surface runoff pathways. Therefore, it is necessary to identify the critical source areas (CSAs) or the sensitive regions of pollutants from agricultural lands in order to control and mitigate ANPS pollution effectively. In this study, an improved method integrating the Phosphorus Index (PI) and the Soil Topographic Index (STI) was applied to predict the risk of phosphorus loss and delineate the CSAs of phosphorus in a typical agricultural watershed in TGRA, China. The results showed that using a STI threshold value of 8.5, the HSA was identified 22.08% of watershed areas. The intersection of above two parts account 5.47% of the total watershed, compared with 24.41% of watershed areas based on an existing approach that uses just one criterion. As the results showed , the CSAs of phosphorus loss were mainly located near streams, with high or relatively high soil phosphorus contents or phosphorus fertilizer, or intense soil erosion are observed. The calculated results are in good agreement with the actual situation. Since the approach is based on GIS, and is a relatively simple application uses readily available geospatial data, therefore the technic could be used to improve cost-effectiveness and provide a useful screening tool for water resource managers responsible for identifying and remediating critical NPS source areas.