[Identifying critical source areas for non-point phosphorus loss in Chaohu watershed]

Agricultural non-point phosphorus (P) pollution is an important cause of eutrophication in many freshwater systems. Identifying areas that at high risk for P loss to surface water in watershed and concentrating management efforts on these minimal portions of lands are better measures than implementing general strategies over a broad area. A modified version of P index was used to assess P loss risk and identify critical source areas in Chaohu watershed at a regional scale. Soil P sorption index and degree of P sorption saturation were introduced as source factors to show the inherent ability of P transport in soil-water interface. Distance from sources to Chaohu Lake was also considered as a new transport factor to show the influence from sources to final receiving water. The ranking schemes were modified according to available data and scale of study area. The soil P sorption indexes show mild spatial variations in watershed. However, the values are relative low which implies higher loss risk. Distinct spatial variations are found in the degree of soil P sorption saturation. More than 40% of the watershed is saturated over 25% by P. The results show prominent spatial variations of non-point P pollution risk index in Chaohu watershed. Highest risk areas, take up about 5% of the watershed, spread near the downstream parts of main rivers to Chaohu Lake. These should be regarded as critical source areas to be treated in priority. It indicates that P index is a rapid and simple tool to identify critical source areas of non-point P pollution at the regional scale and it enables managers to implement best management practice (BMPs) in a high priority to minimize P loss to sensitive watercourses.