An Integrated Approach to Identify Critical Source Areas of Agricultural Nonpoint-Source Pollution at the Watershed Scale.

Nonpoint sources are difficult to control because their nutrient contribution from different parts of a watershed can vary substantially. Identifying critical source areas of nutrient loss is an important step in watershed pollution mitigation programs. This study sought to use an integrated index model to differentiate between subbasins that serve as critical source areas of N and P nonpoint sources of pollution in China's Tiaoxi watershed. In contrast with previous N and P indices, multiple sources of pollution (i.e., agronomic activity, domestic wastewater, livestock farming, and aquaculture) were considered. Nitrogen and P source factors (i.e., N and P annual export loads) and transport factors were multiplied to determine the overall risk of nutrient loss in the integrated index model. Critical source areas were identified by a higher nutrient loss index. Of the 92 subbasins within the Tiaoxi watershed, 13 were determined to be critical sources for N, 10 for P, and seven for both N and P. Critical source area identification corresponds well with water quality data from the subbasins. The results show the potential use of the integrated index model for prioritizing and targeting watershed pollution mitigation activities at the subbasin level.