Identification method and application of critical load contribution areas based on river retention effect

For the entire watershed, the critical source areas (CSAs) and the critical load contribution areas (CLCAs) are two completely different concepts. The CLCAs can reflect the impact of river retention effects on pollutant transmission. In this study, an integrated modelling approach was developed for those complex watersheds by combining two models: MECM (modified export coefficient model) and SWAT (Soil and Water Assessment Tool). A case study was performed in a typical rural area-Miyun Reservoir watershed, China. The simulated results indicated that anthropogenic pollution is the main source of pollutants in most townships, including livestock breeding, rural activities, and crop cultivation. It spreads upstream with the outlet of the basin as the center, and the transport efficiency decays regularly, so the location of the pollution source is closely related to its transport efficiency. The river retention effect has a significant retardation effect on the transportation of pollutants, more than half of the pollutant load will be deposited in the river network. Generally, the CLCAs are concentrated in the area where the transport efficiency and pollutant load are relatively high, which is quite different from the spatial distribution of the CSAs. The research results fully excavated the transmission path and process of pollutants, especially the process of river migration, which helps to improve the scientific configuration of management practices.