Identification of critical source areas and delineation of management subzones of non-point source pollution in Jing River Basin

Critical source area (CSAs) identification and subzone delineation could aid in streamlining watershed management. However, how to build a comprehensive evaluation index system based on the identification of CSAs is the key to solving the problem of management zoning, and this issue has not been fully reported yet. We applied the soil and water assessment tool to investigate non-point source (NPS) pollution for two periods from 1967 to 1990 and from 2000 to 2020 in the Jing River Basin, identify the CSAs in each period using four methods, and delineate the watershed management subzones using K-means cluster analysis. Results showed that, from 1970 to 1990, the annual average sediment yield in the basin was 46.91 Mg/ha/year, while from 2000 to 2020, it decreased by 53.24% to 22.01 Mg/ha/year. Between 2000 and 2020, the average load of total phosphorous (TP) pollution was 0.168 kg/ha/year, which was around 92.1% less than that from 1967 to 1990. Although the areas with the most severe total nitrogen (TN) load (15–18 kg/ha/year) had been treated, the pollution intensity was still relatively serious, and there was a trend of large-scale diffusion. The comprehensive evaluation index could effectively identify CSAs under the triple superposition of sediment yield, TN, and TP, which could be defined as one of the important indicators affecting the management subzones of the Jing River Basin. The watershed was divided into four different management zones, namely: northern loess hills and ravines ecological restoration zone, the priority management subzone in the middle west, the middle-eastern forested ecological conservation area, and the southeastern pollution control zone. This study could provide reference and specific directions for decision makers in efficient and sustainable watershed NPS pollution management at the scale of sub-watershed and management subzone.