Watershed integrated protection and restoration strategy based on land use optimization under nitrogen and phosphorus reduction targets-A case study of Quanmin Reservoir watershed in Guang'an City

The Quanmin Reservoir watershed in Guang'an City, Sichuan Province, located in the upper reaches of the Yangtze River, is selected as the study area. Integrating land use data from the 2021 national land survey, this study analyzes the spatial characteristics of land use patterns. By collecting township-level data on fertilizer application, the Nutient Delivery Ratio module of the InVEST model is applied to quantify the input and output amounts of nitrogen and phosphorus pollutants. Two land management scenarios, cropland protection and ecological conservation, are developed to simulate nutrient reduction outcomes. Based on the simulation results, a set of ecological protection and restoration strategies is proposed to improve water quality within the watershed. The results show that: (1) land use in the Quanmin Reservoir basin is dominated by cropland, primarily concentrated in the central and southern regions. Permanent based farmland accounts for up to 83.5 % of the cropland area, indicating that agricultural non-point source pollution is a prominent issue. (2) The annual application of nitrogen and phosphorus fertilizers in the basin is estimated at 3450 t and 987 t, respectively, with Yuechi County contributing 74.6 % of nitrogen and 60.4 % of phosphorus pollutants loads. Simulation results show that total nitrogen and phosphorus outputs are 957.70 t and 192.57 t, respectively. The spatial distribution patterns of both pollutants closely align with the distribution of cropland, confirming the dominant role of agricultural activities in driving nutrient pollution. (3) Scenario analysis indicates that establishing forest buffer zones around lakes, rivers, and ditches can reduce the output of nitrogen and phosphorus pollutants to a certain extent. Under the cropland protection scenario, the reduction ratios of nitrogen and phosphorus pollutants are 0.40 % and 0.49 %, respectively, while under the ecological protection scenario, it can reach 1.07 % and 1.24 %, respectively. This suggests that the ecological conservation scenario has significantly better nitrogen and phosphorus interception than the cropland conservation scenario, which is more realistic.