The impacts of environmental and socioeconomic factors on the non-point source pollution in a small watershed in the Three Gorges Reservoir Area, China

Determining the impacts of environmental and socioeconomic factors on nitrogen (N) and phosphorus (P) loss in the watershed is critical to reducing non-point source (NPS) pollution. This paper, we set 13 sampling points in the main stream and tributaries of watershed and sampled every two weeks from 2018 to 2020 to monitor the total nitrogen (TN) and total phosphorus (TP) concentration in the waterbodies. Twenty-six potential influencing factors affecting the nitrogen and phosphorus loss in the watershed were selected. The partial least squares regression (PLSR) was used to determine the relationship between TN and TP concentrations in the watershed and the 26 selected potential influencing factors. The results showed that the mean TN concentrations and mean TP concentrations in the dry season (11.42 mg·L⁻¹ and 0.09 mg·L⁻¹, respectively) were both less than those in the wet season (13.20 mg·L−1 and 0.11mg·L−1, respectively). The optimal PLSR model explained 69.6%, 73.1% and 66.1% of the TN concentration variability, and 65.7%, 79.5% and 67.4% of the TP concentration variability during annual, dry season and wet season, respectively. According to the importance of the variables in the predicted value (VIP), topographic wetness index (TWI), planting structure (PS), interspersion and juxtaposition index (IJI), Orchard land use (OP), nitrogen fertilizer application (NF), per capita income (INCOME) and catchment area (AREA) were the key factors affecting TN concentration, whereas topographic wetness index (TWI), interspersion and juxtaposition index (IJI), population density (POP), slope gradient (SLOPE) and hypsometric integral (HI) were the key controlling factors of TP concentration. In addition, TN concentration was affected by cropland land use (CP) during the dry season and proportion of labor (LABOR) and per capita agricultural land area (ALA) during the wet season. TP concentration was affected by mean patch size (AREA_MN), phosphate fertilizer application (PF) and patch density (PD) during the dry season and residential area (RP) and values during the wet season. This study illustrates the impact of environmental and socioeconomic factors on NPS pollution, and can be used as a guide for effective NPS pollution control and water quality management.