Spatiotemporal Patterns and Driving Factors of Water Self-Purification Capacity in the Source Area of the Western Route of the South-to-North Water Diversion

The water self-purification capacity in the source area of the Western Route of the South-to-North Water Diversion Project of China was investigated. This research aimed to promote the coordinated development of water source protection and water diversion benefits. Based on water quality tests conducted in the upstream section of the water source area, the results indicate that the water quality in the upper reaches of the water source region is generally good. However, total nitrogen at some sites in the Dadu River basin exceeded 1.0 mg/L, and the mean total nitrogen concentration in the Dadu River basin was higher than that in the Yalong River basin, while total phosphorus levels remained low in both basins. The nitrogen removal of the Yalong River Basin and the Dadu River Basin attained a peak of 34,654 t in 2012. The nitrogen removal was more than ten times that of phosphorus. Among the prefectures, the water self-purification service value ranks as follows: Garzê > Liangshan > Aba > Golog. The correlations between water quality indicators and driving factors-including land use types, GDP, precipitation, population, and temperature were discussed. In terms of spatial drivers, barren land (r = 0.41) and grassland (r = 0.47) are the major land-use contributors to TN. The model was found which can be used to quantify management priorities and predict potential risks associated with nitrogen pollution. After water diversion, the removal of nitrogen and phosphorus in the water source area declined, which was not linearly related to the amount of water diverted.