Water quality improvements and amplified climate impacts from the South-to-North Water Diversion Project, China

Abstract

It is essential to quantify the effect of inter-basin water diversion projects on water quality. However, determination of the driving mechanisms controlling temporal changes in water quality within the lakes and reservoirs receiving diverted water (i.e., the beneficiary areas) is typically plagued by a lack of long-term, spatially continuous monitoring data. This study developed a stacking machine learning model that was coupled with a Shapley additive explanation algorithm to investigate the roles of major meteorological factors, human activities, and the volume of diverted water on the water quality of beneficiary lakes and reservoirs before and after water diversion by the South-to-North Water Diversion Project (SNWDP). An analysis of the spatiotemporal variations and trends in six water quality parameters was presented, including total nitrogen (TN), total phosphorus (TP), Chlorophyll a (Chl a), ammonia nitrogen (NH₃-N), and chemical oxygen demand (COD_Mn, and COD_Cr). Results demonstrate that except for TN and Chl a, the water quality of the most beneficiary lakes and reservoirs improved after the SNWDP began supplying water. Approximately 42 %, 69 % and 48 % of the beneficiary lakes and reservoirs experienced a significant decrease in TP (p < 0.05), NH₃-N (p < 0.05), and COD_Mn (p < 0.05) concentrations, respectively. The SHAP model suggests that anthropogenic predictors contributed more to water quality improvements (40.39–45.98 % for TP, 30.17–36.60 % for Chl a, and 42.01–53.80 % for NH₃-N) than did either the climatic and geographical drivers. The contribution rates of water diversion to Chl a and NH₃-N were higher than the other parameters, reaching 11.78 % and 15.74 %, respectively. The sensitivity and contribution rates of climatic factors to water quality increased (except for TN) after water diversion. The information provided herein on the impacts of water diversions on the water quality of beneficiary lakes and reservoirs is needed to assist in the management of subsequent water diversion projects.