Spatiotemporal variations in water quality and their relationships with extreme rainfall in the Danjiangkou Reservoir from 2017 to 2023

Objective In recent years, global extreme rainfall events have become increasingly frequent. Revealing the spatiotemporal variation characteristics of water quality in the Danjiangkou Reservoir area and its response to extreme rainfall is of great significance for ensuring water quality safety. Methods In this study, we made use of the daily rainfall data spanning the period from 2017 to 2023, together with the monthly water quality monitoring data that were collected from a total of 31 monitoring stations. A series of analytical approaches were adopted, which included the calculation of multiple extreme rainfall indices, the application of the seasonal Mann Kendall trend test for detecting monotonic trends, the use of Sen’s slope estimator for quantifying the magnitudes of the detected trends, and the performance of Pearson correlation analysis to evaluate the relationships between the extremization of rainfall and the various water quality indicators. Through the integration of these methods, we were able to reveal the trends of spatiotemporal water quality variation and their differential associations with extreme rainfall. Results The results obtained from this study are summarized as follows: 1) Over the entire study period, the majority of the extreme rainfall indices exhibited a general upward tendency, which clearly indicates that the characteristics of precipitation extremization became more pronounced. 2) The temporal and spatial evolution of water quality showed significant differences across the study area. More specifically, the range of fluctuations in water quality within the reservoir area was generally smaller than that observed in the tributary rivers. Among all the water quality parameters examined, total nitrogen (TN) exhibited a significant increasing trend in both the reservoir area and the tributaries, thus making it the most prominent factor exerting pressure on water quality. In contrast, the concentrations of ammonia nitrogen (NH₃ N) and total phosphorus (TP) remained relatively stable throughout the study period, without showing any notable trends. 3) The response of water quality to extreme rainfall displayed clear regional differentiation. For instance, water temperature was found to be sensitive to variations in rainfall. Regarding pH, it was mostly positively correlated with extreme rainfall in the reservoir area, while in the tributaries the correlation was predominantly negative. Dissolved oxygen (DO) generally exhibited a decreasing trend with increasing rainfall, whereas the organic matter indicators, including CODMn and COD, showed increasing tendencies. As for total nitrogen (TN), it was negatively correlated with increased rainfall in both the reservoir and the tributaries; however, the specific rainfall indices that dominated the correlation differed between the two water bodies. Conclusions In general, extreme rainfall influences water quality by altering runoff processes, pollutant transport, and dilution effects, leading to differentiated responses between the reservoir and its tributaries. These findings provide a scientific basis for water quality risk identification and zoned management in the water source area of the Middle Route of the South-to-North Water Diversion Project.