Quantifying Impacts of Human Activities and Climate Change on Runoff Variation: A Case Study of Songhua River Basin, China

The Songhua River Basin (SRB), ranking third largest in China in terms of both runoff volume and basin area, has experienced frequent disasters and drastic changes in runoff since the early 20th century. Many studies have analyzed the causes of runoff reduction; however, the spatiotemporal differences in runoff contributions and their underlying mechanisms remain poorly understood, which are crucial for regional water resources management and effective utilization. This study used the Mann-Kendall rank correlation trend test, continuous wavelet analysis, cumulative anomaly, and the slope change ratio of cumulative quantities (SCRCQ) method to explore the runoff changes characteristics and spatiotemporal differences of the contributions of climate change and human activities to runoff changes across three sub-basins of the SRB. The results show that: 1) runoff from 1955 to 2022 in all the three sub-basins exhibit a statistically significant decreasing trend at 0.05 significant level. 2) Four abrupt change points in runoff were detected in Nenjiang River Basin (NRB) and the mainstream of the SRB (MSRB), whereas only two change points in the Second Songhua River (SSRB). 3) Runoff and precipitation series of the NRB and MSRB exhibit similar multi-timescale cycle characteristics with the most dominated cycles of 45-58 yr. In contrast, it is 12-18 yr for SSRB. 4) Anthropogenic activities are the primary factor leading to in the reduction of runoff in NRB (74.33%-91.67%) and MSRB (50.11%-102.12%), whereas it is only 5.38%-33.12% in SSRB. This is attributed to the uneven distribution of regional climate and human activities in the entire SRB. 5) With the growing demand for water diversion for agricultural irrigation, anthropogenic activities in the NRB and MSRB have increased. However, the opposite is found in SSR, where the increased influence of precipitation on runoff and water conservation policies are identified.