Spatiotemporal Evolution Law and Attribution of Dry Season Runoff in the Middle Reaches of Xijiang River

The dry season is a critical period for water resource regulation in the Xijiang River Basin. Therefore, research on the runoff patterns of the main and tributary rivers in the middle reaches of the Xijiang River during the dry season can provide reference for decision-making by governments and relevant departments at all levels in the basin. In this study, the trends, mutations, and periodic components of dry season runoff are explored using the Mann Kendall rank correlation test, ordered clustering analysis, Morlet wavelet analysis, and other methods. The spatial distribution characteristics are analyzed by the Kriging interpolation method. The effects of precipitation and human activities on the runoff are studied with the cumulative slope change rate method. The results showed that the dry season runoff of the main and tributary streams in the middle reaches of the Xijiang River shows an insignificant upward trend. There is no significant mutation in the dry season runoff series at Taiping Station, Jinji Station, Pingle Station, and Wuzhou Station. The dry season runoff series mainly exhibits cycles of about 3 years, 6-7 years, 15-19 years, and 32-36 years. The dry season runoff variation of main and tributary streams is uneven and asynchronous. The dry season runoff in the Xijiang River basin increased from the 1960s to the 1980s, decreased in the 1990s, and continued to decrease in the 2000s for tributary streams while starting to increase for the main stream, but saw a significant increase again in the 2010s. During the same period, the dry season runoff of different rivers also varied, with the main stream of the Xijiang River having the highest dry season runoff, the tributary Guijiang River having the highest dry season runoff, and the Mengjiang River having the lowest. The dry season runoff events of the tributary Guijiang and Mengjiang Rivers are prone to anomalies, while the dispersion coefficient of dry season runoff at the Wuzhou station on the main stream is in the low value range. The dry season runoff in the middle reaches of the Xijiang River shows an insignificant decreasing trend, with the decreasing trend gradually diminishing from the main stream to the tributary Beiliu River. The increased regulation of water conservancy projects and the implementation of soil and water conservation measures since the 21st century are the main reasons for the changes in dry season runoff in the tributaries of the Xijiang River, including the Mengjiang River and the Guijiang River, as well as the main stream of the Xijiang River. However, precipitation is an important factor contributing to the changes in dry season runoff in the Beiliu River. The analysis results can provide scientific and reliable technological support for the scientific management, development, protection, and administration of the Xijiang River basin, and offer references for decision-making by party committees, governments, and relevant departments at all levels in the basin.