Abstract: In the aim of exploring the characteristics of the spatio-temporal changes of water quality in middle-lower Hanjiang River in wet season and dry season, a comprehensive survey was carried out on the middle-lower Hanjiang River (Danjiangkou-Wuhan reach) during the wet season in June 2019 and the dry season in January 2020. Eighteen sampling sections in the main stream and two sampling sections in the tributaries were arranged. The water quality was evaluated by using the overall pollution index and the overall trophic level index. The results indicated that the overall pollution index of water in the mainstream of the middle-lower Hanjiang River had no significant difference between wet season and dry season; in sectional scale, yet, the pollution of most sampling sections in wet season was more severe than that in dry season. Spatially, the change trends along the river during wet season and dry season were similar. Sections in Xiangyang, Hanchuan, and Wuhan were subjected to severe pollution, mainly total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH₃-N), among which TN concentration exceeded the limit of class II standard (GB 3838-2002) in all sections in both wet season and dry season, apparently larger in wet season, and TP concentration of most sections in wet season was greater than that in dry season, with 27.8% sections exceeding the limit of class II standard, and NH₃-N concentration of all sections were below the limit of class II standard in dry season, while exceeding the limit only in a few sections in wet season. Moreover, the overall trophic level in middle-lower Hanjiang River presented an increasing trend along the river, ranging from moderate nutrition to mild eutrophication. Except for larger fluctuations in dry season, the overall trophic level index saw no significant difference between wet season and dry season. The water quality in the middle-lower Hanjiang River was mainly affected by point source pollution and non-point source pollution, the confluence of tributaries, and water conservancy projects. The research findings offer scientific basis for water resources protection and sustainable development in the middle-lower Hanjiang River.

Key words: middle-lower Hanjiang River, water quality, wet season, dry season, spatio-temporal variation