Abstract: Hanjiang River is the largest tributary of the Yangtze River in the middle basin, and water pollution has become increasingly serious and the ranges of polluted areas have expanded in recent years. With the development of economy and society and the completion of the first phase of the middle route of the south-to-north water diversion project in 2014, the water consumption in the river basin increases rapidly and the water has been transferred. The pressure of water resources and water ecological environment of Hanjiang basin becomes increasingly apparent, and the management of the basin is facing greater challenges nowadays. Based on the monthly water quality monitoring data in three key monitoring control stations (i.e., Xiangyang, Xiantao, Hankou Stations) from 1998 to 2011, the Seasonal Kendall Test was used to detect long-term trends of pollution indicators in the middle and lower reaches of the main stream of Hanjiang River. In terms of salt permanganate index and ammonia nitrogen, it showed a more statistically significant decreasing trend in the three key monitoring control stations. In terms of total phosphorus, it showed a more statistically significant decreasing trend in Xiantao and Hankou stations, but a non-statistically significant decreasing trend in Xiangyang station. In terms of BOD, it also showed a non-statistically significant decreasing trend in the three key monitoring control stations. In terms of dissolved oygen, it showed a more statistically significant decreasing trend in Xiangyang station but a more statistically significant increasing trend in Xiantao station and a non-statistically significant increasing trend in Hankou station. Ammonia nitrogen showed the largest decrease. The accuracy of the model has been verified by the superposition index method. In terms of temporal trends, water pollution is changing from heave pollution to moderate pollution gradually, and In terms of spatial distribution, the water quality in the middle reaches of Hanjiang River is better than that of the downstream. It shows that the water pollution of the middle and lower reaches of Hanjiang river main stream is under control. The Hurst exponent method was applied to analyze the sustainability of trends of pollution indicators. The sustainability trends are positive for all the pollution indicators. Therefore, the trend of water quality change in the middle and lower reaches of Hanjiang River will be consistent with the 14 years before. The water quality can be improved significantly with water environmental protection and management by relevant departments.The research provides technical support for water environment protection and planning.

Key words: the seasonal kendall test, superposition index, hurst exponent