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A reasonable evaluation index system is very important for the scientific assessment of the water environment carrying capacity (WECC). The framework system of driving force-pressure-stateresponse-economic benefit (DPSRE) and one-vote veto mechanism were proposed, which was used to construct an evaluation index system for WECC. The analytic hierarchy process (AHP) was employed to analyze the WECC of Changxing County in Taihu Lake Basin, China, during the years from 2010 to 2018 and different water periods (wet, flat, and dry) in 2018. The results showed that the WECC of Changxing County fluctuatingly increased from 2010 to 2018 and the evaluation grade varied from critical overload to loadable, with the maximum value of 0.71 (loadable) in 2016 and the minimum value of 0.50 (overload) in 2015 and 2018. In addition, the WECC was loadable during the wet (0.75) and flat (0.74) water period, while overload in the dry water period (0.50) in 2018. Through analysis the influence weights of indicators, development and utilization ratio of water resources, domestic sewage treatment compliance rate, output tax of per unit emission right, vegetation coverage rate of riverbank, aquatic plants coverage rate of river were the major indicators affecting the WECC. By analysis the evaluation results, the improvement of WECC was attributed to the increase of economic benefit subsystem. The water quality compliance rate of regional exit section (S0) did not reach 100% was the real reason for the overload status of WECC in 2015, 2018 and the dry water period (2018). Ecological degradation of river and point and non-point source pollution were the most constraints to the WECC in Changxing County.

Assessment of county water environmental carrying capacity (WECC) is one of the target requirements of the action plan for prevention and control of water pollution. By combining the theory of the WECC and an analytic hierarchy process (AHP), an evaluation index system for WECC in the 7 aquatic eco-functional zones in Changxing County was developed that considers multiple interaction among water resources, water environment, water ecology, and land ecological functions. The model was tested with data including 14 indicators observed for 2018. The results indicated that the spatial distribution of WECC in each aquatic eco-functional zone manifested some geographical regularity, the water environment is in good status in zones 1, 2, fair status in zones 5, 7, and poor status in zones 3, 4 and 6. In four WECC subsystems, water environment and water ecology subsystem had significant impact on the WECC. By analysis the evaluation results, the degradation of river water ecology, the failure of water quality in aquatic eco-functional zones, and point and non-point source pollution were the main factors that restrict the WECC of Changxing County, and they were also the key to improving the WECC of Changxing County.

Investigating the coupling relationship between water environment and social economy is the hot issue in the sustainability study of river basin. Ganjiang River is the largest tributary of Poyang Lake watershed in China. Based on the social economic and water environment status quo in Ganjiang River basin from 1999 to 2016, this research builds the coupling coordinate model, and uses this model to analyze the coupling degree and coordinate degree between the social economic development and the water environmental quality by constructing evaluate formulate, comprehensive utilization of principal component analysis method and coupling model. The results indicate that for the time node of 2006, the period from 1999 to 2006 is the social economic lag phase and the period from 2007 to 2016 is the water environment lag phase; in the Ganjiang River basin, the coupling degree and the coordinate degree between social economy and water environment are not ideal from 1999 to 2016. If this basin continues the existing development speed, the pressure of water environment brought by the social and economic development will become very huge in the next years.

Assessing water environmental safety is important to safeguard sustainable development of social economic and ecological environment in Raohe river basin. In this study, based on the theory of assessing water safety, considering social indicators, economic indicators, resource indicators, environmental pressure indicators, environmental protection indicators and environmental status indicators, we select 20 indicators to establish reasonable indicator system and use fuzzy comprehensive evaluation method to assess water safety situation of Raohe river basin in a long time sequence from the year 2005 to 2016. We divide the security level into five degrees: very safe, safe, basically safe, unsafe and dangerous; get the evaluation results from degree of membership of these five security levels; and then analyze the trends of water environmental safety. The evaluation results show that water environment of Raohe river basin is in basically safe degree.