Aquatic Eco-functional Regionalization And River Health Assessment At Watershed Scale Based On Distributed Hydrological Model

River health is crucial for the implementation of river ecological and social values.Under the influences of global warming and human activities,river health problems such as water shortage,water pollution and water ecological degradation have become increasingly prominent and even threaten human existence and social-economic development.As an important method for monitoring and diagnosing river ecological problems,and guiding river management,river health assessment has been widely implemented all over the world.As an important unit of water resources,water environment and water ecological management,river health assessment at the watershed scale could better balance the development and protection of rivers.Due to the spatial variations of natural conditions such as terrain,climate,hydrology,as well as the spatial imbalance of social and economic activities at watershed scale,the ecological characteristics of rivers and the human activities influences also have significant spatial variations.However,the study that systematically evaluating the spatial variations and overall status of river health at watershed scale under comprehensively considering the spatial variations of river ecological characteristics and human influences have not been well conducted.Therefore,the paper proposed the regional-comprehensive river health assessment method based on aquatic eco-functional regionalization at watershed scale.The distributed hydrological model SWAT was applied to obtain the spatial indexes of aquatic eco-functional regional delineation and river health assessment.Additionally,in order to improve the accurate simulation of the spatial indexes,the impacts of watershed subdivision on the model simulations were explored.The Jinjiang watershed in the south-eastern coast of China was taken as an example,and three aspects of research were conducted as follows:(1)The influences of watershed partition on the simulations of water,sediment and their spatial variations in SWAT model were explored,and determined a partition threshold,under which SWAT model could really reflect the spatial variations of water,sediment and water quality in Jinjiang watershed.(2)According to the regionalization index of hydrology which was obtained from the SWAT simulations,and other regionalization indexes such as topography and climate etc.,Jinjiang watershed was partitioned into two levels of aquatic eco-functional regions.(3)The aquatic eco-functional regions were taken as the assessment unit of river health,and the assessment indexes for each eco-functional region was obtained mainly from SWAT simulations,combined with some remote sensing and statistical survey data.In addition,three mathematical assessment methods were used to evaluate the river health at each aquatic eco-functional region,and then the overall river health for Jinjiang watershed was evaluated.The results indicated that:(1)Streamflow was not found to be affected by watershed subdivision,but hillslope sediment generation and channel sediment routing was seriously affected.The partition thresholds for hillslope sediment generation and channel sediment routing were different,and the hillslope sediment generation was finer than the channel sediment rouging.Under the partition threshold of 40.42 km~2 of average sub-watershed area,the hillslope sediment generation and channel sediment routing could be effectively simulated.The changes in hillslope sediment were primarily due to the hydrological response unit(HRU)area,which was very sensitive to watershed subdivision;The changs for channel sediment routing were primarily due to the sensitivity of channel dimensions in the model to watershed subdivision,which then altered the hydraulic radius.(2)SWAT model had good performance for the spatial simulations of daily runoff,monthly sediment and daily TN,ammonia nitrogen at Jinjiang watershed.(3)The Jinjiang watershed could be delineated into three classes of aquatic ecological regions(including five aquatic ecological regions),and five classes of aquatic eco-functional regions(including 18 aquatic eco-functional regions),such as forest water source conservation,garden soil and water conservation,agricultural development and water environment maintenance,hydrologic regulation and storage of reservoirs,and urban development support.(4)For the river health in 18 aquatic eco-functional regions,there were seven aquatic eco-functional regions were at healthy state,eight were at sub-healthy state,and three were at unhealthy state.(5)For the whole Jinjiang watershed,the rivers were at healthy state.The most prominent river problems were the unreasonable planting of tea and agricultural cultivation,which leaded to serious soil erosion and non-point source pollution,thus affecting the water environment.Additionally,in the middle and upper reaches of the watershed,especially in the middle reaches,a large number of dam,small hydropower and other water conservancy and hydropower projects have been built,which have significant influences on the connectivity of the river.Above all,the paper firstly explored the influences of watershed partition on the spatial variations of water,sediment simulations,and determined the partition threshold.Based on this,the regional and comprehensive assessment method of river health integrated SWAT model was proposed to implement the comprehensive evaluation of river health under considering the spatial variations of the river ecological characteristics and human activities impacts.The spatial variations of river health in Jinjiang watershed,and its overall health status were explored,which have important theoretical and application values.