River Health Evolution And Remediation Effect Assessment Of Bailong River Trunk Stream In Gansu

Bailong River is an important river in southern Gansu Province.In order to diagnose the health status of the Bailong River,based on the hydrological characteristics and water quality changes of the Bailong River,this paper constructed a river health evaluation index system.The comprehensive index method and normal cloud model were used to evaluate the health status of the Gannan and Longnan sections of the Bailong River in 2011,2015 and 2019.Then,the evolution process of river health in the main stream of Bailong River since 2011 is analyzed.At the same time,according to the evaluation results,the corresponding restoration countermeasures were proposed and the restoration effects were evaluated,and the main conclusions were drawn as follows:（1）The annual runoff distribution of each hydrological station in the main stream of Bailong River is basically the same.The annual average runoff of Baiyun,Zhouqu and Wudu stations from June to October accounts for 59.75%,64.97%and 63.40%of the annual average runoff,respectively.The average annual runoff from July to September accounted for 39.05%,41.19%and 39.84%of the average annual runoff,respectively.The results of Kendall and Spearman rank correlation test showed that the annual runoff of Baiyun,Zhouqu and Wudu stations showed a significant decreasing trend from 1956 to 2019.The analysis of Mann-Kendall mutation test showed that the annual runoff of Baiyun and Zhouqu hydrological stations changed abruptly in 1990,and that of Wudu hydrological stations changed abruptly in 1987.（2）From 2011 to 2019,the change rates of DO,COD_Mn,COD,BOD₅,NH₃-N and TP in the water quality monitoring sections of Diabu,Dongjiang and Bikou were-1.45%～-14.07%,-45.26%～-35.23%,-38.90%～-27.52%,263.64%～364.15%,-68.52%～-24.31%and 178.91%～291.55%.Compared with the superposition section,the annual mean change rates of DO,COD_Mn,COD,BOD₅,NH₃-N and TP are-1.22%,-21.84%,-2.45%,-7.69%,-23.92%and-9.64%.（3）Combination weighting method was adopted to determine the weight of each index of river health evaluation system in Bailong River trunk stream.The criterion layer of hydrology and water resources has the greatest influence on river health,with a weight of 0.3105.The degree of ecological discharge satisfaction,fish biological loss index and variation degree of discharge process are the main indexes affecting river health,with weights of 0.0975,0.0917 and 0.0868,respectively.（4）Composite index method and Normal cloud model were used to evaluate the health status of the Bailong River trunk stream.In 2011,the health evaluation scores of Gannan Section,Longnan Section and Bailong River main stream were 66.77,63.35 and 64.88 respectively,while in 2015,the health evaluation scores of Gannan Section,Longnan Section and Bailong River main stream were 74.03,65.70 and 69.44 respectively.In 2019,the health evaluation scores of Gannan Section,Longnan Section and Bailong River trunk stream were 74.27,72.03 and 73.03,respectively,and the river health grade was"healthy".（5）The analysis of river health evolution shows that the health status of the Bailong River has gradually improved from 2011 to 2019.The total score of river health condition increased from64.88 to 73.03,with a change of 12.57%.The score of criterion layer of hydrology and water resources increased from 59.40 to 76.77,with a change range of 29.25%.The score of physical structure criterion layer decreased from 43.07（2011）to 38.67（2015）,and then increased to 39.90（2019）,with a range of-7.36%.The water quality criterion layer score increased from 65.87 to72.77,with a range of 10.47%;The overall score of social service function criterion layer increased from 83.25 to 96.21,with a change of 15.57%.（6）Based on the evaluation results,the paper puts forward the countermeasures to ensure the discharge of the hydropower station and the artificial reproduction and release of fish.R2-Cross method and ecological discharge degree index are used to evaluate the downstream discharge of Bailong River mainstream hydropower station in 2020.The results show that the downstream discharge of diversion hydropower station meets the ecological discharge requirements of the reduced water reach after taking measures to guarantee the downstream discharge of hydropower station,and the ecological discharge degree index value ranges from 25.39% to 201.65%.