Assessment of potential ecological risk due to hydrological flow regime modification in the Hanjiang River basin 1950-2017

River regulation by the construction of reservoirs represents one of the greatest challenges to the natural hydrological regime and ecological health of riverine systems. The Danjiangkou (DJK) Reservoir is the largest reservoir on the Hangjiang River and started operation in 1967 and was upgraded in 2012 to provide water resource for the South-North water transfer project through central China. However, the effect of the reservoir operation on the downstream hydrological regime and ecological health of the Hanjiang River after the upgrade (increase in dam wall height) has not been examined thus far. The daily discharge series from four stations along the main stem of the Hanjiang River, including a control site were examined from 1950-2017.  the study period was divided into three periods based on the different stages of the reservoir operation: i)1950-1966, ii) 1967-2012 and iii) 2012-2017. The nature of the hydrological alteration and the ecological risk of diversity (Shannon diversity index) during different periods were investigated. The result clearly indicate that the DJK reservoir has significantly modified the hydrological regime in the middle and downstream section of the Hanjiang River, with most significant modifications recorded immediately downstream of the reservoir. Using the Range of Variability Approach, after the increase of reservoir capacity at Huangjiagang the river ecological flow requirements could not be fully guaranteed and the frequency and intensity of ecodeficit increased. Biodiversity downstream of the dam was significantly reduced at Huangjiagang (Huangzhuang) and there was an increased risk of ecosystem degradation during both the second and third time period.