Calculation of instream ecological water requirements under runoff variation conditions: Taking Xitiaoxi River in Taihu Lake Basin as an example

Climate change and human activity can cause remarkable hydrological variation. Traits of hydrological series such as runoff before and after the change points could be significantly different, so the calculation of instream ecological water requirements (EWRs) is confronted with more challenges. Taking the Xitiaoxi River (XTXR) in the upper reach of the Taihu Lake Basin as an example, this paper investigates the calculation of EWRs using the range of variability approach (RVA) under changing environment. The change point diagnosis of the natural and observed runoff series are conducted for XTXR. Then, differences in the hydrological alternation indicators and instream EWRs processes obtained from various daily runoff series are compared. It was found that the natural and observed annual runoff series in XTXR from 1957 to 2018 both show significant variations, and the change points are in 2007 and 1999 respectively. If runoff data before the change points or all runoff data are used, the instream EWRs obtained from natural runoff are significantly lower than those obtained from the observed runoff. At the monthly time step, EWRs differences within a year mainly occurred from May to August. Also, calculation results of the instream EWRs are strongly related to the selected period of runoff series. The EWRs obtained using runoff series after the change points have rather acute fluctuation within a year. Therefore, when the RVA method is used under changing environment, the instream EWRs should be prudently determined by comparing different calculation results on the basis of river runoff restoration and variability analysis. To a certain extent, this paper enriches our understanding about the hydrological method for EWRs estimation, and proposes new ideas for future research on EWRs.