River network-based index to clarify transmission of hydrological drought in reservoir-regulated basins

Study region: Hanjiang River basin (HRB), China.Study focus: Understanding the transmission of hydrological drought along river networks is of extreme importance for monitoring, forewarning, and mitigation of drought. However, it is extremely difficult to do so under the situation of complex river networks, particularly in the presence of spatially distributed reservoirs. Taking advantage of hydrological reasoning information within a hierarchical river network structure, this study proposed a nonstationary standardized streamflow index (NSSIrn) to clarify the transmission of hydrological drought in reservoir-regulated basins. Then, the generalized regression model based on the normal distribution was used to clarify the transmission of hydrological drought along reservoir-regulated river networks.New hydrological insights for the region: The NSSIrn aided with river network information enables reservoir-induced alteration in drought transmission to be better clarified under nonstationary conditions. The NSSIrn-based drought characteristics determined throughout the HRB showed that droughts that occurred in the downstream region were more severe and longer lasting than those that occurred in the upstream region during 1957-2019. Moreover, regulation of the Danjiangkou reservoir weakens the correlation between upstream and downstream hydrological droughts, i.e., roughly one quarter of upstream drought events did not develop into drought events downstream of the reservoir. This study offers a valuable reference for evaluating the effect of other interventions on drought transmission in human-modified basins.