Evolution of hydrological drought under the regulation of two reservoirs in the headwater basin of the Huaihe River, China

Reservoir regulation has a significant effect on streamflow regimes and thus hydrological drought characteristics. The impacts of two or more cascade reservoirs on upstream and downstream droughts may be different due to various geographical and topographical characteristics and water management activities. In this study, evolution of hydrological drought features, such as duration and magnitude, under the regulation of two cascade reservoirs in the Shahe River basin of the Huaihe River of China is analyzed as an example. The effect of the two reservoirs regulation on the drought severity is revealed by correlation analysis and comparisons of the standardized streamflow indices (SSI) of the reservoir inflow and outflow, and the standardized reservoir storage indices (SRSI). Additionally, the time lags between the standardized precipitation index and the hydrological drought indices (SSI and SRSI) are analyzed with the purpose of revealing the response of hydrological anomalies to meteorological variations. Results indicate that: (1) the multi-months dependence of streamflow and storage on climatic conditions causes longlasting hydrological anomalies, which reduces hydrological drought frequency; (2) the upstream reservoir leads to a marked decrease in streamflow drought severity, particularly during the severe drought. Although the downstream reservoir leads to an overall increase in the drought severity, it mitigates the severe and extreme droughts; (3) the reservoir storage functions to smooth streamflow variation in that it reduces the drought frequency and extends the duration.