A framework to assess the cumulative impacts of dams on hydrological regime: A case study of the Yangtze River

Comparisons of flow time series between preimpact and postimpact periods have been widely used to determine hydrological alterations caused by reservoir operation. However, preimpact and postimpact periods might also be characterized by different climatological properties, a problem that has not been well addressed. In this study, we propose a framework to assess the cumulative impact of dams on hydrological regime over time. The impacts of the Three Gorges Dam (TGD) on the flow regime of the Yangtze River were investigated using this framework. We reconstructed the unregulated flow series to compare with the regulated flow series during the same period (2010 to 2015). Eco-surplus and eco-deficit and the Indicators of Hydrologic Alteration (IHA) parameters were used to examine hydrological regime change. Among 32 IHA parameters, Wilcoxon signed-rank test and principal component analysis identified the October median flow, 1- and 3-day maximum flows, 1-day minimum flow, and rise rate as representative indicators of hydrological alterations. Eco-surplus and eco-deficit showed that the reservoir also changed the seasonal regime of the flows by reducing autumn flow and increasing winter flow. Changes in annual extreme flows and October flows lead to negative ecological implications downstream of the TGD. Ecological considerations should be taken into account during operation of the TGD in order to mitigate the negative effects on the fluvial ecosystem in the middle reach of Yangtze River. The framework proposed here could be a robust method to assess the cumulative impacts of reservoir operation over time.