Incorporation of Optimal Limited Ecological Curves into the Operation Chart of Cascade Hydropower Systems to Alleviate Ecological Damages in Hydrological Extremes

Most ecological operation charts of hydropower stations have focused on the average ecological benefits over a long period of time, while the possible ecological damage caused by flood or drought is often overlooked or averaged out. This study proposed a new hydropower-ecological operation chart of cascade hydropower stations, in which limited ecological curves were introduced and optimized to alleviate the negative impacts caused by drought or flood events on fish habitat and to maintain the long-term average habitat quality without reducing the power generation. The optimal ecological discharge range at a given ecological conservation target was determined from the weighted usable area-discharge curve using the physical habitat simulation model, and then the upper and lower limited ecological curves were obtained by reverse calculation, which together with the conventional operation chart (COC) formed the ecological operation chart (EOC). The limited ecological curves were further optimized with the goal of reducing the ecological damage frequency in wet and dry extremes, and then incorporated into COC to form the optimized ecological operation chart (OEOC). A case study was performed with Jasajiang (JS) and Madushan (MDS) cascade reservoirs on the Yuan River in southwestern China. The results show that the EOC that takes into account the ecological benefits can reduce the ecological damage frequency compared to the COC, but potentially at the expense of the overall ecological benefit. However, further optimization of limited ecological curves in OEOC makes it possible to obtain higher short-term ecological benefit and lower ecological damage frequency with the loss of lower overall ecological benefit. Specifically, OEOC is helpful to reduce the ecological damage frequency and improve the power generation and overall ecological benefit at an ecological target of 60 ~ 80%. Notably, at an ecological target of 80%, OEOC results in a 4.1% increase in power generation and a 11.25% decrease in ecological damage frequency for JS-MDS cascade reservoirs compared with that of COC, respectively.