Adapting the operation of two cascaded reservoirs for ecological flow requirement of a de-watered river channel due to diversion-type hydropower stations

Alteration of hydrological regime due to reservoir operation has significant impacts on the river ecosystem, in particular for diversion-type hydropower stations that make part of the natural river channel considerably de-watered. Maintaining ecological flow is considered an effective way to remediate the damages. However, it may jeopardize the socio-economic benefit of reservoirs, for example reduction of power production. The conflicting objectives bring a dilemma to water managers in defining the reservoir operation policy. This study considers two cascaded reservoirs in the Yalong River, and develops an optimization model to explore trade-offs between hydropower generation and ecological flow requirement. The objective function of the model is the total power production. In addition to conventional constraints on the water level and discharge, the model takes into account downstream ecological flow requirement, defined as a hydrograph representing the daily flow requirement to conserve the habitats of the indigenous fish Schizothorax chongi (S. chongi). The optimization model allows establishing a relation between the target of fish habitat conservation and the corresponding loss of hydropower production, so as to propose compromise solutions. Although the model has only been applied in the paper to one case study, its generic formulation allows applying it to a broad range of cascaded reservoir configurations. Moreover, extensions towards cases with ecological flow requirements for more than one species are straightforward.