Are the thresholds of ecological flows established by the water management authorities aligneed with the natural regime of rivers in Mediterranean regions?

Maintaining ecological flow is a need for enviromental  sustainability of regulated rivers  in managed  water bodies, whatever the different and competitive water demands (urban supply, irrigation, energy, or ecology). This is a challenge that the hydropower sector has to deal with, in areas subjected to complex meteorology with long drought periods followed by intense flood episodes, like those affected by the Mediterranean climate, specially in run of river facilities. These facts point towards the introduction of new regulatory frameworks to push towards a minimum river flow regime more adapted to the naturally variable hydrological regime of streamflow in Mediterranean regions. This question is not trivial and this challenge becomes even more critical in a climate change scenario in which hydropower energy is markedly affected not only by the market but also  by a priori more extreme meteorological events. In this paper, the compliance of the minimum river flow regime established by the Water Authorities in ten water bodies with hydropower generation (6 run-of-river plants and 4 reservoir plants) in southern Spain has been analyzed using historical streamflow and reservoir inflow data series, respectively and assuming that these values  closely represent the natural regime upstream the hydropower plans. For each case, the compliance of ecological regime thresholds during the last 10 hydrological years (September 2010 - August 2020) was analyzed at daily scale. In addition,  an hourly scale analysis was added when hourly data series were available. The results obtained showed that the natural regime of the watercourses hardly complies with the conditions established in the state law on compliance with ecological flows in all of the cases, and that, therefore, these flows were oversized. This overestimation is extremely unbalanced in some of the bodies studied, as in the case of the Cala reservoir, where even a reduction of 90% of the ecological flow, cannot be met by one of the 4 conditions on more than 60% of occasions.

The results highlight the needs for a balanced and updated environmental flow regime in regions with large natural variability in order to support the sustainability of hidropower, especially in run of river facilities.