Enhanced reservoir system management using adaptive environmental flows

The intense hydrological variability and the increased frequency of drought events in Mediterranean basins entail major challenges for achieving an effective balance between water-supply reliability and ecological conservation in regulated river systems. Within this context, this study examines the performance of an adaptive minimum environmental flow (MEF) regime aimed at enhancing drought resilience in a three reservoirs system of the Rivera de Huelva catchment (southern Spain), which constitute a critical component of the water-supply infrastructure for the metropolitan area of Seville.

An adaptive MEF regime, that reflects the natural hydrological variability, is defined based on a statistical approach and using historical streamflow data. This adaptive MEF regime was also variable between meteorological drought and no-drought situations using the Standardized Precipitation Index (SPI). To evaluate the impact of this new MEF regime, reservoir operation was simulated for the 2001/02–2024/05 period under two management scenarios: (1) the MEF regime currently implemented, and (2) the new proposed adaptive MEF regime. Based on observed daily inflows and operational releases, the analysis quantifies the individual impact of each flow regime on reservoir storage dynamics. Reservoir-specific water scarcity situations   - pre-alert, alert, emergency - were established fixing different threshold to the normalization of observed storage volume. This procedure is consistent with the framework of the River Basin Drought Management Plan, and therefore it enables standardized characterization of water scarcity at the reservoir scale. Finally, a comparative analysis, based on the fraction of months allocated to each state was carried out.

The adaptive regime resulted in consistently higher storage levels across the three reservoirs, with mean increases which may vary from 5 to 10 hm³, depending on the specific reservoir. The effects are particularly notable in the smallest reservoir, where the incidence of alert and emergency states is reduced to approximately one third of that observed under the current regime, which indicates a substantial reduction in the frequency of emergency conditions under the adaptive regime.

Overall, the results demonstrate that the implementation of adaptive environmental flows can simultaneously reinforce supply-security outcomes and strengthen ecological resilience, thereby offering a robust and scalable strategy for reservoir management under increasing drought pressure.

Acknowledgments: This work has been funded by the project CONV 39-27 UCO-EMASESA, in the framework of the TED/934/2022-PCAU00006, funded by MITECO and by European Union NextGenerationEU/PTR.