Assessing Climate Change and Reservoir Impacts on Upper Miño River Flow (NW Iberian Peninsula) Using Neural Networks

Climate change is altering the global hydrological cycle, which, combined with human interventions, such as reservoir operation, further disrupts river flows. Given the heterogeneity and importance of these impacts, and the particularities of each basin, regional studies are essential to assess local vulnerabilities. This study focuses on the upper Miño basin (NW Iberian Peninsula), together with the Belesar reservoir, to evaluate projected changes in streamflow between historical (1985–2014) and future (2070–2099) periods under the SSP5-8.5 and the SSP2-4.5 scenarios. Neural networks were applied to model the hydrological cycle, estimating flow from temperature and precipitation data, as well as to simulate reservoir operation, achieving successful validation. Results for SSP5-8.5 reveal a projected intensification of the hydrological cycle, with the 10th percentile (defining low-flow conditions) projected to decrease by approximately −10%, while the 99.997th percentile (defining high-flow conditions) is expected to increase by about +5%. Mean streamflow is projected to decline by more than −15%. Under the more moderate SSP2-4.5 scenario, changes are less pronounced, with the low-flow percentile expected to decrease by roughly −5% and mean streamflow showing a projected decline not reaching −15%. In contrast, the high-flow percentile exhibits an opposite trend, with a projected decrease of about −30% relative to the historical period. The analysis of reservoir operation was conducted under the most extreme emission scenario (SSP5-8.5), to assess its regulatory capacity under the harshest projected hydrological conditions. Results show that reservoir operation helps moderate the projected impact by redistributing water from wetter to drier periods, more than doubling projected summer flows downstream relative to upstream, and lowering winter flows, with the one-year return period value (99.997th percentile) projected to be reduced by approximately −15% by reservoir operation. Although natural future conditions are projected to become more critical, both the adoption of a more moderate emission pathway and an adequate reservoir operation will contribute to alleviating the most adverse hydrological impacts.