Ecological Flow Deficit in a Context of Institutional Rigidities and Climate Change: The Case of Mapocho Alto, Central Chile

Climate change impacts on runoff, coupled with population and production growth, pose significant risks to aquatic ecosystems. These risks are heightened in countries with rigid institutional frameworks that prevent water extraction from adapting to ecological requirements. Central Chile presents a particularly compelling case due to the coexistence of private water rights, challenges in establishing ecological flows, projected reductions in runoff, and the high country’s population share. This study aims to determine current and future ecological flow deficits using two indicators: the accumulated water volume deficit and the frequency of runoff falling below ecological flow thresholds. Given the absence of defined ecological flows in some basins and uncertainties about future water system operations, an original methodology tailored to the Chilean context is proposed. This analysis focuses on the Mapocho Alto system (five basins), which is highly affected by water extraction and outdated ecological flow definitions. Results indicate that annual deficit volumes during the historical period are low across basins and are concentrated between September and November. Under climate change scenarios (three basins), the deficits remain relatively stable in two basins but shift in January–July. However, in the Arrayán en la Montosa basin, climate change significantly increases the deficit volumes and frequencies due to imbalances between the natural water supply and water demand. The conclusions underscore the necessity of addressing institutional constraints, such as static ecological flow definitions, to prevent severe ecosystem issues in basins where runoff is projected to decline while demand remains constant or increases, a concern applicable to other countries with similar institutional frameworks.