Dry getting drier - The future of transnational river basins in Iberia

Study region: Main international rivers of Iberia (SW Europe): Douro, Tagus and Guadiana. Study focus: Iberia has long suffered from water scarcity which will worsen with projected reductions in rainfall and increases in temperature. Nonetheless, there has been almost no research concerning the future discharges of these rivers. We examine an ensemble of climate model projections from CMIP5 RCP 8.5 and use two downscaling methods to produce a range of changes in discharge using a physically-based, spatially-distributed hydrological model (SHETRAN) for historical (1961-1990) and future (2040-2070) periods. New hydrological insights for the region: There is uncertainty in the sign of change in high (winter) discharges but most model runs show decreases in monthly, seasonal and annual discharges for all basins; especially for medium and low discharges, with all but one run showing future decreases. The magnitude of these decreases varies significantly for different CMIP5 ensemble members. However, autumn shows the biggest decreases (reaching-61% for the Douro,-71% in the Tagus, and -92% for the Guadiana) and the reductions are consistently larger for the Guadiana. This is the first study to explore a wide range of possible futures for these international basins. We show that, despite uncertainties in model projections, there is common behavior with reductions in mean and especially in low discharges which will have important implications for water resources, populations, ecology and agriculture.