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

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.