River regimes and recent hydrological changes in the Duero basin (Spain)

River regimes are key tools in water management as they enable prediction of minimum and maximum flows throughout the year. In the Mediterranean region, flow regimes are very sensitive to climatic variations, and are likely to change with the declining precipitation and higher temperatures predicted for the future. This study explores variability in river regimes of the Duero basin (northern Spain), and changes therein over the period 1961–2006. Principal component analyses were used to classify the main types of flow regimes across the basin, and to identify the most common patterns in the monthly streamflow trends. Three types of river regime were identified: (1) a rainfall-based regime with a peak flow in winter, spatially corresponding to the middle and lower reaches of the rivers; (2) a snow–rainfall regime in the headwaters, with high flows in spring; and (3) a human-induced regime, with peak flows in summer because of release of water stored in spring. There was a marked reduction in river discharge in winter and spring over the course of the study period, leading to significant changes in both the magnitude and timing of flows. These results indicate that river regimes have changed in the direction of reduced flow volumes and shifts in peak flows. The causes appear to be decreasing winter precipitation and a reduction in the snowmelt component, presumably due because of increasing temperatures in winter and spring.