Reservoir Management in the Duero Basin (Spain): Impact on River Regimes and the Response to Environmental Change

The climatic conditions of the Iberian Peninsula result in an imbalance between water availability and demand, which is largely managed through the many dams that were built during the 20th century. However, dam operations modify the natural functioning of rivers and related subsystems. In this study we investigated the effect of reservoirs on river regimes in the Duero basin, which is one of the largest river basins in Spain. This involved calculation of a modified impoundment ratio index, and assessment of the correlations between monthly inflows and outflows. Water resources in the basin have decreased markedly during the last five decades, so we also studied how patterns of management have adapted to less water availability in the region. A significant correlation was found between the level of impoundment and the alteration of river regimes by dams. The degree of regulation was highly dependent on annual inflows into the reservoir, and consequently alterations to river regimes were more intense during dry years. The basic pattern of flow regulation involved the storage of water during winter and spring in preparation for high water demand in summer, when natural flows are low. A combination of trend and cluster analyses revealed three responses of reservoir managers to decreasing inflows during the study period: (i) for several reservoirs the level of storage was reduced; (ii) for many reservoirs, particularly those for hydropower production, the storages were increased; and (iii) for the remainder the storage levels were maintained by adjusting the outflows to the decreasing inflows. The results suggest the absence of a common approach to reservoir management, and the dominance of other interests over environmental concerns, particularly in the context of hydrological change in the basin.