Response to climate variability of large, regulated, basins

The response of hydrological systems to antecedent climatic conditions is interesting for understanding the complex relationship between climate and water resources. This is especially true in highly regulated river catchments, where human regulation interferes with the intrinsic natural complexity of large catchments hydrology. In a study recently published in Journal of Hydrology we analyzed the response of monthly runoff to precedent climatic conditions at temporal scales of 1 to 48 months in 88 catchments of the Ebro basin in northeast Spain. The standardized precipitation evapotranspiration index (SPEI) was used to summarize the climatic conditions at different time scales, and was correlated with the standardized streamflow index (SSI) calculated at the mouth of each catchment. The Ebro basin encompasses a gradient from Atlantic to Mediterranean climates, and has remarkable complexity in topography, geology and land cover. The basin is highly regulated by dams, which were built to produce hydropower and supply water for agriculture. These characteristics explainwhy sub-basins of the Ebro River basin respond in differing ways to precedent climatic conditions. Three main sub-basin groups were distinguished on the basis of the correlation of their streamflow responses to different time scales of the SPEI: (1) sub-basins correlated with short time scales of the SPEI (24 months), which generally corresponded to unregulated headwater areas; (2) sub-basins correlated with long time scales of the SPEI (1020 months), corresponding to catchments where groundwater reserves plays a major hydrological role; and (3) subbasins correlated with medium time scales of the SPEI (610 months). The latter occur in the lower sectors of the Ebro basin where dam regulation has a significant influence on the hydrological characteristics. In addition to the three main sub-basin groups, other streamflow responses associated with seasonal factors were identified, particularly those related to snowpack and the various management strategies applied to reservoirs.

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