Reservoir-induced hydrological changes in the Ebro River basin (NE Spain)

The Ebro River and its tributaries (North-Eastern Spain) are regulated by over 187 dams, with a total capacity equivalent to 57% of the total mean annual runoff. We analysed 38 gauging records from 22 rivers that, by virtue of location within the drainage network and period of record, would reflect reservoir-induced hydrological changes. Most showed reduction in flood magnitude, with Q2 and Q10 reduced over 30% on average, more in rivers with higher values of the impounded runoff index, (calculated as reservoir capacity divided by mean annual runoff). Annual runoff did not show strong trends, but the variability of mean daily flows was reduced in most cases due to storing of winter floods and increased baseflows in summer for irrigation. Monthly flows ranged from virtually no change post-dam to complete inversion in seasonal pattern, the latter due to releases for irrigation in the summer, formerly the season of lowest flows.