Dam effects on droughts magnitude and duration in a transboundary basin: The Lower River Tagus, Spain and Portugal

This study examines the effects of a large dam on hydrological droughts in the transboundary Tagus River, central Spain and Portugal. The magnitude and duration of droughts are analyzed by comparing a monthly drought index calculated for the flow series upstream and downstream of the Alcantara reservoir. The dam was built in 1969, and the reservoir is the second largest in Europe (3,162 hm(3)). Water management in the area is complex because of large seasonal and interannual variability in the flow regime, which is characteristic of Mediterranean environments. This paper demonstrates that, as a result of exploitation of the Alcantara reservoir, (1) during periods of water scarcity, the releases in winter and spring are reduced dramatically and the magnitude and duration of summer low flow show a slight increase and (2) the nature of droughts along the Tagus river basin downstream of the dam has shown severe changes since construction of the dam. In fact, during the predam period (1943-1969), droughts were longer and more intense in the Spanish part of the basin than that in the Portuguese part. Since the construction of the Alcantara dam, however, the Portuguese part of the basin has experienced more severe droughts than did the upstream part in terms of both magnitude and duration.