Local effects on the water balance in flood plains induced by dam filling in Mediterranean environments

Dams are common structures in order to guarantee water supply and control flash floods in Mediterranean mountainous watersheds. Even though they are known to modify in space and time the natural regimen of natural flows, little has been said about local effects on the ecosystem along the river banks upstream the dam. In 2002, Rules dam (southern Spain) started to function. This work presents the effects of the dam filling on the water balance in flood plains. The influence of the enhanced soil moisture in the surroundings of the free surface of the reservoir on the vegetation cover status was analyzed and related to meteorological agents and topographic features, before and after the construction of the dam. Meteorological, topographic, soil and land use data were analyzed in the contributing area of the dam, together with Landsat TM images during the period 1984-2010 to derive NDVI values. Results showed higher NDVI values (close to 20-30%) once the dam was filled and NDVI values in very dry years similar to the ones obtained in medium-wet years prior to the construction. Besides, NDVI values after the filling of the dam proved to be highly related to meteorological variables. Principal Component Analysis (PCA) was carried out in order to identify individual and combined interactions of meteorological and dam-derived effects. 85% of the total variance can be explained with the combination of three Principal Components (PC) in which the first one includes the combination of NDVI, meteorological (rainfall) and hydrological variables (interception, infiltration, evapotranspiration from the soil), whilst the second and third PC mainly include topographic features. These results quantify the dam influence along the river banks and the superficial recharge effects in dry years.