283-286 Garcia-Ruiz.p65

The terms ∆soil and ∆snow can be ignored when evaluating big catchments on a yearly basis under a Mediterranean climate with dry summers. It is assumed that the soil and snow water storage are almost completely exhausted by the end of the hydrological year. The term ∆aquifers can also be neglected when the global water balance is not greatly affected by subterranean circulation. Equation 1 then becomes: R = P – ET Eq. 2 Thus, the main variables that control water yield in a basin are precipitation, interception and evapotranspiration. These factors are closely related to changes in climate and land-use/plant cover. Many studies have focused on the former despite works on land use by Shachori et al. (1) and Hewlett (2). Nevertheless, a number of recent reports on basins consider the hydrological effects of felling and clearcutting (see 3–8), reforestation (9–11), and forest fires (12). All of these effects are more difficult to assess as the size of the basin increases, due to the large variability of rainfall and basin characteristics (13). The effects of climate and land use should be clearly separated when managing and forecasting discharge. This is especially true in Mediterranean areas which are affected by rapid economic changes and are extremely dependent on mountain water resources. This paper deals with the role of climate evolution and land-use changes on the evolution of water resources in the Central Spanish Pyrenees. It also provides a good example of the challenges involved in managing water.