Hydrological models of interflow in three Iberian mountain basins

Interflow is the shallow sub-horizontal water flow through the saturated/unsaturated zone generally of a short transit time. It is the main component of the water balance in mountain regions. The study of interflow requires the development of appropriate conceptual models. Its quantification involves challenges due to: (1) the lack of methods to detect and measure it; (2) the possibility of mistaking interflow with local short-time groundwater flow. Therefore, the estimation of interflow is subjected to uncertainties, which in turn lead to errors in the evaluation of other hydrological components. A physically based semi-empirical model of interflow is presented here. It has been implemented in the water balance codes VISUAL-BALAN and GIS-BALAN and tested in three Iberian mountain basins: (1) the Valiñas river basin in the NW of Spain; (2) the Bernesga basin in the North of Spain; and (3) the Zêzere river basin at the Serra da Estrela in central Portugal. Model results show that the mean annual interflow in these mountain basins ranges from 34 to 55 % of the annual precipitation. These basins share the following features, which favour interflow: steep slopes and low-permeability layers in the soil. It can be concluded from this study that the evaluation of groundwater resources in mountain basins requires the proper estimation of interflow.