ABSTRACT
In the historical records there are some remarkably intense rainfalls in Mediterranean areas. In the 18-19 October storm of 1973, that traversed from Malaga to Benidorm in south-east Spain, daily totals of 250mm were commonly recorded; and in the flooding of the Biescas campsite in the Spanish Pyrenees on 7 August 1996, 250mm fell in the Barranco de Arías in the valley of the River Gallego (Figure 5.2). The basin is 18.8km2, with the highest elevation at 2,189m and its lowest point at 940m, and mainly lying between 1,200 and 1,600m. It is largely covered with meadows and pine forests. Storms developed in the Ebro depression to the south of the Pyrenees and when the humid hot air from the Mediterranean arrived at the
Box 5.1 How runoff is generated We can imagine the soil as represented by a tank with a perforated lid and an outlet at the bottom (Figure 5.1). The storage capacity (C) of the tank depends on its depth and porosity. Deep soils with a high porosity have good storage capacity. Rain falling on the perforated lid passes through the surface as infiltration at a rate in mm h1 and moves down through the unsaturated soil to the saturated zone. If the rainfall intensity (mm h1) is higher than the infiltration rate, then the rainfall runs off. This process is called Hortonian overland flow (HOF). After time, the spare capacity in the tank (CS) reduces as more infiltration occurs until (CS)0. Spare capacity approaches zero and this slows down the entrance of water into the soil. When all the storage is filled, the arrival of more water causes saturated overland flow (SOF). The relative role of these two mechanisms varies downslope because throughflow, if it occurs at all in Mediterranean soils, brings the lower parts of the slope to saturation earlier. This is called the partial contributing-area concept.
