SOIL HYDROLOGICAL RESPONSE UNDER SIMULATED RAINFALL IN THE DEHESA LAND SYSTEM (EXTREMADURA, SW SPAIN) UNDER DROUGHT CONDITIONS

Soil hydrology was investigated in the Guadelperalon experimental wate rshed in order to determine the influence of land use and vegetation c over on runoff and infiltration within the Dehesa land system. Five so il-vegetation units were selected: (1) tree cover, (2) sheep trials, ( 3) shrub cover, (4) hillslope grass and (5) bottom grass. The results of the simulated rainfall experiments performed at an intensity of 53. 6 mm h(-1) during one hour on plots of 0.25 m(2), and the water drop p enetration time test indicate the importance of water repellency in th e Dehesa land system under drought conditions. Low infiltration rates (c. 9-44 mm h(-1)) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast pe nding and greater runoff rates were observed. The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conj unction with the soil water repellency, induces high runoff rates (15- 70 per cent). In spite of this, macropore fluxes were found in differe nt locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runof f fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed. (C) 1998 John Wiley & Sons, Ltd.