Surface hydrology in the dehesa ecosystem, Guadalperalón study catchment

Abstract

Research was carried out in the Guadalperalón catchment in Extremadura, Spain, between 1990 and 1997 in order to understand the hydrological and erosional processes operating in semi-arid open woodlands with silvo-pastoral landuse (dehesas). The present paper presents a summary of the major results of the project, including investigations of runoff production at different spatial scales, rainfall interception of holm oaks, and the water balance of the catchment. At the hillslope scale variations of runoff generation could be related to existing soil-vegetation units, which are typical for the peneplain landscape of SW Iberia. Low runoff coefficients are observed below tree canopies, in areas with improved pastures, as well as at footslopes and small valley bottoms with fluvio-colluvial sediment fill. Antecedent soil moisture conditions are shown to be of major importance. The shallow and degraded soils have low infiltration capacity, and, additionally, during prolonged droughts, when ground cover is strongly reduced due to continuous grazing, water repellency was observed. Large amounts of overland flow are produced during events preceded by high rainfall amounts and saturated soil conditions. In this situation all spatial units show high runoff coefficients. As a consequence antecedent soil moisture conditions are decisive for runoff production. During years with below average rainfall Hortonian overland flow dominates and water flow is discontinuous with reinfiltration in the sediment fill of footslopes and valley bottoms. During years with above average rainfall these areas become saturated, and large amounts of runoff are produced (saturated overland flow). As a result runoff coefficients vary strongly, with very low values during dry years (1.5%) and moderate values during humid years (15.9%).