Rainfall interception by Holm Oaks in Mediterranean open woodland

Abstract

The role of holm oaks (Quercus rotundifolia) in the redistribution of precipitation was studied in the dehesas, a landuse system widespread in the southwestern part of the Iberian Peninsula. It consists of pastureland with an openly spaced tree cover. Measurements of through-flow and stemflow were carried out in the Guadalperalón study catchment, and data of three hydrological years are available on an event basis. The effect of tree pruning on interception was also investigated, an activity carried out in intervals of approximately 10 years. Four trees were selected for monitoring, two non-pruned and two pruned individuals. Throughfall was measured with a total number of 163 raingauges distributed in a regular grid below the tree canopies, and meteorological data was registered in intervals of 5 minutes. On average 26.8% of the annual rainfall is intercepted. The pruned trees intercept clearly less rainfall than the unpruned ones, with 23.6% and 29.9%, respectively. Stemflow is of little importance, representing less than 1% of the annual precipitation. The spatial variation of throughfall is high and is mainly related with morphological characteristics of the tree canopy, though the effect of wind could also be demonstrated. On an event basis, throughfall is mainly controlled by the amount of precipitation and to a minor degree by rainfall intensity, temperature and air humidity. Although individual holm oaks intercept a considerable amount of rainfall, the importance on the water balance of the catchment is small due to the low tree density. With a canopy cover of 8%, the water loss due to interception constitutes only 4% of the annual precipitation.