FACTORS INFLUENCING SURFACE RUNOFF GENERATION IN A MEDITERRANEAN SEMIARID ENVIRONMENT - CHICAMO WATERSHED, SE SPAIN

A deeper knowledge of the hydrological response of semi-arid Mediterra nean watersheds would be useful in the prediction of runoff production for assessing flood risks and planning flood mitigation works. This s tudy was conducted to identify the runoff generation mechanisms and th eir controlling factors at the hillslope scale in a Mediterranean semi -arid watershed. Four zero-order microcatchments were selected to meas ure rainfall and runoff for a three-year period. Two groups of soil we re differentiated with respect to the hydrological response. The fine textured, poorly permeable soils of low organic carbon content had a g reater runoff coefficient (9%) and lower runoff threshold (3.6 mm) tha n more permeable, coarser textured soils of medium organic carbon cont ent (<3%, and 8 mm, respectively). The influence of rainfall character istics on the hydrological response was different. Rain intensity was the major rainfall parameter controlling the runoff response in the mi crocatchments on fine textured, low infiltrability soils with a poor p lant cover, while total rainfall was more closely correlated with runo ff in coarser textured, highly permeable soils with a denser plant cov er. It can be concluded that there are two runoff generation mechanism s: (i) an infiltration-excess overland flow in the more degraded areas with low organic carbon content (<0.5%) and low infiltrability (> 5 m m h(-1)); and (ii) a saturation-excess overland flow in the less degra ded areas with a high organic carbon content (>2%), high infiltrabilit y (>8 mm h(-1)) and covered by a dense plant cover (>50%). (C) 1998 Jo hn Wiley & Sons, Ltd.