The way in which rainfall becomes runoff depends largely on catchment physiograpy, precipitation characteristics and antecedent conditions. In dry climates, poor soil condition, antecedent drought and high intensity rainfall events induce rainfall excess (Horton) overland flow. In wet climates, thicker soils, antecedent wetness and gentle rainfall intensities induce saturation ‘from below’ as well as subsurface runoff generation.

Sub-humid Mediterranean mountain areas are subject to strong seasonal changes in wetness. Annual rainfall rates equal or exceed annual potential evapotranspiration, but water excess during late autumn and spring is compensated by marked deficit in summer. The identification of runoff generating mechanisms is of great importance to understand and forecast the hydrological consequences of environmental changes (climate and land use), as well as to design hydrologically sound land reclamation policies.

In Vallcebre, the  monitoring of internal catchment variables (soil moisture, soil tensiometry and water table) allowed the identification of three main kinds of runoff events, each one dominated by a different runoff generation mechanism.

·         ‘dry’ runoff events occur in summer as a consequence of intense rainfalls over badland surfaces where Horton overland flow and high erosion rates are active. Soil covered areas are able to infiltrate all the rainfall, and water table is very low and discontinuous, in consequence, streams convey low discharges with high sediment concentrations.

·         ‘transition’ runoff events occur at the end of the dry season when large rainfall events of moderate intensity occur. The high amount of precipitation means that some soils are unable to transfer the water in depth, and only perched saturation occurs because of the deep antecedent water table. In consequence, overland flow feeds runoff with moderate discharges and short recession limbs, because water table continues to rise after the end of the runoff events.

·         ‘wet’ events occur during wet seasons when rainfall events of moderate intensity occur. Water table rises and allows the formation of saturated areas ‘from below’,  particularly  near the streams where overland flow feeds runoff. These are the major runoff events, and show long and sustained recession periods.

Runoff coefficients plotted against antecedent depth to the water table for the three different kinds of runoff events observed in Vallcebre. ‘Dry’ events are characterised by infiltration excess, ‘transition ‘events are dominated by soil profile saturation, and ‘wet’ events by saturation excess. Observe that antecedent conditions are a predictor for runoff coefficients only during wet conditions (Latron & Gallart, in preparation).