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Fire is one of the most important determining factors in the evolution of Mediterranean ecosystems. Its effects on soil are plural and diverse, acting on structure, chemical and physical properties, biota etc. Among them, the induced variations on soil water dynamics are of key importance for the recovery of the entire ecosystem and in the soil response to erosion processes. Forest fires are also a factor, triggering the risk of desertification. The aim of this Study is to assess the impact effect of different fire intensities on soil hydrology and on runoff generation.

This work was developed in the Permanent Experimental Field Station of La Concordia with nine plots (4 x 20 m), installed in a calcareous hillside, representative of Mediterranean shrubland areas. Experimental fires, of two intensity levels were carried out. Three plots were burned reaching high fire intensity and three other plots were burned with moderate intensity and the remaining plots were left undisturbed as control. Soil water content, water retention capacity and pF curves were measured together with runoff generation dynamics, in the different plots. The intrinsic characteristics of each rain event occurred up until a year after the fires and runoff generated on them was monitored.

The data obtained was compared with the state of the plots after seven years, during the 2000-2002 period. In this period 24 rain events with runoff generation occurred, with average rainfall intensities (I-30) around 10mm h(-1). Both fire treatments show significant differences with respect to the Control plots, which are reflected in a value of runoff production of 76.84% as an average, less than the burned plots. Between fire treatments, the plots that burned with high intensity, show the highest values of runoff yield. However, infiltration rates do not give significant differences between fire treatments.

In the same way, plots that suffer a high intensity fire show greater values (22.50 cm(3) cm(-3)) on water retention capacity than the other treatments, giving significant differences with the Moderate intensity plots and Control plots. Differences oil this parameter between plots burned with Moderate intensity and the Control plots were observed but they were not statistically significant. The obtained pF curves show the same tendencies, being the greatest water content retained at pF of 2 in the burned plots and in the Control plots at pF of 4.2.

The effect of fire degrading the vegetation cover and by means of the temperature impact on soil, produce changes in its structural characteristics and porosity, affecting soil water distribution and the effective response on water erosion processes.