SOIL-WATER BALANCE AS AFFECTED BY THROUGHFALL IN GORSE (ULEX-EUROPAEUS, L) SHRUBLAND AFTER BURNING

The role of fire in the hydrological behaviour of gorse shrub is studi ed from the point of view of its effects on vegetation cover and throu ghfall. In the first year after fire, throughfall represents about 88% of gross rainfall, whereas in unburnt areas it is 58%. Four years aft er fire, the throughfall coefficients are similar in burnt and unburnt plots (about 60%), The throughfall is not linearly related to vegetat ion cover because an increase in cover does not involve a proportional reduction in throughfall. The throughfall predicted by the two-parame ter exponential model of Calder (1986, J. Hydrol., 88: 201-211) provid es a good fit with the observed throughfall and the gamma value of the model reflects the evolution of throughfall rate. The soil moisture d istribution is modified by fire owing to the increase of evaporation i n the surface soil and the decrease of transpiration from deep soil la yers. Nevertheless, the use of the old root system by sprouting vegeta tion leads to a soil water profile in which 20 months after the fire t he soil water is similar in burnt and unburnt areas. Overall, soil moi sture is higher in burnt plots than in unburnt plots, Surface runoff i ncreases after a fire but does not entirely account for the increase i n throughfall. Therefore the removal of vegetation cover in gorse scru b by fire mainly affects the subsurface water flows.