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274 J. LAVA-ORE ET AL. HYDROLOGICAL RESPONSE OF It. MEDITERRANEAN BASIN 275 chemical properties of the soil, converting organic ground cover to soluble ash, modifying the microclimate through removal of overhead foliage or giving rise to phenomena such as water repellency (Chandler et aI., 1983). . However, even If the general consequences of fire have been largely studied, 10 partIcular the ecological and biological impact and the associated soil alteration, there are relatively few studies evaluating the effect of these changes on the hydrological response (see reviews by Anderson et aI., 1976, and TIedemann et aI., 1979). This relative scarcity of quantitative hydrological studIes merely refle~ts the difficulty of obtaining good quality data to compare hydrologIcal behavIOur before and after a fire. Scientific interest in a burnt area generally starts only after the fire, and therefore it is very difficult to find previous studies and pre-fire data to compare. ~ pos~ible solution, often used in fire studies, consists of monitoring two twlO baslOs: the first preserved in its original state, the second artificially burnt. However, even if this strategy may be interestingly used in other fields the hydrological comparison between twinned basins (supposed hydrologic: ally equivalent a priori) is very often difficult and the consequences are usually dU~lOUS. Of course, a much better strategy is to monitor an experimental baslO, collect data over a long period of time (at least ten years), long enough for the characteristics of its hydrological behaviour to become clear, and then produce an experimental and controlled fire. In OUf case a fortuitous wildfire in an area we had been studying for 23 years gave us the chance to study the hydrological effects of fire. In August 1990 a set of Mediterranean experimental research basins which had been studied since 1966 were partially burnt. The consequences of the wildfire, especially the destruction of the vegetation cover and the alteration of soil, were the only changes produced. Thus the comparison between the pre-fire and the post-fire conditions and between the nested sub-basins can be fairly carried out. The profusion and the quality of the data collected during the pre-fire penod and the possibility of comparing the hydr.ological conse- quences on similar basins differently affected by the fire, lends this study a special relevance. In this paper we present the first hydrological results and consequences observed during the first year after the fire. Two aspects, considered the main hydrological changes by other authors (Rowe et aI., 1954; Brown, 1972; Campbell et aI., 1977; Chandler et aI., 1983, Scott and van Wyn, 1990), have been especially analys~d: (I) the changes in the annual runoff response, and (2) the changes in the ,flood regime. THE STUDY AREA The Real Collobrier is an experimental research basin that has been observed and gauged continuously since 1966 by the CEMAGREF group from Aix-en-Provence, France. This basin (430 14'N, 60 13'E), located in the Maures massif (Var Department, France, see Fig. I) near the Mediterranean sea (about 15 km away), is in fact a set of nested sub-basins with marginal human influence. The measurement network consists of 17 pluviographs, spread over an area of 100 km2, a meteorological station and II flow recorder stations (with basin areas between 70ha and 70km'), some of which are equipped with a water temperature sensor, conductivity sensor and sampler (Lavabre, 1990; Sempere Torres, 1990). In August 1990 a severe fire partially affected this area (more than 8000 ha destroyed in 3 days). Depending on the sub-basin, the portion of burnt surface varies from 0 to 85% (Lavabre et aI., 1991). The present study concerns the Rimbaud basin, a 146ha nested sub-basin with elevations ranging from 470 to 622m (see Fig. I), in which the fire destroyed 85% orthe vegetation. This basin can be considered as geologically homogenous. underlain by slightly altered gneiss. Its thin, sandy soils are of the Ranker type. Shrubby maquis and degraded forest of cork trees, chestnuts and maritime pines had occupied the basin totally before the fire. Table 1 summarises the main characteristics of the basin. The aim of this paper is to analyse and compare the hydrological response of the Rimbaud basin before and after the fire (see Fig. 2). To this end, we considered the 23 year pre-fire period as the reference period defining the normal behaviour for the basin. Some linear regressions and a lumped conceptual model have been calibrated, and used to simulate what the response would have been if the fire had not occurred. Differences between these simulations and the response observed are analysed herein. A com- plementary comparison with the response of the Vaubarnier, a nearby sub- basin of 148 ha which was not affected by the fire (see Fig. I), is also provided. A complementary analysis of chemical and mechanical erosion has been carried out by Martin and Chevalier (1991). CHANGES IN THE ANNUAL RUNOFF RESPONSE First let us consider the hydrological year N as the 12 month period starting on 1 August of the year N. Thus in our study the reference period before the fire comprises the hydrological years 1967-1989 (from August 1967 until July 1990). The study period was the hydrological year 1990 (from August 1990 to July 1991). Note that the fire took place between 22 and 24 August 1990. Nevertheless, no rainfall had been recorded since June and no flow was