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An estimation of the contribution of slope runoff to the Rimbaud stream floods (massif des Maures, Var, France) after the August 1990 forest fire

Hydrological sciences journal 1997 v.42 no.6 pp. 893-907
autumn, floods, forest fires, hydrograph, ions, rain, runoff, sodium, stable isotopes, streams, tracer techniques, vegetation cover, watersheds, France
Following the August 1990 forest fire on the Rimbaud catchment (1.46 km2), studies on hydrological, hydrochemical and erosion response led to the development of a decomposition method in hydrograph separation between the waters originating from slope runoff and those originating from underground flow. Sodium and chloride ions were used as tracers, since they are two elements whose concentrations measured in rainfall do not undergo any change on contact with the ground after the vegetation cover has been destroyed. Tests with 18O were inconclusive. Two of the floods that most typified the post-fire surging characteristics of the Rimbaud stream were chosen for decomposition, one at the beginning of the hydrological year (27 September 1992 with a rainfall of 72.0 mm) and the other towards the end of the autumn rains (9 December 1990 with a rainfall of 87.7 mm). These two flood events show clear differences in the volume of runoff originating from rainfall (total runoff coefficient: 18.6% for September 1992 and 87.5% for December 1990), but similarities in maximum discharge (7.7 and 7.8 m3 s−1) and in the quantities of water yielded by slope runoff during the flood itself. The latter source represented 89% of the peak flow and 76% of the total flow during the September 1992 flood, and 89% of the peak flow and 47% of the total flow during the December 1990 flood.