Investigation of the hydrological processes using chemical and isotopic tracers in a small Mediterranean forested catchment during autumn recharge

Flood generation processes are studied in a small mountainous Mediterranean catchment using natural chemical and isotopic water tracing. The study takes place during the autumn of 1996 for three successive floods of uneven magnitude. The flow resumption following the summer period is a prime opportunity to establish the impact of the soil moisture status on the catchment response. A dynamic scheme of the catchment behaviour and mechanisms is obtained. The combined use of different tracers highlights the speed of the water transfer from the hillslope to the catchment outlet as well as the predominant contribution of short residence time water (current water and previous rain water). Study results show that subsurface drainage through the soil macroporosity is one of the dominant processes in the formation of streamflow generation. This is in keeping with the soilʹs high permeability. Hydrograph separations demonstrate that pre-event water is the major contributor to streamflow. However, with time, a rise in the proportion of direct rainwater is observed, thereby highlighting the impact of the spread of soil saturation on the speed of water transfer and on the expansion of the contributing areas. Thus, for the last flood event (13/15-10), almost a third of the streamwater volume comes from the event rainfall and 50% from the first autumn rain event (19-9). The contributing areas reached 14% of the total catchment surface. A δ18O simulation in shallow subsurface runoff is carried out, at the slope scale, with the use of a simple model. The results demonstrate that this subsurface runoff came from the mixing of the rain eventʹs successive fractions without involving pre-existent water stored in the upper soil layer. For this autumnal flow resumption, the catchment mechanisms can be explained by a two-reservoir system, each geochemically distinct from the other. One is a surface reservoir primarily containing rainwater and the other is a subsurface reservoir made up of pre-event water from earlier rainfall. The contribution of an underground long residence time water is negligible.