Contribution of groundwater and overland flows to storm flow generation in a cultivated Mediterranean catchment. Quantification by natural chemical tracing

Little work has up to now been done on the mechanisms of storm flow generation in Mediterranean cultivated environments. The present work analysed such mechanisms by natural chemical tracing in a small Mediterranean wine-growing catchment (0.91 km2): Roujan, Herault, France. Two autumn runoff events with very different characteristics were studied. The first, a minor one (specific peak flow = 28 l/s/km2), was used to evaluate the sensitivity of the environment to low intensity rainfall. The second was significantly larger (specific peak flow = 944 l/s/km2) and was used to analyse the response of the catchment to heavy downpours. Tracer concentrations at the catchment outlet, for the groundwater of two distinct geomorphological units (depression and plateau) and in an experimental plot are presented. A mixing model involving three reservoirs and two tracers (chloride and nitrate) is then used to estimate the contributions of the three main storm flow components: (a) the pre-event water deriving from the depression groundwater; (b) the event water of the precipitations; and (c) the pre-event water of the plateau groundwater. The event water end member basically corresponds to infiltration-excess overland flow plus direct precipitation on saturated areas. The imprecision of the calculations was estimated by the Monte Carlo method. During both runoff events, there was little variation in the rate at which the stream was fed by pre-existing water deriving from the groundwater, although the water tables rose rapidly. Overland flow dominated in the rapid storm flow. Its contribution varied between 12 and 82% according to the importance of the event. When the water level rose, particularly in the case of the heavy runoff event, the overland flows concentrated in the man-made network of ditches running down towards the main ditch. This wave of overland flow spread, expelling the pre-event water into the ditches located downstream, which were initially fed by the groundwater. With the rapid rise in the level of water in the ditches, ditchwater infiltrated into the groundwater, and the latter then ceased to contribute to the flow in the ditch network.