Hydrological behaviour of sub-Mediterranean alpine headwater streams in a badlands environment

Investigations of flash floods generation processes are very few compared with studies in humid areas. The rapidity and intensity of these processes make difficult any data acquisition on the field. The present study aims to improve our knowledge on these environments by using the facilities of the Draix ERBs (Southern French Alps) where well-documented black marls show a typical badlands morphology with V-shaped gullies. Time-varying components of total flow and flow sources have been investigated from a long-term isotopic and hydrochemical survey of rainwaters, overland waters, soil waters and streamwaters. We present in greater detail two flood events which occurred in three adjacent basins and had contrasting antecedent soil moisture conditions. Isotopic data were used to apply time-based hydrograph separations. Due to the very short transfer time of waters, convolution of the rainfall isotopic signal with a two-parameter transfer function (dispersive transfer function) proved the best way to assess the event isotopic concentration. End Members Mixing Analysis (EMMA) has been carried out using non reactive chemical species to get source areas of runoff. New water participation by direct surface transfer was the main mechanism at the peak discharge. This contribution was found to dominate irrespective of whether the antecedent soil moisture was dry or wet. Nevertheless, up to 20% of subsurface water has been observed at peak discharge for dry pre-event conditions. During the flow recession phases, this subsurface water contribution was greater for wet antecedent conditions. In dry initial conditions, subsurface water was mostly new water, whereas in wet conditions pre-event water could contribute to the flow. A deep groundwater reservoir may take part in the pre-event component in the larger basin but in the smaller basins pre-event water was mostly made of short residence time water (from previous rainfall events) originating from sporadic, highly transmissive shallow hillslope reservoirs (loose detrital cover and upper regolith). Differences regarding the hierarchy of the processes were not found to be simply proportional to the basins' area, nor were they clearly related to the mean slope gradients or to the percentage of vegetal cover. It is suggested that controls on streamflow generation occur at local scale and consist in the spatial organisation and the gradual accumulation downwards of first order basins with their respective physical specificities (mostly orientation and slope gradient).