Etude des processus hydrologiques par l'application du traçage environnemental : association à des mesures effectuées à l'échelle locale et analyse d'incertitude

The main objective of this research is to improve the comprehension of the hydrological behaviour of natural catchments. This study is based essentially on the observation and the measurement of hydrological processes on the Haute-Mentue experimental research basin. The main originality of this work is to associate different types of measurements in order to obtain a better vision of hydrological processes responsible for streamflow generation. First the hydrological behaviour is studied at the catchment scale by the application of environmental tracing. Then hillslope measurements are conducted so as to identify the form of water flows through the catchment. Before the application of the environmental tracing technique on the Haute-Mentue, an uncertainty analysis of hydrograph separation models is proposed. In order to improve their interpretation in terms of hydrological processes, it is essential to estimate systematically their uncertainty. Two types of uncertainty are distinguished: the model uncertainty, which is affected by model assumptions, and the statistical uncertainty, which is due to the temporal variability of the chemical tracer concentrations of the components. The statistical uncertainty is studied using a Monte Carlo procedure. The model uncertainty is investigated by the comparison of alternative hypotheses concerning the chemical concentration of mixing model components and their spatio-temporal variability. This analysis validates in a way the use of hydrograph separation for the study of catchment hydrological behaviour. In fact it indicates that despite the uncertainty, the flow sources which generate the stream flow are clearly identified. However, the precision and the coherence of hydrograph separations can be improved by taking account of the temporal and spatial variability of component chemical concentrations. After this analysis, hydrographs recorded at the outlets of four Haute-Mentue sub-basins are separated by the application of a three-component mixing model based on the silica and calcium concentrations of water. This model allows the distinction of the following three components: direct precipitation, soil water and groundwater. The analysis of hydrograph separations of these four sub-basins allows the study of the spatial and temporal variability of hydrological responses. Concerning the spatial variability, despite the uncertainty of hydrograph separation, it appears that the hydrological behaviour of each sub-basin is different. The spatial variability of hydrological responses, which is observed essentially at the soil water contribution level, seems mainly due to the catchment morphology and geology. In general, soil contribution decreases with the steepness of the soil and the permeability of geological formation. As for the temporal variability of hydrological responses, it seems essentially to be controlled by the catchment humidity. Soil contribution increases with increasing basin humidity. A statist