Abstract
This research work aims to characterize soil hydrodynamic properties of the Cévennes area, a low mountain region known to be prone to flash floods. We thus chose to estimate hydraulic conductivity and water retention properties of the Mualem –van Genuchten model and their spatial variability at the hillslope scale and between multiple hillslope profiles. The objectives are also to evaluate the weathered bedrock permeability, and consequently to enhance our understanding of the soil saturation processes.The soil and bedrock hydrodynamic properties were estimated by inverse modelling of soil moisture from a pilot station and a network of 50 stations distributed on 4 granitic hillslope and 2 schist hillslope. The pilot station was instrumented with 3 soil moisture sensors located at 20, 40 and 60 cm deep whereas the hillslope stations were instrumented with 2 soil moisture sensors located variably from 15 to 45 cm deep. Both soil moisture and precipitation were recorded with a 15 min time step for every station. The inverse modelling procedure is based on the multi-objective genetic algorithm NSGA-II. This procedure was used for every station considering a variably deep soil composed by 2 layers surmounting a 100 cm deep third layer representing the weathered bedrock. Fifteen parameters were calibrated for every station, and were estimated from small selected rainfall periods of 1 to 15 days corresponding to the major rainfall events during the monitoring period. Evapotranspiration has been considered as negligible during those events. The analysis of those retention and conductivity properties shows a very important hydraulic conductivity for the studied soils, ranging from 1000 to 2000 mm/h, and their low retention capacity. The soil saturated water content varies from 0.30 to 0.60 cm3.cm-3, which is a consequence of the important soil heterogeneity at the hillslope scale. We also find that heterogeneity on the estimation of soil depth which varies from 31 to 120 cm. For the weathered bedrock layer, we also found contrasted permeability varying from a few units to a hundred of mm/h. Stations with the shallowest soils and the less permeable bedrock presenting a saturation process at the soil/bedrock contact, as monitored and simulated during the most intense rainfalls.If the estimated soil hydrodynamic properties exhibit important variations at the hillslope scale from a station to another, results showed no specific spatial organization of this variability. The station's position on the hillslope is not decisive to estimate saturated soil moisture, soil depth, soil permeability or water retention capacity. The distribution of those properties, calculated for the 6 studied hillslope is not either correlated to the landscape general characteristic: geology, orientation, land cover.