Décomposition chimique et isotopique d'un hydrogramme de crue d'un torrent méditerranéen - Réflexions méthodologiques

Abstract

La décomposition chimique et isotopique d'un hydrogramme de crue d'un petit torrent méditerranéen (Cannone, Corse, France), a été tentée dans un cas simple, en confrontant les résultats des deux méthodes et en discutant leurs limites. L'outil isotopique permet de bien différencier l'eau préexistante et l'eau nouvelle. L'analyse de la variation des éléments chimiques met en évidence la participation d'eau du soi, sans qu'il soit possible de quantifier sa contribution; elle démontre que la composition de la pluie n'est pas conservative; le rôle du pluviolessivage est mis en évidence, conjointement à celui du transfert dans la zone non-saturée. Ainsi les décompositions chimiques des hydrogrammes (en particulier celles réalisées avec la conductivité) reposent le plus souvent sur des approximations théoriquement contestables et conduisent à des erreurs notables, à l'exception toutefois de celles réalisées avec la silice.Problems of environmental water quality such as transfer of pollutants and ecosystem acidification call for a new insight to the path and the contact time of water in the different subsurface reservoirs. An isotopic and chemical hydrograph separation is carried out for a flood in a small upland watershed in the Mediterranean region (Cannone, Corsica Island, France) with special attention to a comparison between the two methods and discussion of their limits. Here, old and new water are clearly distinguished by isotopic composition. The pattern of the dissolved constituents of stream water shows the contribution of ground water, often assumed to be negligible. The non-conservative behaviour of rain-water chemistry during its path to the stream channel due to enrichment by throughfall and leaching of soils, is the main obstacle to chemical separation. Chemical separation, especially through specific conductance seems generally to be hazardous, except perhaps for dissolved silica. Checking the variation of all dissolved constituents would prevent questionable approximations

The Floodscale experiment in the small catchment of Valescure, France: An overview of the isotopic and geochemical data base

“…Travi et al () monitored one flood event in the Rimbaud sub‐catchment of the crystalline Real Collobrier in the Massif des Maures (South of France) and used δ 18 O and Cl − for a two‐component hydrograph separation. Loye‐Pilot and Jusserand () also monitored one flood event in a small catchment in Corsica, using SiO 2 concentrations in a two‐component separation method. They claimed that other chemical elements are not conservative, because of the throughfall effect possibly altering dissolved elemental ratios across the vegetation cover.…”

A seepage face model for the interaction of shallow water tables with the ground surface: Application of the obstacle-type method

“…Since the contributing area represents only some portion of the watershed, a first important issue in hillslope hydrology is to determine the spatial extension of the contributing area and its temporal evolution resulting from the dynamics of subsurface flows. A further important issue is to determine the relative importance of surface and subsurface water in storm hydrographs and to compare these different contributions with results obtained by flow hydrograph separations (Loye-Pilot and Jusserand, 1990;Iorgulescu, 1997;Marc et al, 2001;McGlynn and McDonnell, 2003).…”