Chemical and isotopic variations in throughfall in a mediterranean context

The chemical and isotopic influence of interception is obtained by comparing the composition of incident rain, throughfall and stemflow, at the plot scale in the Real Collobrier catchment (southern France). Measurements made from March to Nov. 1993, on 450 samples, show that enrichment ratios vary significantly from storm to storm and from one chemical to the other. However, the mode ratio (NO3- and K+ excepted) remains around 2.5 or 3.5 and appears representative of Mediterranean climate. The Na-Cl relationship in incident rain preserved in throughfall, and the specific evolution of K+ and NO3- under vegetation are of practical hydrological interest. The observed isotopic changes range from -0.5 to +1.4 parts per thousand and are in good agreement with theo-retical values calculated in other mediterranean contexts. The enrichment by isotopic fractionation due to evaporation can be enhanced or reduced by an active rain-selection canopy process.