Modification of the isotopic composition of rainfall by throughfall and stemflow: The case of Scots pine and downy oak forests under Mediterranean conditions

Most hydrological studies based on stable water isotopes (O and D) use the isotopic composition of rainfall as input signal. Although stable water isotopes are conservative tracers, previous studies have shown that canopies modify the isotopic composition of rainfall. At present, there is a scientific agreement about the factors involved in isotopic modification, but the effect of each factor and the magnitude of the isotopic shift are still not clear. In this study, we analyse at an interevent and intraevent basis the spatio-temporal differences between the isotopic composition of rainfall, throughfall, and stemflow for two different species (Pinus sylvestris L. and Quercus pubescens Willd). The aim of the study is to analyse the isotopic modification that takes place in throughfall and stemflow and how meteorological variables and structural forest characteristics influence the observed changes. Rainfall and throughfall were sampled by a combination of bulk and sequential collectors, whereas stemflow was collected only by bulk collectors. Results showed that the isotopic modification occurred in both directions, although stemflow was consistently more enriched than throughfall. Despite the contrasting canopy structures, no significant differences between species were found. Moreover, the intraevent analysis suggested that all fractionation factors could occur during one event, but evaporation or isotopic exchange would have a higher impact at the beginning of rainfall, whereas canopy selection processes would be more important at the end of rainfall. Our results emphasize the importance of considering the isotopic composition of throughfall and stemflow in isotope-related studies in forested catchments.