Stable water isotopes in the atmosphere/biosphere/lithosphere interface: Scaling-up from the local to continental scale, under humid and dry conditions

Abstract

At and near the land surface, the precipitation is partitioned into fluxes of percolation, surface runoff and evapo-transpiration. These processes are accompanied by changes in the stable isotopic composition, primarily the result of isotopic fractionation between the water molecules associated with the partial evaporation from open surface and soilwaters. Following such an evaporative process the residual water is marked by an enrichment of the heavy isotopes and decrease of the d-excess parameter, with the complementary opposite effect on the atmospheric moisture. Transpiration, on the other hand, returns the water essentially unfractionated to the atmosphere. This results in a change of the amount of water which is unaccompanied by a change in the isotopic composition and which has been widely used to distinguish between the transpiration and evaporation fluxes. An understanding of the isotopic composition also requires consideration of a possible selection between precipitation events of different isotopic composition on a short term and seasonal basis. With increasing geographic scale, any local effect of the evaporative isotopic signature can be attenuated in the runoff under an exorheic regime by subsequent rainout and the accumulation of additional surface or sub-surface discharges that have not suffered an evaporative water loss; however, since rainout usually occurs under close to equilibrium conditions, the integrated isotope signature of all evaporation processes is preserved in the atmospheric moisture. In the more arid environment with its endorheic runoff regime, more and more of the surface water is lost by evaporation as the scale of the basin increases, often terminating in highly saline lakes. In contrast to the situation described above, the signature of the d-excess is thus accentuated in the runoff whereas its contribution to the atmospheric moisture diminishes as a larger and larger fraction of the surface water is being evaporated.