ABSTRACT
Surface waters which are exposed to the atmosphere are one of the most widely studied systems and thus can be used as a model to understand the rules of the game involved in the isotopic changes imposed by the hydrologic pathways. Direct evaporation from a bare soil surface is a factor in the water balance of an arid environment, in particular in sandy soils with a high water table. The hold-up in the soil column and transport through it does not, in itself, affect the isotopic composition of the infiltrating waters, except as far as mixing between moving and standing water parcels occurs. However, when evaporation from within the topsoil occurs during dry intervals between rain events, this results in an enrichment of the heavy isotopes in the residual waters. Allison et al., and Yakir et al. who calculated the expected steady-state isotopic composition, often found the overall enrichment in the leaf-waters to fall short of the expected steady-state value.
