Laboratory and field experiments on infiltration and evaporation of soil water by means of deuterium and oxygen-18

Evaporation from bare soils causes an enrichment of deuterium and 18O in the residual soil moisture, mainly at the evaporating front where soil water is transferred to water vapour diffusing through the upper soil layers already dry. Because of relatively small soil moisture contents in the vicinity of the evaporating front the maximum of ''heavy isotope excess'' is not intense enough that it could be used as a tracer mark for subsequent infiltration. Infiltration/evaporation experiments with moist soil columns yielded uniform isotope profiles with a slight, non-peaked isotope excess. These profiles have a fairly low value for estimates of the local soil water balance. If soils, however, are evaporating only as being the case in some Saharian depression areas where fossil groundwater evaporates by capillary rise and water vapour diffusion through the unsaturated zone, well developed isotope excess profiles are found, from which accurate numbers for the local groundwater evaporation rate can be derived as field work in the hyperarid Bir Tarfawi area (SW Egypt) has shown. (author)