Oxygen isotopic composition of soil water: Quantifying evaporation and transpiration

The oxygen isotopic composition of soil water provides an extra quantitative dimension in water balance analysis which allows separation of evaporation from transpiration. Spatial and temporal variations in water content and oxygen isotopic composition in soils along an arid to humid transect in Hawaii reflect the processes of recharge by rain, mixing with antecedent moisture, and evapotranspiration. Rainwater is always more depleted in O-18 than is the soil water with which it mixes. Input of O-18-depleted rain increases volumetric water content while lowering the soil-water delta(18)O value. Evapotranspiration occurs continuously, leading to a decrease in the volumetric water content and an increase in the soil-water delta(18)O value. These effects are most pronounced at the soil surface and decrease in a downward direction. The frequency of recharge determines temporal variability of these values within a given depth interval, while differences along the transect are due to climatic parameters. Results of a material balance model indicate that evaporation decreases, transpiration increases, and the ratio of evaporation to transpiration decreases with increasing annual rainfall and decreasing temperature. (C) 1998 Elsevier Science B.V.