As shown in Fig. 6, along the soil profile, the mean value of δ18O becomes increasingly negative with increasing soil depth, and the standard deviation of δ18O decreases from 1.75 to 0.38 at the surface and stabilizes at 20 cm below the surface. δ2H exhibits the same trend as δ18O. It is more of an effect of evaporation on topsoil. As soil depth increases, the influence of evaporation on soil water isotope fractionation diminishes. In the same manner as at the low rainfall point in Hawaii, soil water evaporation intensity is determined by atmospheric evaporative power and soil water conductivity (Jean C.C Hsieh, 1998).
Fig. 6 Variation map of δ18O along the soil profile, one standard deviation is represented by the
bars.