Fig. 5 presents the relationship between δ18O and
δ2H in soil water at different depths in the study
area under continuous evaporation conditions. The results show that the
soil water isotope points all fell near the LMWL. At 0-15 cm, the
compositions of oxygen-18 were mostly higher than -8‰, while at 20-30
cm, the compositions of oxygen-18 were lighter than -7.5‰. Both
δ18O and δ2H in soil water decreased
with increasing depth above 30 cm. A similar pattern has been observed
in other semi-arid and arid regions (Barnes and Allison, 1983; Gat.J.R
1995), since water vapor-dominated transport controls the composition of
isotopes near the surface. Moreover, the slope of the soil water
evaporation line was lower than that of the atmospheric precipitation
line for each profile, and the slope increased overall with depth. The
results suggest that soil water evaporated continuously during the test
period and that water vapor-dominated near-surface transport controlled
isotopic composition. In general, the smaller the slope of the
evaporation line, the stronger the non-equilibrium distribution of soil
water, the greater the evaporation rate, and the smaller the amount of
water remaining in the soil (Zimmermann et al., 1966).