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).