3.4 Quantifying soil water evaporation
The evaporation losses at different depths ranging from 0–30 cm at the sampling sites during the test period were obtained according to Equation 3 in Section 2.3 and are presented in Table 3. Owing to the collapse of the canopy caused by heavy snow in Wuhan on December 7, 2018, the experiment was divided into three periods to ensure continuous-evaporation conditions during each period. In addition, the snowfall only changed the initial water content in the third stage without affecting the test results. As shown in Table 3, the evaporation loss calculated from 2H for the surface layer of 0–5 cm during the period from May 13, 2018 to July 12, 2018 is negative, which may have resulted from the larger overall value of δ2H than that of δ18O, which is likely to cause large errors in the calculation. There were only two pieces of data collected from July 12, 2018 to October 19, 2018 because the numbers of samples at other depths were small, and the instrument could not measure them. It should be noted that if f is 0, the calculated evaporation loss score is exceedingly small; however, this does not indicate that no water evaporated during the test. Fig. 7 shows that the evaporation intensity is stronger closer to the surface layer. As the soil depth increased, the evaporation loss of the soil water gradually decreased, and it tended to become stable at 15–20 cm, which is consistent with the drying front observed in Section 3.3. The evaporation losses at different soil depths were accumulated to obtain the evaporation loss at 5–30 cm from the surface of the sampling point based on δ18O and δ2H. During the periods from May 13, 2018 to July 12, 2018 and January 5, 2019 to April 20, 2019, the calculated evaporation losses based on δ18O were 1 mm and 14 mm, respectively, and they were 0 mm and 13.5 mm, respectively, based on δ2H.