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.