Keywords:
Soil water evaporation; Hydrogen and oxygen stable isotopes;
Craig–Gordon (C–G) model; Evaporation loss; Uncertainty and
sensitivity analysis
1. Introduction
Soil
water evaporation is the process by which water in the soil layer below
the surface to above the groundwater surface is transported to the
atmosphere through the soil and plants by soil water suction. It is an
integral part of the water transfer mechanism between atmospheric water,
plant water, and groundwater.
Soil water evaporation is
affected by several factors such as temperature gradient (Liu et
al. , 2020), soil-water transport mechanisms (Liu et al. , 2019),
soil lithology (Hou et al. , 2018), vegetation (Lichner et
al. , 2020), and salinity (Li et al. , 2021) , and thus its water
transfer mechanisms are complex. The accurate quantification of soil
water evaporation is crucial for the assessment of shallow groundwater
resources (Grimaldi et al. , 2015), prevention and control of
saline-alkali soil (He et al. , 2021), reconstruction of irrigated
areas (Figuerola et al. , 2013), and evaluation of the ecological
water demand (Jiang et al. , 2021), particularly in arid and
semi-arid regions.
Currently, common methods for
quantifying soil water evaporation include direct measurements using
a lysimeter (Annelie et
al. , 2021; Laura et al. , 2021), the formula method (Lehmannet al. , 2019), location flux method (Xing et al. , 2019;
Tingting et al. , 2021), and numerical simulations (Ma et
al. , 2019; Li and Shi, 2021). These methods are simple in application
and differ greatly from the complex and variable natural conditions, and
do not fully consider the vertical transport of water in the soil, which
is not conducive to an in-depth understanding of the evaporation process
and mechanism of soil
water.
Naturally occurring stable isotopes (18O and2H) have been widely used in soil water research, for
instance, in the estimations of regional recharge (Koeniger et
al. , 2016), infiltration and mixing (Stumpp and Maloszewski, 2010; Zhaoet al. , 2013), plant water uptake (Koeniger et al. , 2010;
Gaines et al. , 2016), evaporation (Gonfiantini et al. ,
2018) and soil water transfer (Yang et al. , 2018), and the mutual
transformation of surface water and atmospheric water (Li et al. ,
2021), which is difficult to realize using other techniques. The
Craig–Gordon (C–G) model is commonly used for quantifying evaporation
from open water bodies. It is used to calculate the isotopic composition
of evaporated water using information about the water that undergoes
evaporation (i.e., temperature, relative humidity, and isotopic
composition) (Skrzypek et al. , 2015; Gonfiantini et al. ,
2018). In recent years, the model has also been used to quantify soil
water evaporation at different elevations (Wei et al. , 2015; Yonget al. , 2020). However, in the case of arid and semi-arid regions
that receive no rainfall for a long time, exploring the applicability of
this model and determining the parameter changes that can affect the
model is worth considering.
In this study, we created a continuous-evaporation condition and
monitored the meteorological parameters (temperature and
relative humidity), soil water
fluxes (0–30 cm), and soil-water isotope data collected from Wuhan. The
study period extended from May 2018 to June 2019. The monitoring data
were compiled and analyzed to determine the variability of stable
isotopes at different soil depths and the influencing factors and their
interrelationships. The primary objectives of this study are as follows:
(1) to determine the variation of
hydrogen and oxygen stable isotopes in soil water and soil water fluxes
with soil depth and time in a soil profile under continuous-evaporation
conditions; (2) to test the applicability of the C–G model in
quantifying soil water evaporation under continuous-evaporation
conditions; and (3) to explore the potential uncertainties associated
with the C–G model and the implications for quantifying evaporation
losses. We anticipate that this will enhance our understanding of water
cycle processes, as well as provide an estimate of soil water
evaporation loss on different underlying surfaces. Furthermore, the study
will contribute to a rational and scientific approach to the development
and utilization of soil water resources in arid and semi-arid
environments.