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.