Fig. 1 Location of the study site (red box).
The
evaporation experiment was conducted from May 13, 2018 to June 13, 2019.
During the test, surface vegetation was removed, and a rain shed was
built to prevent rainfall infiltration, thereby maintaining
continuous-evaporation conditions. Every two months, the surface 0-30 cm
soil was sampled at 5 cm intervals by a soil auger, and a total of 42
samples were collected six times. All the samples were stored in a
frozen state (−15℃). Moreover, a portable time-domain reflectometry
soil-water measurement system (MPM-160) was used to measure the
volumetric water content in the soil profiles.
Sample processing and determination were performed at the State Key
Laboratory of Biogeology and Environmental Geology at the China
University of Geosciences (Wuhan). The soil water was extracted using an
automatic vacuum condensation extraction system (LI-2100) and
pre-processed using a needle filter (filter membrane: 0.22 µm). For the
experiment, a vacuum of approximately 2000 Pa was maintained and a
heating temperature of 110°C was used. The vacuum level extraction time
is 8 min, and the first replenishment vacuum time and cycle
replenishment vacuum time are 300 min. The lower limit of the
replenishment vacuum was 1200 Pa, and the upper limit of the
replenishment vacuum was 2500 Pa. Lastly, the compositions of the stable
hydrogen and oxygen isotopes in the samples were measured using a liquid
water isotope analyzer (IWA-45-EP). The measurements were obtained by
feeding an average of one standard sample for every three samples, and
the result obtained for each tested sample and the standard sample was
the average of six feed measurements. The standard error of measurement
was ±0.2‰ and ±0.6‰ for δ18O and
δ2H, respectively. The measured data were expressed in
thousandths relative to the Vienna Standard Mean Ocean Water standard
sample:
(1)
(2)
2.2 Meteorological data andδ18O and
δ2H isotopes in precipitation
During the test period, the daily average temperature, relative
humidity, and evaporation data of Wuhan were obtained from the National
Oceanic and Atmospheric Administration Climate Prediction Center
(https://www.cpc.ncep.noaa.gov/) and NASA Earth Observations
(https://neo.sci.gsfc.nasa.gov/).
Rainfall is the only source of
soil water recharge in the study area. A comprehensive understanding of
the composition of precipitation isotopes in the study area helps us to
determine the initial conditions of soil water isotopes. In this study,
we collected 115 monthly average precipitation isotope data in the Wuhan
area, spanning from 1986 to 2013, with a couple months of missing data,
and the available data were divided into three parts: 23 data from
January 1986 to October 1992, 27 data from January 1996 to May 1998, and
21 data from September 2011 to May 2013. Data from January 1986 to May
1998 were obtained from the International Atomic Energy Agency (IAEA)
(https://nucleus.iaea.org/wiser/index.aspx). The monitoring site is the
Wuhan meteorological station (114.13°E, 30.62°N) and the monitoring
frequency is measured month by month. The standard used was the Vienna
standard average seawater isotope value, and the test precision for
δ18O and δ2H was ±0.1‰ and ±0.8‰,
respectively. Data from September 2011 to May 2013 were obtained from
(Deng et al. , 2016). The monitoring site is located at the Wuhan
University Engineering Department and Wuhan Institute of Biological
Products dormitory (114.36°E, 30.54°N), and the monitoring frequency is
measured month by month. The standard used was the Vienna standard
average seawater isotope value, and the test precision for
δ18O and δ2H was ±0.2‰ and ±2‰,
respectively.
2.3
Quantification of soil water evaporation using the C–G model
When the stable isotope composition of water changes only because of
isotopic fractionation during the evaporation process, and information
about the water undergoing evaporation (ambient temperature, relative
humidity, and isotopic composition) is known, the non-steady-state model
in the C–G model (Bennett et al. , 2008; Horita et al. ,
2008) is often used to calculate the evaporation losses of open water
bodies. Theoretically, this method is also suitable for evaluating soil
water evaporation under continuous-evaporation conditions. In our study,
soil-water evaporation losses under continuous-evaporation conditions
were quantified via three field experiments conducted from May 13, 2018
to July 12, 2018; July 12, 2018 to October 19, 2018; and January 5, 2019
to April 20, 2019.
In this scenario, the evaporation loss fraction of the soil water volume
(f) can be calculated using the following equation (Stephen K. Hamilton,
2005). On multiplying f by the thickness of the soil layer and
subsequently dividing it by the number of test days, we obtain the
corresponding evaporation loss (mm/d) at different depths.
(3)
where δP(soil) is the initial value of the soil-water
sample isotope composition, δL(soil) is the final value
of the soil-water sample isotope composition, δ* is
the limiting factor for the isotope enrichment—this term is also known
as the limit isotope composition
enrichment (‰) (J. R. Gat, 1978)—and m denotes the correlation between
the isotopic composition of evaporated water vapor and the isotopic
composition of liquid water—this term is also known as the enrichment
slope (G.B. Allison, 1982).
The change in the ratio of the difference between the soil-water isotope
composition and the environmental limit isotope composition at the end
and the initial time reflects the change in the remaining soil-water
isotope composition value. A large value (for example, f = 0.40)
indicates that 40% of the initial water content has evaporated.
(4)
where h is the average relative humidity (fraction), ε is the total
isotope fractionation (‰), and
δA is the stable isotopic composition of water in
ambient air (‰).
The most accurate method for determining δA is based on
on-site measurements. However, owing to the difficulty associated with
direct measurements in the field, the δA value is
generally estimated based on the stable isotopic composition of the
local rainfall (Gat.J.R 1995; Gibson and Reid, 2014).
(5)
where is the average of the precipitation isotopes at the time of the
two sampling periods before and after; ε+ and α+ are the equilibrium
enrichment factor and equilibrium fractionation factor that change with
temperature, respectively, as proposed by(Juske Horita, 1994):
(6)
2H: (7)
18O: (8)
where temperature (T) is expressed in Kelvin.
(9)
where is the total isotopic fractionation factor (‰) and is the kinetic
isotopic fractionation factor (‰).
(10)
where n is a constant, which is related to the correlation between the
molecular diffusion resistance and molecular diffusion coefficient and
is usually considered to be 1 for non-mobile air layers (e.g., soil
water evaporation and plant transpiration), θ is the ratio of the
molecular diffusion coefficient to the total diffusion fractionation
coefficient and is generally considered to be 1 for soil water
evaporation. CD is a parameter that describes the
diffusion efficiency of the molecule and has a value of 25.1‰ and 28.5‰
for hydrogen and oxygen, respectively (GONFIANTINI, 1986; John Crusius,
2000).
(11)
In this study, precipitation isotopes are used to reflect the
environmental changes during evaporation, i.e., in Eq. (5). In this way,
the during the test period can be determined by knowing the
precipitation isotope values for both sampling periods. In the absence
of precipitation isotope data for Wuhan during the test period,
precipitation isotope data for Wuhan collected from network monitoring
data from 1986 to 1998 were used to replace precipitation isotope data
during the test period in order to calculate rain. As shown in Fig. 2,
we conclude that precipitation isotope values for the same period from
1986 to 1998 and from 2018 to 2019 can be considered similar if the past
and current monthly averages of precipitation and temperature parameters
in the same month were similar in the same study setting. Therefore, we
used the following equations to calculate δ18O and
δ2H for the months that satisfied this condition.
(12)
(13)
(14)
(15)
In these formulas, n is the total number of data collected in a
particular month; j is the month j; i is the data collected in a
particular month i; rain is the rainfall in mm; T is the temperature in
℃; 18O, 2H, and T are the oxygen 18
isotope value, deuterium isotope value, and temperature, respectively;
and , , , and are the corresponding monthly mean values.