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The study site at Wulanaodu Station was located in the western Horqin Sand Land, northeastern Inner Mongolia, China (42°59′N–43°00′N, 119°37′E–119°39′E). Wulanaodu Station, built in 1975 and affiliated with the Institute of Applied Ecology of the Chinese Academy of Sciences, is one of the monitoring network stations of the Department of Desertification Control, State Forestry Administration of China. The climate of the study area is temperate, semiarid continental monsoonal. The mean annual precipitation is approximately 230 mm, with 70% of this falling during the experiment between June and September. Additionally, the annual open-pan evaporation is approximately 2000 mm. The annual average temperature is 6.2℃, with the minimum monthly mean temperature of −13.74℃ in January and the maximum 25.14℃ in July. The average aridity index is 1.99, and the relative humidity varies between 50%–60%. The annual mean wind velocity is in the range of 3.2–4.1 m/s, and the prevailing wind is northwest in winter and spring and southwest to south in summer and autumn.
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The experiment was designed to observe interactions among the soil, soil water, and groundwater, and it was conducted in 2016 (from June 8th to June 20th and June 25th to August 18th, 2016). As shown in Fig. 1, we arranged two soil observation tanks at Wulanaodu Station, and established the underground observation room in the middle of the two soil tanks. We welded six iron boxes in the two soil tanks, and their length and width were 4.5 m and 1.5 m, respectively. The height was different as the different groundwater table (GWT); initial groundwater table were 70 cm, 80 cm, 130 cm, 140 cm, 190 cm and 200 cm. The six treatments were repeated 3 times each. The 10-cm stone layer was laid at the bottom of each iron box to simulate the underground aquifer. The bottom of the iron box was connected with the groundwater observation. Additionally, the two polyvinylchloride (PVC) pipes of 10 mm diameter on the side of the iron box which was higher than the 30 cm of the box body.
Figure 1. The schematic diagram of an experimental device. The two cylindrical positions are the groundwater observation pipe, and the circular hole in the device is the position of the time domain reflectometry
Holes with 5 cm in diameter were set at the depth of 50 cm, 100 cm and 150 cm on the side of the iron box, which is convenient for inserting the time domain reflectometry (TDR). Finally, the iron box filled with the sand soil that were collected from soil samples.
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The TDR devices (Jinzhou Sunshine Technology, Co. Liaoning of China) were used to measure the soil water content every day. The TDR devices were located at depths of 50 cm, 100 cm and 150 cm; they were below the surface with an attempt to minimize the destruction of topsoil vegetation and soil structure. The position of the groundwater table was measured every day. Evaporation data were observed using E601B pan (Weifang Jinshui Huayu Information Technology Co. LTD, Co. Shandong of China), which we considered to be the potential evaporation.
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The HYDRUS-1D model was used as the benchmark model because it has been validated by analytical techniques and applied research (Šimůnek et al., 2008). In addition, the numerical model package HYDRUS-1D was used to simulate the processes of unsaturated water and evaporation from the soil surface in one-dimensional variably saturated media. The Hydrus-1D program solves the convection-diffusion equation for the saturated and unsaturated water flow and heat and solute transport of the Richards equation (Richards, 1931). The Richards equation is expressed as:
$$\frac{{\partial \theta }}{{\partial t}} = \frac{{\partial }}{{\partial {\textit{z}}}}\left[ {K\left( {\partial } \right)\left( {\frac{{\partial h}}{{\partial {\textit{z}}}} + 1} \right)} \right] - S$$ (1) where θ is volumetric water content (m3/m3), t is time (d), z is vertical coordinate (m) positive downward, K is the unsaturated hydraulic conductivity function of soil (m/d), h is the water pressure head (m), and S is the source/sink term (m3/(m3/d)).
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In water flow module, the soil water characteristic curve is the most basic hydraulic characteristic curve for solving the soil flow equation. Soil hydraulic properties were described using the van Genuchten-Mualem analytical functions (van Genuchten, 1980), expressed as:
$$ {\rm{\theta }}\left( h \right) = \left\{ {\begin{array}{*{20}{c}} {{\theta _r} + \dfrac{{{\theta _s} - {\theta _r}}}{{{{\left[ {1 + {{\left| {ah} \right|}^n}} \right]}^m}}}\;\;\;\;h \le 0}\\ {{\theta _s}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;h > 0} \end{array}} \right. $$ (2) where θ is soil water content (cm3/cm3), h is the water pressure head (m), θr is residual water content (cm3/cm3), θs is the saturated water content (cm3/cm3), α and n are the shape parameters, m is the parameter in the soil water retention function, m =1−1/n. According to the particle analysis data of the experimental site, the model parameter values of reference are shown in Table 1.
Table 1. The soil particle percentage and hydraulic parameters of HYDRUS-1D
Soil particle percentage Hydraulic parameters Soil type Sand/ % Silt/ % Clay / % Bulk density θr θs α n > 0.05 mm 0.05–0.002 mm < 0.002 mm (g/cm3) (cm3 /cm3) (cm3 /cm3) (1/cm) Sand 86.00 13.64 0.36 1.28 0.045 0.41 0.045 2.68 Notes: θr, the residual water content; θs, the saturated water content; α and n, van Genuchten’s shape parameters -
The atmospheric condition was the upper boundary condition; the deep drainage (with groundwater) condition was the lower boundary condition, which was imposed at the soil surface and bottom boundary of the flow domain, respectively (Neuman, 1974). The function of boundary condition is:
$$ \left\{ {\begin{array}{*{20}{c}} {\left| { - K\dfrac{{\partial \theta }}{{\partial x}} - K} \right| \le E}\\ {{h_a} \le h \le {h_s} = 0} \end{array}} \right.\;\;\;\;\;\;x = 0 $$ (3) where E is maximum (potential) rate of infiltration or evaporation under the prevailing atmospheric conditions (L/T), θ is soil water content (cm3/cm3), K is the unsaturated hydraulic conductivity function of soil (m/d), h is the water pressure head (m),
$ {h}_{a} $ is the minimum pressure head allowed under the prevailing soil conditions (m),$ {h}_{s} $ is the maximum pressure head allowed under the prevailing soil conditions (m).The function of deep drainage condition is:
$$ g = {\rm{ }} - A{\rm{exp }}\left( {B|h - GWL0L|} \right) $$ (4) where g is drainage rate (cm /T), A and B is experience parameters, GWL0L is groundwater level (cm).
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The consistency between simulation results and observation data for evaluation using the correlation coefficient R2 is given by the following equation:
$$ {R^2} = \left[ {\dfrac{{\displaystyle\sum\limits_{i = 1}^r {\left( {{C_{{\rm{si}}}} - {{\overline C}_{{\rm{si}}}}} \right)\left( {{C_{{\rm{ob}}}} - {{\overline C}_{{\rm{ob}}}}} \right)} }}{{\displaystyle\sum\limits_{i = 1}^r {\left( {{C_{{\rm{si}}}} - {{\overline C}_{{\rm{si}}}}} \right)} \displaystyle\sum\limits_{i = 1}^r {\left( {{C_{{\rm{ob}}}} - {{\overline C}_{{\rm{ob}}}}} \right)} }}} \right] $$ (5) where r is the total number of observed values used in the calibration and validation process,
$ {C}_{{\rm{ob}}} $ is an observed value,$ {C}_{{\rm{si}}} $ and is a simulated value.$ {\overline {C}}_{{\rm{ob}}} $ and$ {\overline {C}}_{{\rm{si}}} $ are the mean values of the observed and simulated data points, respectively.We use the root mean square error (RMSE) and mean absolute error (MAE) to measure the deviation between the observation data and simulated data.
$$ RMSE=\sqrt{\frac{1}{b}\sum _{i=1}^{b}{\left(f\left({x}_{i}\right)-{y}_{i}\right)}^{2}} $$ (6) $$ MAE=\frac{1}{b}\sum _{i=1}^{b}\left|f\left({x}_{i}\right)-{y}_{i}\right| $$ (7) where b is the total number of observed or simulated values,
$ {x}_{i} $ is an observed value,$ {y}_{i} $ and is a simulated value. -
All meteorological data were obtained from an automated meteorological station (43°02′N, 119°65′E) located near the study site (< 200 m) during the experimental period (June 8 to August 15, 2016), including daily precipitation and daily evaporation. Fig. 2 showed that rainfall events occurred during the experimental period and that the total precipitation was 17.9 cm. The seasonal distribution of precipitation was uneven, and was mainly concentrated in July and August. Three heavy rainfall events occurred during July and included relatively large precipitation on July 21 (5.5 cm), July 25 (4.3 cm) and July 28 (3.7 cm)). The total ET occurred during the experimental period was 53.6 cm.
Figure 2. The precipitation, evapotranspiration (ET), groundwater Table 1 (GWT1) and groundwater Table 2 (GWT2) at Wulandodu Desertification Combating Ecological Station during the study experimental period (June 8 to August 15, 2016)
In this study, we use the static pressure probe of the input hydraulic meter with the digital display meter to measure the groundwater change. During the experimental period, the initial water levels of the devices was 100 cm (groundwater Table 1 (GWT1)) and 110 cm (groundwater table 2 (GWT2)), with the bottom of the device being the datum plane. Affected by precipitation and evaporation, the water table increased respectively 20.2 cm and 25.8 cm, respectively, during the experimental period.
The Effects of Groundwater Depth on the Soil Evaporation in Horqin Sandy Land, China
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Abstract: The interactions between groundwater depth and soil hydrological processes, play an important role in both arid and semi-arid ecosystems. The effect of groundwater depth on soil water variations were neglected or not explicitly treated. In this paper, we combine a simulation experiment and a water flow module of HYDRUS-1D model to study the variation in soil evaporation under different groundwater depth conditions and the relationship between groundwater depth and evaporation efficiency in Horqin Sandy Land, China. The results showed that with an increase in groundwater depth, the evaporation of soil and the recharge of groundwater decrease. In this study, the groundwater recharge did not account for more than 21% of the soil evaporation for the depths of groundwater examined. The soil water content at 60 cm was less affected by the evaporation efficiency when the mean groundwater depth was 61 cm during the experimental period. In addition, the evaporation efficiency (the ratio of actual evaporation to potential evaporation) decreases with the increase in groundwater depth during the experiment. Furthermore, the soil evaporation was not affected by groundwater when the groundwater depth was deeper than 239 cm.
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Key words:
- groundwater depth /
- soil evaporation /
- evaporation efficiency /
- HYDRUS-1D
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Table 1. The soil particle percentage and hydraulic parameters of HYDRUS-1D
Soil particle percentage Hydraulic parameters Soil type Sand/ % Silt/ % Clay / % Bulk density θr θs α n > 0.05 mm 0.05–0.002 mm < 0.002 mm (g/cm3) (cm3 /cm3) (cm3 /cm3) (1/cm) Sand 86.00 13.64 0.36 1.28 0.045 0.41 0.045 2.68 Notes: θr, the residual water content; θs, the saturated water content; α and n, van Genuchten’s shape parameters -
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