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\begin{document}

\title{Attenuation of the contribution of groundwater to a wetland caused by
groundwater overexploitation}



\author[1]{Tianyu Zhou}%
\author[1]{Longcang Shu}%
\author[1]{Chengpeng Lu}%
\author[1]{Bo Liu}%
\affil[1]{Hohai University College of Hydrology and Water Resources}%


\vspace{-1em}



  
  \date{May 26, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
The continuous exploitation of groundwater has made wetland degradation
an ecological and geological environmental problem that cannot be
ignored and which has had impacts on the ecological environment and
human production and life. In this study, with the help of Visual
MODFLOW software, we used numerical simulation technology to simulate
the wetland--aquifer interaction during the multiyear pumping process,
establish a quasi-ideal model of wetlands based on the actual area of
the Baiyangdian Basin, simulate the relationship of water quantity
change between wetlands and piedmont plain aquifers during groundwater
exploitation and its natural recovery process, and quantify the
attenuation of the contribution of groundwater to wetlands caused by
groundwater overexploitation. The results show that the impact of
groundwater overexploitation on wetland degradation is mainly divided
into two parts: one is the reduced base flow from the piedmont plain to
the wetland, and the other is the induced infiltration caused by the
reverse recharge of groundwater from the wetland due to the pumping
effect. At the beginning of pumping, the effect of reduced base flow on
wetland degradation is dominant, but with a longer pumping time, the
effect of induced infiltration on wetland degradation exceeds the effect
of reduced base flow. After stopping pumping, the effect of induced
infiltration on wetland degradation responds instantly and decreases
rapidly, while the effect of reduced base flow on wetland degradation
continues for a long time. The total water reduction of wetlands
increases with increasing hydraulic conductivity, and in actual wetland
areas, if groundwater overexploitation is not restricted or artificial
supply measures are not taken, the amount of wetland water will
gradually decrease until it is exhausted.%
\end{abstract}%



\sloppy


\textbf{Attenuation of the contribution of groundwater to a wetland
caused by groundwater overexploitation}

\textbf{Tianyu Zhou \textsuperscript{1, 2}, Longcang
Shu\textsuperscript{1, 2, *}, Chengpeng Lu\textsuperscript{1, 2}, Bo
Liu\textsuperscript{1, 2}}

1 College of Hydrology and Water Resources, Hohai University, Nanjing
210098, China;

211301010054@hhu.edu.cn(Z.T.Y.)

2 State Key Laboratory of Hydrology-Water Resources and Hydraulic
Engineering, Hohai University, Nanjing 210098, China

* Correspondence:\emph{mailto:lcshu@hhu.edu.cn} ; Tel: +86-138-5194-1641

\textbf{Abstract} : The continuous exploitation of groundwater has made
wetland degradation an ecological and geological environmental problem
that cannot be ignored and which has had impacts on the ecological
environment and human production and life. In this study, with the help
of Visual MODFLOW software, we used numerical simulation technology to
simulate the wetland--aquifer interaction during the multiyear pumping
process, establish a quasi-ideal model of wetlands based on the actual
area of the Baiyangdian Basin, simulate the relationship of water
quantity change between wetlands and piedmont plain aquifers during
groundwater exploitation and its natural recovery process, and quantify
the attenuation of the contribution of groundwater to wetlands caused by
groundwater overexploitation. The results show that the impact of
groundwater overexploitation on wetland degradation is mainly divided
into two parts: one is the reduced base flow from the piedmont plain to
the wetland, and the other is the induced infiltration caused by the
reverse recharge of groundwater from the wetland due to the pumping
effect. At the beginning of pumping, the effect of reduced base flow on
wetland degradation is dominant, but with a longer pumping time, the
effect of induced infiltration on wetland degradation exceeds the effect
of reduced base flow. After stopping pumping, the effect of induced
infiltration on wetland degradation responds instantly and decreases
rapidly, while the effect of reduced base flow on wetland degradation
continues for a long time. The total water reduction of wetlands
increases with increasing hydraulic conductivity, and in actual wetland
areas, if groundwater overexploitation is not restricted or artificial
supply measures are not taken, the amount of wetland water will
gradually decrease until it is exhausted.

\textbf{Keywords:} Wetland degradation; groundwater overexploitation;
numerical simulation; surface water and groundwater; attenuation of
contribution.

\textbf{Introduction}

Wetland degradation refers to the unreasonable, weakened or even lost
structure and function of wetland ecosystems under the influence of
unreasonable human activities or adverse natural factors and leads to
the degradation of system stability, resilience, productivity and
service functions at multiple levels. Wetland degradation includes three
important parts: the degradation of organisms, soil and
water\textsuperscript{{[}1, 2{]}}. Among them, water is decisive in
maintaining the stability and health of wetland
ecosystems\textsuperscript{{[}3{]}}.

The decrease in water resources leads to the shortage of wetland water
replenishment, which is a direct natural
factor\textsuperscript{{[}4{]}}. Human activities are the main driving
factor of wetland water reduction, and they are also one of the main
causes of wetland degradation\textsuperscript{{[}5, 6{]}}. In China, the
main groundwater overexploitation areas are concentrated in North China
and Northeast China. The North China Plain is one of the water-scarce
areas as well as an area of fast socioeconomic development in
China\textsuperscript{{[}7{]}}. Groundwater exploitation has supported
quick economic growth, which has also caused wetland degradation, land
subsidence, sea water intrusion, and other adverse
effects\textsuperscript{{[}8{]}}. Groundwater exploitation in the North
China Plain has changed the groundwater flow pattern from natural
horizontal flow to vertical flow under exploitation conditions, and many
large groundwater cones of depression have been
formed\textsuperscript{{[}9{]}}. By using the backpropagation (BP)
neural network analysis method, Wei, L et al. determined the control
factors affecting the change in shallow groundwater in overexploitation
areas. The results show that the groundwater depth in most areas of the
North China Plain is increasing continuously except for a few areas, and
artificial exploitation is the controlling factor affecting the shallow
groundwater in the North China Plain\textsuperscript{{[}10{]}}.

Baiyangdian Lake is the largest freshwater lake in North
China\textsuperscript{{[}11, 12{]}}, and it is also the largest wetland
ecosystem in the North China Plain. In recent years, the ecological
environment has been severely damaged, and the ecological function of
the Baiyangdian wetland has gradually weakened since
1989\textsuperscript{{[}13{]}}. Insufficient water intake is an
important reason for the deterioration of the Baiyangdian wetland
ecology and its environment, as well as the shrinking of Baiyangdian
Lake\textsuperscript{{[}14{]}}. During the 1990--2017 period, natural
wetlands in Baiyangdian shrank as a whole. Precipitation was the main
influencing factor of the changes in natural wetlands, and remedial
water replenishment measures could only temporarily alleviate the water
crisis in Baiyangdian\textsuperscript{{[}15{]}}. The South-North Water
Transfer Project plays a positive role in alleviating the short supply
of groundwater in the North China Plain as well as in the maintenance
and protection of groundwater\textsuperscript{{[}16{]}}. It will relieve
the groundwater shortage problem, but it cannot eradicate the water
shortage problem, and groundwater will still be the main source of
industrial, agricultural and domestic water
consumption\textsuperscript{{[}17{]}}. At present, research on the main
factors influencing Baiyangdian wetland degradation is mainly divided
into two aspects: climate change and human
activities\textsuperscript{{[}18{]}}. The impact of climate change is
mainly due to the decrease in atmospheric precipitation, which leads to
a decrease in water inflow into the lake. The impact of human activities
includes the construction of upstream water conservancy projects and the
exploitation of groundwater\textsuperscript{{[}19{]}}. The reason for
the decrease in wetland water volume caused by groundwater exploitation
is the interaction between wetland surface water and groundwater.

Rivers, lakes and wetlands interact with groundwater in many
ways\textsuperscript{{[}20, 21{]}}. Zektser, S et al. described the
environmental impact of large-scale groundwater exploitation and its
overexploitation. A case study in the southwestern United States shows
that large amounts of groundwater exploitation can reduce lake
levels\textsuperscript{{[}22{]}}. Yang, Z et al. used multiple field
measurements in the semiarid Bulang subcatchment, part of the Hailiutu
River basin in Northwest China, to identify and quantify groundwater and
surface water interactions, which showed that groundwater and stream
water are essentially one resource and need to be managed
together\textsuperscript{{[}23{]}}. Wang, W. K et al. discovered that
the hydrodynamic processes and ecological effects of groundwater and
river water are controlled by stream--groundwater
transformation\textsuperscript{{[}24{]}}. Zhang Haojia et al. used the
GSFLOW pair to build a coupling model of surface water and groundwater
and studied the conversion relationship between surface water and
groundwater\textsuperscript{{[}25{]}}. Shu Longcang et al.
quantitatively studied the exchange capacity of surface water and
groundwater in the New Bianhe River in Suzhou
City\textsuperscript{{[}26{]}}. Chen Xunhong used numerical simulation
techniques to simulate the interaction between rivers and aquifers in
the process of seasonal groundwater pumping, quantitatively analyzed the
process of total river water depletion in pumping and later pumping
periods, and described the interaction between surface water and
groundwater in this process\textsuperscript{{[}27{]}}.

Research on wetlands mainly focuses on the current status of wetland
degradation and the influence of the reduction in surface water and
atmospheric precipitation on wetland degradation. There are few studies
that quantitatively analyze the relationship between wetland surface
water and groundwater. Therefore, we used the quasi-ideal model of
wetland--aquifer, with the help of the Baiyangdian wetland, to explore
the relationship between wetland surface water and groundwater
overexploitation, reveal the mechanism of water exchange between
groundwater and wetland surface water, and provide a reference for
solving the problem of wetland degradation.

\textbf{Wetland--aquifer interaction} : In the wetland--aquifer system,
when the head of the piedmont plain aquifer is higher than the head of
the wetland, the wetland receives groundwater recharge from the piedmont
plain aquifer, and when there is a head difference and hydraulic
connection between the piedmont plain and the wetland, the groundwater
recharges the wetland. Under natural conditions, the base flow does not
change significantly due to natural variability and is basically stable.
However, when pumping is performed at a location where the head is
higher than the wetland, the groundwater flow field in the area will be
changed. Initially, when the pumping volume is small and the pumping
time is short, the cone of depression is small, which will not change
the recharge relationship between the piedmont plain aquifer and the
wetland. However, the base flow from the piedmont plain aquifer to the
wetland decreases, the amount of water obtained by the wetland gradually
decreases, and a groundwater divide forms between the wetland and the
pumping wells; as the pumping time and the pumping rate increase, the
cone of depression expands, and the divide gradually shifts to the
wetland, and the base flow recharge to the wetland decreases until the
divide expands to the wetland boundary. At the same time, the hydraulic
gradient between the pumping wells and the wetland is reversed, the
wetland's supply from groundwater disappears, and the piedmont plain
aquifer will no longer recharge the wetland, while the wetland will
continuously recharge the piedmont plain aquifer. The process of
recharging the piedmont plain aquifer from the wetland is called induced
infiltration. Therefore, the reduced base flow and induced infiltration
are the reason for the depletion of the total water flow in the wetland,
which leads to wetland degradation. The three processes of
wetland--aquifer interaction are shown in Figure 1(a), (b), and (c).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Process 1: No pumping well\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

(b) Process 2: The existence of a groundwater divide\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

(c) Process 3: Wetland recharge to groundwater

Figure 1.Schematic diagram of wetland-aquifer interaction at different
pumping stages

\textbf{Methods}

In this study, the models were built using Visual MODFLOW software,
which is the most popular and internationally recognized professional
standard visualization software system for 3D groundwater flow and
solute transport simulation and evaluation. The system was developed by
Waterloo Hydrogeology Canada based on the original MODFLOW software and
has a powerful graphic visual interface function. The model area and the
calculation unit can be circled directly on the computer, and the values
for each unit and boundary condition can be assigned directly on the
computer.

In this study, a quasi-ideal model is used to add the hydrogeological
parameters of the actual area to the ideal model. The study area of the
ideal model is a 10 km \selectlanguage{ngerman}× 20 km rectangular area, with 10 km × 12 km of
groundwater recharge area on the left side of the piedmont plain, 10 km
× 8 km of wetland on the right side, and 4 km × 4 km of lake in the
middle of the wetland.

In the ideal model, the western boundary is set as the flux boundary,
the eastern boundary is the constant head boundary, the western boundary
of the wetland completely cuts the phreatic aquifer, the direction of
groundwater flow in the study area is from west to east, and the contour
of the groundwater level is perpendicular to the north and south
boundaries, which is set as the no-flux boundary. The generalized
schematic diagram of the simulation area is shown in Figure 2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Figure 2. Generalized schematic diagram of the simulation area

The actual area on which this quasi-ideal model relies is the plain area
of the Baiyangdian Basin in the North China Plain, wetland degradation
refers to the degradation of the Baiyangdian wetland, and the lake set
up is Baiyangdian Lake. The aquifer in this study area is a phreatic
aquifer, which is divided into one layer in the vertical direction with
a thickness of 60 m. In this model, precipitation infiltration recharge
is the main source of groundwater recharge in the North China Plain. The
coefficient of precipitation infiltration recharge \selectlanguage{greek}α \selectlanguage{english}is approximately
0.17--0.35, which is taken as 0.2; the lateral recharge in the study
area is 25\% of the rainfall infiltration recharge. The average depth of
Baiyangdian Lake is 3.5 m, the elevation of the lake surface is 10.5 m,
and the elevation of the lake bottom is 7 m. The water-bearing medium is
a sand layer, the hydraulic conductivity is set as 25 m/d, the specific
yield \selectlanguage{greek}μ\selectlanguage{english}=0.20, and the bottom of the phreatic aquifer is a clay layer,
taken as the confining bed.

According to the above hydrogeological conceptual model, the groundwater
flow in the study area is generalized to a homogeneous isotropic
submerged two-dimensional transient flow model, and the corresponding
mathematical model is as follows:\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
\(\frac{\partial}{\partial x}\left(K\left(H-B\right)\frac{\partial H}{\partial x}\right)+\frac{\partial}{\partial y}\left(K\left(H-B\right)\frac{\partial H}{\partial y}\right)+W=\mu\frac{\partial H}{\partial t},\left(x,y\right)\in D\) & (1)\tabularnewline
\(H\left(x,y,t\right)\left|\tau_{1}\right.\ =H_{1}\left(x,y,t\right),\left(x,y\right)\in\tau_{1},t\geq 0\) & (2)\tabularnewline
\(K\left(H-B\right)\frac{\partial H\left(x,y\right)}{\partial n}\left|\tau_{2}\right.\ =q\left(x,y,t\right),\left(x,y\right)\in\tau_{2}\) & (3)\tabularnewline
\(H\left(x,y,t\right)\left|t=0\right.\ =H_{0}\left(x,y\right),\left(x,y\right)\in
D\) & (4)\tabularnewline
\bottomrule
\end{longtable}

where K is the hydraulic conductivity; W represents the source-sink
term; D represents the extent of the simulation area; H represents the
groundwater level; B is the elevation of the phreatic aquifer bottom;
H\textsubscript{1} is the initial groundwater level; q is the flow rate;
\selectlanguage{greek}μ \selectlanguage{english}is the specific yield; t is the time; \selectlanguage{greek}τ\selectlanguage{english}\textsubscript{1} is the
Dirichlet condition; and \selectlanguage{greek}τ\selectlanguage{english}\textsubscript{2} is the Neumann condition.

The model is split into 50 rows and 100 columns, and according to the
Zone Budget module in Visual MODFLOW, the wetland and piedmont plain
aquifers are divided into two zones, zone 1 and zone 2, as shown in
Figure 3. This is done to quantitatively calculate the changes in
reduced base flow, induced infiltration and total water reduction due to
pumping during the wetland--aquifer interaction.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 3. Model partitioning diagram

In the numerical simulation of the quasi-ideal model, the simulation
duration is 7200 days, and the time step is 30 days. Different
situations were simulated for comparison. First, we investigated the
groundwater flow between zone 1 and zone 2 by setting up five pumping
wells in the study area and continuously pumping during the simulation
duration of 7200 days. Moreover, to explore the natural recovery ability
of the wetland, the pumping wells were shut down when they were pumped
up to 3600 days, and the recovery of the wetland was analyzed from day
3600 to day 7200 under the condition that a cone of depression had been
formed due to groundwater overexploitation. The model was then analyzed
for parametric uncertainty, and three different values of hydraulic
conductivity K were set during the simulation by describing the effect
of changes in the hydraulic conductivity on wetland water flow through
indicators such as the reduced base flow, induced infiltration and total
water reduction, as well as analyzing the effect of different hydraulic
conductivities on wetland recovery capacity under the condition of
closing the pumping wells at 3600 days.

\textbf{Results}

\textbf{Simulation results when groundwater is not exploited}

First, in the simulation time without pumping wells, since the initial
head values in the study area were artificially set when the model was
established, the overall initial head values decreased from west to
east, while the head values at the eastern constant head boundary were
equal to the initial head values set at the eastern boundary, and the
western boundary was set as a flux boundary to continuously recharge the
entire study area. Therefore, in the simulation time without pumping
wells, the overall groundwater flow direction is from west to east, and
the wetland is sufficiently recharged. At this time, the wetland will
not be degraded, which is the first process of wetland--aquifer
interaction.

\textbf{Simulation results of groundwater exploitation}

Five pumping wells were set up in the study area, and the well locations
and pumping rates are shown in Table 1. The pumping times were all 7200
days. Since the time step was 30 days during the simulation, the Zone
Budget module also output water volumes every 30 days.

Table 1 Location of pumping wells and pumping rate\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Pumping well & X-axis coordinates /m & Y-axis coordinates /m & Pumping
rate /m\textsuperscript{3}/d\tabularnewline
PW1 & 11000 & 3000 & 5000\tabularnewline
PW2 & 11000 & 4000 & 5000\tabularnewline
PW3 & 11000 & 5000 & 5000\tabularnewline
PW4 & 11000 & 6000 & 5000\tabularnewline
PW5 & 11000 & 7000 & 5000\tabularnewline
\bottomrule
\end{longtable}

When the pumping time was 90 days, only the water level near five
pumping wells dropped slightly, and a small groundwater flow to the
wells could be observed, forming five small cones of depression, as
shown in Figure 4. The water level at the location of the pumping wells
was 8.0 m. The overall groundwater flow direction was still from west to
east. According to the Zone Budget module, the base flow from zone 1 to
zone 2 was 3023.1 m\textsuperscript{3}/d, which is 100.6
m\textsuperscript{3}/d less than the 3123.7 m\textsuperscript{3}/d at
the simulation time of 60 days. The reason is that the installation of
intermediate pumping wells leads to a reduction in water flow to the
wetland (zone 2), which can be described as reduced base flow. At this
time, a groundwater divide has formed between the pumping wells and the
wetland (Figure 4, red dotted line), the water level in the wetland and
cone of depression\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 4. Contour of groundwater level for 270 days of pumping

is relatively low, and the water level in the middle location is higher
than that on the two sides. As the pumping continues, the pumping time
reaches 270 days, and the base flow from zone 1 to zone 2 is reduced to
1525.8 m\textsuperscript{3}/d. At this time, the area of the cone of
depression also increases with the pumping time. The water level at the
location of the pumping wells also decreased to 7.0 m, as shown in
Figure 5. The figure also shows that the hydraulic gradient has reversed
in the middle of the wetland and at the pumping well, and the
groundwater flow direction has changed from east to west, which means
that the groundwater divide has gradually spread to the wetland with
increasing pumping time (Figure 5, red dotted line), and this process is
the second process of wetland--aquifer interaction.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Figure 5. Contour of groundwater level for 270 days of pumping

When the pumping time is 300 days, the base flow from zone 1 to zone 2
becomes 1313.8 m\textsuperscript{3}/d; at the same time, some water
flows from zone 2 to zone 1 with a volume of 41.12
m\textsuperscript{3}/d, which means that the water from the wetland
recharges the piedmont plain aquifer, and the process can be described
as induced infiltration. The area of induced infiltration is initially
distributed in the center at the junction of zones 1 and 2. This is
because the pumping wells are located in the middle of the study area,
and the extent of the cone of depression formed during pumping is also
roughly distributed in the middle. The inversion of the hydraulic
gradient in the middle of the study area causes the groundwater flow
direction to change from east to west, while the edge is relatively less
affected by pumping, which can be clearly observed in Figure 5. Another
reason for setting the pumping wells in the middle of the study area is
that the south and north boundaries of the model are no-flux boundaries,
and the contour of the groundwater level is perpendicular to the south
and north boundaries. If the pumping wells are located too close to the
no-flux boundaries, with the increase in pumping time, the formed cone
of depression will definitely affect the groundwater flow direction at
the south and north boundaries, which does not match the set boundary
conditions.

As the pumping process continues, the area where the hydraulic gradient
is inverted becomes increasingly larger, while the infiltration volume
of zone 2 flowing to zone 1 increases, and the base flow of zone 1
flowing to zone 2 decreases. The base flow from zone 1 to zone 2 is
reduced to 0.69 m\textsuperscript{3}/d when pumping is carried out for
1440 days. At this time, due to the reversal of the hydraulic gradient,
the infiltration from zone 2 to zone 1 increased to 2661.5
m\textsuperscript{3}/d, and the water level at the location of the
pumping wells dropped to 5.0 m. When pumping was carried out for another
30 days and the simulation duration was 1470 days, the base flow from
zone 1 to zone 2 became 0, which means that the groundwater divide
formed between the wetland and the piedmont plain due to pumping had
spread to the wetland boundary, as shown in Figure 6. The groundwater in\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Figure 6. Contour of groundwater level for 1470 days of pumping

the piedmont plain area is no longer recharged to the wetland, and the
amount of water recharged from the wetland to the piedmont plain aquifer
was 2702.9 m\textsuperscript{3}/d. This process is the third process of
wetland--aquifer interaction. Figures 7 and 8 show the relationship
between base flow and induced infiltration with time. In the next
numerical simulation, the third process is continued, and the induced
infiltration from the wetland to the piedmont plain aquifer gradually
increases. The induced infiltration increases to 6386.4
m\textsuperscript{3}/d at 7200 days.

To visually observe the groundwater divide formed between the piedmont
plain and the wetland due to groundwater exploitation, three calculation
points are set up to observe the water level change between the piedmont
plain and the wetland in order to determine the specific location of the
groundwater divide. The coordinates of the five calculation points are
shown in Table 2.

Table 2 Location of head observation wells\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
Well number & X-axis coordinates/m & Y-axis coordinates/m\tabularnewline
OB1 & 9200 & 5000\tabularnewline
OB2 & 9500 & 5000\tabularnewline
OB3 & 11800 & 5000\tabularnewline
\bottomrule
\end{longtable}

Through the simulation results and the simulated water level of the
calculation point, we can observe the formation of the groundwater
divide and the gradual expansion of the divide to the wetland. The
profile of model row 25 and the enlarged local position are shown in
Figure 7 (to better observe the groundwater divide movement, the model
profile has been magnified 120 times vertically).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Figure 7. Model 25 row profile and local enlarged position

When the pumping time is 30 days, as shown in Figure 8 (a), the location
of the groundwater divide is 500 m from the pumping well. The water
levels of the three calculation points are 8.74 m, 8.91 m, and 8.19 m,
which could quantitatively describe the formation of the groundwater
divide. When pumping is carried out for 150 days, as shown in Figure 8
(b), the location of the groundwater divide gradually approaches the
wetland boundary, 1700 m away from the pumping well. When the pumping
time is 270 days, as shown in Figure 8 (c), the groundwater divide
extends to the wetland boundary, and the surface water of the wetland
begins to recharge the groundwater. In the remainder of the simulation
time, wetland surface water supplies groundwater to the piedmont plain.
Table 3 shows the groundwater levels between the pumping well and the
wetland at the three calculation points at different simulation times.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/imagf0/imagf0}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

(a)30d (b)60d (c)90d\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
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\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

(d)270d (e)1500d (f)7200d

Figure 8. Water level between pumping well and wetland at different
simulation time

Table 3 Water level at different simulation time\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Calculation point & Water level(m) & Water level(m) & Water level(m) &
Water level(m) & Water level(m) & Water level(m)\tabularnewline
& 30d & 60d & 90d & 270d & 1500d & 7200d\tabularnewline
OB1 & 8.74 & 8.35 & 8.10 & 7.18 & 5.24 & 2.57\tabularnewline
OB2 & 8.91 & 8.64 & 8.44 & 7.61 & 6.03 & 3.15\tabularnewline
OB3 & 8.19 & 8.25 & 8.27 & 8.32 & 7.57 & 5.24\tabularnewline
\bottomrule
\end{longtable}

Figure 9 shows the change in water quantity in the wetland aquifer
system, where the reduced base flow Q\textsubscript{2} is obtained from
the following equation:

Q\textsubscript{2}=Q\textsubscript{0}-Q\textsubscript{1}

where Q\textsubscript{0} is the base flow when groundwater is not
exploited, Q\textsubscript{1} is the base flow when groundwater is
exploited, and Q\textsubscript{2} is the reduced base flow.

Total water reduction is the sum of reduced base flow and induced
infiltration and represents the total water reduction in the wetland,
which is obtained from the following equation:

Q\textsubscript{4}=Q\textsubscript{2}+Q\textsubscript{3}

where Q\textsubscript{4} is the total water reduction in the wetland,
Q\textsubscript{2} is the reduced base flow, and Q\textsubscript{3} is
the induced infiltration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

(a) Base flow reduction process (b) Reduced Base flow increase process\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image18/image18}

\end{center}
\end{figure}

(c) Infiltration increase process (d) Total water reduction process

Figure 9. Water quantity change when pumping water all the time

where Q\textsubscript{1} is the base flow, Q\textsubscript{2} is the
reduced base flow, Q\textsubscript{3} is the induced infiltration, and
Q\textsubscript{4} is the total water reduction.

To simulate the natural recovery of the wetland when groundwater pumping
is stopped after groundwater exploitation, we made some changes based on
the original model. The simulation duration of the model is still 7200
days, and the pumping duration is 3600 days. When the simulation is
carried out for 3600 days, the five pumping wells are closed until the
end of the full simulation. After pumping is stopped, the water exchange
between the wetland and the piedmont plain aquifer is similar to the
inverse of the three wetland--aquifer interaction processes during the
pumping period.

When pumping was stopped at 3600 days, the base flow from zone 1 to zone
2 was 0, the infiltration from zone 2 to zone 1 was 4480.1
m\textsuperscript{3}/d, and the water level at the location of the
pumping wells was 4.0 m. When pumping was stopped, the induced
infiltration gradually decreased, and after 840 days of pumping
cessation, the infiltration from zone 2 to zone 1 became 104.75
m\textsuperscript{3}/d. At this time, the base flow from zone 1 to zone
2 changes from 0 to 6.45 m\textsuperscript{3}/d, which means that the
groundwater cone of depression formed due to pumping is gradually
recovering and the water level is gradually rising. The base flow also
increases slowly from 0. When pumping was stopped for 930 days, the
induced infiltration became 0, and the base flow became 130.11
m\textsuperscript{3}/d. Until the end of the simulation, the base flow
slowly recovered to 1960 m\textsuperscript{3}/d.

Similarly, in the numerical model for the cessation of pumping after
3600 days, we plotted the curves of base flow, reduced base flow,
induced infiltration and total water reduction with time, as shown in
Figure 10. It is possible to observe when the base flow and infiltration
appear and disappear, which could quantitatively describe the base flow
and infiltration.

To investigate the effects of different hydraulic conductivities on the
reduced base flow, induced infiltration and total water reduction in
this quasi-ideal model, different hydraulic conductivities K=15, 20, and
25 m/d were used in the original model, and the pumping duration was
also set to 3600 days. Pumping was stopped after 3600 days to observe
the changes in the reduced base flow, induced infiltration and total
water reduction under different conditions.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image19/image19}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image20/image20}

\end{center}
\end{figure}

(a) Base flow reduction process (b) Reduced Base flow increase process\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image21/image21}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image22/image22}

\end{center}
\end{figure}

(c) Infiltration increase process (d) Total water reduction process

Figure 10. Water quantity change when stopping pumping for 3600 days

At 3600 pumping days, when K = 15, 20, and 25 m/d, the base flow from
zone 1 to zone 2 decreased to 0 for 1260 days, 1320 days and 1470 days,
and the infiltration times were 330 days, 300 days and 300 days. The
infiltration volumes were 7.56 m\textsuperscript{3}/d, 9.11
m\textsuperscript{3}/d and 41.12 m\textsuperscript{3}/d, respectively.

In the 3600 days when pumping was stopped, when K=15, 20, and 25 m/d,
the base flow from zone 1 to zone 2 reappeared for 5340 days, 4740 days,
and 4440 days, respectively, and the time when the induced infiltration
was 0 was 5430 days, 4860 days and 4530 days, respectively.

The cone of depression formed during pumping is also related to the
difference in hydraulic conductivity. When K=15, 20, and 25 m/d, the
water levels in the water level observation wells located 30 m away from
the pumping wells are 2.73 m, 4.14 m, and 5.11 m, respectively, when
pumping for 1500 days, which means that the smaller the hydraulic
conductivity is, the faster the water level in the center of the cone of
depression drops.

Similarly, we plotted base flow, reduced base flow, induced
infiltration, and total water reduction versus time for the three
different hydraulic conductivities, as shown in Figure 11. In the
reduced base flow curve with time, the greater the hydraulic
conductivity of the aquifer, the greater the reduced base flow, and the
earlier the reduced base flow decreases when pumping is stopped at 3600
days, and the reduced base flow\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image23/image23}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image24/image24}

\end{center}
\end{figure}

(a) Base flow reduction process (b) Reduced Base flow increase process\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image25/image25}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image26/image26}

\end{center}
\end{figure}

(c) Infiltration increase process (d) Total water reduction process

Figure 11. Water quantity change of different hydraulic conductivity

volume until the end of the simulation is also proportional to the size
of the hydraulic conductivity. Different from the reduced base flow, the
value of induced infiltration is inversely proportional to the size of
the hydraulic conductivity, and the time when the induced infiltration
becomes zero after pumping is stopped is also inversely proportional to
the size of the hydraulic conductivity. In general, the total water
reduction is directly proportional to the size of the hydraulic
conductivity.

Under the condition that water is pumped throughout the simulation
period, the groundwater divide between the pumping wells and the wetland
caused by groundwater exploitation in the wetland--aquifer interaction
model is quantitatively portrayed based on the water exchange between
zone 1 and zone 2, and the base flow from the piedmont plain aquifer to
the wetland is continuously reduced until it becomes zero, while the
reverse groundwater recharge from the wetland is induced to the piedmont
plain aquifer. During the full simulation period, the reduced base flow
increased rapidly in the early stage and was relatively flat in the
later stage. The reduced base flow responded to pumping immediately,
while the induced infiltration began to occur only when the groundwater
divide spread to the wetland boundary and groundwater in zone 2 flowed
to zone 1, with a certain lag. The longer the pumping time, the more
dominant the effect of induced infiltration on wetland degradation.

In the model with cessation of pumping after 3600 days, the situation
for the first 3600 days was the same as that of pumping for the entire
time. The natural recovery process of the wetland after pumping
cessation is an inverse wetland--aquifer interaction process. The
gradual reduction of induced infiltration from zone 2 to zone 1 and the
recovery of base flow from zone 1 to zone 2 characterize both the
reappearance of the groundwater divide between the wetland and the
pumping well and the disappearance of the groundwater divide with
increasing pumping time. The groundwater flow direction in the whole
simulation area resumed to flow from west to the east again. After the
cessation of exploitation, the effect of reduced base flow on wetland
degradation shows a certain lag after pumping cessation. In contrast,
the effect of induced infiltration on wetland degradation responds
instantly. At the end of the simulation, the only cause of wetland
degradation is the reduced base flow.

Numerical simulations with different hydraulic conductivities showed
that the effect of reduced base flow on wetland degradation is
proportional to the hydraulic conductivity in the process of groundwater
exploitation and natural recovery of wetlands. However, the inflection
point at which the reduced base flow decreases with increasing pumping
time is inversely proportional to the hydraulic conductivity, and the
effect of induced infiltration on wetland degradation decreases with
increasing hydraulic conductivity in this model. The total water
reduction of wetlands increases with increasing hydraulic conductivity.

\textbf{Conclusion}

(1) Groundwater exploitation will form a cone of depression, and the
water level around it will flow toward the center of the pumping well,
which leads to the inversion of the groundwater hydraulic gradient
between the wetland and the piedmont plain, and the base flow of the
piedmont plain aquifer recharging the wetland decreases rapidly. A
groundwater divide is formed between the wetland and the piedmont plain.
With the increase in pumping time, the cone of depression continuously
expands, and the groundwater divide gradually moves toward the wetland
until it expands to the wetland boundary. At this time, the surface
water of the wetland begins to recharge the piedmont plain groundwater
and induces infiltration from the wetland to the piedmont plain. After
pumping stops, the water level of the original cone of depression
gradually rises due to recharge from the western boundary, and the
lowest point of the cone of depression level gradually moves toward the
wetland until the groundwater flow direction in the study area returns
to west--east.

(2) The impact of groundwater overexploitation on wetland degradation is
mainly divided into two parts. One part is the reduced base flow from
the piedmont plain aquifer to the wetland, which leads to the reduction
in groundwater recharge received by the wetland. The other is the
expansion of the cone of depression, which induces the groundwater in
the wetland to infiltrate into the piedmont plain aquifer, leading to a
reduction in wetland surface water. The reduction in base flow tends to
occur rapidly and correspondingly with groundwater exploitation, while
induced infiltration has a certain lag. At the beginning of pumping, the
effect of reduced base flow on wetland degradation dominates, but with
pumping time, the effect of induced infiltration on wetland degradation
exceeds that of reduced base flow. After stopping pumping, the effect of
induced infiltration on wetland degradation responds immediately and
decreases rapidly, while the effect of reduced base flow on wetland
degradation still increases briefly for some time and then slowly
decreases but its effect lasts for a long time.

(3) The different hydraulic conductivities of aquifers also have an
impact on wetland degradation. The effect of reduced base flow on
wetland degradation is proportional to the hydraulic conductivity, and
the effect of induced infiltration on wetland degradation decreases with
increasing hydraulic conductivity. In general, according to the
numerical simulations under different hydraulic conductivities, the
total water reduction of wetlands increases with increasing hydraulic
conductivity. In actual wetland areas, if groundwater exploitation is
not restricted or artificial supply measures are not taken, the amount
of wetland water will gradually decrease until it is exhausted.

\textbf{Declarations}

\textbf{Funding:} The research work was funded by the Key Technologies
and Application Demonstration of Groundwater Over-Extraction Control and
Protection in Huang-Huai-Hai Region (No. 2021YFC3200502).

\textbf{Competing Interests:} The authors have no relevant financial or
non-financial interests to disclose.

\textbf{Availability of data and materials:} All data and materials used
in the manuscript comply with field standards and they are available.

\textbf{Authors' Contributions:} All authors contributed to the
research. Tianyu Zhou wrote the paper by himself and finished the
literature review and data analysis, Longcang Shu proposed the idea of
this research and tutored the process of the research, Chengpeng Lu and
Bo Liu proposed the idea of this research and joined the process of the
research. Four authors read and approved the final manuscript.

\textbf{Acknowledgements:} This work is based on the project ``Key
Technologies and Application Demonstration of Groundwater
Over-Extraction Control and Protection in Huang-Huai-Hai Region''. We
gratefully acknowledge the helpful comments and constructive suggestions
by reviewers and editors.

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