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[1, 2]. Among them, water is decisive in
maintaining the stability and health of wetland
ecosystems[3].
The decrease in water resources leads to the shortage of wetland water
replenishment, which is a direct natural
factor[4]. Human activities are the main driving
factor of wetland water reduction, and they are also one of the main
causes of wetland degradation[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[7]. Groundwater exploitation has supported
quick economic growth, which has also caused wetland degradation, land
subsidence, sea water intrusion, and other adverse
effects[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[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[10].
Baiyangdian Lake is the largest freshwater lake in North
China[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[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[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[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[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[17]. At present, research on the main
factors influencing Baiyangdian wetland degradation is mainly divided
into two aspects: climate change and human
activities[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[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[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[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[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[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[25]. Shu Longcang et al.
quantitatively studied the exchange capacity of surface water and
groundwater in the New Bianhe River in Suzhou
City[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[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.
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).