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).