(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.