Control Mechanism Of Groundwater And Soil Water Dynamics In Discharge Areas In Arid Region

As the interface between the vadose zone and the saturated zone,the dynamics of water table is controlled by the land surface process,and it also controls the water movement in the vodase zone and the multi-scale groundwater flow system in deep.In groundwater discharge areas,groundwater in the shallow part is closely coupled with atmospheric conditions through the vodase zone,while groundwater in deep part is controlled by the fluctuating waterlevel in recharge area.In this study,the Wudu lake watershed in Ordos basin was selected,and several groundwater and soil water monitoring points were established in the recharge and discharge areas.Based on the mutli-year field monitoring data,under the arid and semiarid climate the formation mechanism of unsaturated and saturated zones including multi-depth hydrodynamic features in the discharge area was discussed.The traditional Tothian basin uses the multi-year average water level as its upper boundary.Based on the field phenomenon that the fluctuation of recharge area is more intense than that of discharge area,this study improves the upper boundary to seasonally fluctuating groundwater table in recharge area,deduces the analytical solution of seasonal variation of unsteady flow,and depicts the dynamic characteristics of deep groundwater in discharge area.The results show that the unsteady flow field will be distorted,and the stagnation points moves in a circular motion.The groundwater fluctuation in the discharge area is controlled by the dynamic groundwater level in the recharge area.The amplitude of hydraulic head increases with depth,and the artesian phenomenon in the discharge area decreases or even disappears when the groundwater level in the recharge area decreases.Meanwhile,the distribution of recharge and discharge area changes seasonally.After investigating the effect of the watertable dymanics on the groundwater flow system,this study further combines the soil water dynamic data to compare and analyze the vertical water content response time characteristics of soil profiles at different depths in the discharge area.There is a seasonal phenomenon that soil water does not infiltrate into the watertable,however,the watertable has been raised in the field.By using FEFLOW,the two-dimensional soil water-groundwater model simulates that,under the influence of rainfall events,infiltration forms a groundwater mound in shallow depth preferentially.The groundwater mound transverse transfer process becomes one of the sources of deep soil water fluctuations in this area,and the vertical variation pattern of water content fluctuation amplitude is different at different soil profiles.Based on the lateral movement of water mounds controlling the deep soil water dynamics in the discharge area of this study area,a one-dimensional atmospheric boundary-fluctuating water level model was established by using HYDRUS-1D to analyze the influence of flucatuating water table on evaporation and recharge fluxes.Based on the sensitivity analysis in theoretical models of water table depth,climatic conditions and soil texture and vegetation distribution,the universality of the two-directional damping of soil water content in the field is verified.This research enhances the understanding of formation and characteristics of groundwater and soil water dynamics in the arid region.