A Study On Water Transport In Vadose Zone Controlled By Precipitation And Groundwater Table Fluctuation

In shallow groundwater areas,there is a complex interaction between atmospheric water,soil water and groundwater.When studying the infiltration and recharge of groundwater,the predecessors generally adopted the unit gradient lower boundary condition which could not consider the capillary rise of groundwater.What kind of boundary conditions can be used to reasonably describe the soil water movement in the shallow groundwater area is an urgent problem to be solved.In this paper,the Wudu Lake in the Ordos Basin is selected as the study area.The DK2 and DK0 soil monitoring profiles are taken as examples to study the complex interactions between precipitation,soil water and groundwater.After precipitation,the lag time and uplift range of different depth water content uplifts are counted.In summer,the DK2 profile has the longest lag time near 70 cm,which is expressed as “two-way transmission”.In the fall,precipitation infiltration can reach the groundwater table,showing a "one-way transmission".Comparing DK2 profile with two DK0 profiles,it is considered that the "two-way transmission" of water is the result of the interaction of strong evaporation and groundwater fluctuation,which is independent of upper atmospheric conditions.The corresponding conceptual model is established,and then the numerical model of boundary conditions under independent water level fluctuation is established by using Hydrus-1D software.Models with a unit gradient lower boundary condition and with a fixed head lower boundary condition are also constructed for comparison for scenario simulation,and the results are compared and analyzed.The results of the variable head model show that during the evaporation phase,the capillary water zone drops due to the drop of the groundwater table,causing the water to transfer downwards.At this time,there will be a zero flux surface in the deep layer of the unsaturated zone,which will limit the upward water flux to satisfy the evapotranspiration near the surface of the soil.In the fixed head lower boundary model,there is an upward water flux on the soil profile to support the evaporation of the surface soil,resulting in a much larger upward water flux,which will overestimate the actual surface evaporation.In the unit gradient model,the water flows downward and the water content decreases,causing the water content near the surface to fall to a level that is insufficient to support evaporation,and the actual surface evaporation will be underestimated.The lower boundary of the variable head is the most suitable boundary condition for shallow groundwater in the study area.A conceptual model of correct groundwater recharge/evaporation can be established to obtain meaningful recharge/evaporation estimation results.This study enhances our understanding of precipitation infiltration,evaporation and soil water flow in arid regions,and also provides an important basis for how to set the lower boundary conditions in the one-dimensional flow model.