Observations And Modeling Studies On Vadose Zone Water In The Badain Jaran Desert

The groundwater source of the Badain Jaran Desert(BJD)has been controversial for a long time,among which two of the focus points are groundwater recharge and groundwater discharge,which are closely related to the vadose zone water.Therefore,the observation and modeling studies on vadose zone water in the BJD are of great significance for ascertaining the groundwater source and protecting the water resources and environment of this area.Based on the data from historical literatures,field observations and laboratory experiments,the infiltration recharge and the groundwater evaporation in the BJD are simulated and analyzed from different aspects.Based on the collected data from soil water observations at different depths and measurements of the hydraulic parameters,the variability characteristics of the aeolian sands in the BJD are analyzed with Monte-Carlo simulation by using the coupling stochastic model of the soil water retention characteristics.And on this basis,the vertical distribution of the soil water potential is reconstructed and the results indicate that the hydraulic gradient is approximately 1.0 in the downward infiltration zone,which basically satisfy the free infiltration condition under the force of gravity.According to the Richards equation,the distributions of volumetric water content with respect to different infiltration rates are assessed with Monte-Carlo simulation.The reasonable variation range of the infiltration rate is determined from the comparison of statistical properties between simulated and observed data.Results show that the most possible range of the infiltration rate is 15~34 mm/yr.According to the monitoring results of temperature,negative pressure head and water content in the vadose zone of the BJD within a depth of 3 m for many years,the amplitudes of temperature and moisture content in the shallow vadose zone decrease obviously with the increase of depth.For the deep vadose zone with a depth of more than 3 m,using observation results at 3 m as boundary condition,the downward transmission of seasonal fluctuation is analyzed by the use of Hydrus-1D model and an analytic model proposed by previous studies respectively.Both models also provide seasonal fluctuation results of infiltration flux,which present a similar trend that the amplitude decreases with the increasing depth.Taking the amplitude of negative pressure head of less than 1 cm as the critical condition that the fluctuation is negligible,the penetration depth of seasonal fluctuation is about 47 m,below which the infiltration flux stabilizes at a level of 30.7±4 mm/yr.The groundwater evaporation under 54 different scenarios which are designed based on the climate background,the characteristics of aeolian sandy soils and the typical vegetation types at different groundwater depths of the BJD are numerically simulated.The simulation results show that the annual average groundwater evaporation decreases with the increase of the depth to water table non-linearly.This nonlinear relationship can be accurately fitted with a newly proposed empirical formula.The quantity of the groundwater evaporation in the lake concentration area of the BJD is assessed according to the above fitting results,and the evaluation results show that the total amount of the groundwater evaporation is significantly larger than that of the evaporation from the lake surface,and the former is 2.5~2.6 times as large as the latter,which must be considered in the analysis of the water balance.