Numerical Modeling Of Soil Moisture Movement And Salt Transport In The Vadose Zone Of The Badain Jaran Desert

The water movement in the unsaturated zone has immediate impact on the groundwater recharge, and the characteristics of the salt transport in the vadose zone can be used for further study of calculating the infiltration recharge in the historical time, which is the basic of the chloride mass balance(CMB) method. Based on the soil moisture field-monitoring data, the physical characteristics and the hydraulic parameters of the soil which are measured by laboratory experiments, the numerical simulation is made by Hydrus-1D, and then study the water movement in the vadose zone of the Badain Jaran Desert from different angles, as well as the salt movement characteristics has been carried on the preliminary discussion.Some vertical sections are excavated in different locations near Sumujilin Lakes in the Badain Jaran Desert to determine the water content in different depths by portable TDR(time domain reflector), and the soil water potential is calculated, the results illustrate two points: 1. The overall moisture content increases with the increasing depth, and its dispersion degree is inversely proportional to the depth; 2. The shallow soil water moves upwards that presents the state of evaporation, which represents the characteristic of the no-precipitation period, and the deep soil water moves downward which presents the infiltration state. On the basis of the monitoring data from different references, the diagrams which reflect the variation of the moisture content with depth are drawn. The dispersion characteristic and the general trend of the data are also analysed. The results show that: Without the influence of the groundwater, the water content in the vadose zone varies within the scope of 10%; The water content which has the maximum probability density is 5%; the dispersion degree of the water content decreases with the increase of depth; If the depth is more than 1 m, it is substantially infiltrated state.The long term monitoring data of soil water shows that the moisture content at the depth of 1 m presents slight fluctuation with the varied seasons, and it can be fitted by a sinusoidal function with the periodical time of one year. According to this feature, a one-dimensional vertical unsaturated water flow model with the depth below 1 m and above 101 m is established by Hydrus-1D to model the distribution of the water content at different time, with the upper boundary moisture content sinusoidal periodical changes over time. The simulation results show that: The fluctuation amplitude of the water content decreases with the increased depth; the infiltration flux at the bottom of the model is very steady and it is directly proportional to the saturated hydraulic conductivity. It give an illustration that the groundwater recharge in the Badain Jaran Desert may be very steady, and it mainly depends on the hydraulic conductivity of the sand layer.According to some results of the application of CMB technique in the Badain Jaran Desert by some formers, one-dimensional vertical transport of chloride ion within the depth scope of 20 m is modeling by Hydrus-1D, and the maximum simulation time is about 800 years. The simulation results show that: The uncertainty of the initial water content and the initial concentration profile has some influence on the simulation results within 400 years; With the increasing dispersion coefficient, the curve of the chloride concentration with depth gradually becomes smooth; The speed rate of the downward migration of the peak value of the chloride concentration is mainly influenced by the hydraulic conductivity; By contrast, the impact of the diffusion coefficient can be ignored. These factors need to take into account when use the CMB method.