Simulations on soil water variation in arid regions

Abstract

Significant soil water variation has often been found in top few meters of arid soils. Understanding soil water variation in these soils is crucial to groundwater recharge estimation, rainfall runoff process, risk assessment, and water resource management. A soil hydrologic model (SHM) was developed for simulating soil water movement in the vertical direction using time steps of minutes to days. To account for the dual processes of matrix and macropore flow, a parameterization scheme of dual processes has been adopted to derive effective hydraulic conductivity used in the SHM simulation. The integral-balance model based on water flux at different degrees of water saturation used to calculate the macropore conductivity is more useful in quantitatively integrating the macropore contribution to the dynamic soil water fluxes. The SHM, successfully applied to humid and semi-arid regions and validated at the Nevada Test Site (NTS) in this study, was used to evaluate soil water variation in an arid region, the NTS. Soil texture effects on soil water content have been evaluated; results indicate that higher hydraulic conductivity soils have less soil water content. A representative vegetation type at the NTS— Larrea tridentate is included to simulate the effects of vegetative cover on the soil water content. The simulations show that the bare soils have higher soil water content than the vegetated soils, which is consistent with observations and other modeled results. Due to low precipitation at the NTS for much of the year, effects of the macropore flow on soil water content are insignificant. However, the macropore flow could be an important factor influencing the soil water content during high precipitation events.