Water Distribution in an Arid Zone Soil: Numerical Analysis of Data from a Large Weighing Lysimeter

Abstract

Although desert soils cover approximately one third of the Earth's land surface, surprisingly little is known about their physical properties and how those properties affect the ecology and hydrology of arid environments. The main goal of this study was to advance our understanding of desert soil hydrodynamics. For this purpose, we developed a process-based component within HYDRUS-1D to describe the moisture dynamics of an arid zone soil as a function of water fluxes through the soil surface. A modified van Genuchten model for the dry end of the soil water retention curve was developed to better capture the basic flow processes for very dry conditions. A scaling method was further used to account for variabilities in water retention because of changes in the bulk density vs. depth. The model was calibrated and validated using hourly soil moisture, temperature, and mass data from a 3-m-deep weighing lysimeter of the Scaling Environmental Processes in Heterogeneous Arid Soils facility at the Desert Research Institute (Las Vegas, NV). Measurements and simulations during a 1-yr period agreed better under precipitation (wetting) than under evaporation (drying) conditions. Evaporation was better simulated for wet than for dry soil surface conditions. This was probably caused by vapor-phase exchange processes with the atmosphere, which were unaccounted for and need to be further explored. Overall, the model provides a promising first step toward developing a more realistic numerical tool to quantify the moisture dynamics of arid ecosystems and their role in climate change, plant growth, erosion, and recharge patterns.

Analysis of water vapor adsorption in soils by means of a lysimeter and numerical modeling

“…As our objective is to study the processes involved in water vapor adsorption, it is important to simulate the processes in their more fundamental form, especially the turbulent vapor fluxes at the soil surface. Therefore, we do not want to use the lysimeter data directly for a prescribed flow boundary condition, as was done, for instance, by Dijkema et al (2018). Instead, we calculate these fluxes through a more fundamental relationship (Equation 17), and we use the lysimeter data for calibration.…”

Surface Evaporation in Arid Regions: Insights From Lysimeter Decadal Record and Global Application of a Surface Evaporation Capacitor (SEC) Model

“…We used the soil water characteristic parameters listed in Table 2 of Dijkema et al () with values: θ res = 0.07 m 3 /m 3 , θ sat = 0.292 m 3 /m 3 , α = 1.57 m, n = 2.57, and saturated hydraulic conductivity K sat = 0.48 m/day. The estimated soil evaporation characteristic length for this soil L C is 0.57 m (for mean potential evaporation of 5.5 mm/day; see below).…”

“…The water storage dynamics of a desert soil were monitored in large weighing lysimeter facility Scaling Environmental Processes in Heterogeneous Arid Soils near Las Vegas, NV, USA (Chief et al, ; Dijkema et al, ). The lysimeters with an inner diameter of 2.26 m and depth 3.00 m were filled with local desert soil (93% sand, 5.5% silt, and 1.5% clay).…”

“…For this study, we have used water content measurements from depths of 0.10, 0.25, 0.50, 1.00, 2.00, and 2.50 m. Additional information including lysimeter mass, precipitation, radiation, wind velocity, and air temperature and humidity were used to estimate surface evaporation and for model testing. The study uses data collected from October 2008 to 2017 from lysimeter 1 for which hydraulic properties were measured and reported in Dijkema et al ().…”

“…Before we applied the SEC model for arid regions and global scale, we use a decade‐long record from deep desert lysimeters (Dijkema et al, ) to reveal the processes that control the partitioning of rainfall water in dry areas. The specific objectives of this study were (1) to predict surface evaporation and the partitioning to deep percolation and transpiration for arid regions, (2) to use the SEC formalism to identify soil conditions and rainfall thresholds for the onset of capacitor leakage and water protection from surface evaporation, and (3) to test the model approach using decadal measurements records from bare soil lysimeter.…”