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Soil Moisture and Evapotranspiration Modeling - RVDE

Activities > Soil Moisture and Evaporation Modeling

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Zabriskie Point, Death Valley National Park. Photograph by David Miller, USGS.

Short- and long-term maps of meteorological conditions, such as precipitation, air temperature, wind speed, relative humidity, and solar radiation, are needed to better develop an understanding of the recoverability and vulnerability of the Mojave Desert ecosystem. In particular, these maps are needed as input for a detailed daily to hourly soil moisture model, which can be used to calculate run-on, runoff, soil moisture, plant water uptake, evapotranspiration (including evaporation from bare soils), and deep percolation.

Soil moisture is an integral part of understanding the vulnerability and recoverability of wind erosion, water erosion, soil crust development, fire fuel development, and compaction. Statistical and geostatistical models are being developed based on meteorological data (for example, precipitation, air temperature, solar radiation, etc.) from existing weather stations both temporally and spatially. These meteorological models use the same time step as the originally collected data, usually daily to hourly, and convert them to spatial maps at the scale of the digital elevation model (usually 30 m or larger).
Another soil moisture model is also being developed that uses the Richards' equation for soil liquid water movement, Ficks' law for vapor flow, and Fourier's law for heat flow. This soil moisture model includes runoff generation, vapor flow, and condensation caused by water potential gradients (including those developed by thermal gradients).

These models are also being combined to develop a complete water balance at daily to hourly time steps for large and small areas within the Mojave Desert.

Highlights and Key Findings

Preliminary maps of precipitation and soil moisture were prepared for the Mojave Preserve using data from 1949 to 1999. These, in turn, were used to help develop new maps for recoverability and vulnerability of wind erosion, soil crust development and soil compaction.

Potential Evapotranspiration Model

A potential evapotranspiration model of the Mojave ecosystem has been developed and calibrated. The model calculates daily potential evapotranspiration at each pixel in the digital elevation model. The basis for this model is a detailed solar radiation model, which calculates time of sunrise, time of sunset, and solar radiation loads throughout the solar day. The solar radiation model includes the topographic effects of slope, aspect, and blocking ridges and the seasonal changes in atmospheric turbidity, ozone, precipitable water, and albedo.

High desert vegetation at Granite Pass, Mojave National Preserve. Photograph by David Miller, 2002.

Soil Moisture Model

The potential evapotranspiration model is used as the basis for a Mojave ecosystem soil moisture model. The soil moisture model calculates all components of the water balance for all defined soil layers. The model is 3-dimensional. During periods of rainfall, snowfall, and snow melt, the model routes excess soil moisture in a 2-dimensional overland surface flow model. Once runoff routing has been accomplished, the model reverts to 1- dimension for each pixel in the digital elevation model. The 1-dimensional model uses a Gardner-Jury drainage function (a simplified Richard's equation) and the modified Priestely-Taylor model for evapotranspiration and root water uptake.

Basin Characterization Model

The potential evapotranspiration model is also used as the basis for a basin characterization model for the Mojave Desert ecosystem. This model is a simplified version of the soil moisture model, and uses a monthly time step to calculate water balance and determine the potential for recharge or runoff generation. The model is used to classify basins on their potential to generate recharge and whether or not the basin is dominated by in-place (mountain block) recharge or runoff (mountain front/ephemeral stream) recharge.

Humidity Index of the Mojave Ecosystem

Using the previous three models developed as part of the RVDE project, the UNESCO (United Nations Education, Scientific and Cultural Organization) Humidity Index was calculated for each digital elevation model pixel (270 meters) for the Mojave Desert. The model identifies whether the area is hyper-arid, arid, semi-arid, dry subhumid, or humid. In addition to the humidity index each pixel has a unique value that allows relative comparisons within the same humidity index.

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