Modeling Soil Moisture in the Mojave 

Desert

By David M. Miller, Debra Hughson, and Kevin M. Schmidt 

Open-File Report 2008-1100

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark Myers, Director 

U.S. Geological Survey, Reston, Virginia 2008 

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Miller, D.M., Hughson, D. and Schmidt, K.M. , 2008, Modeling Soil Moisture 

in the Mojave Desert.: Geological Survey Open-File Report 2008-1100, 6 

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Modeling Soil Moisture in the Mojave Desert

Open-File Report 2008-1100

The Mojave Desert is an arid region of southeastern California and parts of Nevada, 

Arizona, and Utah; the desert occupies more than 

25,000 square miles (fig. 1). Ranging from below 

sea level to over 5,000 feet (1,524 m) in elevation, 

the Mojave Desert is considered a “high desert.” 

On the west and southwest it is bounded by the 

Sierra Nevada, the San Gabriel, and the San 

Bernardino Mountains. These imposing mountains intercept moisture traveling inland from the 

Pacific Ocean, producing arid conditions characterized by extreme fluctuations in daily temperatures, strong seasonal winds, and an average 

annual precipitation of less than six inches. The 

Mojave Desert lies farther south and at a lower 

elevation than the cooler Great Basin Desert and 

grades southward into the even lower and hotter 

Sonoran Desert.

In hot dry landscapes, such as the Mojave Desert, the 

competition for water, and thus survival, is intense. Even though 

dry most of the time, the countless washes, gullies, and rills, and 

the vegetation associated with them, attest to the importance of 

precipitation in the desert ecosystem (figs. 2 and 3). Because 

streams, springs, and wetlands exist only in a tiny fraction of the 

desert, virtually all organisms make use of water stored in soils.

Addressing the questions of how water moves into and 

through the soil and how plants take advantage of water is critical to understanding how biota survive in these water-limited 

lands. The ability of plants and animals to utilize moisture 

in desert soils is largely dependent upon four factors: (1) the 

amount and timing of precipitation events, (2) the rate of 

evaporation, (3) the distribution of plant cover and relative plant 

water needs, and (4) the characteristics of the soil itself. While 

desert moisture dynamics are driven primarily by precipitation 

and evaporation, soil attributes such as texture and structure are 

fundamental to how water is utilized by plants. Because soil 

moisture is a vital element of arid ecosystem processes, the US 

Geological Survey is creating models of soil moisture, which 

are valuable tools that can be used in making land management 

decisions.

U.S. Department of the Interior

U.S. Geological Survey

Figure 1. Location of the Mojave Desert region and the principal 

mountain ranges that block storm precipitation.

Figure 2. This active dry wash 

demonstrates that, although dry 

most of the time, streams flow 

repeatedly in the desert. Plant life is 

limited in washes because of floods 

and abrasion by flowing sand and 

gravel, but plants next to the wash 

are larger than normal and probably 

take advantage of supplemental 

water from streamflow. This photo 

by D. M. Miller, USGS, was taken 

after a wet winter and spring; the 

abundant grasses and flowers next 

to the wash have responded to the 

favorable conditions.

Mojave Desert

Ecoregion

Los Angeles

Las Vegas

AZ

Sierra NV UT

Nevada

Mtns.

San Gabriel

Mtns. San Bernardino

Mtns.

CA

Pacific

Ocean

March 2008
Background

As desert soil develops over time (the process of pedogenesis), the texture of the upper part of geologic deposits changes, 

primarily with addition of windblown silt and clay but also with 

weathering, salt deposition, and incorporated plant remains. The 

soil develops layers, or horizons, which tend to be parallel to 

the ground surface. Each soil horizon has unique characteristics 

that differentially affect moisture infiltration and retention. In 

the Mojave, wind-blown dust, which slowly migrates downward 

into coarse stony deposits, transforms each horizon into a progressively finer-textured deposit (fig. 4). The surface is capped 

by a pavement of stones.

The rate of rain-water flow through the soil horizons is 

determined, in part, by each horizon’s hydraulic conductivity. Hydraulic conductivity, a measure of the rate water flows 

through a material, is a function of the number of pores or 

spaces between soil particles and aggregates. The pore space 

depends upon the particle size, shape, structure, and packing. 

Sands, for example, have a granular structure and more pore 

space, and thus higher hydraulic conductivities than clays, 

which have a platy structure and less pore space (fig. 5). 

Much of the Mojave Desert is made up of coarse-grained, 

stony alluvial fan materials (fig. 6). Moisture drains readily 

by gravity in coarser soils, while fine-grained soils have pores 

small enough to hold water by capillary force, which reduces 

conductivity (figs. 5 and 7). This inhibits gravity drainage and 

holds the moisture in place, where it can be used by plants. 

Some soil layers with very low conductivities can act as barriers, preventing water infiltrating to lower soil horizons.

Infiltration and soil moisture redistribution depend not only 

on soil physical properties but also on the location of vegetation. 

The relationship between soil moisture and plants is a challenging field of active research. While the uptake of soil moisture 

by plants depends on the subsurface distribution of active roots, 

those roots alter the soil properties, changing infiltration and 

soil moisture retention. In addition, soil moisture can be transported from wetter to drier soil through a plant’s xylem (tubelike plant cells that primarily conduct water and nutrients from 

roots to leaves) via a process known as hydraulic redistribution. 

Soil moisture is also affected by evaporation from the surface. Drying of the uppermost soil layer pulls moisture upward 

due to capillary pressure, but as the soil dries this mechanism 

for water flow becomes very inefficient. Water vapor continues to migrate, creating a diurnal moisture content cycle as the 

upper layer is dried during the day and re-wetted at night by 

upward vapor flow. 

Based on these principles, an optimal location for a longlived desert perennial shrub, such as Larrea tridentata (creosote 

bush), is where it can take advantage of both brief, intense summer thunderstorms and soil moisture held in fine-textured soils 

from gentle winter precipitation (fig. 8). A successful strategy, 

therefore, is to have deep roots in granular soils and shallow 

roots in fine-textured soils. Stream flow may also add extra 

water to soils under and near washes. This explains why perennial plants along the margins of active washes in alluvial fans 

grow larger than on the older surfaces between active washes 

(fig. 2).

Figure 3. Flash flood in a channel of the Avawatz Mountains 

alluvial fan as it exits onto California State Highway 127 in Silurian 

Valley. Photo April 30, 1999 by D. M. Miller, USGS.

Figure 4. Pedogenesis. Simplified illustration of soil 

development in Pleistocene-age alluvial fan deposits typical of 

the eastern Mojave Desert. Soil horizons are denoted as follows: 

Av—vesicular A, a silt-rich cap containing small cavities, or 

vesicles, which forms under desert pavement; Bw—cambic B, 

characterized by weak cementation, as well as the addition of silt, 

minor amounts of clay, and a slight reddish color; Bt—argillic B, 

characterized by stronger cementation, a strong reddish color, 

as well as the addition of substantial silt and clay; Bk—calcic B, 

characterized by progressive addition of calcium carbonate that 

first coats clasts and later fills in the matrix.

Stony desert pavement

Av (vesicular) horizon with

weak vertical soil structures

Bk (calcic) horizon

with fractures and

thin layering at the top

Bt (argillic) clay-rich

horizon with strong

vertical soil structures

Bw (cambic) horizon with

weak vertical soil structures
Soil Moisture Modeling

Scientists are developing soil moisture models to represent 

and interpret data collected in the field (figs. 8 and 9). By using 

properties inferred from surficial geology and rainfall distribution, numerical models can be used to predict soil moisture at 

various depths and its availability for plant growth. Soil characteristics, including the degree of pedogenesis and residual moisture from past rainfall, along with intensity of rainfall, determine the proportion of moisture that runs off over the ground 

surface and the proportion that infiltrates into the upper soil 

horizon. Volumetric soil moisture content, the volume of water 

relative to the total volume of soil, is a function of particle size 

distribution (figs. 4 and 5), water and air pressure, the amount of 

rainfall, and time. The model partitions precipitation into runoff 

and infiltration and then accounts for downward redistribution 

of the infiltrated moisture. Modeling of soil moisture is based 

on laws of the physics of water flow (i.e., Richards Equation) 

used for estimating the vertical movement of water into an 

initially dry porous soil. Hence, we can forecast the depth of 

infiltration, the relative water content, and also the redistribution 

of water after rainfall stops.

Because soil moisture is vital to plant survival in arid ecosystems, these models help guide land management decisions. 

Figure 5. Soil Structure and Hydraulic Conductivity. Soil structure refers to the aggregation of soil particles and the arrangement of these 

aggregates into structured groups that influence the movement of moisture. Granular structure, usually less than 0.5 cm in diameter, promotes 

the most rapid water infiltration. Blocky structure is comprised of irregular blocks, usually 1.5–5.0 cm wide. Prismatic structure is characterized 

by vertical columns that can be several cm high. With prismatic structure, movement of the water in the soil is mainly vertical, and the supply of 

water to plant roots may be poor. Columnar structure consists of columns of soil with a salt “cap” at the top and is found in some desert soils. 

Platy structure, usually found in certain soil horizons and in compacted soil, is made up of thin flat plates of soil that lie horizontally. Platy structure 

can restrict root and water penetration.

Figure 6. Telescope Peak above Hanupah Canyon alluvial fan in 

Death Valley National Park, California as seen from Dantes Peak 

in the Black Mountains. The different colors, from pale tan (active 

washes) to dark brown (old varnished deposits), can be used to 

map different soil characteristics. Photo by D. M. Miller, USGS.

Figure 7. Large coarse soil particles allow moisture to move 

to deep positions, while small fine particles hold moisture in 

shallower positions through capillary force. The various soil types 

have different soil moisture storage characteristics that plants and 

soil biota take advantage of, including where, how much, and how 

long the moisture is retained.

Coarse soil

large particles

large pores

Fine soil

small particles

small pores

Granular Blocky Prismatic Columnar Platy

Rapid flow Moderate flow Slow flow
Soil moisture characteristics help 

explain why some areas provide a 

more suitable habitat for rare plants, 

important species such as Yucca 

brevifolia (Joshua tree), and biological soil crusts. In order to protect 

threatened and endangered species, 

such as the desert tortoise, land 

managers need to be able to recognize suitable habitats, which includes 

understanding plant communities that 

provide forage for the animals. An 

understanding of soil moisture is also 

essential for restoring disturbed areas 

such as abandoned mine lands; land 

managers must determine the most 

suitable plants for revegetation and 

the proper irrigation methods so the 

plants survive. In addition, our ability 

to predict soil moisture availability 

under future rainfall scenarios may 

govern success in restoring desert 

plant communities.

David Miller,

US Geological Survey 

Debra Hughson,

National Park Service

Kevin Schmidt,

US Geological Survey

Design and layout by Tina Kister

0.35

0.30

0.20

0.15

0.10

0.05

0

0 5 10 15 20

Time elapsed (hrs)

Volumetric soil moisture 0.25

3 cm

10 cm

25 cm

50 cm

Model

Figure 8. Effect of Soil Permeability. These graphs of 

model simulations illustrate the contrasting soil moisture 

profiles for coarse-grained and fine-grained soils and for two 

model rainfalls: an intense, 1-hour summer thunderstorm and 

a gentle, 24-hour winter rain. In coarse soil, on the left, water 

infiltrated 15 cm at the end of the summer storm (1 hour) 

and more deeply after 24 hours. During the winter storm, 

however, rain falls gently throughout the day and infiltrates 

deeply into the coarse soil, providing more soil moisture 

than is provided by the brief summer storm. For the fine soil 

on the right, rain falls too rapidly during the summer storm 

to completely infiltrate (some runs off into streams), and 

the wetting front infiltrates only to about 7 cm, even after 24 

hours. The winter storm penetrates more deeply into the fine 

soil because none of the rain runs away during the gentle 

rainfall, but it only penetrates to about 20 cm. Plants with a 

shallow root system may prefer the fine soil, while those with 

deep roots will do better in coarse soils.

Figure 9. This graph compares the soil moisture recorded during a rain event (February 3, 2004, 

which lasted 2.5 hours and totaled 7.6 mm) and model simulations. Soil moisture probes buried at 3, 

10, 25, and 50 cm in Pleistocene-age deposits (with strong soil horizons) record shallow infiltration 

only, with the shallowest probe wetting rapidly and then drying slowly, while the 10-cm deep probe 

becomes wet more slowly. Moisture does not penetrate to 25 cm. This real-world example provides 

validation for the models.

Volumetric soil moisture

Fine soil

(Vesicular A)

Depth (cm)

0

5

10

15

20

25

30

35

40

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.35

Coarse soil

(Active wash)

24-hr Winter storm

24-hr Summer storm

1-hr Summer storm

Recoverability and Vulnerability of Desert Ecosystems

Soil moisture models are being developed through the Recoverability and Vulnerability of 

Desert Ecosystems (RVDE) program, which is an interdisciplinary, ecosystem-based program designed to create scientific tools for land managers to use as they work to conserve 

and restore desert landscapes.

See also http://geography.wr.usgs.gov/news/docs-04/RVDE_FS058-03.pdf

For more information please contact:

David Miller 

US Geological Survey

345 Middlefield Road, MS-973