Ground-Water Flow Modeling by the U.S. Geological Survey 

in Nevada: Uses and Approaches

By Wayne R. Belcher and Alan H. Welch

Often these uses of water compete with one another. For 

instance, pumping of ground water for agriculture can lower 

water levels in natural springs, thereby threatening the existence 

of aquatic animals and plants. Or pumping of ground water in 

rural areas for municipal water supply can limit the amount of 

water available for agriculture.

Tools are needed to analyze the effects of these competing 

interests in water and to allow land managers and planners to 

balance the various uses. A ground-water computer model is 

one such tool that can be used to examine effects from pumping on water levels, spring flows, and flow paths (directions of 

ground-water flow). These effects can affect water availability, 

water-dependent ecosystems, and the movement of contaminants. A computer model can be a valuable tool for managing 

the water resources and for addressing complex socio-economic 

and political issues resulting from conflicting water uses. 

A

B

C

D

Figure 1. Uses of ground water in Nevada: A, Large crater resulting from nuclear testing at the Nevada Test Site in Nye County; B, 

Urbanization in Pahrump Valley, Nye County; C, Agricultural irrigation in Amargosa Desert, Nye County; D, Crystal Pool spring, Ash Meadows 

National Wildlife Refuge, Nye County. (Photo credit: U.S. Department of Energy, National Nuclear Security Administration.)

Introduction 

Water is a precious commodity, especially in the arid southwest region of the United States, where there is a limited supply 

of surface water and ground water. Growing populations have 

increased the demands on this limited supply of water. In addition to water supply, many other uses for ground water exist in this 

region (fig. 1). Some of these uses include:

Agriculture

Mining and other commercial activities

Municipal and domestic water supply

Defense-related activities (water supplies for military 

installations and nuclear testing activities)

Aquatic habitat for endangered and threatened species

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•

•

•

•

Fact Sheet 2006–3138

November 2006

U.S. Department of the Interior

U.S. Geological Survey
Figure 2. Conceptual model of how ground water flows between recharge areas in mountains and discharge areas in valleys with 

withdrawals from pumping (modified from Faunt and others, 2004b).

Ground-Water Flow in Nevada

To model a ground-water system, scientists first must study 

the system to understand directions and rates of ground-water 

flow, creating a “conceptual model”. The conceptual model of 

ground-water flow in the Great Basin (which contains most of 

Nevada) consists of several interconnected, complex groundwater flow systems (fig. 2). Ground-water flow occurs in three 

subregions (local, intermediate, and regional). The relatively 

shallow and localized systems are superimposed on deeper, 

regional systems. Regional ground-water flows predominantly 

through thick Paleozoic carbonate rocks that are folded and 

faulted; more localized flow occurs in basin-fill and volcanicrock units. Faults and folds can place aquifers (permeable rock) 

and confining units (low-permeable rock) next to each other 

or create zones where ground-water flow can be enhanced or 

restricted. Ground-water flow also can be affected by changes 

occurring within the rock including, but not limited to, the 

amount of welding in volcanic tuffs, changes in the type of sediments making up sedimentary rocks, and the chemical composition of the rock caused by the flow of hot water. 

Ground water from the regional and local systems is 

discharged naturally by spring flow and evapotranspiration (the 

combination of evaporation and plant transpiration), and by 

pumping for agricultural, commercial, and domestic uses. 

What is Ground-Water Flow Modeling?

Models are simplified descriptions of physical reality and 

can be verbal descriptions, graphical representations, physical models, or mathematical equations. Mathematical equations, such as the ground-water flow equation, can be solved 

efficiently by computer programs. The ground-water flow 

equations are solved on a computer by two main methods—by 

finite differences and finite elements. Both of these methods 

solve the ground-water flow equations for a conceptual model 

(fig. 3A) across a model grid. To model a ground-water system 

(fig. 3A), it is divided into discrete units, or cells, consisting of 

a grid system with one or more layers or dimensions (fig. 4). 

Finite-difference grids are regular rectangular grids (fig. 3B)

and finite-element grids are irregular polygonal subdivisions 

(fig. 3C); these grid types reflect the mathematical techniques 

used to solve the ground-water flow equations. These grids can 

represent one-, two-, or three-dimensional systems (fig. 4).

Computerized numerical methods are used with the 

appropriate input to define the ground-water system to create 

a computer (or numerical) model. For ground-water systems, 

model input consists of the areal extent, thickness, and altitude 

of aquifers (water-bearing units) and confining units (units that 

restrict ground-water flow) and the hydrologic properties of 

these units. Model input also consists of the amount, the location, and by which method water enters (recharges) and leaves 

(discharges) the modeled system. The definition of the amount, 

the location, and the method by which water enters or leaves the 

ground-water system describes the “boundary conditions”. 

When the ground-water flow equation is solved for each 

cell, the water level and flow is calculated for each cell. The 

equation can be solved for equilibrium (steady state) or changing (transient) conditions. These modeled water levels and (or) 

flows then can be compared to measured water levels in wells 

and (or) measured flows. Model calibration consists of adjusting 

model inputs until there is a good match between the modeled 

and measured water levels/flows. The need to calibrate model 

inputs by trial and error is reduced greatly by parameter estimation, a mathematical process that helps calculate model inputs 

that best represent the natural system. 

Dry playa

Permeable rock

table Water

Springs

FAULT

Low-permeability rock

Regional

Inte ediate

Local

Discharge area

Dry Playa

Recharge area

Recharge area

Discharge areaPumping well Pumping well

Ground-water flow

Playa boundary

Recharge area boundary line

Fault

EXPLANATION

rm

2
Figure 3. Aquifer showing A, well field and aquifer boundaries; B, 

finite-difference grid; and C, finite-element grid used in computer 

modeling of ground-water systems (modified from Mercer and 

Faust, 1980).

The modular, finite-difference ground-water flow modeling computer program, MODFLOW, developed by the U.S. 

Geological Survey (USGS), is a computer program for simulating common features in ground-water systems (Harbaugh and 

McDonald, 1996; Harbaugh, 2005). MODFLOW originally 

was constructed in the early 1980’s and has evolved continually 

since then with development of many new packages to model 

different ground-water processes. Currently, MODFLOW is the 

world’s most widely used program for simulating ground-water 

flow. More information on MODFLOW can be found in Leake 

(1997).

Ground-Water Flow Modeling in Nevada

Regional-Scale Models 

The conceptual model presented in figure 2 is the basis of 

several computer models that describe regional ground-water 

flow in Nevada. Regional-scale models examine larger groundwater systems that incorporate multiple basins and the groundwater flow between them. A regional-scale model of the carbonate-rock province of the Great Basin was developed by Prudic 

and others (1995). The Death Valley regional ground-water 

flow system model (Belcher, 2004; Faunt and others, 2004a) is 

another example of a regional-scale model of ground-water flow 

in Nevada (fig. 5). 

Local-Scale Models

Local-scale models examine smaller flow systems in 

more detail, typically with a smaller grid size and area than a 

regional-scale model. Local-scale models enable scientists to 

model a ground-water flow system in detail, capturing information that normally would be smoothed out in a regional model. 

The choice of a regional-scale or local-scale model depends on 

the scope of the scientific study. For instance, in the Amargosa 

Desert area in southern Nevada (figs. 5 and 6B), water levels 

from pumping can change rapidly over short distances and 

the variable geology in the basin-fill deposits greatly affects 

ground-water flow. Accordingly, a model with a smaller “local” 

grid size is needed in this type of setting (fig. 6). Boundary 

conditions for the edges of a local-scale model can be obtained 

from a regional-scale model when the regional-scale model area 

encompasses the local-scale model area. 

Linking Regional- and Local-Scale Models

There are two ways to link local models to regional models. The first way to link them is to produce a separate local 

model that scientists obtain the boundary conditions for the 

edges of a local model from a regional model. Typically, this 

linkage is one way and only water levels or flows (but not both) 

are specified at the boundary. 

The second way to link local- and regional-scale models 

is to produce an “embedded” model. The embedded model is 

a local model that is “coupled” to the regional model, allowing 

information on ground-water flow to be transferred between 

them, in effect calibrating both models simultaneously. The 

local model has a finer-scale grid of an area within a regionalscale model. The regional-scale model is known as the “parent” 

grid, while the embedded model (the coupled local model) is the 

“child” grid (fig. 6A). The embedded model is coupled to the 

Figure 4. Example of a finite-difference model grid for simulating 

three-dimensional ground-water flow (from Leake, 1997).

Columns

2 3 4 5 6 7 8 9 10

1

1 2 3 4 5

Rows

6 7 8 9

Cells inside of 

aquifer system

Cells outside of 

aquifer system

Layer 1

Layer 2

Layer 3

Well field

aquifer boundaries

 finite difference 

grid block

block-center node

source-sink node

$y $x

b

model point

source-sink node

irregular

finite-element

b

A

B

C

3
Figure 5. Death Valley regional ground-water flow system model grid (modified from Faunt and others, 2004a), with the Yucca Flat and 

Amargosa embedded model study areas outlined.

MODEL COLUMN 

MODEL ROW 

20 40 60 80 100 120 140 160

194

180

160

140

120

100

80

60

40

20

0 20 40 MILES

50,000-meter grid based on Universal Transverse 0 40 80 KILOMETERS

Mercator projection, Zone 11. Shaded-relief base

from 1:250,000-scale Digital Elevation Model;

sun illumination from northwest at 30 degrees

above horizon

Active cells

Inactive cells Death Valley regional ground-water

 flow system model grid boundary

Death Valley regional ground-water

 flow system model boundary

EXPLANATION

36

37

38

117 116 115

CALIFORNIA

NEVADA

San Bernardino Co 

Inyo Co 

Clark Co 

Lincoln Co 

Esmeralda Co Nye Co 

Amargosa Desert

Pahrump

Valley

Death

Yucca 

Mtn

Pahute

Mesa

Valley 

Devils

Hole

Penoyer

Valley

Nevada Test Site

Amargosa Model Study Area

Yucca Flat Model Study Area

Furnace

Creek

Tecopa

Beatty

Shoshone

Pahrump

Las

Vegas

4
Figure 6. A, The linking of parent (regional-scale) grid and the child (local-scale) grid of an embedded model (modified from Mehl and Hill, 

2005) with B, the southern Amargosa model grid embedded in the Death Valley regional ground-water flow system model grid.

A

B

EXPLANATION

Amargosa embedded model study area

Nevada test site boundary

Parent grid nodes

Shared parent grid node along boundary

Shared child grid node along boundary

Child grid nodes

Child

Parent

Parent

Child

Death Valley regional 

ground-water flow system 

model grid

5
Faunt, C.C., D’Agnese, F.A., and O’Brien, G.M., 2004b, 

Hydrology, chap. D in Belcher, W.R., ed., Death Valley 

regional ground-water flow system, Nevada and California—Hydrogeologic framework and transient ground-water 

flow model: U.S. Geological Survey Scientific Investigations Report 2004–5205, p. 137–163, http://pubs.usgs.

gov/sir/2004/5205/

Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological 

Survey modular ground-water model— The Ground-Water 

Flow Process: U.S. Geological Survey Techniques and Methods 6-A16, variously paged, http://pubs.usgs.gov/tm/2005/

tm6A16/

Harbaugh, A.W., and McDonald, M.G., 1996, User’s documentation for MODFLOW-96, an update to the U.S. Geological 

Survey modular finite-difference ground-water flow model: 

U.S. Geological Survey Open-File Report 96–485 56 p.

Leake, S.A., 1997, Modeling ground-water flow with MODFLOW and related programs: U.S. Geological Survey Fact 

Sheet 121–97, 4 p., http://pubs.usgs.gov/fs/FS-121-97/

Mehl, S.W., and Hill, M.C., 2005, MODFLOW-2005, the U.S. 

Geological Survey modular ground-water model—Documentation of shared node local grid refinement (LGR) and the 

Boundary Flow and Head (BFH) Package: U.S. Geological 

Survey Techniques and Methods 6-A12, 68 p., http://pubs.

usgs.gov/tm/2006/tm6a12/

Mercer, J.W., and Faust, C.R., 1980, Ground-water modeling 

- an overview: Ground Water, v. 18, no. 3, p. 212-227. 

 Prudic, D.E., Harrill, J.R., and Burbey, T.J., 1995, Conceptual 

evaluation of regional ground-water flow in the carbonaterock province of the Great Basin, Nevada, Utah, and adjacent 

states: U.S. Geological Survey Professional Paper 1409–D, 

102 p. 

For additional information contact:

Director

USGS, Nevada Water Science Center

2730 N. Deer Run Road

Carson City, NV 89701

email: GS-W-NVpublic-info@usgs.gov

http://nevada.usgs.gov

regional model using a Local Grid Refinement (LGR) package 

for MODFLOW (Mehl and Hill, 2005). The advantage of LGR 

is that it uses a two-way linking procedure to balance water 

levels and flows across the boundary between the two models. 

Embedded models of underground test areas at Yucca Flat on 

the Nevada Test Site and the southern part of Amargosa Desert, 

both in Nye County, Nevada, currently are being developed 

(fig. 5).

Role of the USGS in Water Resources

The USGS maintains a neutral, unbiased approach to scientific data collection, interpretation, and presentation. One of 

the strengths of the USGS is the ability to carry out large-scale, 

multi-disciplinary investigations that increase our understanding of Earth and provide managers with the tools and information they need to address issues of social concern. Unlike many 

other government agencies, the USGS is not a regulatory or 

management agency and does not manage lands and natural 

resources and is not an advocate for public policies. The role 

of the USGS is to supply data and scientific interpretation 

that allows managers to make informed decisions about how 

resources are managed for society. 

Maintaining a neutral, unbiased standing on management issues for natural resources has allowed the USGS to be 

considered an authority in the field of earth sciences within the 

Federal Government. USGS data and interpretations are used 

in judicial cases concerning resource management because we 

maintain an unbiased stance. Extensive quality assurance and 

review procedures assure that science is accurate and 

interpretations remain neutral. 

References Cited

Belcher, W.R., ed., 2004, Death Valley regional ground-water 

flow system, Nevada and California—Hydrogeologic framework and transient ground-water flow model: U.S. Geological Survey Scientific Investigations Report 2004–5205, 408 

p, http://pubs.usgs.gov/sir/2004/5205/

Faunt, C.C., Blainey, J.B., Hill, M.C., D’Agnese, F.A., and 

O’Brien, G.M., 2004a, Transient numerical model, chap. F in 

Belcher, W.R., ed., Death Valley regional ground-water flow 

system, Nevada and California—Hydrogeologic framework 

and transient ground-water flow model: U.S. Geological Survey Scientific Investigations Report 2004–5205, p. 257–352, 

http://pubs.usgs.gov/sir/2004/5205/

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