A Three-Dimensional Numerical Model

of Predevelopment Conditions in the

Death Valley Regional Ground-Water

Flow System, Nevada and California

By Frank A. D’Agnese, Grady M. O’Brien, Claudia C. Faunt,

Wayne R. Belcher, and Carma San Juan

Denver, Colorado

2002

Prepared in cooperation with the

OFFICE OF ENVIRONMENTAL RESTORATION AND WASTE MANAGEMENT,

U.S. DEPARTMENT OF ENERGY,

NATIONAL NUCLEAR SECURITY ADMINISTRATION,

NEVADA OPERATIONS OFFICE, under

Interagency Agreement DE–A108–96NV11967

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4102
District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046

For additional information write to:

Chief, Yucca Mountain Project Branch

U.S. Geological Survey

Box 25046, Mail Stop 421

Denver Federal Center

Denver, CO 80225–0046

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

Copies of this report can be purchased 

from:

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225
CONTENTS III

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 2

Purpose and Scope....................................................................................................................................................... 4

Site Description ..................................................................................................................................................................... 5

Physiography ............................................................................................................................................................... 5

Climate......................................................................................................................................................................... 5

Soils and Vegetation..................................................................................................................................................... 6

Geologic Setting .......................................................................................................................................................... 7

Proterozoic and Paleozoic Time ........................................................................................................................ 8

Mesozoic Time .................................................................................................................................................. 8

Tertiary and Quaternary Time............................................................................................................................ 8

Tertiary and Quaternary Tectonics..................................................................................................................... 10

Hydrology .................................................................................................................................................................... 10

Land and Water Use..................................................................................................................................................... 13

Ground-Water System............................................................................................................................................................ 13

System Boundaries ...................................................................................................................................................... 13

Hydrogeologic Framework .......................................................................................................................................... 14

Hydrogeologic Units.......................................................................................................................................... 14

Hydrogeologic Structures .................................................................................................................................. 16

Source, Occurrence, and Movement of Ground Water................................................................................................ 17

Northern Death Valley Subregion...................................................................................................................... 18

Central Death Valley Subregion ........................................................................................................................ 21

Pahute Mesa–Oasis Valley Ground-Water Basin..................................................................................... 23

Ash Meadows Ground-Water Basin ........................................................................................................ 23

Alkali Flat–Furnace Creek Ground-Water Basin..................................................................................... 23

Southern Death Valley Subregion...................................................................................................................... 24

Ground-Water System Budget Components................................................................................................................ 26

Estimates of Ground-Water Discharge .............................................................................................................. 26

Estimates of Recharge ....................................................................................................................................... 29

Numerical Model of Regional Ground-Water Flow .............................................................................................................. 32

Numerical Modeling Difficulties, Simplifications, and Assumptions......................................................................... 32

Numerical Model Selection ......................................................................................................................................... 34

Nonlinear Regression Objective Function ......................................................................................................... 34

Parameter Definition.......................................................................................................................................... 35

Parameter Sensitivities....................................................................................................................................... 35

Model Construction ..................................................................................................................................................... 36

Grid Definition................................................................................................................................................... 36

External Boundary Conditions .......................................................................................................................... 36

Representation of the Hydrogeologic System ................................................................................................... 39

Hydrogeologic Units................................................................................................................................ 39

Hydrogeologic Structures ........................................................................................................................ 39

Evapotranspiration and Spring Flow ....................................................................................................... 39

Recharge .................................................................................................................................................. 40

Observation Data ............................................................................................................................................... 42

Hydraulic-Head Observations.................................................................................................................. 44

Ground-Water Discharge Observations ................................................................................................... 48

Model Calibration........................................................................................................................................................ 49

Conceptual Model Variations ............................................................................................................................ 49

Location and Type of Boundary Conditions............................................................................................ 49

Definition of Discharge Areas ................................................................................................................. 50

Definition of Recharge Areas .................................................................................................................. 50

CONTENTS
IV CONTENTS

Variations in Interpretation of Hydrogeologic Framework ..................................................................... 52

Confining Units ............................................................................................................................. 53

Carbonate Rocks............................................................................................................................ 54

Volcanic Rocks .............................................................................................................................. 65

Basin-Fill Units ............................................................................................................................. 78

Vertical Anisotropy........................................................................................................................ 82

Horizontal Flow Barriers ............................................................................................................... 82

Observation Data Review and Reweighting ...................................................................................................... 84

Model Evaluation ........................................................................................................................................................ 85

Evaluation of Simulated Hydraulic Head and Ground-Water Discharge .......................................................... 85

Evaluation of Model Fit........................................................................................................................... 86

Spatial Distribution of Unweighted and Weighted Residuals ................................................................. 86

Distribution of Weighted Residuals Relative to Weighted Simulated Values ......................................... 90

Normality of Weighted Residuals and Model Linearity.......................................................................... 90

Evaluation of Estimated Parameter Values and Sensitivities............................................................................. 95

Evaluation of Simulated Water Budgets............................................................................................................ 103

Summary of Model Evaluation.......................................................................................................................... 103

Model Limitations and Potential Future Improvements........................................................................................................ 103

Observation Limitations .............................................................................................................................................. 103

Quality of Observations..................................................................................................................................... 103

Interpretation of Observations ........................................................................................................................... 104

Representation of Observations......................................................................................................................... 104

Hydrogeologic Framework Limitations ...................................................................................................................... 105

Complex Geometry............................................................................................................................................ 105

Complex Spatial Variability............................................................................................................................... 105

Flow Model Limitations .............................................................................................................................................. 105

Physical Framework Representation ................................................................................................................. 105

Hydrologic Conditions Representation ............................................................................................................. 105

Summary................................................................................................................................................................................ 106

References Cited.................................................................................................................................................................... 109

FIGURES

1–19. Maps showing:

1. Graphic and prominent topographic features of the Death Valley region, Nevada and California................... 3

2. Desert climatic zones of the Death Valley region ............................................................................................. 6

3. Weather regimes of the Death Valley region..................................................................................................... 7

4. Tectonic features of the Death Valley region and vicinity ................................................................................ 9

5. Structural-physiographic subsections of the Death Valley region .................................................................... 11

6. Hydrographic areas of the Death Valley region ................................................................................................ 12

7. Interpreted subregions, ground-water basins, and associated flow paths of the Death Valley regional 

ground-water flow system................................................................................................................................. 15

8. Hydrogeologic units of the Death Valley region............................................................................................... 18

9. Hydrogeologic features interpreted as potential flow barriers.......................................................................... 19

10. Northern Death Valley subregion...................................................................................................................... 21

11. Central Death Valley subregion......................................................................................................................... 22

12. Southern Death Valley subregion...................................................................................................................... 25

13. Ground-water discharge areas in the Death Valley regional flow system......................................................... 28

14. Net infiltration for the Death Valley regional flow system using INFIL .......................................................... 33

15. Location of Death Valley regional flow system model grid.............................................................................. 37

16. Death Valley regional flow system model boundary conditions and constant-head cells................................. 38

17. Delineation of evapotranspiration units and zones in the Oasis Valley discharge area, Nevada....................... 41

18. Model cells representing drains used to simulate ground-water discharge from the Death Valley regional

flow system model ............................................................................................................................................ 42
CONTENTS V

19. Recharge zone multiplication array representing hydrogeologic units at the water table ................................ 43

20. Graph showing distribution of standard deviation values assigned to hydraulic-head observations to determine

weights from the premodeling observation analysis and the calibrated model ........................................................ 48

21–41. Maps showing:

21. Recharge simulated in the Death Valley regional flow system model.............................................................. 53

22. Definition of lower clastic confining unit hydraulic-conductivity zone parameters and unit thickness........... 55

23. Definition of Precambrian granites and metamorphic rocks hydraulic-conductivity zone parameters and 

unit thickness. ................................................................................................................................................... 56

24. Definition of Tertiary-Jurassic intrusives hydraulic-conductivity zone parameters and unit thickness ........... 57

25. Definition of hydraulic-conductivity parameters and unit thickness for Mesozoic volcaniclastic and 

sedimentary rocks, upper clastic confining unit, and lower clastic confining unit thrust-fault units .............. 58

26. Definition of hydraulic-conductivity parameters and unit thickness for upper carbonate aquifer and 

lower carbonate aquifer thrust-fault units ........................................................................................................ 63

27. Definition of lower carbonate aquifer hydraulic-conductivity zone parameters and unit thickness................. 64

28. Definition of Belted Range aquifer hydraulic-conductivity zone parameters and 

unit thickness .................................................................................................................................................... 68

29. Definition of Timber Mountain/Pahute Mesa basal confining unit hydraulic-conductivity zone 

parameters and unit thickness ........................................................................................................................... 69

30. Definition of Timber Mountain/Pahute Mesa basal aquifer hydraulic-conductivity zone parameters and 

unit thickness .................................................................................................................................................... 70

31. Definition of Bullfrog confining unit hydraulic-conductivity zone parameters and 

unit thickness. ................................................................................................................................................... 71

32. Definition of Paintbrush/Calico Hills tuff cone hydraulic-conductivity zone 

parameters and unit thickness ........................................................................................................................... 72

33. Definition of Timber Mountain aquifer hydraulic-conductivity zone parameters and unit thickness.............. 73

34. Definition of volcanic aquifer hydraulic-conductivity zone parameters and unit thickness............................. 74

35. Definition of volcanic confining unit hydraulic-conductivity zone parameters and unit thickness ................. 75

36. Definition of undifferentiated volcanic rocks hydraulic-conductivity zone parameters and unit thickness..... 76

37. Definition of Quaternary-Tertiary playa deposits hydraulic-conductivity zone parameters and unit thickness 79

38. Definition of Tertiary sediments/Death Valley section hydraulic-conductivity zone parameters 

and unit thickness.............................................................................................................................................. 80

39. Definition of Quaternary-Tertiary valley-fill alluvium hydraulic-conductivity zone parameters and 

unit thickness .................................................................................................................................................... 81

40. Hydraulic-head residuals (observed minus simulated) and simulated potentiometric surface for 

model layer 1 .................................................................................................................................................... 88

41. Hydraulic-head weighted residuals (observed minus simulated) and simulated potentiometric surface for

model layer 1 .................................................................................................................................................... 89

42–55. Graphs showing:

42. Ground-water discharge residuals with expected observed discharge variation............................................... 90

43. Simulated and observed ground-water discharge for major discharge areas.................................................... 91

44. Observed and simulated ground-water discharge ............................................................................................. 91

45. Ground-water discharge weighted residuals..................................................................................................... 92

46. Weighted residuals relative to weighted simulated equivalent ......................................................................... 93

47. Normal probability plot of weighted residuals ................................................................................................. 94

48. Normal probability plot of weighted residual against correlated normal number............................................ 94

49. Parameters defining hydraulic conductivity for Precambrian granites and metamorphic rocks and 

Tertiary-Jurassic intrusives; A, Composite scaled sensitivity; B, Hydraulic conductivity assigned for each 

parameter and expected parameter-value range................................................................................................ 96

50. Parameters defining hydraulic conductivity for the upper clastic confining unit and lower clastic 

confining unit, hydrogeologic units greater than 300 meters below land surface; A, Composite scaled 

sensitivity; B, Hydraulic conductivity assigned or estimated for each parameter and linear 95-percent

confidence interval for estimated parameters ................................................................................................... 97
VI CONTENTS

51. Parameters defining hydraulic conductivity for the upper clastic confining unit and lower clastic 

confining unit, hydrogeologic units less than 300 meters below land surface; A, Composite 

scaled sensitivity; B, Hydraulic conductivity assigned for each parameter, and linear 95-percent confidence

interval for estimated parameter........................................................................................................................ 98

52. Parameters defining hydraulic conductivity for the lower carbonate aquifer and the upper 

carbonate aquifer; A, Composite scaled sensitivity; B, Hydraulic conductivity assigned or estimated 

for each parameter, expected parameter-value range, and linear 95-percent confidence interval for estimated

parameters ......................................................................................................................................................... 99

53. Parameters defining hydraulic conductivity for undifferentiated volcanics, volcanic aquifer, volcanic 

confining unit, and Timber Mountain/Pahute Mesa volcanic rocks; A, Composite scaled sensitivity; 

B, Hydraulic conductivity assigned or estimated for each parameter, expected parameter-value range, 

and linear 95-percent confidence interval for estimated parameters................................................................. 100

54. Parameters defining hydraulic conductivity for basin fill: Quaternary-Tertiary valley-fill alluvium, 

Quaternary-Tertiary playa deposits, and Tertiary sediments/Death Valley section; A, Composite scaled 

sensitivity; B, Hydraulic conductivity assigned or estimated for each parameter, expected parameter-value 

range, and linear 95-percent confidence interval for esimated parameters....................................................... 101

55. Parameters defining hydraulic conductivity for lower clastic confining unit thrusts, Mesozoic 

volcaniclastic and sedimentary rocks, and lower carbonate aquifer thrusts; A, Composite scaled sensitivity; 

B, Hydraulic conductivity assigned for each parameter and expected parameter-value range ......................... 102

TABLES

1. Hydrogeologic units used in the Death Valley regional flow system model and in previous U.S. Department 

of Energy models in the Death Valley region.......................................................................................................... 17

2. Divisions of the Death Valley regional ground-water flow system......................................................................... 20

3. Description of flow observations with observed discharge and coefficient of variation used in the calibrated 

model....................................................................................................................................................................... 30

4. Area-altitude classes modified from Maxey and Eakin (1950)............................................................................... 31

5. Thickness and depth to top of each model layer..................................................................................................... 36

6. Description of hydrogeologic features defined as horizontal flow barrier package parameters ............................. 40

7. Description of recharge parameters defined for the recharge zones shown in figure 19 ........................................ 44

8. Possible “UseFlag” entries used to attribute water-level measurements in the Death Valley regional flow 

system database....................................................................................................................................................... 45

9. Possible “ConditionFlag” entries used to attribute water-level measurements in the Death Valley regional 

flow system database............................................................................................................................................... 46

10. Possible “MethodFlag” entries used to attribute water-level measurements in the Death Valley regional flow 

system database....................................................................................................................................................... 47

11. Accuracy assigned to water-level measurements based on method of measurement ............................................. 47

12. Drain parameter values and composite scaled sensitivities..................................................................................... 51

13. Recharge parameter descriptions, values, and composite scaled sensitivities ........................................................ 52

14. Hierarchical structure of hydrogeologic unit parameters and major events guiding parameter delineation........... 59

15. Description of lower clastic confining unit (LCCU) horizontal hydraulic-conductivity parameters...................... 61

16. Description of Precambrian granites and metamorphic rocks (pCgm) horizontal hydraulic-conductivity

parameters ............................................................................................................................................................... 62

17. Description of Tertiary-Jurassic intrusives (TJi) horizontal hydraulic-conductivity parameters............................ 62

18. Description of lower carbonate aquifer (LCA) horizontal hydraulic-conductivity parameters .............................. 66

19. Description of Paintbrush/Calico Hills tuff cone (TC) horizontal hydraulic-conductivity parameters .................. 77

20. Description of Timber Mountain/Pahute Mesa basal confining unit (TBCU) horizontal hydraulic-conductivity

parameters ............................................................................................................................................................... 77

21. Description of Timber Mountain/Pahute Mesa basal aquifer (TBQ) horizontal hydraulic-conductivity parameters77

22. Description of volcanic aquifer (VA) horizontal hydraulic-conductivity parameters............................................. 77

23. Description of volcanic confining unit (VCU) horizontal hydraulic-conductivity parameters............................... 77

24. Description of undifferentiated volcanic rocks (VU) horizontal hydraulic-conductivity parameters..................... 78

25. Description of Quaternary-Tertiary valley-fill alluvium (QTal) horizontal hydraulic-conductivity parameters .... 82
CONTENTS VII

26. Description of Quaternary-Tertiary playa deposits (QTp) horizontal hydraulic-conductivity parameters............. 83

27. Description of Tertiary sediments/Death Valley section (TSDVS) horizontal hydraulic-conductivity parameters 83

28. Vertical anisotropy parameter descriptions, values, and composite scaled sensitivities......................................... 84

29. Horizontal flow barrier parameter values and composite scaled sensitivities ........................................................ 85

30. Water budget for the simulated values in the calibrated model compared with estimates from this study ............ 87

CONVERSION FACTORS AND VERTICAL DATUM

Temperature: Degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) by the formula oF = (1.8×oC)+32. 

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

SYMBOL DEFINITIONS AND ACRONYMS

Multiply By To Obtain

Length

millimeter (mm) 0.03937 inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

Area

square hectometer (hm2) 2.471 acre

square kilometer (km2) 0.3861 square mile

Volumetric Rate

cubic meter per day (m3/d) 0.2961 acre-foot per year

cubic meter per day (m3/d) 0.0004087 cubic foot per second

cubic meter per day (m3/d) 0.1835 gallon per minute (gpm)

3D Three dimensional

CSS Composite scaled sensitivity

CV Coefficient of variation

DEM Digital elevation model

DRN Drain 

DVRFS Death Valley regional flow system (model domain)

ET Evapotranspiration

GIS Geographic information system

HFB Horizontal flow barrier

HFM Hydrogeologic framework model

HUF Hydrogeologic-unit flow

INFIL Net infiltration model

LCA Lower carbonate aquifer

LCCU Lower clastic confining unit

LVVSZ Las Vegas Valley shear zone

Mvs Mesozoic volcaniclastic and sedimentary rocks

NTS Nevada Test Site

NWIS National Water Information System

pCgm Precambrian granites and metamorphic rocks

QTal Quaternary-Tertiary valley-fill alluvium
VIII CONTENTS

QTp Quaternary-Tertiary playa deposits

SWNVF Southwest Nevada Volcanic Field

TBA Belted Range aquifer

TBCU Timber Mountain/Pahute Mesa basal confining unit

TBQ Timber Mountain/Pahute Mesa basal aquifer

TC Paintbrush/Calico Hills tuff cone

TCB Bullfrog confining unit

TJi Tertiary-Jurassic intrusives

TMA Timber Mountain aquifer

TSDVS Tertiary sediments/Death Valley section

UCA Upper carbonate aquifer

UCCU Upper clastic confining unit

UGTA Underground Test Area

UTM Universal Transverse Mercator

VA Volcanic aquifer

VCU Volcanic confining unit

VU Undifferentiated volcanic rocks

YMP Yucca Mountain Project
ABSTRACT 1

A Three-Dimensional Numerical Model of 

Predevelopment Conditions in the Death Valley 

Regional Ground-Water Flow System, 

Nevada and California

By Frank A. D'Agnese, Grady M. O’Brien, Claudia C. Faunt, Wayne R. Belcher, and 

Carma San Juan

ABSTRACT 

In the early 1990’s, two numerical models 

of the Death Valley regional ground-water flow 

system were developed by the U.S. Department of 

Energy. In general, the two models were based on 

the same basic hydrogeologic data set. In 1998, 

the U.S. Department of Energy requested that the 

U.S. Geological Survey develop and maintain a 

ground-water flow model of the Death Valley 

region in support of U.S. Department of Energy 

programs at the Nevada Test Site. The purpose of 

developing this “second-generation” regional 

model was to enhance the knowledge and understanding of the ground-water flow system as new 

information and tools are developed. The U.S. 

Geological Survey also was encouraged by the 

U.S. Department of Energy to cooperate to the 

fullest extent with other Federal, State, and local 

entities in the region to take advantage of the 

benefits of their knowledge and expertise.

The short-term objective of the Death 

Valley regional ground-water flow system project 

was to develop a steady-state representation of the 

predevelopment conditions of the ground-water 

flow system utilizing the two geologic interpretations used to develop the previous numerical 

models. The long-term objective of this project 

was to construct and calibrate a transient model 

that simulates the ground-water conditions of the 

study area over the historical record that utilizes a 

newly interpreted hydrogeologic conceptual 

model. This report describes the result of the 

predevelopment steady-state model construction 

and calibration.

The Death Valley regional ground-water 

flow system is situated within the southern Great 

Basin, a subprovince of the Basin and Range 

physiographic province, bounded by latitudes 

35 degrees north and 38 degrees 15 minutes north 

and by longitudes 115 and 118 degrees west. 

Hydrology in the region is a result of both the arid 

climatic conditions and the complex geology. 

Ground-water flow generally can be described as 

dominated by interbasinal flow and may be 

conceptualized as having two main components: a 

series of relatively shallow and localized flow 

paths that are superimposed on deeper regional 

flow paths. A significant component of the 

regional ground-water flow is through a thick 

Paleozoic carbonate rock sequence. Throughout 

the flow system, ground water flows through 

zones of high transmissivity that have resulted 

from regional faulting and fracturing.

The conceptual model of the Death Valley 

regional ground-water flow system used for this 

study is adapted from the two previous groundwater modeling studies. The three-dimensional 

digital hydrogeologic framework model developed for the region also contains elements of both 

of the hydrogeologic framework models used in 

the previous investigations. As dictated by project 

scope, very little reinterpretation and refinement 

were made where these two framework models 
2 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

disagree; therefore, limitations in the hydrogeologic representation of the flow system exist. 

Despite limitations, the framework model 

provides the best representation to date of the 

hydrogeologic units and structures that control 

regional ground-water flow and serves as an 

important information source used to construct 

and calibrate the predevelopment, steady-state 

flow model.

In addition to the hydrogeologic framework, a complex array of mechanisms accounts 

for flow into, through, and out of the regional 

ground-water flow system. Natural discharges 

from the regional ground-water flow system occur 

by evapotranspiration, springs, and subsurface 

outflow. In this study, evapotranspiration rates 

were adapted from a related investigation that 

developed maps of evapotranspiration areas and 

computed rates from micrometeorological data 

collected within the local area over a multiyear 

period. In some cases, historical spring flow 

records were used to derive ground-water 

discharge rates for isolated regional springs.

For this investigation, a process-based, 

numerical model was developed to estimate net 

infiltration. This result can be used to represent 

recharge by assuming that water that infiltrates 

past the “root zone” ultimately becomes groundwater recharge. Net infiltration based on this 

water-balance approach, however, suggests that 

recharge in the region exceeds the estimated 

ground-water discharge for the basin by almost a 

factor of four. The net infiltration model contains 

some assumptions and was limited in regard to 

soil moisture, bedrock permeability, and precipitation distributions that result in these higher than 

expected values. Despite this limitation, the net 

infiltration model does provide a good starting 

point for estimating regional ground-water 

recharge in the ground-water flow model.

The Death Valley regional ground-water 

flow system was simulated using a three-dimensional steady-state model that incorporated a 

nonlinear least-squares regression technique 

to estimate selected model parameters. The 

numerical modeling program MODFLOW-2000 

was used in creating a finite-difference model 

consisting of 194 rows, 160 columns, and 

15 layers. The grid cells were oriented northsouth and were of uniform size, with side dimensions of 1,500 meters. The layers span thicknesses 

of 50 to 300 meters. The model grid encompasses 

about 70,000 square kilometers.

The required initial model parameter values 

were supplied by discretization of the threedimensional hydrogeologic framework model and 

digital representations of the remaining conceptual model components. The three-dimensional 

simulation and corresponding sensitivity analysis 

supported the hypothesis of interactions between 

a relatively shallow local and subregional flow 

system and a deeper dominant regional system 

controlled by the carbonate aquifer.

During calibration of the model, techniques 

available in MODFLOW-2000 allowed for estimation of a series of parameters that provided a 

best fit to observed hydraulic heads and flows. 

Numerous conceptual models were evaluated to 

test the validity of various interpretations about 

the flow system. Only those conceptual model 

changes contributing to a significant improvement 

in model fit, as indicated by a reduction in the 

sum of squared errors, were retained in the final 

optimized model. The final model was evaluated 

to assess the likely accuracy of simulated results 

by comparing measured and expected quantities 

with simulated values. Evaluation of the model 

indicates that although the model is clearly an 

improvement on previous representations of the 

flow system, there is an indication of important 

uncertainties and model error.

INTRODUCTION

In the early 1990’s, two numerical models of the 

Death Valley regional ground-water flow system 

(fig. 1) were developed by the U.S. Department of 

Energy (DOE). One model was developed collaboratively for the Yucca Mountain Site Characterization 

Office (DOE/YMSCO) and the Nevada Operations 

Office Hydrologic Resource Management Project 

(DOE/NV-HRMP). Another was developed for the
INTRODUCTION 3

KERN CO 

SAN BERNARDINO CO 

INYO CO

CLARK CO 

LINCOLN CO NYE CO 

ESMERALDA CO

CALIFORNIA

NEVADA

Amargosa

Rive r

Valley

Last Chance Range

Grapevine Mts

White Mts

Cottonwood Mts

Black Mts

Funeral Mts

Shadow Mts

Granite Mts

Tucki

Mtn

Amargosa

Valley

Spring Mts

Skull Mtn

Specter Range

Nopah Range

McCullogh Range

Pahute 

Mesa

Pahranagat Range

East Pahranagat Range

North Pahranagat Range

Death 

Pahrump

Valley

Indian

Springs

Valley

Las

Vegas

Valley

Timber

Mtn

Black

Mtn

Timpahute 

 Range

Monte Cristo Range

Big Smokey

Valley

Clayton

Valley

Panamint

Valley

Searles

Valley

Indian

Wells

Valley

Owens

Valley

California

Valley

Greenwater

Valley

Owlshead

Mts

Avawatz

Mts

Quail Mts

Fish Lake Valley

Stone

Cabin

Valley

Cactus

Flat

Stonewall

Flat

Stonewall

Mtn

Magruder

Mtn

Gold

Flat

Jackass

Flats

Yucca

Flat

Amargosa

Flat

Frenchman

Flat

Reveille

Valley

Kawich

Valley

Tikaboo

Valley Emigrant

Valley

Desert

Valley

Penoyer

Valley

San Antonio Mts

Monitor

Range

Goldfield

Hill Cactus Range

Reve

ille 

Range

Quinn

Railroad Va

Canyon

Range

lley

Palmetto Mts Montezuma Range

Slate Ridge

Slate Range

Argus Range

Sierra Nevada

Kawich Range

Gold

Mtn

Belted Range

Spotted Range

Sheep Range

Eleana

Range

Pintwater Range

Groom Range

Hiko Range

East Desert Range

Half Pint Range

Amargosa Range

Kingston

Range Mesquite

Mts

Soda

Mts

Ivanpah

Mts

Rainier

Mesa

Calico

Hills

Sarcobatus

Flat

Mojave Desert

Tonopah

Furnace

Creek

Las

Vegas

Tecopa

Beatty

Shoshone

Baker

Saline Valley

Eureka Valley

Inyo Mts

Resting Spring 

Range

Stewart

Valley

 Ash

Meadows

Amargosa

Farms

Striped

Hills Crater

Flat

Yucc

Mtn

a

Bare

Mtn

Oasis Bullfrog

Hills

Valley

Chicago

Valley

Ivanpah Valley

Pana

mint Ra

nge

CALIFORNIA

NEVADA

STUDY

 AREA

35°

118° 117° 116° 115°

38°

37°

36°

EXPLANATION

Yucca Mountain Project 

model boundary

Death Valley regional

flow system model boundary

Underground Test Area

model boundary

Nevada Test Site boundary

0 40

0 80 KILOMETERS 40

80 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

Figure 1. Graphic and prominent topographic features of the Death Valley region, Nevada and 

California.
4 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Nevada Operations Office Underground Test Area 

(DOE/NV-UGTA) subproject of the Environmental 

Restoration (ER) Project (fig. 1). The regional model 

for the DOE/YMSCO and DOE/NV-HRMP, referred 

to in this report as the Yucca Mountain Project (YMP) 

model, was developed by the U.S. Geological Survey 

(USGS) using MODFLOWP (Hill, 1992) and is documented in D'Agnese and others (1997). The model for 

the DOE/NV-UGTA, referred to in this report as the 

UGTA model (U.S. Department of Energy, 1997), was 

developed by the ER Support Services Contractor (IT 

Corporation and its subcontractors) using MODFLOW 

(McDonald and Harbaugh, 1988).

In general, the two models are based on the 

same basic hydrologic data set. The geologic data sets 

differ somewhat in detail of interpretation, but the 

same general hydrogeologic framework is apparent in 

both models. Significant differences of geologic interpretation occur in the hydrogeologic framework where 

data are sparse and results are highly interpretive. Estimates of recharge are also highly interpretive and vary 

significantly throughout the model domains. These 

differences affect both ground-water flow paths and 

flux through the models.

In 1998, the DOE requested that the USGS 

develop and maintain a ground-water flow model of 

the Death Valley regional ground-water flow system in 

support of DOE/YMSCO and DOE/NV programs. 

The purpose of developing this “second-generation” 

regional model was to enhance the knowledge and 

understanding of the Death Valley regional groundwater flow system as new information and tools are 

developed. Furthermore, the USGS has been encouraged by DOE to cooperate to the fullest extent with 

other Federal, State, and local entities in the region, 

including the National Park Service, the U.S. Fish and 

Wildlife Service, the Bureau of Land Management, 

and Nevada and California counties, to take advantage 

of the benefits of their knowledge and expertise.

Short-term objectives of the Death Valley 

regional ground-water flow system project include the 

construction and calibration of a steady-state model 

that approximates predevelopment conditions for the 

Death Valley regional ground-water flow system that 

may (1) provide the foundation and boundary conditions for the site-scale models at Yucca Mountain and 

the Underground Test Areas on the Nevada Test Site 

(NTS), (2) characterize regional three-dimensional 

(3D) ground-water flow paths, (3) define discharge 

and recharge locations, (4) estimate magnitude of 

subsurface flux, and (5) represent the effects of 

regional geologic structural features on regional flow.

Long-term objectives of the Death Valley 

regional ground-water flow system project involve the 

construction and calibration of a transient model that 

simulates the ground-water conditions of the study 

area over time and that could be utilized to: (1) evaluate the effects of changes in system flux, regardless 

of whether the changes are natural or human-induced; 

(2) provide a technical basis for decisions on the quantity of water available for defense and economic development activities on the NTS; (3) determine the 

potential effect of increased offsite water use on the 

NTS water supplies; (4) provide a framework for 

determining effective source plume, ambient trend, 

and point-of-use ground-water quality monitoring 

locations; and (5) provide an opportunity for other 

Federal, State, and local agencies and organizations to 

take a step toward developing a cooperative, regional 

Death Valley Ground-Water Management District.

Purpose and Scope

The purpose of this report is to document 

progress to date of the numerical steady-state groundwater flow model being developed as part of the 

Death Valley regional ground-water flow system 

project. The ground-water flow model simulates 

assumed predevelopment conditions. This groundwater flow model has been constructed from a data set 

synthesized from the previous two DOE regional 

ground-water model investigations. The results of this 

flow model will change as modifications are made to 

incorporate additional hydrogeologic data.

The scope of this study can be summarized as 

follows: 

1. The study is limited to the Death Valley region, and 

the model domain, referred to as the Death Valley 

regional flow system (DVRFS), approximates the 

combined areas of the previous two DOE 

regional ground-water flow models (fig. 1).

2. The details of the hydrogeologic framework are 

limited to a “hard merge” of the existing two 

regional hydrogeologic interpretations. Therefore, no additional reinterpretation of regional 

hydrogeology was conducted. The details of the 
SITE DESCRIPTION 5

hydrogeologic framework “hard merge” effort 

are described in Belcher and others (2002).

SITE DESCRIPTION

The Death Valley region (fig. 1) includes several 

large, prominent valleys: Amargosa Valley, Pahrump 

Valley, and Death Valley. The region also includes 

several major mountain ranges including the Panamint 

Range, the Spring Mountains, the Sheep Range, the 

Amargosa Range, the Kawich Range, the Kingston 

Range, the Pahranagat Range, the Timpahute Range, 

and the Last Chance Range. These major physiographic and geologic features result in a complex 

ground-water flow system.

Physiography

The Death Valley region is situated within the 

southern Great Basin, a subprovince of the Basin and 

Range physiographic province (Fenneman, 1931). 

Late Cenozoic tectonic activity and faulting account 

for much of the topographic relief (Grose and Smith, 

1989). Altitudes range from 86 m below sea level at 

Death Valley to 3,600 m above sea level at Charleston 

Peak in the Spring Mountains. The relief between 

valleys and adjoining mountains locally exceeds 

1,500 m (Bedinger and others, 1989). Most of the 

principal mountain ranges have distinct northwestsoutheast (NW/SE) trends, although the trends of 

intermediate-scale topographic features are quite variable. The ranges occupy only about 25 percent of the 

landscape in the study area (Peterson, 1981). The 

remainder of the landscape is occupied by broad intermontane basins formed from tectonically downdropped grabens. The basins are filled with alluvium 

and some interbedded volcanic deposits that gently 

slope from the valley floors to the bordering mountain 

ranges forming piedmonts (Peterson, 1981).

The valley floors are local depositional centers, 

usually containing playa lakes that act as catchments 

for surface-water runoff (Grose and Smith, 1989). The 

Amargosa River (fig. 1), an intermittent stream whose 

drainage basin encompasses about 15,000 km2

, 

discharges into the south end of the Death Valley saltpan, the largest playa in the study area (Hunt and 

others, 1966). Most of the basins seldom contain 

perennial surface water. Playas and alluvial flats lying 

within these basins constitute about 10 percent of the 

region (Bedinger and others, 1989). Numerous playas 

contain saline deposits that indicate the evaporation of 

surface water and(or) shallow ground water from the 

playa surface. Some of the playas that have been 

deformed by Quaternary faulting contain springs 

where ground water is forced to the surface by juxtaposed lake sediments and alluvial aquifers (Bedinger 

and others, 1989).

Climate

Climatic conditions in the Death Valley region 

are controlled by both altitude and latitude. The 

northern part of the region, including the Cactus, 

Kawich, and Timpahute Ranges (fig. 1), forms part of 

the Great Basin Desert, which is characterized by 

warm, dry summers and cold, dry winters. The 

southern part of the region, including Death Valley and 

the eastern Mojave, forms part of the Mojave Desert, 

which is characterized by hot, dry summers and warm, 

dry winters (Benson and Darrow, 1981). The central 

region around the NTS has been called the Transition 

Desert (Beatley, 1976), which represents a mixing of 

the two climates (fig. 2).

Precipitation in the region is influenced by two 

distinct storm patterns affecting the desert climate—

one in the winter, the other in summer. Winter precipitation (dominantly snow) tends to be of low intensity 

and long duration and covers large areas. In contrast, 

most summer rains result from localized convective 

thunderstorms of high intensity and short duration 

(Hales, 1972; 1974).

Quiring (1965) and French (1983) analyzed the 

distribution of precipitation resulting from the winter 

and summer weather regimes across southern Nevada. 

Quiring (1965) concluded that two sources (one winter 

and one summer) of precipitation can be identified 

(fig. 3). Precipitation resulting from these moisture 

sources, which affect regions south of latitude 

38.5 degrees north, is influenced by major orographic 

and topographic controls. As a consequence, some 

areas of southern Nevada receive a precipitation 

excess while other areas receive a precipitation deficit 

relative to mean annual precipitation (French, 1983).
6 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

RENO

TONOPAH

FRESNO

CEDAR

CITY

SAN

DIEGO

LOS

ANGELES

TUCSON

PHOENIX

35°

40°

120° 115° 110°

NEVADA UTAH

ARIZONA

LAS

CALIFORNIA VEGAS

EXPLANATION

GREAT BASIN WOODLAND AND DESERT

(Sagebrush Desert)

TRANSITION DESERT

MOJAVE DESERT

COLORADO SONORAN DESERT

ARIZONA SONORAN DESERT

DEATH VALLEY REGION

Soils and Vegetation

The soils and vegetation of the Death Valley 

region are controlled to a significant degree by 

climatic, geomorphic, and hydrologic factors. These 

relations are highly variable and complex.

Soils in the Death Valley region typically follow 

a pattern of lithosols on the mountains, medium- to 

coarse-textured soils on alluvial fans and terraces, and 

fine-grained, alluvial soils in the valley floors. In 

general, the soils of the mountains and hills are 

shallow and have a coarse texture with little moistureholding capacity. The soils of the alluvial fans on the 

upper bajadas also are coarse textured but are much 

deeper, so infiltration rates are relatively high. Infiltration rates of the alluvial basin soils are slow because 

the downward movement of water is often impeded by 

indurated calcium carbonate layers (pedogenic 

carbonate), fine-grained playa deposits, and, more

Figure 2. Desert climatic zones of the Death Valley region.
SITE DESCRIPTION 7

35°

36°

37°

38°

39°

40° 120° 118° 116° 114°

MINA

ADAVEN

MERCURY

LAS VEGAS

BOULDER

CITY

BEATTY

SUMMER MOISTURE SOURCE UMMER MOISTURE SOURCE

112° 110°

NEVADA

CALIFORNIA

UTAH

ARIZONA

NEVADA

TEST

SITE

WINTER

MOISTURE

SOURCE

POTENTIAL RAINSHADOW

BOUNDARY

infrequently, by silicified hardpans that form within 

the soils over time (Beatley, 1976).

Vegetation distributions in the Death Valley 

region are influenced by many factors. Water availability and temperature exhibit latitudinal and altitudinal controls that are dominantly climate related. 

Thus, vegetation communities in the region demonstrate both topographic and geographic patterns. 

Mixing of the cold, northern Great Basin Desert 

climate with the warm, southern Mojave Desert 

climate results in a heterogeneous distribution of plant 

associations, rather than distinct homogeneous associations (Beatley, 1976).

Geologic Setting

The Death Valley region has a long and active 

geologic history, including intermittent marine and 

nonmarine sedimentation, large-scale compressive 

deformation, plutonism, volcanism, and extensional 

tectonics (Stewart, 1980; Mifflin, 1988). Knowledge 

of the geologic diversity beneath the alluvial basins is 

indirect in most of the region.

The Death Valley region consists of clastic and 

crystalline rocks of Middle Proterozoic crystalline 

rocks to Early Cambrian clastic rocks; carbonate and 

clastic rocks of Paleozoic age; clastic, volcanic, and 

intrusive rocks of Mesozoic age; varied fluvial, 

paludal, and playa sedimentary deposits of Pliocene 

age; Tertiary volcanic, volcaniclastic, and sedimentary 

rocks; and Tertiary to Quaternary alluvium and colluvium and eolian deposits of Quaternary age (Waddell, 

1982). Literature on detailed geologic studies 

throughout this region is voluminous; yet only a few 

integrative, comprehensive, and summary papers exist 

that cover the entire region and address all of the structural-tectonic-volcanic elements incorporated in the 

framework. Burchfiel and Davis (1981) discussed 

tectonic regimes in the California area, and Stewart 

(1978) discussed the tectonics of the Nevada part of 

the region concisely and comprehensively using structural mechanics principles. Grose and Smith (1989) 

described this geologic complexity and offered insight 

into the hydrogeologic and tectonic controls on 

ground-water flow. Wernicke and others (1988) have 

addressed the Cenozoic extensional history of the 

region; Carr (1990) attempted to integrate the CenoFigure 3. Weather regimes of the Death Valley region (Quiring, 1965).
8 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

zoic volcanic history with the extensional history. 

Most of the study area has undergone deformation, 

and some parts have been nearly continuously tectonically active since the Late Proterozoic (Grose and 

Smith, 1989). 

Structural and tectonic features of the study area 

(fig. 4) reveal a long, complex tectonic evolution. 

Combinations of normal, reverse, and strike-slip 

faulting and folding episodes have resulted in complex 

distributions of rocks. Consequently, diverse rock 

types, ages, and deformational structures are often 

juxtaposed. As a result, subsurface conditions are variable and complex.

Proterozoic and Paleozoic Time

Metamorphic basement rocks of Middle 

Proterozoic age were deposited approximately 1.7 to 

1.4 billion years ago in geosynclinal, orogenic, and 

magmatic arc-type terranes. Sedimentation patterns 

were also influenced by the NE/SW-trending transcontinental arch. During the Late Proterozoic, the study 

area underwent a period of continental margin rifting 

(Grose and Smith, 1989).

This region was a stable, continental margin 

from Late Proterozoic to Devonian time. Late Proterozoic to Early Cambrian continental quartzites and siltstones are overlain by Middle Cambrian through 

Devonian carbonate and calcareous shales in a westward-thickening clastic and carbonate sequence up to 

8,000 m in thickness. The first major Phanerozoic 

tectonic event in the Death Valley region was the 

Antler orogeny. During the Antler orogeny (Devonian 

to Mississippian time), uplift north and west of the 

area resulted in a thick wedge of clastic rocks, derived 

from adjacent highlands, being deposited in a NE/SWtrending foreland basin. Shelf-type carbonate deposition continued during Mississippian time in the southeast part of the region (for example, the Spring 

Mountains). This basin is now defined by the location 

of the Chainman Shale, which dominantly consists of 

relatively impermeable argillites and shales. The 

Antler orogeny also caused eastward thrusting of a 

more than 100,000-m-thick allocthon of deep-ocean 

shales, chert, and volcanic rocks. The leading edge of 

the Roberts Mountain thrust, formed during the Antler 

orogeny, is in the northwestern part of the Death 

Valley region (Grose and Smith, 1989). During Pennsylvanian time, the basin was filled, and shallow 

marine carbonates were deposited on the Mississippian-aged clastic rocks. As a result of these events, 

more than 10,600 m of Paleozoic and Late Proterozoic 

sediments were deposited over the area (U.S. Department of Energy, 1997).

Mesozoic Time

The Sonoma orogeny (Late Permian and Early 

Triassic time) resulted in the overthrusting of deepocean siliceous volcanic rocks toward the continent. 

Structures associated with the Sonoma orogeny occur 

mainly in the northwestern part of the Death Valley 

region (fig. 4). These events created scattered terranes 

of early Mesozoic metasedimentary and metavolcanic 

rocks. The Sevier orogeny (Middle Jurassic and Late 

Cretaceous time) affected the entire area by contraction with regional detachments and was highlighted by 

north-south-trending thrust faulting (including the 

Pahranagat Range, Gass Peak, Lee Canyon, and 

Keystone thrusts within this study area) and simultaneous intrusions of granites of Mesozoic age 

throughout the Death Valley region (Grose and Smith, 

1989) (fig. 4). Wernicke and others (1988) have documented that some of this thrusting activity may be as 

old as the Permian.

Tertiary and Quaternary Time

In contrast to earlier compressional tectonism, 

regional uplift, erosion, volcanism, and extension 

occurred in the Tertiary. As a result, the Death Valley 

region now includes numerous north-south-trending 

valleys containing continental alluvial, paludal, and 

colluvial materials that are interstratified with lava 

flows, tuffs, and tuffaceous sediments. The study area 

underwent intense volcanism during the middle to late 

Tertiary period and was heavily scarred by massive 

volcano-tectonic and caldera depressions and voluminous ash flows of the Southwest Nevada Volcanic 

Field (SWNVF) (Byers and others, 1976; Sawyer and 

others, 1994). Successive eruptions produced at least 

seven large, partially overlapping calderas, which were 

filled with syneruptive welded tuffs and posteruptive 

lava flows and which blanketed surrounding older 

rocks with vast deposits of tuff (U.S. Department of 

Energy, 1997). The SWNVF has significantly affected 

parts of the area either by altering or completely 

removing the preexisting rocks. Meanwhile, water 

levels in pluvial lakes rose and fell in response to 

climate fluctuations, and deposition of basin-fill 
SITE DESCRIPTION 9

118° 117° 116° 115°

WHITE PINE

CO

EUREKA CO LANDER CO CHURCHILL CO

MINERAL

CO

39°38°37°36°35° KERN CO

SAN

BERNARDINO

CO

LINCOLN

CO

CLARK

CO

INYO

CO

TULARE

CO

DVA

MONO

CO

ESMERALDA

CO

NYE

CO

ARIZONA

NEVADA

NEVADA

CALIFORNIA

T T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T 2

3

4

5

6

7

8

9

10

1

11

12

13

14

16

18

17

19

15

TM

E

E

E

W

W

FURNACE CREEK

LAS VEGAS VALLEY

DEATH

VALLEY

GARLOCK

FURNACE CREEK - DEATH VALLEY

EXPLANATION

TM

DVA

10

1 11

1213141516171819

23456789

10

T

E

W

NEVADA TEST SITE

DEATH VALLEY AULACOGEN AREA—Late Precambrian

SHEAR ZONE—

NORTHEAST MARGIN OF WALKER LANE BELT WHERERELATIVELY WELL DEFINED

TM, Towne Pass–Manly Pass

MAJOR STRIKE-SLIP FAULTS—Cenozoic. Arrows indicate

MAJOR THRUST FAULTS OF SEVIER OROGENY, CRETACEOUS

direction of relative movement

,

GOLCONDA THRUST FAULT (LEADING EDGE) OF SONOMAOROGENY—

AND OLDER UNNAMED THRUSTING EVENTS

Late Permian to Early Triassic. Sawteeth on upper

ROBERTS MOUNTAIN THRUST FAULT (LEADING EDGE) OFANTLER OROGENY—

plate

Late Devonian to Early Mississippian.

WEST LIMIT OF MIOGEOSYNCLINAL SHELF; WEST LIMIT OFGRANITIC ROCKS OF CONTINENTAL CRATON— 

Sawteeth on upper plate

Early and

EAST LIMIT OF MIOGEOSYNCLINAL SHELF; WEST LIMIT OFCRATONAL PLATFORM—

Middle Proterozoic

Late Precambrian to Late Cretaceous

Belted Range

Butte Valley

Dry Lake

Gass Peak

Keystone

Last Chance

Lee Canyon

Luning

Leyton Well

Mine Mountain and CP

Muddy Mountain–Glendale

Pahranagat Range

Paradise

Racetrack

Schwaub Peak

Silver Peak

Specter RangeSpotted Range

Wheeler Pass

0 50 100 KILOMETERS0 50 100 MILES

Figure 4. Tectonic features of the Death Valley region and vicinity (modified from Grose and Smith, 1989).
10 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

material continued. Modern alluvial basins have been 

filled with as much as 3,000 m of coarse gravels, 

sands, and localized deposits of playa silt and clay.

Tertiary and Quaternary Tectonics

Superimposed on the earlier pre-Tertiary structural features, the area in and around the NTS began to 

be pulled apart along normal and strike-slip faults 

associated with the formative stages of the modern 

Basin and Range (U.S. Department of Energy, 1997). 

Together with the volcanic features, these attributes 

dominate the topography and physiography of the 

present-day study area (Grose and Smith, 1989). 

According to Dickinson and Snyder (1979), basin and 

range deformation occurred in two phases. The first 

phase began during late Eocene time and ended during 

middle Miocene time and is associated with the deposition of silicic volcanic rocks. The second phase of 

extensional tectonics was characterized by reduced 

volcanic activity and was important in shaping 

present-day topography. Late Cenozoic tilting and 

warping are also evident (Grose and Smith, 1989). 

Tectonic activity in the Basin and Range Province has 

continued to historical times, as indicated by historical 

faulting in the study area. Carr (1984) suggested that 

basin and range deformation has decreased in the last 

few million years because the amount of offset along 

normal faults decreases near the surface.

The basin and range tectonics are superimposed 

on the Walker Lane Belt, a NW/SE-trending, rightlateral, strike-slip shear zone located near the southern 

Nevada-California border (Locke and others, 1940; 

Longwell, 1960; Stewart, 1971, 1978). The Walker 

Lane Belt is part of a megastructure that crosses the 

Basin and Range Province from Texas to Oregon 

(Carr, 1990). The Walker Lane Belt separates the 

NW/SE structural-physiographic trends in the southwestern Great Basin, east of the Sierra Nevada, from 

the predominantly north-south trend of the more 

typical basin and range structure. The Walker Lane 

Belt has long been recognized as an area of active 

faulting containing patterns of faults that are anomalous with respect to the typical fault patterns in the 

Great Basin (Stewart, 1988). The Walker Lane Belt is 

dominated by strike-slip rather than dip-slip faulting; 

and except for caldera structures, large vertical 

displacements are not characteristic (Carr, 1990). The 

Las Vegas Valley shear zone and the Furnace Creek–

Death Valley fault system (fig. 5) are major structural 

features associated with the Walker Lane Belt.

Within the study area, structural trends, styles, 

and tectonic activity (Carr, 1990) of the southern Great 

Basin are diverse. Carr divided the area into three 

major structural-physiographic subsections: the InyoMono, the Walker Lane, and Basin and Range (fig. 5). 

Within these subsections are two NE/SW-trending 

structural zones: the Spotted Range–Mine Mountain 

zone and the Pahranagat shear zone (fig. 5). Winograd 

and Pearson (1976) refer to a major potentiometric 

trough. The location of the trough is probably structurally controlled (Winograd and Thordarson, 1975, 

p. C71–C74) and is roughly coincident with part of the 

Spotted Range–Mine Mountain zone.

In addition to the Spotted Range–Mine Mountain zone and the Pahranagat shear zone, the Walker 

Lane Belt also contains a number of somewhat lessdefined NE/SW-trending structural zones. Because 

they contain highly fractured rocks with potentially 

large transmissivity, these less-defined zones may 

influence regional ground-water flow patterns (Faunt, 

1994; Carr, 1984). These less-defined zones include 

NE/SW-trending structural lineaments from the Bullfrog Hills across the Timber Mountain caldera 

complex (fig. 5), a similar trend from the southern 

Sarcobatus Flat to Black Mountain Caldera (fig. 5), 

and a NE/SW structural-topographic trend from Death 

Valley through the Gold Mountain–Slate Ridge area to 

Stonewall Flat (fig. 5).

Hydrology

In the Death Valley region, perennial streamflow is virtually nonexistent. Several perennial streams 

originate from snowmelt in the high altitudes of the 

Spring and Magruder Mountains. These streams have 

highly variable base flows and in dry years have 

almost imperceptible discharges. Perennial streams 

fed from springs along the lower reaches of the Amargosa River have the most consistent surface-water base 

flows. Despite the small surface-water component, the 

Death Valley region has been delineated on the basis 

of surface-water or topographic divides into hydrographic areas that have been used for descriptive and 

accounting purposes in many surface-water and 

ground-water studies in the Great Basin (Eakin and 

others, 1976) (fig. 6).
SITE DESCRIPTION 11

The ground-water flow systems of the Death 

Valley region are extremely complex. Hydraulic 

compartmentalization may occur throughout the study 

area due to the complex geologic structure. Interbasin 

flows control most of the regional ground-water flow 

system. Most ground-water recharge results from infiltration of precipitation and runoff on the high mountain ranges (Bedinger and others, 1989). Natural 

ground-water discharge is by flow to springs and by 

evapotranspiration (ET) in areas where the water table 

is near the land surface. Human-induced discharge 

occurs in the form of ground-water pumping for agricultural, commercial, and residential uses (Bedinger 

and others, 1989).

Regional ground-water flow patterns do not 

coincide with topographic basins; however, flow 

generally follows the regional topographic gradient 

and water generally moves toward Death Valley. A

Figure 5. Structural-physiographic subsections of the Death Valley region (modified from Carr, 1990).

Las Vegas Valley Shear Zone

Nevada

Test

Site

Death 

CALIFORNIA

NEVADA

Valley

1

2

3

4

6 5

8 7

9 10

11

12

13

A

B

C

Inyo-Mono

Basin and Range

Pahranagat

Shear Zone

Southeast Walker Lane Belt

Northwest Walker Lane Belt

Spotted RangeMine Mountain

Undifferentiated Southern Great Basin

Furnace Creek - Death Valley Fault Zone

Stonewall

Flat

Bullfrog

Hills

35°

118° 117° 116° 115°

38°

37°

36°

Slate Ridge

Gold

Mtn Timber Mtn

Caldera

EXPLANATION

BOUNDARY

MAJOR STRUCTURAL ZONES

MINOR STRUCTURAL ZONES

CALDERAS

Death Valley Regional

Flow System model

boundary

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

A Bullfrog Hills

Sarcobatus Flat

Gold Mountain

Claim Canyon

Ammonia Tanks

Rainier Mesa

Grouse Canyon

Black Mountain

Stonewall Mountain

Mount Helen

Goldfield

Cactus Range

Cathedral Ridge

Bald Mountain

Kawich Range

Quinn Canyon Range

C

B

13

12

11

10

9

8

7

6

5

4

3

2

1

0 40 80 MILES

0 40 80 KILOMETERS
12 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

209

170 148

147

145

146

227B

228

229

230

244 245

241

242 240

243

243

243 XXX

XXX

227A

225

226

144

157

159

158A

168

161

162

163

160

158B

173A

169A

169B

210

211

212

KERN CO

SAN BERNARDINO CO 

INYO CO

CLARK CO 

LINCOLN CO NYE CO

ESMERALDA CO

CALIFORNIA

NEVADA

Amargosa

R i v e r

35°

118° 117° 116° 115°

38°

37°

36°

EXPLANATION

DEATH VALLEY REGIONAL GROUNDWATER FLOW SYSTEM BOUNDARY

NEVADA TEST SITE BOUNDARY

HYDROGRAPHIC AREAS AND NUMBERS—

See facing page for names

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

Figure 6. Hydrographic areas of the Death Valley region (modified from Eakin and others, 1976).
GROUND-WATER SYSTEM 13

Lida Valley

Stonewall Flat

Sarcobatus Flat

Gold Flat

Cactus Flat

Kawich Valley

Emigrant Valley

 A Groom Lake Valley

 B Papoose Lake Valley

Yucca Flat

Frenchman Flat

Indian Springs Valley

Pahrump Valley

Mesquite Valley

Three Lakes Valley (Northern Part)

Tikapoo Valley

 A Northern Part

 B Southern Part

Penoyer Valley

Railroad Valley

 A Southern Part

Pahranagat Valley

Coyote Spring Valley

Three Lakes Valley (Southern Part)

Northern Las Vegas Valley

Mercury Valley

Rock Valley

Fortymile Canyon

 A Jackass Flats

 B Buckboard Mesa

Oasis Valley

Crater Flat

Amargosa Desert

Chicago Valley

California Valley

Lower Amargosa Valley

Death Valley

Valjean Valley

Shadow Valley

No hydrographic area number available

HYDROGRAPHIC AREAS OF THE 

 DEATH VALLEY REGION

144

145

146

147

148

157

158

159

160

161

162

163

168

169

170

173

209

210

211

212

225

226

227

228

229

230

240

241

242

243

244

245

XXX

laterally and vertically extensive carbonate rock 

sequence forms the most significant regional hydraulic 

control because it underlies the area. Most of the 

regional flow is affected by structurally and lithologically induced variations in permeability (Winograd 

and Thordarson, 1975). Regional flow is also affected 

by shallow local flow systems that are controlled by 

recharge and discharge locations and by the complex 

geology. In several places, high mountain ranges 

support local ground-water mounds; the mountain 

ranges may act as hydraulic barriers to subsurface 

flow, preventing interbasinal ground-water flow. The 

net result of these interactions is the compartmentalization of flow into local, subregional, and regional 

subsystems.

Land and Water Use

Most of the land in the Death Valley region is 

owned by the U.S. Government and is administered by 

numerous Federal agencies. Privately owned land is 

scattered throughout the region, but most private 

ownership is concentrated near the agricultural centers 

of Amargosa Valley and Pahrump Valley, the mines 

near Beatty, Nev., and the recreational gateways at 

Shoshone, Tecopa, and Baker, Calif. (fig. 1).

The major land-use activities in the region 

include agriculture, livestock ranching, recreation, and 

mining. Water within the basin mostly is utilized for 

domestic, commercial, agricultural, livestock, military, 

and mining purposes. Water resources in the basin 

directly support the natural diversity protected by the 

National Park Service in Death Valley and the U.S. 

Fish and Wildlife Service at Ash Meadows. These 

areas are protected because of the presence of rare 

desert oases containing endemic wildlife, such as the 

Devils Hole Pupfish (Cyprinidon Diabolis), whose 

continued existence depends on naturally occurring 

spring discharges.

GROUND-WATER SYSTEM

A conceptual model of the Death Valley 

regional ground-water flow system was developed by 

integrating interpretations from the two previous 

ground-water modeling studies (D'Agnese and others, 

1997; U.S. Department of Energy, 1997). The 

following sections describe the boundaries of the 

ground-water flow system; the hydrogeologic framework of the ground-water system, including the hydrogeologic units and structures; the source, occurrence, 

and movement of water through the system; and an 

estimated water budget for the ground-water system 

under predevelopment conditions.

System Boundaries

The boundaries of the Death Valley groundwater flow system were defined by Harrill and Prudic 

Figure 6. Hydrographic areas of the Death 

Valley region (modified from Eakin and 

others, 1976)—Continued.
14 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

(1998) and are shown in figure 7. The ground-water 

system is a 3D body of consolidated and unconsolidated rock that is saturated with ground water. This 

saturated volume of rock is bounded on all sides by a 

“boundary surface” (Franke and others, 1987). The 

boundary surface allows water to either flow into or 

out of the system, such as at the water table, or acts as 

a flow barrier, which allows little or no water to enter 

or leave the system across the boundary surface, such 

as at a ground-water divide or an impermeable mountain block.

The upper boundary surface of the Death Valley 

ground-water flow system is the water table. Under 

natural, or predevelopment conditions, ground water 

moves into the flow system across this boundary as 

regional ground-water recharge or out of the system 

across this boundary at natural ground-water discharge 

sites, such as springs, ET areas, and wet playas. Under 

stressed, or transient conditions, the upper boundary 

surface may fluctuate as a result of changes to groundwater recharge and discharge. The lower boundary 

surface is at a depth where ground-water flow is dominantly horizontal or parallel to the lower surface and 

moves with such small velocities that the volumes of 

water involved do not significantly impact regional 

flow estimates. The depth of this surface may vary and 

generally corresponds with the upper surface of small 

permeability basement rock.

The sides of the ground-water flow system may 

be either no-flow or flow boundaries. When the system 

is observed at steady-state, no-flow conditions exist 

where ground-water movement across the boundary is 

impeded by physical barriers, interpreted flow paths 

parallel to the boundary, or divergence of groundwater flow paths resulting in a ground-water divide. 

Flow paths and ground-water divides both may be 

altered as no-flow boundaries under nonsteady-state or 

steady-state conditions. Most of the Death Valley 

regional ground-water flow system is interpreted as 

having a no-flow boundary resulting from physical 

barriers (fig. 7). No-flow boundaries interpreted as 

resulting from ground-water divides occur in the area 

of northern Las Vegas Valley. Lateral flow boundaries 

exist where ground-water potentiometric gradients 

permit flow across a boundary through fractures or 

higher permeability zones. Lateral flow boundaries 

(underflow) include sections of the delineated 

boundary where ground water is interpreted to potentially flow into the system at southern Railroad Valley, 

Reveille Valley, Stone Cabin Valley, Mud Lake 

(Ralston Valley), Fish Lake and Eureka Valleys, Saline 

Valley, Panamint Valley, Pilot Knob Valley, Pahranagat 

Valley, and northern Las Vegas Valley (fig. 7).

Hydrogeologic Framework

The hydrogeologic framework defines the physical geometry and composition of the surface and 

subsurface materials through which ground water 

flows. The framework defines the fundamental nature 

of the ground-water flow system and includes a definition of the regional topography, the hydrogeologic 

units, and the hydrogeologic structures in the region.

Hydrogeologic Units

In this report, the rocks and deposits forming the 

hydrogeologic framework are grouped into hydrogeologic units. A hydrogeologic unit has considerable 

lateral extent and has reasonably distinct hydrologic 

properties because of its physical (geologic and structural) characteristics. Although all the major geologic 

features are retained, many of the smaller geologic 

units were grouped into larger entities by generalizing 

both lithologic and hydrologic properties of the 

bedrock and basin-fill units.

Winograd and Thordarson (1975) developed a 

nomenclature for hydrogeologic units in the vicinity of 

the Nevada Test Site. In the DVRFS, the entire 

sequence of hydrogeologic units may or may not be 

present in some parts of the study area due to lack of 

deposition, normal or thrust faulting, low-angle extensional faulting, or melting and replacement from 

plutons or caldera formation.

In consolidated rocks, the distinction between 

aquifers and confining units is generally related to 

observations and assumptions of the degree to which 

stratigraphic units tend to be fractured. This fracturing 

is the result of both primary (such as cooling joints) 

and tectonic fractures.

The major units, defined by Winograd and Thordarson (1975), from oldest to youngest are the lower 

clastic aquitard (now termed the lower clastic 

confining unit [LCCU]), the lower carbonate aquifer 

(LCA), the Eleana confining unit (now termed the 

upper clastic confining unit [UCCU]), the upper 

carbonate aquifer (UCA), the tuff aquifers (now 

termed the volcanic aquifers [VA]), and volcanic aquitards (now termed the volcanic confining units
GROUND-WATER SYSTEM 15

CALIFORNIA

NEVADA

Nevada Test Site

Central

Death Valley

Subregion

Southern

Death Valley

Subregion

Northern

Death Valley

Subregion

Amargosa

R i v e r

1

2

3

4

5

6

7

8

10

10

9

8

7

6

5

4

3

2

1

9

35°

118° 117° 116° 115°

38°

37°

36°

EXPLANATION

DEATH VALLEY REGIONAL

FLOW SYSTEM MODEL BOUNDARY

DEATH VALLEY GROUND-WATER FLOW

SYSTEM AND SUBREGION BOUNDARY

(HARRILL AND PRUDIC, 1998)

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

Southern Railroad Valley

Reveille Valley

Stone Cabin Valley (Cactus Flat)

Ralston Valley (Mud Lake)

Fish Lake and Eureka Valleys

FLOW PATHS AND NUMBERS—

Saline Valley

Panamint Valley

Pilot Knob Valley

Pahranagat Valley

North Las Vegas Valley

POTENTIAL FLOW INTO OR BETWEEN SUBREGIONS

FLOW INTO GROUND-WATER FLOW SYSTEM

Figure 7. Interpreted subregions, ground-water basins, and associated flow paths of the Death Valley regional 

ground-water flow system.
16 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

[VCU]), and the alluvial aquifer (QTal). The LCCU 

forms the basement and is generally present beneath 

the other units except in caldera complexes. The LCA 

is the most extensive and transmissive unit in the 

region. It is present in most of the area, although it 

does not control ground-water flow within the caldera 

complexes. The UCCU is present in the north-central 

part of the NTS. It typically impedes vertical flow 

between the overlying UCA and underlying LCA, and 

is associated with many of the large hydraulic gradients in and around the NTS. The VA and VCU form a 

stacked series of alternating aquifers and confining 

units in and around the SWNVF. The VA are moderately transmissive and are saturated in the western 

sections of the NTS. The QTal forms a discontinuous, 

albeit important, aquifer in the region. 

The zones of saturation in the hydrogeologic 

framework may be regional, semiperched, or perched. 

Regional ground-water flow occurs primarily within 

the LCA and VA. Perched ground water is present 

locally throughout the NTS and occurs locally within 

the tuffs wherever confining units compose ridges or 

hills that lie above the regional zone of saturation 

(Winograd and Thordarson, 1975).

The hydrogeologic units contained in the 

DVRFS framework model represent a combination of 

the hydrogeologic units contained in the YMP/HRMP 

hydrogeologic framework model (HFM) (D'Agnese 

and others, 1997, p. 17–20) and the hydrostratigraphic 

units contained in the UGTA Phase I geologic model 

(U.S. Department of Energy, 1997). Both of these 

framework models were loosely based on the hydrogeologic units defined by Winograd and Thordarson 

(1975). Table 1 presents the set of units used in both 

models and the resulting units used in the DVRFS 

HFM described in this report, including thrust-faulted 

units. Figure 8 shows the ground-surface (outcrop) 

distribution of hydrogeologic units. A detailed 

description of the hydrogeologic units included in the 

DVRFS HFM is found in the description of the 3D 

framework model section of the hydrogeologic framework model report (Belcher and others, 2002).

Hydrogeologic Structures

Hydrogeologic structures include faults, fractures, or joints that alter the physical framework of a 

region and, thus, influence ground-water flow. Faults 

in the Death Valley region may affect ground-water 

flow by acting as barriers, conduits, and combined 

conduit/barriers, or by offsetting strata of different 

hydrologic characteristics. Therefore, in addition to 

changes to the flow field resulting from the truncation 

of aquifers and confining units by offsets in the units, 

structures have two effects on ground-water flow: 

direct effects associated with alterations to flow rates 

and ground-water velocities within the faulted zone, 

and indirect effects associated with physical alterations to the flow field in the area near the faulted 

zone.

Caine and Forster (1999) summarized these as 

the fault core and the damaged zone. The fault core is 

the component of a fault zone where tectonics have 

altered the original lithology. In many cases, the core 

will have reduced permeability, relative to that of the 

adjacent damaged zone. The damaged zone is the 

network of subsidiary structures surrounding the fault 

core. They are composed of both open and filled fractures with the geometry and intensity of the fracturing 

resulting in heterogeneity and anisotropy in bulk 

permeabilities. The damaged zone’s permeability is 

typically enhanced relative to both the fault core and 

the undeformed rock. Fault cores typically impede 

fluid flow across the fault, while the damaged zones 

typically conduct flow along the fault zone. So, 

depending on the direction of flow, the fault can be 

either a conduit or barrier (or combined conduit/

barrier) to flow. 

The Death Valley region is typically a series of 

alluvial filled basins that are separated by ranges 

composed of consolidated bedrock. These basins and 

ranges are controlled by faults. In many cases, the 

juxtaposition of aquifers and confining units is sufficient to significantly affect the flow system. The 

regional aquifer can be faulted against a low-permeability material, forcing the water to the surface in the 

form of springs, such as those at Ash Meadows, or 

diffuse discharge.

A subset of the mapped faults in the Death 

Valley region was chosen to be incorporated as relative 

barriers in the flow model. These faults were chosen 

based on their orientation and length. Faults longer 

than 5 km and generally perpendicular to the ground-
GROUND-WATER SYSTEM 17

water flow direction were selected to be incorporated 

as relative barriers (fig. 9). As discussed before, in 

general, faults that are perpendicular to the general 

flow direction act as relative barriers to flow. In the 

Death Valley region these faults are typically oriented 

northwest-southeast. The Death Valley fault zone, 

Furnace Creek fault zone, Gravity fault, and the 

combined Amargosa River fault/Stewart Valley fault 

are all examples of the faults that impede flow and 

were incorporated into the flow model (fig. 9).

Source, Occurrence, and Movement of 

Ground Water

The source, occurrence, and movement of 

ground water in the Death Valley regional groundTable 1. Hydrogeologic units used in the Death Valley regional flow system model and in previous U.S. Department of 

Energy models in the Death Valley region

Hydrogeologic

unit

UGTA unit

(U.S. Department 

of Energy,

1997)

YMP unit

(D'Agnese and 

others, 1997)

Description of primary component

QTal AA QTvf Alluvium/Valley fill

QTp --- Qp Playa deposits

VU VU QTv, Tv Undifferentiated volcanic rocks

VA VA QTv, Tv Volcanic aquifer–southern Nevada Test Site

VCU VCU QTv, Tv Volcanic confining unit–southern Nevada Test Site

TMA TMA QTv, Tv Timber Mountain aquifer

TC TC QTv, Tv Timber Mountain–Paintbrush/Calico Hills tuff cone

TCB TCB QTv, Tv Timber Mountain–Bullfrog confining unit (nonwelded tuffs)

TBA TBA QTv, Tv Timber Mountain–Belted Range aquifer (welded tuffs)

TBCU TBCU QTv, Tv Timber Mountain–Basal confining unit (nonwelded tuffs)

TBQ TBQ QTv, Tv Timber Mountain–Basal aquifer (welded tuffs)

TSDVS TSDVS Tvs Tertiary sediments/Death Valley section

Mvs --- Mvs Mesozoic volcaniclastic and sedimentary rocks

Mvs_LC --- Mvs Mesozoic volcaniclastic and sedimentary rocks–Lee Canyon 

thrust

Mvs_KS --- Mvs Mesozoic volcaniclastic and sedimentary rocks–Keystone 

thrust

UCA LCA3 --- Upper carbonate aquifer

UCCU UCCU ECU Upper clastic confining unit

LCA LCA P2 Lower carbonate aquifer

LCCU LCCU P1 pCgm Lower clastic confining unit

LCA_T2 LCA_T1 --- Lower carbonate aquifer–Specter Range and Wheeler Pass 

thrusts (upper plate)

LCCU_T2 LCCU_T1 --- Lower clastic confining unit–Schwaub Peak, Specter Range, 

and Wheeler Pass thrusts (upper plate)

LCA_LC --- --- Lower carbonate aquifer–Lee Canyon thrust

LCA_GP --- --- Lower carbonate aquifer–Gass Peak thrust

LCA_T1 LCA_T2 --- Lower carbonate aquifer–Schwaub Peak, Specter Range, and 

Wheeler Pass thrusts (lower plate)

LCCU_GP --- --- Lower clastic confining unit–Gass Peak thrust

LCCU_T1 LCCU_T2 --- Lower clastic confining unit–Schwaub Peak, Specter Range, 

and Wheeler Pass thrusts (lower plate)

pCgm LCCU pCgm Precambrian granites and metamorphic rocks

TJi I TJg Tertiary-Jurassic intrusives
18 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

CALIFORNIA

NEVADA

35°

118° 117° 116° 115°

38°

37°

36°

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection, Zone 11

QUATERNARY-TERTIARY VALLEY 

 FILL—Alluvial, colluvial, ash

 fall, and lake deposits

QUATERNARY-TERTIARY VOLCANIC

 ROCKS—Rhyolitic, andesitic,

 and basaltic lava flows

QUATERNARY-TERTIARY PLAYA

 DEPOSITS—Lakebed deposits

 of silt and clay

TERTIARY SEDIMENTS AND

 VOLCANICLASTIC ROCKS—Tuffs 

 and tuffaceous clastic rocks

MESOZOIC SEDIMENTARY 

 AND METAVOLCANIC 

 ROCKS—Dominantly sandstones

TERTIARY-LATE JURASSIC

 INTRUSIVE ROCKS—Crystalline 

 granitic rocks

PALEOZOIC CARBONATE 

 ROCKS—Upper and lower carbonate 

 aquifers (limestones, dolomites, and 

 calcareous shales)

PALEOZOIC-PRECAMBRIAN 

 CLASTIC ROCKS—Lower clastic

 confining unit (conglomerates,

 argillites, and quartzites)

PRECAMBRIAN IGNEOUS AND 

 METAMORPHIC ROCKS—Crystalline 

 rocks–gneisses, schists, and migmatites

PALEOZOIC CLASTIC ROCKS— 

 UPPER CLASTIC CONFINING

 UNIT—Dominantly shales

DEATH VALLEY REGIONAL FLOW

 SYSTEM MODEL BOUNDARY

EXPLANATION

HYDROGEOLOGIC UNITS—

QTal

Tv

TSDVS

TJi

Mvs

LCA/UCA

LCCU

pCgm

QTp

UCCU

water flow system has been described by numerous 

authors and was summarized by D'Agnese and others 

(1997, p. 62–69). That conceptual model is adapted for 

this investigation.

According to D'Agnese and others (1997) 

ground-water flow in the DVRFS is most easily 

described in terms of subregions. For convenience, the 

subregions are subdivided into ground-water basins 

and sections (fig. 7 and table 2). These boundaries are 

used for conceptual ease and descriptive purposes 

only, and these subregions, ground-water basins, and 

sections do not define known discrete independent 

flow systems. Much remains to be done to refine these 

subregions.

Northern Death Valley Subregion

Ground water in the Northern Death Valley 

subregion is derived from precipitation on high altitudes of the Montezuma Range and the Palmetto, 

Gold, and Stonewall Mountains. An unknown volume 

of ground water also may be entering the subregion 

across the system boundary from Mud Lake (Ralston 

Valley), Fish Lake and Eureka Valleys, and Saline 

Valley (fig. 7).

Figure 8. Hydrogeologic units of the Death Valley region (modified from Faunt and others, 1997).
GROUND-WATER SYSTEM 19 35

118

°

° 117° 116° 115°

38°

37°

36°

Universal Transverse Mercator projection, Zone 11.Shaded-relief base from 1:250,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon

0

0 40 80 MILES

40 80 KILOMETERS

EXPLANATION

Death 

CALIFORNIA NEVADA

Valley

Nevada Test Site boundary

HORIZONTAL FLOW BARRIERS—

DEATH VALLEY REGIONALFLOW SYSTEM MODEL BOUNDARY

NEVADA TEST SITE BOUNDARY

AMARGOSA RIVER FAULT

SOUTHERN DEATH VALLEY FAULT ZONE

COMBINED AMARGOSA RIVER FAULT/ STEWART VALLEY FAULTBARE MOUNTAIN FAULTFLUORSPAR CANYON DETACHMENT FAULT

BULLFROG HILLS DETACHMENT

CRATER FLAT FAULT

UNNAMED TRANSITION ZONE TO  SOUTHERN EXTENSION OF FURNACE  CREEK FAULT ZONE; NORTHEAST SIDE OF CENTRAL DEATH VALLEY "IGNEOUS  RHOMBOCHASM"

FURNACE CREEK FAULT ZONEGRAVITY FAULTHIGHWAY 95 FAULT

PAHRUMP-STEWART VALLEY-STATE LINE FAULT ZONE

UNNAMED SOUTHERN EXTENSION OF FURNACE CREEK FAULT ZONE; EAST SIDE OF DEATH VALLEY "IGNEOUS RHOMBOCHASM"

SOLITARIO CANYON FAULT

PAHRUMP-STEWART VALLEY-STATE LINE FAULT ZONE

SHEEPHEAD FAULT ZONE

GREENWATER VALLEY FAULT ZONECENTRAL DEATH VALLEY–FURNACE CREEK FAULT ZONE

DEATH VALLEY FAULT ZONE

EASTERN (AND SOUTHERN) LIMIT OF ELEANA FORMATION INVOLVED IN FOOTWALL THRUST DUPLEX OF THE BELTED RANGE THRUST

UNNAMED SPLAY OF THE LAS VEGAS VALLEY SHEAR ZONE

KEANE WONDER FAULT

LAS VEGAS VALLEY SHEAR ZONEPROJECTED SUBSURFACE TRACE OF THE CP THRUST

NORTHERN DEATH VALLEY–FURNACE CREEK FAULT ZONEWESTERN BOUNDARY OF THE SILENT CANYON CALDERA COMPLEX, CORRESPONDING TO THE NORTHERN PART OF THE THIRSTY CANYON LINEAMENT

Figure 9. Hydrogeologic features interpreted as potential flow barriers.
20 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Ground water recharged on the mountains in the 

northwestern part of this subregion is interpreted to 

move toward the central axes of adjacent valleys. In 

this subregion, water levels indicate that much of the 

ground-water flow appears to be controlled by the 

NE/SW-trending structural zones described by Carr 

(1984). Deep, regional interbasinal flow is unlikely 

because the subregion is mostly underlain by relatively 

impermeable, shallow Tertiary intrusive granites and 

crystalline Proterozoic rocks. The regional carbonate 

aquifer is believed to be extensive and continuous only 

in the southeastern part of the subregion (Grose, 

1983), near western Pahute Mesa and southern Sarcobatus Flat (fig. 10). Four dominant hydrostratigraphic 

sections contain the majority of flow in the basin: the 

Lida-Stonewall section, the Sarcobatus Flat section, 

the Grapevine Canyon–Mesquite Flat section, and the 

Oriental Wash section (fig. 10).

The Lida-Stonewall section (fig. 10) potentially 

receives underflow from Mud Lake and recharge from 

areas along the northern boundary of the subregion 

and transports it south to Sarcobatus Flat and Death 

Valley. This section contains playa areas at Stonewall 

Flat and near Lida Junction. The trace amounts of 

ground water evapotranspired in these areas are 

believed to be derived mostly from local surface water. 

The Sarcobatus Flat section (fig. 10) contains 

discharge areas at Sarcobatus Flat and Coyote Hole 

playas. These areas are believed to evapotranspire 

ground water that has moved along subregional flow 

paths. At Coyote Hole playas, impedance of groundwater flow by bedrock at shallow depths immediately 

south of the playas results in evapotranspiration of 

ground water that may originate on western Pahute 

Mesa (D'Agnese and others, 1997). As with the LidaStonewall section, a NE/SW-trending lineament may 

affect regional ground-water flow patterns. For 

example, ground water originating from Cactus and 

Gold Flats in the northeast may be forced to the 

surface and discharge at areas on, or adjacent to, the 

Sarcobatus Flat playa. Uncertainty also exists in the 

potential for ground-water flow through the Bullfrog 

Hills into the Amargosa Valley.

The Grapevine Canyon–Mesquite Flat section 

contains a major discharge area that includes Grapevine and Staininger Springs (at Scotty’s Castle). 

Discharge at these sites appears to originate as ground 

water that has flowed from the northeast to the southwest past Sarcobatus Flat (fig. 10) (D'Agnese and 

others, 1997). The springs also may result from structural and topographic controls. The intersection of the 

small-permeability, NW/SE-trending Death Valley 

fault with an apparently large-permeability NE/SWtrending structural zone (Carr, 1984) may result in the 

truncation of the large-permeability zone causing 

ground water to discharge. Ground water that does not 

discharge in Grapevine Canyon continues past this 

discharge area to flow down the axis of Death Valley 

to discharge at Mesquite Flat. Additional ground water 

that may enter the flow system from Saline Valley is 

also interpreted to discharge at Mesquite Flat.

The Oriental Wash section includes a small 

spring area at Sand Spring in northern Death Valley. 

These comparatively low-temperature and smallvolume springs appear to be discharging locally 

derived ground water recharged on the dominantly 

granitic mountains to the north. Ground-water flow is 

apparently directed toward the springs along the axis 

of Oriental Wash, which is associated with a NE/SWtrending structural zone (Carr, 1984), and the

Table 2. Divisions of the Death Valley regional 

ground-water flow system

NORTHERN DEATH VALLEY SUBREGION

Lida-Stonewall section

Sarcobatus Flat section

Grapevine Canyon–Mesquite Flat section

Oriental Wash section

CENTRAL DEATH VALLEY SUBREGION

Pahute Mesa–Oasis Valley ground-water basin

Southern Railroad Valley section

Kawich Valley section

Oasis Valley section

Ash Meadows ground-water basin

Pahranagat section

Tikaboo Valley section

Indian Springs section

Emigrant Valley section

Yucca–Frenchman Flat section

Specter Range section

Alkali Flat–Furnace Creek ground-water basin

Fortymile Canyon section

Amargosa River section

Crater Flat section

Funeral Mountains section

SOUTHERN DEATH VALLEY SUBREGION

Pahrump Valley section

Shoshone-Tecopa section

California Valley section

Ibex Hills section

Valjean Valley section
GROUND-WATER SYSTEM 21

discharges occur along the northern terminus of the 

Death Valley fault. Some ground water moving along 

this flow path may also bypass Sand Spring and flow 

down the axis of Death Valley toward Mesquite Flat. It 

is also inferred that ground water that potentially may 

enter the system as underflow from Eureka Valley may 

likewise flow down the axis of Death Valley toward 

Mesquite Flat (fig. 10).

Central Death Valley Subregion

In the Central Death Valley subregion, the dominant flow paths historically have been associated with 

major regional or subregional discharge areas. In this 

subregion, flow paths have traditionally been grouped 

into three ground-water basins, each containing 

several sections (fig. 11): Pahute Mesa–Oasis Valley 

ground-water basin, Ash Meadows ground-water

Figure 10. Northern Death Valley subregion.

Amargosa

River CALIFORNIA

NEVADA

A

B

C

D

Last Chance Range

Stonewall

Flat

Mesquite

Flat

Stonewall

Mtn

Gold

Mtn

Pahute 

Mesa

Cactus

Flat

Gold

Flat

Saline

Valley

Coyote

Hole

playas Sarcobatus Flat

Grapevine Mts

Panamint Range

Salt

Creek

Springs

Oriental Wash

Grapevine

Springs

Cottonwood Mts

Amargosa

Valley

Beatty

Lida

Junction

Montezuma Range

Palmetto

Mts

Staininger

Springs

Sand

Spring

Grapevine

Canyon

Bullfrog

Hills

118° 117° 116° 38°

37°

36° Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

EXPLANATION

GROUND-WATER SECTIONS—

LIDA-STONEWALL SECTION

SARCOBATUS FLAT SECTION

GRAPEVINE CANYON–MESQUITE FLAT SECTION

ORIENTAL WASH SECTION

ARROWS DESIGNATE DOMINANT REGIONAL

FLOW PATH ASSOCIATED WITH GROUND-WATER

SECTION DISCUSSED IN TEXT 

LOCATION OF SPRING

LOCATION OF POPULATED PLACE

SUBREGION BOUNDARY

GROUND-WATER SECTION BOUNDARY

POTENTIAL FLOW INTO OR

BETWEEN SUBREGIONS

0 20 40 MILES

0 20 40 KILOMETERS

D

C

B

A

Death 

Valley
22 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

CALIFORNIA

NEVADA

Amargosa Rive r

Furnace

Creek

Pahrump

Mercury

Kawich Valley

Emigrant Valley

Penoyer

Valley

Kawich Range Belted Range

Stone

Cabin

Valley

Cactus

Flat

Tikaboo Valley

Pahranagat Valley

Indian Springs Valley

Las

Vegas

Valley

Spotted Range

Sheep Range

Indian

Springs Cactus

Springs

Spring Mts

Pahrump

Valley

Specter

Range

Pahute 

Mesa

Yucca

Flat

Frenchman

Flat

Yucca Mtn

Timber

Mtn

Amargosa

Narrows

Shoshone

Mtn

Cottonball

Basin

Middle

Basin Ash Meadows

Funeral Mts

Sarcobatus Flat Bullfrog

Hills

Crater

Flat

Oasis Valley

Death Valley

Panamint Val

ley

Alkali

Flat

Carson Slough

Fortymile Canyon

1c

1b

2b

3b

2d

3d

2e

2f

2c

1a

2a 3a

Gold

Flat

Reveille

Valley Railroad Valley

Amargosa

Valley

Nopah Range

Beatty

Bare3c

Mtn Grapevine Mts

Badwater

Basin

117° 116° 115°

38°

37°

36°

ARROWS DESIGNATE DOMINANT REGIONAL

 FLOW PATH ASSOCIATED WITH GROUND-WATER

 SECTION DISCUSSED IN TEXT

LOCATION OF SPRING

LOCATION OF POPULATED PLACE

SUBREGION BOUNDARY

GROUND-WATER SECTION BOUNDARY

POTENTIAL FLOW INTO OR BETWEEN SUBREGIONS

GROUND-WATER BASINS AND SECTIONS—

(1) PAHUTE MESA–OASIS VALLEY

 GROUND-WATER BASIN

SOUTHERN RAILROAD VALLEY

KAWICH VALLEY SECTION

OASIS VALLEY SECTION

(2) ASH MEADOWS GROUND-WATER BASIN

PAHRANAGAT SECTION

TIKABOO VALLEY SECTION

INDIAN SPRINGS SECTION

EMIGRANT VALLEY SECTION

YUCCA–FRENCHMAN FLAT SECTION

SPECTER RANGE SECTION

(3) ALKALI FLAT–FURNACE CREEK 

 GROUND-WATER BASIN

FORTYMILE CANYON SECTION

AMARGOSA RIVER SECTION

CRATER FLAT SECTION

FUNERAL MOUNTAINS SECTION

a

b

c

a

b

c

d

e

f

a

b

c

d

0 20 40 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

0 20 40 KILOMETERS

EXPLANATION

Figure 11. Central Death Valley subregion.
GROUND-WATER SYSTEM 23

basin, and Alkali Flat–Furnace Creek ground-water 

basin (D'Agnese and others, 1997).

Pahute Mesa–Oasis Valley Ground-Water Basin

Ground water in the Pahute Mesa–Oasis Valley 

basin is derived dominantly from infiltration in the 

Kawich and Belted Ranges and Pahute Mesa 

(D'Agnese and others, 1997). Additional recharge may 

occur as regional ground water flows across system 

boundaries from southern Railroad, Reveille, and 

Stone Cabin Valleys (#1, #2, and #3 in fig. 7; fig. 11). 

Because the western boundary of this basin is poorly 

defined, ground water in the western part of the basin 

(parts of Cactus and Gold Flats) may flow toward the 

eastern part of Sarcobatus Flat. This ground-water 

basin has three dominant sections: the southern Railroad Valley section, the Kawich Valley section and the 

Oasis Valley section (fig. 11).

Ground water in the southern Railroad Valley 

section originates either as recharge on the flanking 

mountains or as underflow from northern Railroad 

Valley (D'Agnese and others, 1997). Ground water in 

the section dominantly flows south and southwest 

toward Kawich Valley (fig. 11).

Ground water in the Kawich Valley section may 

flow toward a potentiometric-surface trough located 

under western Pahute Mesa (Waddell and others, 

1984). A possible regional fault or fracture zone has 

been described at this location (Blankennagel and 

Weir, 1973). Water flowing along this trough toward 

Oasis Valley is a primary feature of this section. At 

Oasis Valley, ground water is discharged by evapotranspiration and spring flow. Ground water that does 

not discharge within Oasis Valley flows through the 

subsurface at the Amargosa Narrows south of Beatty 

and into the Alkali Flat–Furnace Creek ground-water 

basin (Waddell, 1982) (fig. 11). Small amounts of 

ground water in Oasis Valley also may flow toward 

Crater Flat under Bare Mountain.

Ash Meadows Ground-Water Basin

The Ash Meadows ground-water basin is the 

largest basin in the Central Death Valley subregion 

(Waddell, 1982). Much of the ground water in this 

basin is derived from infiltration on the mountain 

ranges that surround the basin. Additional recharge 

may occur as regional ground water flows across 

system boundaries at Pahranagat Valley (Winograd 

and Thordarson, 1975) (#9, fig. 7). This basin is 

subdivided into six sections (fig. 11): Pahranagat 

section, Tikaboo Valley section, Indian Springs 

section, Emigrant Valley section, Yucca–Frenchman 

Flat section, and Specter Range section.

Ground water recharged on the mountain areas 

of this ground-water basin travels mostly toward the 

center of the basin approximately located near the 

Spotted Range. Ground water in Tikaboo Valley, 

Emigrant Valley, and Yucca and Frenchman Flats is 

interpreted as flowing toward a potentiometric trough 

(D'Agnese and others, 1998). Regional and subregional ground water recharged on the Sheep Range 

and Spring Mountains also flows into this potentiometric trough from the south and east, thereby contributing to ground-water flow past Indian Springs Valley 

(fig. 11).

The potentiometric-surface trough may be a 

zone of high permeability associated with the Spotted 

Range–Mine Mountain structural zone (Carr, 1984; 

D'Agnese and others, 1998) and is believed to include 

numerous regional faults and fractures (Winograd and 

Thordarson, 1975). The trough is bounded on the 

south and southeast by the Las Vegas Valley shear 

zone (fig. 9). The shear zone may contain low-permeability material, possibly fault gouge (Winograd and 

Thordarson, 1975), causing discharge at Indian and 

Cactus Springs. The flow paths along the trough are 

directed through the Specter Range area until they 

encounter a NW/SE-trending fault at Ash Meadows. 

This fault causes much of the ground water to be 

discharged as spring flows and evapotranspiration 

(Winograd and Thordarson, 1975). Ground water that 

does not discharge at Ash Meadows flows into the 

Alkali Flat–Furnace Creek ground-water basin where 

it mixes with ground water moving along regional and 

subregional flow paths (fig. 11).

Alkali Flat–Furnace Creek Ground-Water Basin

In this basin, ground water is derived from infiltration on Pahute Mesa, Timber Mountain, Shoshone 

Mountain, and the Grapevine and Funeral Mountains. 

Additional recharge to this basin may occur as interbasinal ground-water flow across basin boundaries 

from Sarcobatus Flat, Oasis Valley, and Ash Meadows. 

The Alkali Flat–Furnace Creek ground-water basin is 

divided into four sections (fig. 11): Fortymile Canyon 

section, Amargosa River section, Crater Flat section, 

and Funeral Mountains section.
24 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Locally recharged ground water moves toward 

discharge areas in the southern parts of the basin. 

Dominant ground-water flow paths appear to mimic 

surface-water flow. The surface-water flow seems to 

be structurally controlled along the Amargosa River 

and Fortymile Wash and in Crater Flat (Grose and 

Smith, 1989). In the northwestern portions of the 

basin, subregional ground-water movement is dominantly lateral and downward toward regional flow 

paths (Czarnecki and Waddell, 1984; Sinton, 1987; 

and Kilroy, 1991). Near Yucca Mountain, however, 

gradients are dominantly upward into the volcanic 

units (Luckey and others, 1996). In the south-central 

portions of the basin, near the Nevada-California 

border (fig. 11), regional ground-water movement is 

dominantly upward from carbonate units into the 

subregional system and toward discharge areas along 

the Amargosa River, Carson Slough, and Alkali Flat 

(Czarnecki and Waddell, 1984; Czarnecki, 1990).

In the southern Amargosa Valley, regional 

ground-water movement is toward the southwest and 

south. This ground water may either flow through fractures in the southeastern end of the Funeral Mountains 

and discharge at Furnace Creek or flow southward and 

discharge at Alkali Flat (Czarnecki and Waddell, 1984; 

Czarnecki and Wilson, 1991). Once past the springs at 

Furnace Creek, the ground water flows toward the 

saltpan and is discharged either by stands of mesquite 

on the lower part of the Furnace Creek fan or by evaporation from the Death Valley saltpan. Additional 

ground water entering Death Valley as underflow from 

Panamint Valley may also discharge at the saltpan.

Ground water that moves along shallow flow 

paths, but does not discharge at the Amargosa River, 

Carson Slough, or Ash Meadows, moves southward 

toward Alkali Flat where it discharges as spring flow 

and evapotranspiration. Walker and Eakin (1963) estimated that approximately 1,700 m3/d of ground water 

flows past Alkali Flat into the southern Death Valley 

subregion.

Southern Death Valley Subregion

In general, ground water in the Southern Death 

Valley subregion is derived primarily from infiltration 

on the Spring Mountains and small inputs from the 

Nopah, Kingston, and Greenwater Ranges (fig. 12). 

Additional minor ground-water flow into this subregion may occur across the boundary from the 

Alkali Flat–Furnace Creek basin south of Alkali Flat, 

and farther south from Pilot Knob Valley (fig. 7). The 

subregion contains five sections (fig. 12): Pahrump 

Valley section, Shoshone-Tecopa section, California 

Valley section, Ibex Hills section, and Valjean Valley 

section.

Ground water recharged on the Spring Mountains moves toward Pahrump Valley. Historically, 

springs discharged at Manse and Bennett Springs 

along the base of the broad alluvial fans at the foot of 

the Spring Mountains. Pumping of ground water in the 

valley had caused these springs to cease flowing. More 

recently (late 1990’s) Manse Spring has begun to flow 

again, perhaps due to changes in agricultural practices 

in the valley. Ground water in the Pahrump Valley 

section flows dominantly along subregional flow paths 

either to the west toward Stewart Valley and the 

northern end of Chicago Valley, or to the southwest 

toward California Valley. Because the Nopah Range is 

composed of low-permeability quartzite rocks in the 

subsurface (Grose, 1983), it is believed to cause a 

bifurcation in ground-water flow. Some of the ground 

water flowing toward the south and west is discharged 

at Stewart and Pahrump Valley playas. Some of this 

ground water may also discharge at the southern end 

of Ash Meadows at Big, Bole, and Last Chance 

springs (Winograd and Thordarson, 1975, p. 91; 

Peterman and Stuckless, 1992a, p. 70; Peterman and 

Stuckless, 1992b, p. 712). Ground-water flow that 

continues toward Chicago Valley, within the Shoshone-Tecopa section, mixes with ground water 

flowing from south of Alkali Flat and ultimately 

discharges as spring flows and ET in an area between 

the towns of Shoshone and Tecopa. In the California 

Valley section, ground water that flows south from 

Pahrump Valley discharges south of Tecopa at springs 

along the Amargosa River canyon in the Sperry Hills 

and at China Ranch (W.C. Steinkampf, U.S. Geological Survey and W.E. Werrell, National Park Service, 

written commun., 1996).

Ground water that does not discharge at the 

Shoshone-Tecopa area may continue flowing to the 

southwest into the Ibex Hills section to be discharged 

as spring flow and ET in the Saratoga Springs area, 

which includes adjacent areas of shallow ground water 

along the flood plain of the Amargosa River. Some 

additional ground water may enter the basin from 

Valjean and Shadow Valleys into California Valley and 

discharge at Saratoga Springs. A small amount of flow 

may continue north past Saratoga Springs to discharge 

at the Badwater Basin saltpan (fig. 12). Additional
GROUND-WATER SYSTEM 25

Amargosa

River

CALIFORNIA

NEVADA

A

B

C

D

E

Badwater Basin

Ibex Hills

Salt Springs

Hills

Amargosa River

Canyon

 Ash Meadows

Alkali

Flat

Nopah Range Stewart Valley

Spring Mts

Pahrump

Valley

Bennett

Spring

Saratoga

Springs

Manse

Spring

California

Valley

Chicago Valley

Tecopa

Shoshone

China

Ranch

Greenwater Range

Kingston

Range

Mesquite

Valley

Silurian

Valley

Valjean

Valley Shadow

Valley

Sperry

Hills

115° 117° 116°

36°

35°

GROUND-WATER SECTIONS—

LOCATION OF SPRING

LOCATION OF POPULATED PLACE

A

B

C

D

E

0 20 40 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

0 20 40 KILOMETERS

Pahrump Valley Section

Shoshone-Tecopa Section

California Valley Section

Ibex Hills Section

Valjean Valley Section

EXPLANATION

ARROWS DESIGNATE DOMINANT REGIONAL

 FLOW PATH ASSOCIATED WITH GROUND-WATER

 SECTION DISCUSSED IN TEXT 

SUBREGION BOUNDARY

GROUND-WATER SECTION BOUNDARY

POTENTIAL FLOW INTO OR

BETWEEN SUBREGIONS

Figure 12. Southern Death Valley subregion.
26 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

ground water entering Death Valley as underflow 

from Pilot Knob Valley may also discharge at the 

saltpan.

Ground-Water System Budget 

Components

Because inflow and outflow volumes are poorly 

defined for many areas in the Death Valley region, the 

water budget for the ground-water flow system is difficult to compute. The large size of this basin precludes 

the comprehensive and accurate assessment of all 

inflows to and outflows from the system. Despite this 

difficulty, several investigators have attempted to estimate water budgets for various parts of the flow 

system (Walker and Eakin, 1963; Rush, 1968; Malmberg and Eakin, 1962; Glancy, 1968; Malmberg, 1967; 

Hunt and others, 1966; Miller, 1977; Harrill, 1986; 

Waddell, 1982; Rice, 1984; Harrill and others, 1988; 

Dettinger, 1989; D'Agnese and others, 1997). These 

investigators used different boundaries to define the 

various basins in the flow system. Therefore, little if 

any of the data can be adequately compared.

Under natural, steady-state, predevelopment 

conditions, ground-water recharge occurs through 

infiltration of precipitation and spring flows and as 

underflows from adjacent basins, while ground-water 

discharge occurs through springs and ET. These 

processes account for the only ground-water system 

inflows and outflows. A discussion of how these 

ground-water flow system components were estimated 

for the simulation follows.

Estimates of Ground-Water Discharge

In arid regions such as the DVRFS, ET is a 

major process by which water is discharged from the 

ground-water flow system. Water stored within 

shallow unconfined aquifers is available for use by the 

local vegetation and at the water table is in contact 

with the atmosphere. Much of the water being evaporated or transpired from the DVRFS is constrained to 

local areas of ground-water discharge. The vast 

majority of the water evapotranspiring within these 

discharge areas originates as spring or seep flow or as 

diffuse upflow from underlying regional confined 

aquifers.

Evapotranspiration has been estimated for most 

of the major discharge areas of the DVRFS (Malmberg 

and Eakin, 1962; Walker and Eakin, 1963; Pistrang 

and Kunkel, 1964; Hunt and others, 1966; Malmberg, 

1967; Cardinalli and others, 1968; Glancy, 1968; 

Rush, 1968; Winograd and Thordarson, 1975; Miller, 

1977; Harrill, 1986; Czarnecki, 1997; D'Agnese and 

others, 1997; Laczniak and others, 1999; Laczniak and 

others, 2001). Most ET estimates were computed by 

multiplying the area of the land surface determined to 

be undergoing evapotranspiration by some average ET 

rate estimated for the area’s soil and vegetation. 

Discrepancies in these estimates typically result from 

differences in methods used to delineate areas or in 

methods used to estimate ET rates.

Many of the earlier estimates were developed as 

part of reconnaissance studies of the ground-water 

resource. Typically these studies were constrained to 

one or two discharge areas and relied on ET rates 

determined for vegetation from arid and semiarid areas 

outside the DVRFS. The most recent and comprehensive study of ET in the DVRFS is by Laczniak and 

others (2001). The primary focus of their study was to 

estimate, in a consistent manner, the average annual 

ET from the major areas of natural ground-water 

discharge within the Death Valley region shown in 

figure 13.

Laczniak and others (2001) identified and delineated ET areas in the DVRFS based on a classification 

of spectral satellite data. Each was classified into ET 

units that represent a unique combination of soil and 

vegetation conditions. As part of their study, these 

investigators estimated an average annual rate of ET 

for each identified ET unit within a discharge area. 

Estimates were computed from micrometeorological 

data collected within the local area over a multiyear 

period. To estimate the ground-water component of 

annual ET, they estimated a ground-water ET rate by 

subtracting that water assumed to originate from local 

precipitation. Their estimates of ground-water ET 

rates and spatial distributions of ET units were used in 

this study to develop an overall estimate of groundwater discharge from the DVRFS.

For discharge areas not evaluated in Laczniak 

and others (2001), ET distributions or average volumetric discharge from regional springs was developed 

from data available in published and in-progress 

studies. In Pahrump Valley, a map showing the distribution of phreatophytes in 1959–61 (Malmberg, 1967, 

pl. 3) was assumed to be a reasonable representation of 

the ET units prior to the start of ground-water development in the Pahrump Valley discharge area. Ground-
GROUND-WATER SYSTEM 27

water ET rates for these units were estimated from 

rates for similar ET units found in nearby areas given 

in Laczniak and others (2001). An ET-unit map developed from spectral imagery as part of an ongoing 

study of evapotranspiration in Death Valley (DeMeo 

and others, 1999) was used to delineate ET units in the 

Death Valley discharge area. Ground-water ET rates 

for these units were computed from micrometeorological data being collected in Death Valley as part of 

the ongoing study. In Penoyer Valley, the distribution 

and rate of ET used in the DVRFS model was taken 

directly from the UGTA regional ground-water flow 

model (U.S. Department of Energy, 1997) (fig. 13).

Four isolated areas where large volumes 

of ground-water discharge can be accounted for by 

spring flow are also simulated in the DVRFS. These 

include: Grapevine and Staininger Springs located 

near Scotty’s Castle; Texas, Travertine, and Nevares 

Springs located at Furnace Creek Ranch; Indian and 

Cactus Springs located in Indian Springs, Clark 

County, Nevada; and Corn Creek Springs also at the 

north end of Las Vegas Valley (fig. 13).

Discharge volumes for these springs are taken 

from published reports. Discharge at Staininger 

Spring, the water supply for the Scotty’s Castle area in 

Death Valley, has been reported in several publications 

(Ball, 1907; Waring, 1915; Waring, 1965; Rush, 1968; 

and Miller, 1977). Miller (1977) reports a measurement of 188 gallons per minute (gpm) (1,025 m3/d) in 

1971 and references three other measurements, 

187 gpm (1,019 m3/d) in 1924, 180 gpm (981 m3/d) in 

1958, and about 200 gpm (1,090 m3

/d) in the spring of 

1967 as reported by Rush (1968). Ball (1907) reports 

approximately 600,000 gal/d (417 gpm, 2,271 m3/d). 

Waring (1915) reports a discharge of 10 gpm (54 

m3/d) that was estimated or reported. Waring (1965) 

reports a discharge of 30 gpm (163 m3

/d). Mendenhall 

(1909) mentions the spring has an unlimited supply of 

good water but does not report a discharge. The actual 

measurements reported by Miller (1977) and Rush 

(1968) are probably the best to estimate an average 

discharge. The mean of the four measurements 

[(188+187+180+200)/4 = 188.75] is approximately 

190 gpm (1,035 m3/d).

Discharge at Grapevine Springs has only been 

measured by Miller (1977). Miller reported an aggregate discharge of 450 gpm (2,450 m3/d) for the dozen 

or more springs and seeps. Miller also included 

evapotranspiration from 150 acres of phreatophytes in 

the discharge. Ball (1907) and Mendenhall (1909) 

mention the spring by name but do not report a 

discharge. Rush (1968) estimates the spring discharge 

of 20 gpm (109 m3/d) by guessing. Perhaps the best 

estimate for discharge is presented by Miller, 

450 gpm (2,450 m3/d), and that value is used in the 

DVRFS simulation.

Discharge at Texas Spring has changed since 

the spring was improved by tunneling into the spring 

area. Waring (1915) lists a discharge of 25 gpm 

(136 m3/d). Pistrang and Kunkel (1964) report a 

National Park Service estimate of 0.28 ft3

/s 

(685 m3/d) in 1926. The tunnel was constructed 

between 1926 and 1941. Pistrang and Kunkel report a 

National Park Service discharge estimate of 0.38 ft3

/s 

(930 m3

/d) in 1941. Pistrang and Kunkel report 

numerous discharges from 1956 to 1963 measured by 

the USGS with an 18-inch rectangular weir. The 

1956–63 discharges generally ranged from 0.47 to 

0.50 ft3/s (1,150 to 1,223 m3/d). Miller (1977) reports 

a discharge of 210 gpm (1,145 m3/d) on December 8, 

1976. Discharges for 1989–92 (LaCamera and Westenburg, 1994), 1993 (Hale and Westenburg, 1995), 

1994 (Westenburg and LaCamera, 1996), 1995 

(LaCamera and others, 1996), 1996 (LaCamera and 

Locke, 1997) were measured by the National Park 

Service with a flume and by the USGS with a current 

meter. Measurements for 1990–92 were revised 

(LaCamera and Locke, 1997). All of these measurements are effectively within 15 percent of 1,220 m3

/d 

and that value is used in the DVRFS simulation.

Pistrang and Kunkel (1964) report on Travertine 

Springs during the period 1956–64. Data collected by 

the USGS ranged from 1.68 to 1.94 ft3

/s (4,111 to 

4,747 m3/d). Miller (1977) attributed short-term 

fluctuations in spring discharge to the condition and 

efficiency of the collection system and to the variation 

in ET losses in the collector area. Miller reports a 

long-term discharge of 850 gpm (4,633 m3

/d) during 

the period 1956–72, with no apparent change in longterm trend. Miller reports only a flow of 700 gpm 

(3,815 m3/d) from an aggregate of springs for January 

6, 1977. Pistrang and Kunkel data are for 10 springs, 

while Miller’s 1977 measurement is for 3 springs. No 

explanation is given for the change in the number of 

springs. Miller’s 1977 measurement cannot be directly 

compared to Pistrang and Kunkel’s data or to Miller’s 

long-term average. All of Pistrang and Kunkel’s data 

fall within 10 percent of Miller’s long-term average 

except for the March 6, 1958, measurement. One 

month later the spring was cleaned out and was within
28 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

CALIFORNIA

NEVADA

Amargosa

Rive r

Mojave Desert

Indian

Springs

Cactus

Springs

Grapevine

Springs

Nevares

Springs

Texas

Spring

Travertine

Springs

Corn Creek

Springs

Penoyer

Valley

Scotty's Castle Area

35°

118° 117° 116° 115°

38°

37°

36°

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

DEATH VALLEY REGIONAL 

 FLOW SYSTEM BOUNDARY

PHREATOPHYTE AREAS DELINEATED

 BY MALMBERG (1967)

EVAPOTRANSPIRATION AREAS

 (LACZNIAK AND OTHERS, 2001)

NEVADA TEST SITE BOUNDARY

Figure 13. Ground-water discharge areas in the Death Valley regional flow system.
GROUND-WATER SYSTEM 29

10 percent of the average discharge. The low discharge 

on March 6, 1958, can probably be neglected. Miller’s 

long-term average 850 gpm (4,633 m3

/d) is probably a 

good estimate of average discharge.

Pistrang and Kunkel (1964) report on the 

Nevares Spring discharge during 1951 and 1956–57. 

Average discharge during 1956–57 was 0.6 ft3

/s 

(1,470 m3

/d) for the main spring area. Discharge at the 

remaining Nevares Springs areas totaled 0.12 ft3

/s 

(290 m3

/d). Hunt and others (1966) report the 

combined discharge of five points collected by the 

USGS. The discharge was 0.73 ft3

/s (1,790 m3/d) in 

1951 and 0.72 ft3/s (1,760 m3/d) in 1957. Miller 

(1977) reports a combined discharge of 260 gpm 

(1,420 m3

/d) for Nevares Spring and a nearby 

unnamed spring. Miller’s discharge is comparable to 

Pistrang and Kunkel’s and is within 10 percent of the 

1956–57 average of Pistrang and Kunkel. The 

discharge value 0.77 ft3/s (1,884 m3/d) is probably the 

best discharge for all the combined Nevares Springs 

and is used in the DVRFS simulation.

Carpenter (1915) was the first to estimate 

discharge at Indian and Cactus Springs. He reports a 

discharge of 0.91 ft3

/s (2,230 m3/d) on December 15, 

1912. Maxey and Jameson (1948) estimated 400 gpm 

(2,180 m3

/d), Malmberg (1967) estimated 700 acrefeet per year (acre-ft/yr) (2,365 m3

/d), and Rush 

(1970) estimated 500 acre-ft/yr (1,690 m3/d). The 

discharge estimates of Maxey and Jameson and Malmberg are within 10 percent of Carpenter’s discharge. 

Rush’s estimate appears to be over 20 percent underestimated with respect to the 1912 measurement or it 

represents a decline in discharge due to other groundwater withdrawals in the Indian Springs area. Rush 

(1970) has no discussion on the subject; he just reports 

the estimate. A good average estimate for Indian and 

Cactus Springs discharge used in the DVRFS simulation is 2,240 m3/d.

Carpenter (1915) also has the first reported 

discharge for Corn Creek Spring; he reports a 

discharge of 0.2 ft3

/s (490 m3/d) on December 8, 1912. 

Malmberg (1965) reports yearly discharge values of 

240 acre-ft/yr (811 m3

/d) for 1946 and 200 acre-ft/yr 

(676 m3

/d) for each year from 1947 to 1955. From 

1985–94 and 1997, daily discharges are reported in the 

USGS annual Water Resources Data report for Nevada 

(Pupacko and others, 1988; Pupacko and others, 

1989a; Pupacko and others, 1989b; Pupacko and 

others, 1990; Bostic and others, 1991; Garcia and 

others, 1992; Hess and others, 1993; Emett and others, 

1994; Clary and others, 1995; and Bonner and others, 

1998). Malmberg’s discharge estimate of 676 m3

/d for 

1947–55 appears to be a good estimate. The majority 

of the 1985–94 and 1997 discharge data are within 

15 percent of this estimate. Carpenter’s 1912 discharge 

is low compared to Malmberg’s estimate. There is 

no supporting information to evaluate the 1912 data 

and compare it to the later data. The 1985–94 and 

1997 discharge data show some variation with time 

but generally are within 15 percent of Malmberg’s 

discharge. An average estimated discharge of 

676 m3/d is used in the DVRFS simulation. 

A summary of discharge observation descriptions and observed ground-water discharge rates in the 

DVRFS are presented in table 3. Figure 13 shows all 

ground-water discharge areas simulated in the 

DVRFS.

Estimates of Recharge

The major source of recharge to the regional 

ground-water flow system is from precipitation on the 

higher mountains within the region. The regional flow 

system is also recharged by some interbasin flow from 

outside of the DVRFS. Some recharge also results 

from recycled irrigation and domestic waters as well 

as seepage of spring discharge back into the groundwater system (Rice, 1984). These recycled components are considered to be small compared to regional 

infiltration and interbasin flux volumes and are 

neglected (Waddell, 1982, p. 14).

Empirical, water-balance, and distributedparameter methods have been used to characterize the 

location and amount of recharge in the Death Valley 

region. Each method attempts to characterize the 

complex array of factors controlling recharge; each 

has limitations.

An empirical precipitation-recharge relation was 

developed by Maxey and Eakin (1950) from water 

mass-balance estimates for basins in southern and 

eastern Nevada. They suggest that the annual precipitation amount and the percentage of precipitation that 

becomes ground-water recharge increases with 

increasing altitude. Depending on the valley, Maxey 

and Eakin (1950) assumed that no recharge occurs 

where mean annual precipitation is less than about 

200 millimeter (mm) or altitude is lower than 

1,524 m. Above 1,524 m in altitude, they assigned an 

increasing percentage of precipitation that was 

assumed to become recharge to a ranked series of 

305-m altitude intervals (table 4).
30 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Table 3. Description of flow observations with observed discharge and coefficient of variation used in the calibrated 

model

Observation description Observation 

name

Observed discharge, 

in cubic meters

per day

Coefficient of 

variation

Ash Meadows, Amargosa Flat G-AM-AMFLT 6,019 10.32

Ash Meadows, Carson Slough G-AM-CARSL 498 0.50

Ash Meadows, central area G-AM-CENTR 21,444 10.15

Ash Meadows, upper drainage G-AM-UPDRN 3,219 0.15

Ash Meadows, northern area G-AM-NORTH 19,499 10.14

Ash Meadows, southern area G-AM-SOUTH 10,085 10.23

Chicago Valley G-CHICAGOV 1,452 10.36

Corn Creek Springs G-CORNCREK 676 1.00

Death Valley, Badwater basin area G-DV-BADWT 5,019 0.28

Death Valley, Confidence Hills area G-DV-CONFI 6,651 0.28

Death Valley, Cottonball basin area G-DV-COTTN 3,547 0.28

Death Valley, Furnace Creek alluvial fan G-DV-FRNFN 10,185 0.28

Death Valley, Mesquite Flat area G-DV-MESQU 29,075 0.28

Death Valley, Middle basin G-DV-MIDDL 2,587 0.28

Death Valley, Mormon Point area G-DV-MORMN 7,225 0.28

Death Valley, Nevares Springs G-DV-NEVAR 1,884 0.15

Death Valley, Saratoga Springs area G-DV-SARAT 6,535 0.28

Death Valley, Texas Spring G-DV-TEXAS 1,220 0.15

Death Valley, Travertine Springs G-DV-TRVRT 4,633 0.10

Death Valley, western alluvial fans G-DV-WESTF 13,637 0.28

Franklin Well area G-FRANKWEL 1,182 0.50

Franklin Lake, eastern area G-FRNKLK-E 411 1.00

Franklin Lake, northern area G-FRNKLK-N 2,254 1.00

Franklin Lake, southern area G-FRNKLK-S 711 1.00

Grapevine Springs, Scotty’s Castle area G-GRAPE-SC 1,035 0.50

Grapevine Springs, spring area G-GRAPE-SP 2,450 0.20

Indian Springs and Cactus Springs G-INDIANSP 2,240 0.10

Oasis Valley, Beatty area G-OV-BEATY 2,774 10.13

Oasis Valley, Coffer’s Ranch area G-OV-COFFR 5,343 10.19

Oasis Valley, middle Oasis Valley area G-OV-OASIS 3,157 10.10

Oasis Valley, Springdale area G-OV-SPRDL 8,113 10.10

Pahrump Valley, Bennett Spring area G-PAH-BENT 16,753 0.50

Pahrump Valley, Manse Spring area G-PAH-MANS 5,375 0.50

Penoyer Valley area G-PENOYERV 12,833 0.50

Sarcobatus Flat, Coyote Hills area G-SARCO-CH 1,503 10.55

Sarcobatus Flat, northeastern area G-SARCO-NE 30,421 10.46

Sarcobatus Flat, southwestern area G-SARCO-SW 11,960 10.53

Shoshone basin, northern area G-SHOSH-N 2,259 10.16

Shoshone basin, southern area G-SHOSH-S 4,831 10.15

Stewart Valley, predominantly playa area G-STEWRT-P 995 0.16

Stewart Valley, predominantly vegetation area G-STEWRT-V 2,381 10.56

Tecopa basin, Amargosa Canyon area G-TC-AMCAN 3,394 10.13

Tecopa basin, China Ranch area G-TC-CHRNC 1,784 0.50

Tecopa basin, Resting Spring area G-TC-RESTS 2,537 10.16

Tecopa basin, Sperry Hills area G-TC-SPERY 1,341 0.50

Tecopa basin, central area G-TC-TECOP 12,221 10.12 1

 Value provided by Geotrans, Inc. (written commun., September 1999)
GROUND-WATER SYSTEM 31

Other investigators working in the Great Basin 

developed similar area-altitude relations for their 

studies (Walker and Eakin, 1963; Miller, 1977; Malmberg and Eakin, 1962; Malmberg, 1967; Winograd and 

Thordarson, 1975; Harrill, 1986). Each investigator 

noted that the method was completely empirical and 

ignored differences in lithology, soils, climate, vegetation, and topographic factors such as exposure, aspect, 

and slope.

Rice (1984) developed a relatively detailed 

recharge calculation as part of a regional modeling 

study. Rice employed a water-balance method that 

utilized average annual precipitation distributions 

derived from Quiring's (1965) regressions and potential ET estimates developed from empirical methods. 

These computations showed that recharge occurs if the 

altitude is greater than 1,675 m and the annual precipitation is greater than 254 mm. Because these two 

criteria meet the plant requirements of pinyon pine and 

juniper, Rice placed the area of recharge to coincide 

with that plant zone and plant zones of higher altitude 

and moisture. More recent investigations by West 

(1989) have placed the lower altitude limits of regional 

recharge at the Mixed Shrub-Transition Zone where 

blackbrush flourishes. West noted that these communities represent the altitudinal zone at which winter 

recharge of soil moisture is normally balanced by 

summer ET loss. Therefore, in wet years some 

regional recharge may occur in these areas.

Recent investigators have attempted to refine the 

Maxey-Eakin method by adapting it to make it more 

sensitive to the physical land-surface factors affecting 

recharge. For the YMP regional model, D'Agnese and 

others (1997) used comparative hydrogeologic data 

sets to enhance the Maxey-Eakin method. Several 

recharge indicators were identified, including (1) 

elevation, (2) slope aspect, (3) rock type, and (4) vegetation. These data sets were classified into four maps 

describing recharge potential on a five-point scale, 

where 1 represents low recharge potential and 5 represents high recharge potential. The four recharge-potential maps were then grouped to develop a final map 

containing six classes: high, high-moderate, moderate, 

moderate-low, low, and no recharge potential. The 

areas most likely to have high recharge potential were 

those that possessed favorable conditions for all four 

recharge indicators.

The six recharge-potential classes were similar 

to those used in the Maxey-Eakin method in that they 

were used in the ground-water flow model to represent 

a percentage of average annual precipitation that is 

recharged. However, during calibration of the YMP 

model, parameter sensitivities indicated that perhaps 

only four zones representing high, moderate, low, and 

no recharge potential could be estimated given the 

model constraints supplied by hydraulic head and 

spring flow observations. In the final YMP regional 

model, recharge occurred in zones of 0 percent, 

1 percent, 3 percent, and 23 percent average annual 

precipitation.

For the UGTA regional model, the amount of 

recharge was calculated using a modification of the 

Maxey-Eakin method. In and near the NTS, the 

recharge was modified to account for runoff and 

recharge below stream channels and in distributary fan 

deposits. This primarily resulted in simulated recharge 

beneath Fortymile Canyon and Fortymile Wash. The 

Maxey-Eakin coefficients and ranges then were 

adjusted to produce a recharge rate equal to the algebraic sum of the other model fluxes, while producing 

recharge in lower elevation areas such as Shoshone 

Mountain and the Bullfrog Hills.

For this investigation, a distributed-parameter, 

process-based, numerical, net infiltration model developed by Hevesi and others (2002) is used. Their 

numerical model, INFIL, was developed using components of the mass-balance equation. The numerical 

model is quasi-three-dimensional and uses quasimass-balance processes, real or stochastically simulated precipitation, parameters describing the physical 

setting, and hydrologic properties of the Death Valley 

region to approximate actual conditions. The model is 

calibrated by matching simulated daily mean streamdischarge rates to the measured daily mean discharge 

resulting from episodic runoff events at calibration 

basins. In addition to measured stream discharge, the 

model was calibrated using volumetric water content 

data from neutron boreholes that were used to calcuTable 4. Area-altitude classes modified from Maxey 

and Eakin (1950)

[m, meters; datum is sea level; >, greater than; <, less than]

Area-altitude

class

Precipitation 

percentage that 

becomes recharge

> 2,439 m 25

2,134 – 2,439 m 15

1,829 – 2,134 m 7

1,524 – 1,829 m 2

< 1,524 m 0
32 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

late flux. The model results in a calculated distribution 

of net infiltration for the region. In this manner the 

results of the model substantiate a conceptual model of 

net infiltration that is based on the physical setting and 

hydrologic properties of the region. The distribution of 

regional net infiltration can be used to represent 

regional ground-water recharge by assuming that 

water that is simulated to infiltrate past the “root zone” 

ultimately becomes ground-water recharge (fig. 14).

Net infiltration for the DVRFS based on the 

INFIL model developed by Hevesi and others (2002), 

however, suggests that estimated recharge in the 

DVRFS exceeds the estimated ground-water discharge 

for the basin by almost a factor of four. Limitations of 

the net infiltration model that may contribute to this 

overestimation include:

1. Processes between the root zone and the water table 

are not taken into account,

2. Defined bedrock permeability could be unrealistically large,

3. ET was underestimated in upland areas with thin 

soil,

4. Precipitation record for 1980–95 used in the model 

was higher than the long-term average,

5. Rainfall extrapolation algorithm overestimated 

rainfall in many areas, 

6. Soil moisture properties were overly generalized,

7. Errors in saturated and unsaturated hydraulicconductivity estimates may have affected certain 

soil types and climate conditions, and

8. All natural-system processes and conditions, many 

of which act to reduce infiltration, could not be 

simulated with the simplifications in the model.

The net infiltration model was sensitive to simulated soil and rock properties, precipitation rates, and 

simplifications. These model limitations tended to 

overestimate net infiltration. Ultimately, rates of net 

infiltration could be overestimated because significant 

quantities of water could be impeded or redistributed 

by relatively unknown processes in the unsaturated 

zone before crossing the water table; therefore, some 

means of evaluating the potential effects of this INFIL 

model simplification must be developed during the 

DVRFS model simulation.

NUMERICAL MODEL OF REGIONAL 

GROUND-WATER FLOW

Development of a numerical model allows for a 

more detailed analysis of the movement of ground 

water through the numerous hydrogeologic units that 

constitute the shallow, local, and subregional groundwater-flow system components and the dominant 

regional flow system controlled by the carbonate 

aquifer. The resulting DVRFS model is necessarily 

large and complex. Calibration of this model by 

strictly trial-and-error methods was judged to be both 

ineffective and inefficient; therefore, nonlinear regression methods are used to estimate parameter values 

that produce the best fit to observed heads and flows. 

Related methods are used to evaluate model results.

Numerical Modeling Difficulties, Simplifications, and Assumptions 

Previous studies by Prudic and others (1993) 

and Waddell (1982) showed that it is difficult to utilize 

computer models to effectively describe ground-water 

flow in an area as geographically large and geologically complicated as the Death Valley region. Prudic 

and others (1993) reiterated that many arguments can 

be invoked concerning the validity of the assumptions 

and hydrologic values used in simulating groundwater flow when such complex geology and hydrology 

are involved.

Inevitably, simplifications and assumptions 

must be used to adapt the complex conceptual model 

for numerical simulation. The assumptions and simplifications used to develop the DVRFS model include 

the following:

1. Ground water in the region flows through fractures 

and solution openings of consolidated rocks, as 

well as in porous valley-fill alluvium. However, 

fracture-flow simulation is impractical at a 

regional scale; therefore, a porous medium simulation is used. This assumption is probably 

reasonable because the (1,500-m cell size) model 

grid used to simulate regional flow results in the 

averaging of hydraulic properties over 2.25-km2

areas. Despite the seemingly large size of the 

model grid, one grid cell is quite small when 

compared to the overall area of 69,840 km2

. 

Some of the factors considered when determining 

an appropriate model-cell size were computational efficiency, appropriately representing the 

available data, and ability to effectively simulate 

regional-scale ground-water flow.

2. The model simulation assumes steady-state conditions prior to development, in which estimates of 

recharge (calculated from data gathered between 

1980 and 1995) equal estimates of natural 

discharge prior to ground-water development.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 33

Death 

CALIFORNIA

NEVADA

Valley

35°

118° 117° 116° 115°

36°

37°

38°

NEVADA TEST SITE

 BOUNDARY

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon

RECHARGE GRID (millimeter per year)

0.01–1

1–5

5–10

10–25

25–50

50–100

100–150

150–200

DEATH VALLEY

 REGIONAL FLOW 

 SYSTEM BOUNDARY

Figure 14. Net infiltration for the Death Valley regional flow system using INFIL 

(Hevesi and others, 2002).
34 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

That is, the model does not include ground-water 

withdrawals. Whether current recharge equals 

natural discharge is unknown. Three conditions 

exist, however, that may violate this assumption. 

First, the regional flow system still may be undergoing a drying-out sequence following a wetter 

climate cycle related to the late Pleistocene 

(Prudic and others, 1993). As a result, current 

ground-water levels and discharge rates may not 

be in equilibrium with present-day recharge and 

interbasinal flux rates. Second, the flow system 

may experience seasonal fluctuations that are 

reflected in the target data but not accounted for 

in the simulation. A resulting annual average 

condition is simulated. Third, hydraulic-head, 

spring-flow, and other data used in model calibration were collected over an interval of many 

years, and these data are affected by seasonal and 

yearly changes to the ground-water flow system. 

While data analysis was undertaken to evaluate 

hydraulic-head observations for many of these 

conditions, in many cases insufficient data were 

available to make a reasonable conclusion.

3. Horizontal isotropy within each model cell is used. 

This assumption is reasonable for cells corresponding to basin fill, but may be unreasonable 

for cells corresponding to consolidated rocks. 

Where flow is through fractures, the fractures 

may have a preferred orientation that could 

produce a greater hydraulic conductivity in one 

direction. While the model is capable of simulating a different horizontal anisotropy on a cellby-cell basis, it can do so only along the defined 

row-column orientation. Also, because the region 

contains numerous regional fracture patterns, it 

was concluded that no single orientation was 

dominant over the entire model domain. 

Numerical Model Selection

The numerical modeling code used in this study 

is MODFLOW-2000 (Harbaugh and others, 2000; Hill 

and others, 2000). As documented by Harbaugh and 

others (2000), MODFLOW-2000 is a version of the 

U.S. Geological Survey 3D, finite-difference modular 

ground-water flow modeling code, MODFLOW 

(McDonald and Harbaugh, 1988; Hill, 1992) in which 

nonlinear regression may be used to estimate flowmodel parameters that result in the best fit to measured 

hydraulic heads and flows.

MODFLOW-2000 is a block-centered finitedifference code that views a 3D flow system as a 

sequence of layers of porous material organized in a 

horizontal grid or array. The horizontal grid is generated by specifying array dimensions in the x and y 

directions. In the DVRFS model, model layers are 

defined as being confined, so that layer thickness is 

constant. Although the top layer in the simulated flow 

system is clearly unconfined, defining its thickness 

from an estimated potentiometric-surface map and 

representing it as confined produces a good approximate thickness and is much more efficient numerically. Also, for a steady-state simulation, model layers 

that are simulated as confined closely approximate that 

of an unconfined layer.

Nonlinear Regression Objective Function

Nonlinear regression estimates parameter values 

by finding the values that minimize the weighted sum 

of squared residuals objective function, S(b), which is 

calculated as:

S(b) = (y-y')TW (y-y') (1)

where 

b is an np  1 vector containing parameter values;

np is the number of parameters estimated by 

regression; 

y and y' are n  1 vectors with elements equal to 

observed and simulated (using b) values 

respectively (for the DVRFS model, the 

observed and simulated quantities are 

hydraulic heads and ground-water 

discharge);

y-y' is a vector of residuals, defined as the observed 

minus simulated values;

n is the number of measured and simulated hydraulic 

heads and flows;

W is an n  n weight matrix; and

T superscripted indicates the transpose of the vector.

For the DVRFS model, the weight matrix is 

diagonal, with each diagonal entry equal to the inverse 

of the estimated variance of the observation “measurement” error, where “measurement” error is defined 

more broadly than might be expected, in that some 

types of model error are included. This weighting will 

result in parameter estimates with the smallest 

possible variance if (1) the estimated variances and the 
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 35

model are accurate, (2) the model is effectively linear, 

and (3) the number of observations is asymptotically 

large (Bard, 1974). In addition to variances, 

MODFLOW-2000 permits the designation of standard 

deviations or coefficients of variation, from which 

variances are calculated as described by Hill and 

others (2000, p. 39–40, 53, 57, 65). These indicators of 

measurement precision are determined on the basis of 

an analysis of likely measurement error.

In MODFLOW-2000, initial parameter values 

are assigned and then are changed using a modified 

Gauss-Newton method (Hill, 1998, p. 7–13) such that 

equation 1 is minimized. The resulting values are 

called optimal parameter values. This procedure is 

repeated for each conceptual model considered.

Parameter Definition

Parameters may be defined to represent most 

physical quantities of interest, such as hydraulic 

conductivity and recharge. MODFLOW-2000 allows 

these spatially distributed physical quantities to be 

represented using zones over which the parameter is 

constant, or using more sophisticated interpolation 

methods. In either case, multipliers or multiplication 

arrays can be used to modify parameter values in a 

known way.

Parameter Sensitivities

As part of the regression, sensitivities are calculated as: y'

i

/bj, the partial derivative of the simulated 

hydraulic head or flow, y'

i

, with respect to the jth estimated parameter, bj

, using the sensitivity-equation 

method as described by Hill and others (2000, p. 67–

70). Because the ground-water flow equations are 

nonlinear with respect to many parameters, sensitivities calculated for the same parameter for different sets 

of parameter values will be different.

Besides being used in the regression calculations, sensitivities are useful to the modeler because 

they reflect how important each measurement is to the 

estimation of each parameter. The composite scaled 

sensitivity (CSS) is a statistic that summarizes all the 

sensitivities for one parameter and, therefore, indicates 

the cumulative amount of information that the 

measurements contain toward the estimation of that 

parameter. Because they are dimensionless, composite 

scaled sensitivities can be used to compare the amount 

of information provided by various types of data for 

different types of parameters. Using the weight matrix 

W in equation 1, composite scaled sensitivity for 

parameter j, CSSj

, is calculated as:

CSSj ={[ i=1,nWi(y'

i

 / bj)

2bj2]/n}1/2 (2)

Parameters with large CSS values relative to 

those for other parameters are likely to be easily estimated by the regression; parameters with smaller CSS 

values are likely to be difficult or impossible to estimate. For some parameters, the available measurements may not provide enough information for 

estimation. In this circumstance, the parameter value 

will need to be set by the modeler, or more head and 

flow measurements will need to be added to the 

regression. Parameters with values set by the modeler 

are called unestimated parameters. Composite scaled 

sensitivities calculated for different sets of parameter 

values will be different (Hill, 1998, p. 42), but in this 

work they are rarely different enough to indicate that a 

previously unestimated parameter can subsequently be 

estimated.

An alternative to setting a parameter value is to 

use prior information on the parameter. This alternative is especially important when evaluating prediction 

uncertainty because it allows measures of uncertainty 

in model predictions, such as confidence intervals, to 

reflect uncertainty in the unestimated parameter. Prior 

information was not used in the calibration of the 

DVRFS model but can be included later as needed for 

prediction uncertainty.

Confidence intervals on the estimated parameter 

values can be calculated using linear (first-order) 

theory with sensitivities calculated for the optimal 

parameter values (Hill and others, 1998, p. 523). 

Parameters with a large CSS tend to have small confidence intervals. Confidence intervals are useful when 

trying to decide how many parameters are needed to 

represent, for example, the hydraulic-conductivity 

distribution. If, for example, four zones are considered 

to be important, but the regression yields estimates 

that are within the confidence intervals related to two 

or more of the zones, it is likely that fewer zones are 

adequate. If the regression using fewer zones yields a 

similar model fit to the measurements, the available 

measurements are insufficient to distinguish between a 

model with four zones and one with fewer zones.
36 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Model Construction

 The 3D hydrogeologic data sets describing the 

DVRFS were discretized to develop the input arrays 

required for MODFLOW-2000. Because the various 

3D hydrogeologic data sets were developed at grid cell 

resolutions ranging from 100 to 1,500 m, their discretization to a common, larger grid cell resolution inevitably results in further simplification of the flowsystem conceptual model and hydrogeologic framework model. This resampling and simplification of the 

3D hydrogeologic data sets was apparent in (1) definition of the model grid, (2) assignment of boundary 

conditions, and (3) definition of model parameters.

Grid Definition

The model consists of 194 rows, 160 columns, 

and 15 layers. The 465,600-cell model is oriented 

exactly north-south and encompasses about 

70,000 km2 (fig. 15). The lower, left-corner origin 

of the grid was located at UTM coordinates 

(X;Y = 437,000; 3,928,000 m). Grid discretization 

along both rows and columns was set to 1,500 m. The 

15 model layers represent hydrogeologic units at 

depths and thicknesses described in table 5. Model 

layer thickness increases with increasing depth, which 

allows for higher resolution at the top of the flow 

model where hydrologic and geologic conditions are 

more constrained by field observations.

External Boundary Conditions

The boundaries of the DVRFS model differ 

somewhat from the two previous DOE regional 

ground-water flow model studies (fig. 1). The 

currently defined boundaries for this flow model 

reflect those that are generally included in most 

previous investigations. There are, however, some 

notable exceptions. The northern boundary conforms 

mostly to the boundary of the Death Valley regional 

flow system (fig. 7) defined by Harrill and Prudic 

(1998). Differences occur where the flow model 

boundary is aligned to include Penoyer Valley and the 

southern part of southern Railroad Valley to be consistent with the boundary of the UGTA regional model 

(fig. 1; fig. 7). The western boundary is coincident 

with the boundary defined by Harrill and Prudic 

(1998) and differs from previous regional groundwater flow models in that it extends the boundary to 

the surface-water divide along the Cottonwood Mountains and Panamint Range. The southern boundary 

includes areas southeast of Panamint Valley and west 

of the Owlshead Mountains (fig. 1) that were interpreted by Bedinger and others (1989) to contribute to 

the ground-water system. The southeastern boundary 

of the flow model is aligned approximately along the 

crest of the southern Spring Mountains to be coincident with the Keystone thrust. The HFM results 

suggest that the Keystone thrust forms an effective 

barrier between LCA units in the hanging wall and 

Mesozoic clastic rocks in the footwall. The southern 

boundary also excludes Silurian, Valjean, and Shadow 

Valleys from the flow model. For simulation purposes, 

the eastern boundary of the DVRFS flow model has 

been defined to coincide with the Gass Peak thrust. 

This flow model boundary was defined such that the 

effectiveness of the Gass Peak thrust as an impermeable boundary could be evaluated. Likewise, the flow 

model boundary was extended along the axis of 

Pahranagat Lakes (fig. 16). The western half of Pahranagat Valley is included in the simulation and Pahranagat Lakes were used as constant-head boundaries 

to allow the numerical simulation to calculate whether 

a ground-water divide between these two valleys is a 

justifiable conclusion.

Table 5. Thickness and depth to top of each model layer

Layer

Layer

thickness,

in meters

Depth to top of layer 

below top of flow 

model, in meters

1 50 0

2 50 50

3 50 100

4 100 150

5 100 250

6 100 350

7 100 450

8 100 550

9 100 650

10 100 750

11 150 850

12 200 1,000

13 250 1,200

14 250 1,450

15 300 1,700
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 37

Death 

CALIFORNIA

NEVADA

Valley

35°

118° 117° 116° 115°

38°

37°

36°

Universal Transverse Mercator projection, Zone 11.

Shaded-relief base from 1:250,000-scale Digital Elevation 

Model; sun illumination from northwest at 30 degrees 

above horizon 0 40 80 MILES

0 40 80 KILOMETERS

NEVADA TEST SITE BOUNDARY

EXPLANATION

MODEL GRID BOUNDARY

MODEL GRID CELLS

Figure 15. Location of Death Valley regional flow system model grid.
38 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

.

MODEL COLUMN

MODEL ROW

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

190

194

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

Pahranagat

Lakes

Mud Lake

Salt Creek

Cactus Flat Reveille Valley

Railroad Valley

36°

37°

38°

117° 116° 115°

NEVADA TEST SITE BOUNDARY

EXPLANATION

MODEL GRID BOUNDARY—No-flow conditions

ACTIVE MODEL GRID CELLS

INACTIVE MODEL GRID CELLS

CONSTANT-HEAD CELL WITH NAME—Color in 

 cells for differentiation only. Cell names in nonitalic

 type were tested during calibration, but not simulated 

 in calibrated model

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 16. Death Valley regional flow system model boundary conditions and 

constant-head cells.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 39

The extent of active and inactive flow model 

cells are similarly defined. The extent of defined active 

cells—that is, those cells within the model boundary—

is the same in all layers (fig. 16). In layer 1, all lateral 

boundaries were designated as no-flow except along 

the eastern boundary at Pahranagat Lakes, the northern 

boundary at Mud Lake and Cactus Flat, and the 

southern boundary at Salt Creek. Cells in these locations were defined as constant-head boundaries. 

Hydraulic-head values assigned to Pahranagat Lakes 

reflect the perennial lake levels supported by groundwater flow out of the system. At Mud Lake and Cactus 

Flat, hydraulic heads interpreted from water levels in 

wells were used to represent the water table immediately north of the model boundary. At Salt Creek, the 

land-surface elevation was assigned as the hydraulichead value to represent the water table expressed at the 

land surface as ground-water discharge. There are only 

no-flow boundaries in the remaining model layers. 

During calibration several additional constant-head 

boundaries were evaluated at Reveille Valley and 

southern Railroad Valley (fig. 7 and fig. 16).

Representation of the Hydrogeologic System

Hydrogeologic Units

The discretization of the HFM into numerical 

flow-model input arrays involved the export of 

hydrogeologic units from the framework model into a 

series of surfaces that defined the top of each hydrogeologic unit. These surfaces were utilized by the 

MODFLOW-2000 hydrogeologic-unit flow (HUF) 

package (Anderman and Hill, 2000) to define the 

geometry of the hydrogeologic units for the flow 

model.

The MODFLOW-2000 (Harbaugh and others, 

2000) HUF package (Anderman and Hill, 2000) 

allows incorporation of the framework model into 

the flow model. This was done by inputting the top 

altitude and thickness grids developed from the 

framework model as flow-model arrays. This represents a significant increase in the efficiency of linking 

the framework model and the flow model. Essentially, 

the HUF package allows the flow model itself to 

resample the framework model’s hydrogeologic units 

into the flow-model grid, with HUF determining 

which hydrogeologic unit is in each flow-model layer. 

This gives the user great flexibility in the ability to 

change the flow-model geometry without the tedious 

process of manual resampling efforts. 

The altitudes of the top of the cells constituting 

each hydrogeologic unit from the framework model 

were exported. This produced a series of grids that 

only contain altitude values where a hydrogeologic 

horizon exists in the 3D model. It is important to note 

that because the altitude values contained in the hydrogeologic unit grids are from the horizontal center of 

the cells representing the 3D model, the limits of these 

grids are somewhat different than the original grids 

used to construct the model. The grids used to 

construct the model are node-based with the altitudes 

being assigned to the points existing at the corners of 

the cells constituting the model. The exported grids are 

cell-based with the altitude being assigned to the 

center of the cell constituting the model.

Variations in the hydraulic properties of hydrogeologic unit parameters are defined using ZONE 

arrays. Zone arrays distinguish between regions within 

a hydrogeologic unit that have different hydraulic 

conductivity values.

Hydrogeologic Structures

In this model, the horizontal-flow barrier 

(HFB) package (Hsieh and Freckleton, 1993) was used 

to simulate relatively thin, vertical, low-permeability 

geologic features that impede the horizontal flow of 

ground water. The features are approximated as a 

series of explicitly represented relative barriers situated on the boundaries between pairs of adjacent cells 

in the finite-difference grid.

The subset of mapped faults chosen to be incorporated as relative barriers in the flow model was 

sampled into a “stair-step” pattern of cell boundaries 

that closely approximate the location of the mapped 

faults. The locations of these cell boundaries were then 

specified for input to MODFLOW-2000. For 

simplicity, the faults were assumed to extend through 

the entire thickness of the flow model and were 

assigned a thickness of 1 m. The conductances of these 

relative barriers were originally assigned relatively 

small values on the order of 10–6 m2/d. A description 

of all faults simulated with the HFB package is 

provided in table 6. 

Evapotranspiration and Spring Flow

In the DVRFS model, ground-water discharge, 

including ET and spring flow, is simulated using the
40 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

drain (DRN) package (McDonald and Harbaugh, 

1988). The DRN package simulates ground-water 

discharge through a head-dependent boundary. Ground 

water discharges from the cell in which the drain is 

specified when the simulated head in the cell rises 

above the drain elevation. 

For the DVRFS model, each discharge area was 

subdivided into one or more zones. These zones define 

subareas within the main ground-water discharge area 

typically associated with a major spring or spring 

group. Figure 17 illustrates this zonal breakdown in 

the Oasis Valley discharge area. Spatial distributions 

of ET units obtained from Laczniak and others (2001), 

Malmberg (1967), U.S. Department of Energy (1997), 

and DeMeo and others (1999) were overlaid on the 

model grid. The overlay was used to compute the area 

of an ET unit within model nodes and ET zones of a 

discharge area. The grid definition of all discharge 

areas simulated in the DVRFS model is presented in 

figure 18.

Recharge

To define ground-water recharge using the RCH 

package, the net infiltration model provided by Hevesi 

and others (2002) was resampled to a 1,500-m grid. In 

the initial model configuration, recharge was defined 

using one parameter that varied the net influx of 

ground water by a multiplicative factor for the entire 

model domain. Subsequent model runs utilized a zone 

array of up to 13 zones representing the major rock 

types within the hydrogeologic framework model that

Table 6. Description of hydrogeologic features defined as horizontal-flow barrier (HFB) package parameters

Parameter

name Feature description

HFBRestSpr Unnamed southern extension of Furnace Creek fault zone; east side of central Death Valley “igneous rhombochasm”

HFBAmargos Amargosa River fault

HFBAshMead Combined Amargosa River fault/Stewart Valley fault

HFBBareMtn Fluorspar Canyon detachment fault

HFB_DeathV Death Valley fault zone

HFBEagleMt Unnamed transition zone to southern extension of Furnace Creek fault zone; northeast side of central Death Valley 

“igneous rhombochasm”

HFBFurnace Furnace Creek fault zone

HFB_LVVSZ Las Vegas Valley shear zone

HFBPahrump Pahrump-Stewart Valley–State Line fault zone

HFBTecopa Greenwater Valley fault zone

HFBTecPass Sheephead fault zone

HFBGravity Gravity fault

HFB_IndSpr Unnamed splay of the Las Vegas Valley shear zone

HFB_Pahute Western boundary of the Silent Canyon caldera complex, corresponds to the northern part of the Thirsty Canyon 

lineament

HFB_KeaneW Keane Wonder fault

HFB_CDVFCF Central Death Valley–Furnace Creek fault zone

HFB_Saratog Southern Death Valley fault zone

HFB_NDVFCF Northern Death Valley–Furnace Creek fault zone

HFBCraterF Crater Flat fault

HFBSolitar Solitario Canyon fault

HFBHwy95 Highway 95 fault

HFBBareMtF Bare Mountain fault

HFBStewart Pahrump-Stewart Valley–State Line fault zone

HFB_Eleana Eastern (and southern) limit of Eleana Formation involved in footwall thrust duplex of the Belted Range thrust

HFB_NCFYM Projected subsurface trace of the CP thrust

HFBBullfrog Bullfrog Hills detachment
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 41

0

0 1 2 3 KILOMETERS

1 2 3 MILES

116°50'

36°52'30"

36°57'30"

37°02'30"

36°55'

37°

37°05'

116°47'30" 116°45' 116°42'30" 116°40' 116°37'30"

BEATTY

EXPLANATION

DOMINANT CONDITION 

 IN AREA

MODEL DISCHARGE 

 OBSERVATION 

 DESCRIPTION

OPEN WATER

SUBMERGED AQUATIC 

 VEGETATION

DENSE WOODLAND 

 VEGETATION

DENSE MEADOW AND

 WOODLAND VEGETATION

DENSE TO MODERATELY 

 DENSE GRASSLAND 

 VEGETATION

SPARSE TO MODERATELY 

 DENSE SHRUBLAND 

 VEGETATION

SPARSE GRASSLAND 

 VEGETATION

MOIST BARE SOIL

COFFER'S RANCH AREA

SPRINGDALE AREA

MIDDLE OASIS VALLEY AREA

BEATTY AREA

BOUNDARY OF OASIS 

 VALLEY DISCHARGE 

 AREAS

MODEL GRID

SPRING

DRAIN

CELLS

ET

ZONE

Figure 17. Delineation of evapotranspiration (ET) units and zones in the Oasis Valley discharge area, Nevada (Laczniak 

and others, 2001).
42 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

occur at the estimated water table (fig. 19). Parameters 

were defined as multiplicative factors for net infiltration and they are described in table 7. The net effect is 

a modification of the INFIL rates provided by Hevesi 

and others (2002) by some factor that is dependent on 

the rock type at the water table. This modification is 

interpreted as representing processes that would otherwise redirect infiltrating water between the root zone 

and the water table.

Observation Data

Measured hydraulic heads and ground-water 

discharges and their corresponding weights were used 

by MODFLOW-2000 during parameter estimation to

Figure 18. Model cells representing drains used to simulate ground-water discharge 

from the Death Valley regional flow system model.

36°

37°

38°

117° 116°

G-PENOYERV

G-SARCO-NE

G-SARCO-SW

G-GRAPE-SP

G-GRAPE-SC

G-SARCO-CH

G-OV-COFFR

G-OV-OASIS

G-OV-BEATY

G-DV-MESQU

G-DV-NEVAR

G-DV-TEXAS

G-DV-TRVRT

G-AM-UPDRN

G-AM-AMFLT

G-AM-CENTR

G-AM-NORTH

G-AM-CARSL

G-FRANKWEL

G-FRNKLK-N

G-SHOSH-N

G-SHOSH-S

G-TC-RESTS

G-TC-AMCAN

G-TC-CHRNC

G-TC-TECOP G-TC-SPERY

G-FRNKLK-S

G-STEWRT-P

G-STEWRT-V

G-CHICAGOV

G-FRNKLK-E

G-PAH-BENT

G-PAH-MANS

G-AM-SOUTH

G-DV-COTTN

G-DV-FRNFN

G-DV-MIDDL

G-DV-BADWT

G-DV-WESTF

G-DV-MORMN

G-DV-CONFI

G-DV-SARAT

G-INDIANSP

G-CORNCREK

G-OV-SPRDL

NEVADA TEST SITE BOUNDARY

MODEL CELLS REPRESENTING DRAINS WITH

OBSERVATION NAME—Observation descriptions

provided in table 3. Color in cells for differentiation only

EXPLANATION

MODEL GRID BOUNDARY

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

G-SARCO-NE
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 43

36°

37°

38°

117° 116°

NEVADA TEST SITE BOUNDARY

EXPLANATION

MODEL GRID BOUNDARY

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

1. BASIN FILL

2a. VOLCANICS - TIMBER MOUNTAIN

2b. VOLCANICS - ALL OTHER

3a. CRYSTALLINE - UPPER TJi AND pCgm

3b. CLASTIC - UPPER UCCU

3c. CLASTIC - UPPER LCCU AND Mvs

3d. CLASTIC - LCCU THRUSTS

4a. CRYSTALLINE - LOWER TJi AND pCgm

4b. CLASTIC - LOWER UCCU

4c. CLASTIC - LOWER LCCU

5a. CARBONATES (WEST)

5b. CARBONATES (SPRING MOUNTAIN)

5c. CARBONATES (NORTHEAST)

RECHARGE ZONE—

Figure 19. Recharge zone multiplication array representing hydrogeologic units at the 

water table.
44 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

provide values to define the objective function for the 

model simulation. The series of analyses conducted to 

obtain the initial set of hydraulic-head and groundwater discharge weights is described below.

Hydraulic-Head Observations

Although a large set of water-level data exists 

for the DVRFS, not all of these data are appropriate for 

use in the predevelopment, steady-state model. Only 

those water levels representing regional steady-state 

conditions existing prior to the onset of pumping, or 

water levels exhibiting no response to pumping, were 

used.

The USGS National Water Information System 

(NWIS) databases in Las Vegas, Nev., and San Diego, 

Calif., were the ultimate source of all water-level and 

borehole data used in the DVRFS model. As part of 

the modeling activity, new or previously omitted borehole and water-level data were entered into these 

NWIS databases. These databases were believed to be 

the most comprehensive data sources for water levels 

in the Death Valley region.

Each water-level measurement was attributed in 

the project database to identify appropriate data for the 

flow model. Three attributes were assigned to each 

water-level measurement. These attributes define the 

appropriate use of the data (“UseFlag”), the hydrologic conditions the data represented (“ConditionFlag”), and the methods used for analysis 

(“MethodFlag”). The attributes and their definitions 

are presented in table 8, table 9, and table 10.

Water levels in boreholes with only one waterlevel measurement were automatically attributed with 

a UseFlag of “Insufficient data.” These data were 

subsequently reviewed to ensure that they were 

reasonable for inclusion in the flow model. If these 

data did not appear to represent regional ground-water 

conditions, the attribute was modified to indicate the 

appropriate use.

Hydrographs were generated for each borehole 

with more than one water-level measurement. By 

reviewing well hydrographs and additional spatial data 

contained in the accompanying DVRFS database, the 

appropriate UseFlag and ConditionFlag for each 

water-level measurement were determined and automatically stored in the DVRFS database. Once all 

water levels were attributed with a UseFlag, database 

queries were used to select the appropriate water levels 

for further analysis.

Well-construction data were used to determine 

the depth interval where each borehole was open to the 

flow system. Boreholes used as observations had to 

have a definable depth interval associated with the 

hydraulic-head measurement. All boreholes with 

water-level measurements attributed with a UseFlag of 

“Steady state (regional scale)” or “Insufficient data” 

were included in this analysis. If a site had multiple 

intervals where water could enter the borehole the 

uppermost and lowermost depths were used. If 

construction data were not available, the mean water 

level and the bottom of the well were used to define 

the open depth interval associated with the water level. 

If a well or hole depth was not available, the head 

observation was not used. Boreholes excluded from

Table 7. Description of recharge parameters defined for the recharge zones shown in figure 19

Parameter name Description

RCH_zone1 Recharge on basin-fill units

RCH_zone2a Recharge on volcanic Timber Mountain units

RCH_zone2b Recharge on volcanic units outside of the Timber Mountain area

RCH_zone3a Recharge on crystalline units TJi and pCgm, depths less than 300 meters

RCH_zone3b Recharge on clastic unit UCCU, depths less than 300 meters

RCH_zone3c Recharge on clastic units LCCU and Mvs, depths less than 300 meters

RCH_zone3d Recharge on clastic units LCCU thrusts

RCH_zone4a Recharge on crystalline units TJi and pCgm, depths greater than 300 meters

RCH_zone4b Recharge on clastic unit UCCU, depths greater than 300 meters

RCH_zone4c Recharge on clastic unit LCCU, depths greater than 300 meters

RCH_zone5a Recharge on carbonate units LCA and UCA in the western part of the model

RCH_zone5b Recharge on carbonate units LCA and UCA in Spring Mountain area

RCH_zone5c Recharge on carbonate units LCA and UCA in the northeastern part of the model
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 45

the model because of no available hole depth were 

located in areas with other head observations, so no 

additional attempts were made to assign an open depth 

interval.

Hydraulic-head observation weights are a 

measure of the observation error. A total observation 

error expressed as a standard deviation was assigned to 

each head observation. MODFLOW-2000 calculates 

the appropriate weight for use in the parameter-estimation process (equation 3). In this discussion “observation error” is analogous to the MODFLOW-2000 input 

variable “STATISTIC,” expressed as a standard deviation, which is assigned to each head observation. More 

detailed discussions on observation weights can be 

found in Hill (1998) and Hill and others (2000).

(3)

where

weight = relative accuracy of head observation used to calculate the weighted 

residual, and

STATISTIC = total observation error estimate 

expressed as a standard deviation.

An estimate of observation error was calculated 

using accuracy data stored in the database. The potential errors considered include those associated with 

(1) land-surface altitude, (2) borehole location (latitude and longitude), and (3) measurement method. 

Each error was expressed as a variance, and they were 

summed to obtain the total observation variance. This 

variance was then converted to a standard deviation 

and used as the observation error.

Land-surface altitude errors were often the 

largest errors affecting the head observation accuracy. 

Land surface was the datum used to calculate the head 

observation in meters above sea level. If this datum 

was in error by 10 m, the water-level altitude was in 

error by at least 10 m. The error magnitude was based 

on the method used to determine the land-surface altitude. If a topographic map was used to locate the borehole, the error was based on the map scale. If a global 

positioning system (GPS) was used to determine landweight 1

STATISTIC2 = ---------------------------------

Table 8. Possible “UseFlag” entries used to attribute water-level measurements in the Death Valley regional flow system 

database. The “UseFlag” indicates data suitability for various types of hydrologic studies and general hydrologic conditions

UseFlag attribute Description

Steady State (regional scale) These data represent predevelopment and(or) reasonable steady-state hydrologic conditions that reflect 

the regional potentiometric surface.

Steady State (local scale) These data represent reasonable steady-state hydrologic conditions of a localized potentiometric 

surface.

Transient (regional scale) These data represent postdevelopment and(or) consistently changing hydrologic conditions due to 

pumping, recharge, and so forth, in boreholes that monitor the regional potentiometric surface.

Transient (local scale) These data represent postdevelopment and(or) changing hydrologic conditions due to pumping, 

recharge, and so forth, in boreholes that monitor a localized potentiometric surface or in areas where 

the hydrologic conditions do not affect the regional flow system.

Localized These data represent localized hydrologic conditions that are not appropriate for regional studies.

Insufficient data There are not sufficient data to make a definitive determination on the hydrologic conditions or suitability of the data.

Suspect These data appear to be suspicious, in error, or affected by unnatural conditions that preclude their use in 

studies of the natural hydrologic flow system.

Nonstatic level These data are affected by testing or construction activities and the water level does not represent static 

regional flow-system conditions.

Superseded These data were collected prior to site reconfiguration or modification, and subsequent data collected at 

this location, possibly under different site IDs, are more appropriate for hydrologic investigations.

Duplicate These data have been entered more than once because of different parameter codes or data entry error. 

The data should not be used for statistical and related analyses.

None These data are known to be erroneous, not representative of the hydrologic system, or are missing critical information and should not be used in any hydrologic investigation.
46 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

surface altitude, the error was based on the accuracy of 

the GPS system.

Borehole-location (latitude and longitude) error 

introduces additional land-surface altitude error 

because the correct altitude cannot be obtained if the 

borehole is not accurately located. This potential error 

increases if the borehole is located in a steep-slope 

area. A simple method was developed to convert these 

horizontal errors into vertical errors. The land-surface 

slope was used as an approximation for the potentiometric-surface slope, and the slope at each head-observation location was calculated using a digital elevation 

model (DEM). Location-accuracy data stored in the 

database were used if available, but if no data were 

available it was assumed that the borehole location 

error was  1 minute of longitude. Horizontal borehole-location error was converted to a vertical accuracy (eq. 4):

(4)

where

Z = vertical error due to horizontal borehole-location accuracy

X = horizontal borehole-location accuracy, and

Z  (X tan )

Table 9. Possible “ConditionFlag” entries used to attribute water-level measurements in the Death Valley regional flow 

system database. Water-level measurement “ConditionFlag” indicates the hydrologic condition(s) that the data represent and 

supports the “UseFlag” determination

ConditionFlag attribute Description

Reported perched water The site and/or data were identified as representing localized perched-water conditions in a 

published report.

Suspected perched water Based on the available data and professional judgment the data appear to represent localized 

perched-water conditions.

Testing area The data appear to be affected by nuclear testing or other explosive detonations.

Declining trend The data appear to be part of a discernible, overall downward trend. Possible causes include nearby 

pumping, decreased recharge, and drainage.

Rising trend The data appear to be part of a discernible, overall rising trend. Possible causes include recovery 

from pumping, precipitation event(s), and proximity to recharge areas.

Pumping/recovery The measurement was made while the site, or nearby site, was being pumped or was recovering 

from pumping.

Pumping area The site is located in an area that has been impacted by ground-water withdrawals.

Equilibration The water level appears to be part of a discernible, overall trend that is approaching an equilibrium 

level, either higher or lower than the initial measurement.

Undeveloped The data do not accurately represent hydrologic conditions because of insufficient borehole 

development.

Dry The site was dry at the time of the measurement. The water level has dropped below the bottom of 

the borehole or access tube.

Suspect The water level is grossly different in magnitude or trend from other data at the same or nearby 

sites. Data may have been measured inaccurately or entered incorrectly.

Erratic/Unstable The data are erratic, due to unknown causes, and probably do not represent actual hydrologic conditions. The validity of the data cannot be determined.

Insufficient data There are not sufficient data to make a determination on the hydrologic conditions at the time of the 

measurement.

Consistent The data appear to be part of a reasonably consistent level or cyclic trend maintained at the site.

Precipitation response The data appear to be responsive to localized precipitation events, which might suggest a localized 

hydrologic system.

Missing Critical associated data are missing that make the data useless. For example, no date with a waterlevel measurement.

Anomalous – high The water level is higher or larger in magnitude than equivalent data at this site or nearby sites.

Anomalous – low The water level is lower or smaller in magnitude than equivalent data at this site or nearby sites.

Other The data do not fit into any other category. A description of the hydrologic condition should be 

added to the remark field.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 47

 = land-surface slope at borehole location (used as 

an approximation of the potentiometric-surface 

slope). 

Measurement method was a relatively minor 

source of error associated with hydraulic-head observations. An accuracy was assigned to each water-level 

measurement based on the measurement method 

stored in the database (table 11). Each observation 

used the mean accuracy of all measurements with a 

UseFlag of “Steady state (regional scale)” or “Insufficient data.”

Water-level measurement variability was 

analyzed and was found to be an inconsequential 

source of error. By definition water-level measurements selected for inclusion in the flow model had 

minimal variability. These measurements were 

selected because they represented a consistent water 

level. The mean of all appropriate measurements sufficiently accounted for the natural water-level variability. 

A minimum observation error was assigned 

because some observations had standard deviations 

that were too small given the flow-model construction. 

It was unreasonable to restrict the parameter-estimation process to match head observations within 2 m. 

Head observations with a UseFlag of “Insufficient 

data” were assigned a minimum standard deviation of 

5 m, and “Steady state (regional scale)” observations 

were assigned a minimum standard deviation of 3 m. 

A summary of the initial standard deviations assigned 

to head observations is presented in figure 20.

Table 10. Possible “MethodFlag” entries used to attribute water-level measurements in the Death Valley regional flow 

system database. Water-level measurement “MethodFlag” indicates the analytical methods used to analyze the data and to 

determine the “UseFlag” and general hydrologic conditions

MethodFlag attribute Description

Graphical Data were visually inspected using graphical methods such as hydrographs, so that trends and anomalous data could be identified.

Spatial Data were analyzed to determine if they agreed with regional trends, spatial hydrologic conditions, 

and(or) were reasonable given data from nearby sites.

Supporting Data were analyzed and compared to supporting data types such geology, precipitation, recharge, pertinent reported information, and so forth.

Construction Data were analyzed and compared to the site construction and configuration, which may affect the 

measured data value. 

Criteria Data were analyzed and compared to various criteria that are indicative of specific hydrologic conditions. For example, there is generally no single factor or type of data that is available to determine if 

a water level is perched, so several types of data can be used collectively to make a determination.

Reported Data were reported in a published document as having the hydrologic condition(s) specified by the 

“ConditionFlag.”

Query The data were selected by a database query.

Table 11. Accuracy assigned to water-level measurements 

based on method of measurement

Method description Accuracy,

in feet

Airline measurement 2.0

Analog or graphic recorder 2.0

Calibrated airline measurement 1.0

Calibrated electric-tape measurement 0.5

Electric-tape measurement 0.5

Estimated 50 percent of depth-towater value

Interpreted from geophysical logs 3.0

Manometer measurement 1.0

Nonrecording gage 2.0

Other 2.0

Pressure-gage measurement 1.0

Reported—method not known 1.0

Steel-tape measurement 0.5
48 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Ground-Water Discharge Observations

Laczniak and others (2001; appendix) presented 

a formal analysis assessing the uncertainty of their 

estimates of ground-water ET using a Monte Carlo 

simulation approach. The Monte Carlo simulation 

generated statistics of ground-water ET from each 

discharge area by using randomly generated values of 

the variables used to compute ground-water ET. These 

variables included (1) the area of each ET unit, (2) the 

average annual ground-water ET rate of each ET unit 

within a discharge area, and (3) the average annual 

precipitation of each discharge area. In generating 

random values for these variables, the simulation 

assumed that each variable was independent and 

normally distributed about a mean equal to the value 

used by Laczniak and others (2001) to compute 

ground-water ET. Values generated for each parameter 

were constrained by a probability distribution defined 

in terms of a coefficient of variation (CV), where CV 

is the standard deviation divided by the mean. Values 

of CV used to generate ET rates were determined from 

ranges given in Laczniak and others (2001) describing 

the expected variation of the ET rate within each ET 

unit. CV values were computed on the assumption that 

ranges represented 2 standard deviations of a normal 

population. CV values used to generate annual precipitation rates within a discharge area were determined 

from statistics determined from long-term measurements made by the National Weather Service at 

weather stations within or near each discharge area. 

CV values used to generate ET-unit areas were 

assumed to be 10 percent.

Uncertainty for each discharge area was calculated from the mean and standard deviation generated 

by 1,000 realizations of a Monte Carlo simulation and 

was expressed in terms of a CV (Laczniak and others, 

2001). A higher CV implies a greater uncertainty in 

the estimate of ground-water discharge. Simulated CV 

values ranged from 0.10 to 0.56. Based on this analysis, ground-water ET estimates for Oasis Valley 

(10 percent) and Tecopa basin, central area 

(12 percent) are the most certain and those for 

Stewart Valley (56 percent) and Sarcobatus Flat 

(46–55 percent) are the least certain. In general, 

discharge areas dominated by open playa and those 

with fewer ET units were the most uncertain. Final CV 

values used in the DVRFS model are listed in table 3. 

A similar approach was used to determine CV values 

for ground-water discharge rates not developed by the 

Laczniak and others (2001) study.

Figure 20. Distribution of standard deviation values assigned to hydraulic-head observations to 

determine weights from the premodeling observation analysis and the calibrated model.

PREMODELING ANALYSIS

CALIBRATED MODEL

250

200

150

100

50

0

0–3 3–5 5–10 10–15 15–25 25–50 50–100

NUMBER OF OBSERVATIONS

HYDRAULIC-HEAD OBSERVATION STANDARD DEVIATION, IN METERS
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 49

Model Calibration

During calibration, a number of conceptual 

models were evaluated using the regression methods in 

MODFLOW-2000. A best fit to hydraulic-head and 

ground-water discharge observations was calculated 

for each conceptual model. Evidence of model error or 

data problems was investigated after each model run. 

These analyses were used in conjunction with independent hydrogeologic data to modify, and hopefully 

improve, the existing conceptual model, observation 

data sets, and weighting.

Conceptual Model Variations

Four major types of conceptual model variations 

were evaluated to test those components of the 

ground-water system about which the least is known. 

These modifications included changes to (1) the location and type of flow-system boundary conditions, 

(2) the definition of discharge areas, (3) the definition 

of recharge areas, and (4) the configuration of hydrogeologic framework features. For each change in the 

conceptual model, a new set of parameters was estimated using MODFLOW-2000, and the resulting new 

simulated hydraulic heads and ground-water 

discharges were compared with observed values. Only 

those conceptual model changes contributing to significant (>10 percent) improvement in model fit during a 

given optimization run, as indicated by a reduction in 

the sum of squared errors (equation 1), were retained 

in the final optimized model. Variations in hydrogeologic framework interpretation contributed most to 

improving the numerical model fit.

Location and Type of Boundary Conditions

During model calibration several different 

boundary configurations were simulated to evaluate 

flow and no-flow assumptions along the northern 

model boundary. Limited water-level data made it 

difficult to determine if ground-water divides exist in 

these areas. Previous investigations have suggested 

southward ground-water flow from many of the adjacent northern basins (Eakin, 1962; Rush, 1968, 1970). 

Constant-head cells were evaluated in the Mud Lake, 

Cactus Flat, Reveille Valley, and Railroad Valley areas 

(fig. 16).

In early simulations, the northern part of the 

model did not have adequate amounts of water 

entering the system as natural infiltration to support 

hydraulic heads and ground-water discharges. Because 

the recharge model provided by Hevesi and others 

(2002) was considered a “high-end” estimate of 

recharge, a water source other than infiltration was 

required to raise water levels and increase flow to 

ground-water discharge areas. Four groups of 

constant-head cells were simulated to test hypotheses 

of interbasinal flow from these northern regions.

Constant-head cells along the northern model 

boundary (fig. 16) were evaluated with the cells 

defined in multiple configurations, including layer 1, 

and layers 1–15, 1–10, and 1–5 (table 5). The volume 

of water flowing into the model with cells in multiple 

layers was excessive in most cases. In the calibrated 

model a reasonable volume of water entered the model 

with the constant-head cells in layer 1 for Mud Lake 

and Salt Creek. The Cactus Flat area, however, needed 

constant-head cells in layers 5–10 to allow a reasonable volume of water into the model. In the calibrated 

model water exited the flow model at the layer 1 

constant-head cells in the Pahranagat Lake areas 

(net 21,400 m3/d).

The Mud Lake constant-head group (fig. 16) 

consisted of 2 cells in layer 1. Few water-level data 

were available to determine the altitude of the constant 

heads. The two nearest boreholes with water-level 

measurements were approximately 10 km north and 

30 km south of the model boundary. Water levels in 

these boreholes indicated a southward gradient into the 

model. The cell elevations were set about 15 m below 

the water level at the closest model head observation. 

Water entering the model through these constant-head 

cells was important for maintaining hydraulic heads 

throughout the northwestern part of the model and 

for discharge at Sarcobatus Flat and Grapevine Spring. 

These constant-head cells were used in the final 

calibrated model and there was a net inflow of 

16,500 m3/d.

Constant-head cells were evaluated in Cactus 

Flat because of low hydraulic heads in the Cactus Flat 

and Gold Flat areas as well as low discharge at Sarcobatus Flat and Grapevine Spring. This constant-head 

group (fig. 16) consisted of 5 cells in layer 1. Water 

levels near the model boundary indicate that prior to 

development a ground-water divide probably existed. 

These constant-head cells were quite effective at 

improving hydraulic heads or ground-water discharge 

residuals. For these reasons this constant-headboundary group was included in the final flow model 

configuration, and there was an inflow of 25,500 m3/d.
50 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

The Reveille Valley constant-head boundary 

(fig. 16) consisted of 4 cells in layer 1. There were no 

water-level data in this valley, so it was not possible to 

estimate the hydraulic gradient. A thick section of 

permeable LCA is present in Reveille Valley, so inflow 

was highly sensitive to the constant-head altitude. 

Inflow through the Reveille Valley constant-head 

boundary did not noticeably improve hydraulic heads 

or ground-water discharge residuals in the northern 

areas so this group of constant-head cells was removed 

from the model.

The southern Railroad Valley constant-headboundary group (fig. 16) consisted of 4 cells in layer 1. 

Several boreholes are present in this area, and the 

water-level-data qualities vary greatly. As a result, 

determining predevelopment ground-water conditions 

was difficult. Simulations including a constant-head 

boundary at southern Railroad Valley resulted in 

decreased discharge at Penoyer Valley. Flow out of the 

model at southern Railroad Valley resulted from simulated hydraulic heads being higher than observed 

water levels in the three wells in this area. Simulation 

problems in this area could be the result of hydrogeologic conceptual model uncertainties, not knowing 

accurate hydraulic gradients, and not knowing if 

southern Railroad and Penoyer Valleys are actually 

part of the simulated flow system. These constanthead cells were removed from the model.

Definition of Discharge Areas

Discharge areas in the DVRFS flow model were 

initially defined using one parameter for each drain in 

the uppermost layer. These parameters are defined as 

conductance. During the calibration process it became 

apparent that discharge from bedrock units was not 

being adequately simulated. Discharge from layer 1 

drains was insufficient to match observed discharge 

rates and disproportionately intercepted water from 

shallower flow paths. To more appropriately simulate 

discharge from the deep regional ground-water flow 

paths, drains were moved to deeper model layers. The 

drain cells selected to represent this deeper component 

of flow were selected based on relative temperatures of 

discharging spring water. Warm-water springs are 

considered to represent deep ground-water flow. The 

model layer for these drains was selected based on the 

uppermost occurrence of the LCA. If no LCA was 

present in the selected model cell, the drain was placed 

in layer 10. Drains discharging from layer 1 are 

denoted by parameter names starting with a “G” and 

from deeper than layer 1 with an “S” (table 12). 

Conductance values and composite scaled sensitivities 

(equation 2) for all drain parameters are provided in 

table 12.

Definition of Recharge Areas

Recharge in the DVRFS flow model was 

initially defined using one parameter that varied the 

net inflow of ground water by a multiplicative factor 

between 0 and 1 for the entire model domain. The 

initial CSS value for this parameter was high, generally within the top five most sensitive parameters. 

Parameter estimation for this recharge parameter 

resulted in a value of 0.20, indicating that the total 

recharge provided by Hevesi and others (2002) potentially was overestimated by a factor of four. As previously stated, this overestimation is believed to result 

from INFIL model simplifications and not accounting 

for processes occurring between the root zone and the 

water table.

During many of the early model runs, the 

consistently high CSS value for the recharge multiplier 

suggested that adequate observations existed to 

describe recharge using additional model parameters. 

Given INFIL limitations, it was, therefore, assumed 

that the materials at the water table were reasonable 

indicators of materials between the water table and 

land surface (excluding soils). Consequently, the 

major rock types occurring at the water table were 

used to define additional recharge parameters. Parameters were defined as multiplicative factors for recharge 

occurring on each major rock type including basin fill, 

volcanic, clastic, crystalline, and carbonate rock types 

(fig. 19 and table 7).

Parameters defining recharge on the carbonate 

rocks contributed the largest volumes of water to the 

ground-water flow system. High recharge rates on the 

Spring Mountains were necessary to properly simulate 

discharge in Pahrump Valley, Shoshone/Tecopa basins, 

Amargosa Valley, and Indian Springs (fig. 1). 

Carbonate rocks in the northeastern part of the model, 

however, did not require as much recharge to simulate 

observed water levels and flows. Ultimately, three 

recharge parameters were defined for recharge on 

carbonate rocks in the Spring Mountains (simulated 

mean recharge of about 24 mm/yr) and in the northeastern (simulated mean recharge of about 6 mm/yr) 

and central/western (simulated mean recharge of about
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 51

2 mm/yr) parts of the model domain. In the final calibrated model, recharge on the Spring Mountains was 

55 percent of the net infiltration array, whereas 

recharge on the northeastern and central/western areas 

was about 25 percent and 12 percent of the net infiltration array provided by Hevesi and others (2002), 

respectively (table 13). This reduction of net infiltration seems reasonable considering that the composition of the carbonate rocks is quite variable among 

these three regions of the flow model.

Parameters defining recharge on volcanic rock 

areas also were further subdivided. This was done to 

determine if the undifferentiated volcanic (VU) rocks 

had less actual recharge than the INFIL model 

suggests. The rationale was that the VU rocks are 

significantly older than most of the volcanic rocks in 

the Timber Mountain and Pahute Mesa areas and 

potentially had altered zones that could impede infiltrating waters. Ultimately, recharge parameters were 

defined for the Timber Mountain and Pahute Mesa 

volcanic units (RCH_zone2a) and for all other 

volcanic units (RCH_zone2b) (fig. 19). In the final 

calibrated model, recharge on the volcanic rocks was 

left at approximately 100 percent of the original estimates (table 13). This high level of agreement is 

reasonable considering that the INFIL model was originally developed and tested on the volcanic rocks of 

Yucca Mountain (Hevesi and others, 2002).

Recharge rates equal to net infiltration rates 

applied to confining units with smaller values of 

hydraulic conductivity (K) in the DVRFS flow model 

typically resulted in hydraulic-head mounding. This 

inconsistency likely resulted from an overestimation 

of natural infiltration in the INFIL model because rock 

units were assigned permeabilities that were larger

Table 12. Drain parameter values and composite scaled 

sensitivities

Parameter name

Conductance, in 

meters squared 

per day

Composite 

scaled

sensitivity

G-AM-AMFLT 10 0.224

G-AM-CARSL 100 0.149

G-AM-CENTR 100 0.675

S-AM-CENTR 10 1.5610–2

S-AM-UPDRN 100 0.205

G-AM-NORTH 100 0.239

S-AM-NORTH 10 6.0010–2

G-AM-SOUTH 1,000 0.347

S-AM-SOUTH 10 4.3910–3

G-CHICAGOV 100 0.196

G-CORNCREK 100,000 1.6110–4

G-DV-BADWT 1,000 7.2910–3

G-DV-CONFI 1,000 1.2410–2

G-DV-COTTN 1,000 1.1010–2

G-DV-FRNFN 1,000 7.2910–4

G-DV-MESQU 1,000 3.3510–3

G-DV-MIDDL 1,000 5.9310–4

G-DV-MORMN 1,000 5.5110–3

S-DV-NEVAR 10,000 2.6910–4

G-DV-SARAT 1,000 2.1810–2

S-DV-SARAT 1,000 0

S-DV-TEXAS 100 0.298

S-DV-TRVRT 10,000 1.6810–4

G-DV-WESTF 1,000 2.3510–3

G-FRANKWEL 100 5.2810–2

G-FRNKLK-E 100 6.7410–2

G-FRNKLK-N 100 0.186

G-FRNKLK-S 100 1.3810–2

S-GRAPE-SC 10 2.5010–2

S-GRAPE-SP 10 0.154

G-INDIANSP 100 2.9310–2

S-INDIANSP 100 2.4010–2

G-OV-BEATY 100 0.318

S-OV-BEATY 10 9.1010–5

G-OV-COFFR 100 0.505

S-OV-COFFR 10 6.5410–2

G-OV-OASIS 100 0.185

S-OV-OASIS 10 3.4010–2

G-OV-SPRDL 100 0.334

S-OV-SPRDL 10 5.2710–2

G-PAH-BENT 1,000 9.5410–2

G-PAH-MANS 1,000 2.7310–2

G-PENOYERV 10,000 6.0710–3

G-SARCO-CH 100 0.174

G-SARCO-NE 100 0.261

G-SARCO-SW 100 0.102

G-SHOSH-N 100 8.3010–2

G-SHOSH-S 100 0.202

Table 12. Drain parameter values and composite scaled 

sensitivities—Continued

Parameter name

Conductance, in 

meters squared 

per day

Composite 

scaled

sensitivity

S-SHOSH-S 10 5.9510–3

G-STEWRT-P 1,000 6.3210–2

G-STEWRT-V 1,000 1.0910–2

G-TC-AMCAN 10 5.9310–2

G-TC-CHRNC 10 1.2710–2

G-TC-RESTS 1,000 0.190

S-TC-RESTS 10 0

G-TC-SPERY 10 7.3310–2

G-TC-TECOP 1,000 1.5910–2
52 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

than reasonable. This may result from infiltrating 

water moving past the root zone but being diverted 

before reaching the water table by smaller K rock 

units. Bedinger and others (1989) account for a similar 

situation in metamorphic and crystalline rocks in the 

Death Valley region. They suggest that the hydraulic 

conductivity of metamorphic and crystalline rocks at 

depths less than 300 m below land surface is significantly larger than rocks at depths greater than 300 m. 

To test this interpretation in the DVRFS flow model, 

the HFM was modified to represent confining units 

that occur within 300 m of land surface (“upper” units) 

and those that occur at depths more than 300 m below 

land surface (“lower” units). Then, recharge parameters were defined by grouping recharge rates occurring 

on “upper” and “lower’” confining units. Recharge 

parameters with this configuration allowed net infiltration to be reduced slightly more on smaller K “lower” 

confining units. However, the effective recharge on all 

of the clastic and crystalline rocks was nearly zero 

(table 13). Only slight amounts of recharge were 

required to maintain the ground-water mounds occurring in these areas. 

Variations in Interpretation of Hydrogeologic Framework

Four general types of changes were made to the 

hydrogeologic conceptual model during calibration. 

These include: (1) splitting major rock types into more 

detailed and complex hydrogeologic units, (2) subdividing hydrogeologic units into spatial regions or 

zones, (3) adding vertical anisotropy parameters, and 

(4) differentiating hydrogeologic-unit parameters 

based on depth. Initially, the hydrogeologic conceptual 

model used in the DVRFS flow model was simplified 

to represent four major rock types including confining 

units (K1), carbonate rocks (K2), volcanic rocks (K3), 

and basin-fill units (K4). These rock types were 

considered homogeneous and isotropic and were 

represented by K parameters. Then, the hydrogeologic 

framework representation was evaluated by calculating hydraulic heads and ground-water discharges 

using MODFLOW-2000. Sensitivities were calculated 

for parameters in each conceptual model representation. Parameters with high CSS values were further 

divided. For example, the carbonate rock K parameter 

was subdivided first because of its high sensitivity. 

This logic was followed for all K parameters that had 

high enough sensitivities. Model fit generally 

improved as K parameters were subdivided into more 

specific hydrogeologic units. Some parameters created 

during this process had low CSS values. The assigned 

value for these parameters did not influence calculated 

heads and flows significantly; however, defining these 

parameters helped in the overall understanding of how 

the ground-water flow system functions.

Table 13. Recharge parameter descriptions, values, and composite scaled sensitivities

Parameter 

name Description Parameter value

Composite 

scaled

sensitivity

RCH_zone1 Basin-fill units 0.0 3.6810–2

RCH_zone2a Volcanic units–Timber Mountain and Pahute Mesa area 1.0 2.31

RCH_zone2b Volcanic units–All other 1.0 2.15

RCH_zone3a Crystalline units–Upper 300 meters of TJi & pCgm 0.005 2.7410–3

RCH_zone3b Clastic units–Upper 300 meters of UCCU 0.05 1.0610–3

RCH_zone3c Clastic units–Upper 300 meters of LCCU & Mvs 0.01 2.91

RCH_zone3d Clastic units–LCCU thrusts 0.05 6.0210–2

RCH_zone4a Crystalline units–TJi & pCgm deeper than 300 meters 0.001 6.7510–2

RCH_zone4b Clastic units–UCCU deeper than 300 meters 0.01 16.8

RCH_zone4c Clastic units–LCCU deeper than 300 meters 0.00003 1.21

RCH_zone5a Carbonate units in central/western area 0.12 0.174

RCH_zone5b Carbonate units in Spring Mountains area 0.55 2.01

RCH_zone5c Carbonate units in northeastern area 0.25 1.66
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 53

0.37–1.10

1.10–3.65

3.65–11.00

11.00–18.26

18.26–25.57

25.57–36.53

36.53–73.05

73.05–109.58

109.58–146.10

146.10–182.63

0–0.37

37°

36°

38°

117° 116°

NEVADA

TEST SITE

BOUNDARY

EXPLANATION

MODEL GRID

BOUNDARY

0 40

0 80 KILOMETERS 40

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

RECHARGE, IN MILLIMETERS 

 PER YEAR—

Confining Units

The confining-unit rock type consists of crystalline and clastic rocks. Crystalline hydrogeologic units 

include Precambrian granites and metamorphic rocks 

(pCgm) and Tertiary-Jurassic intrusives (TJi). Clastic 

hydrogeologic units consist of the upper clastic 

confining unit (UCCU), lower clastic confining unit 

(LCCU), and the Mesozoic volcaniclastic and sedimentary rocks (Mvs).

Initially, all rocks interpreted to be confining 

units were grouped into one K parameter. The sensitivity of this parameter was lower than the three other 

major rock-type parameters. This may be a result of 

Figure 21. Recharge simulated in the Death Valley regional flow system model.
54 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

there being smaller volumes of water flowing through 

these smaller K rocks and few hydraulic-head observations present in these hydrogeologic units. This coarse 

representation of the hydrogeologic framework 

resulted in a simulated head distribution that generally 

matched the location of hydraulic gradients. The simulated hydraulic gradients, however, were in many 

places not as well defined because many smaller K 

rocks were lumped together. When various types of 

smaller K rocks were differentiated into different K 

parameters (for example, crystalline and clastic units) 

a closer match to large hydraulic gradients resulted.

Smaller K rocks also control the direction and 

magnitude of dominant regional ground-water flow 

paths. In many cases, deeply buried, crystalline rocks 

form the core of mountain ranges in the region. These 

rocks are generally barriers to interbasin flow and, in 

some areas, affect flow to discharge areas. Also, 

clastic rocks occur higher in the section than crystalline rocks and can also form mountain-range blocks to 

regional flow, such as in the Funeral Mountains and 

Sheep Range (fig. 11). In some cases, the further 

differentiation of these clastic and crystalline rocks 

improved not only model fit to hydraulic heads but 

also model fit to flows out of the model at important 

regional ground-water discharge areas.

Zone arrays and parameters also were used to 

refine the distribution of K parameters for clastic and 

crystalline rocks (figs. 22, 23, 24, and 25). Hydraulicconductivity parameter zones were delineated to 

isolate largely continuous areas of a hydrogeologic 

unit or to isolate areas with potentially similar structural deformation.

The hydrogeologic reasoning supporting this is 

that isolated or relatively continuous sections of rocks 

could be compositionally distinct or have undergone 

similar structural deformation. Also, different forms of 

structural deformation tend to affect the hydraulic 

properties of ductile and brittle rocks differently. For 

example, shear-zone deformation tends to reduce the 

hydraulic conductivity of ductile rocks, such as the 

clastic units, but may increase the hydraulic conductivity of brittle rocks, such as the crystalline units. 

Extensional deformation tends to increase the 

hydraulic conductivity of these units by creating or 

enhancing secondary permeability.

Crystalline and clastic K parameters defined by 

spatial zones had varying degrees of impact on the 

flow model. For example, the large hydraulic gradient 

from the Grapevine Mountains into Death Valley was 

simulated more accurately when the LCCU K parameter in the Grapevine Mountains was zoned along 

faults bounding the Grapevine Mountains (fig. 22). By 

better defining this large hydraulic gradient, more 

reasonable volumes of water flowing through the 

Grapevine Mountains to discharge points in northern 

Death Valley were simulated. Similarly, LCCU parameters defined by zones in areas coincident with structural basins also controlled intrabasin and interbasin 

flow to discharge areas (fig. 22).

Although the pCgm and LCCU parameters 

generally have smaller K, larger K values were 

required to allow ground water to flow from Pahrump 

Valley to Shoshone and Tecopa Valleys. CSS values 

for the pCgm K parameters were generally low, 

suggesting that there were insufficient data supporting 

these parameters. Also, K for most pCgm parameters 

was small, which would limit the flow through this 

unit and tend to lower model sensitivity. Even though 

model sensitivity was generally low for the pCgm and 

TJi K parameters, they play an important role in 

directing interbasin and intrabasin flow (figs. 23 and 

24). A summary of how confining unit parameters 

were subdivided is presented in table 14. A description 

of LCCU, pCgm, and TJi parameters is presented in 

table 15, table 16, and table 17.

Carbonate Rocks

The carbonate-rock group consists of the upper 

carbonate aquifer (UCA) (fig. 26) and the lower 

carbonate aquifer (LCA) (fig. 27). These hydrogeologic units were initially grouped into one K parameter, and the CSS value was more than four times 

greater than the parameters defining the other major 

rock types. This high sensitivity was expected because 

the LCA is the most extensive and prolific aquifer in 

the region. Because of this high sensitivity, the K 

parameter defining carbonate rocks was subdivided 

into two K parameters for the UCA and LCA hydrogeologic units. The LCA remained highly sensitive. 

As a result, the four HFM model units describing LCA 

rocks contained in thrust sheets were then separated 

into individual K parameters. On a regional scale, the 

K parameters defining LCA thrust units are relatively 

unimportant; however, these K parameters can influence flow patterns on a local scale. For example, 

discharges at Nevares, Texas, and Travertine Springs 

were strongly influenced by the K of the LCA_T1 

parameter (fig. 26). Because the LCA hydrogeologic
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 55

K1222ZoneL10

K1221ZoneU10

K1222ZoneL7

K1221ZoneU7

K1221ZoneU12

K1222ZoneL12

K1221ZoneU2

K1222ZoneL2

K1221ZoneU3

K1222ZoneL3

K1221ZoneU1

K1222ZoneL1

K1221ZoneU11

K1222ZoneL11

K1221ZoneU12

K1222ZoneL12

K1221ZoneU6

K1222ZoneL6

K1221ZoneU5

K1222ZoneL5

K1221ZoneU7

K1222ZoneL7

K1221ZoneU4

K1222ZoneL4

K1221ZoneU8

K1222ZoneL8

K1221ZoneU9

K1222ZoneL9

K1221ZoneU10

K1222ZoneL10

36°

37°

38°

117° 116°

LCCU THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

LCCU ZONES

WITH PARAMETER

NAMES 

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

EXPLANATION

K1221ZoneU7

K1222ZoneL7

MODEL GRID

BOUNDARY

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 22. Definition of lower clastic confining unit (LCCU) hydraulic-conductivity 

zone parameters and unit thickness.
56 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K1112Zone2

K1114Zone4

K1116Zone6

K1111Zone1

K1118Zone8

K1113Zone3

K1117Zone7

K1115Zone5

K1111Zone1

36°

37°

38°

117° 116°

pCgm THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

pCgm ZONES

WITH PARAMETER

NAME

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K1118Zone8

EXPLANATION

MODEL GRID

BOUNDARY

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 23. Definition of Precambrian granites and metamorphic rocks (pCgm) 

hydraulic-conductivity zone parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 57

K1121TJIL2

K1121TJIU2

K1121TJIL1

K1121TJIU1

36°

37°

38°

117° 116°

TJi THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

TJi ZONES

WITH PARAMETER

NAMES

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K1121TJIU2

K1121TJIL2

EXPLANATION

MODEL GRID

BOUNDARY

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 24. Definition of Tertiary-Jurassic intrusives (TJi) hydraulic-conductivity zone 

parameters and unit thickness.
58 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

36°

37°

38°

117° 116°

NEVADA TEST

SITE BOUNDARY

UCCU ZONES WITH

PARAMETER NAME

K1211UPCUP

K1211UPCUP

K1212UPCLO

K1212UPCLO

LCCU—THRUSTS ZONES

WITH PARAMETER

NAME

K1223LOCTH

K1223LOCTH

Mvs ZONES WITH

PARAMETER NAME K123MESCLA

K123MESCLA

K123MESCLA

EXPLANATION

MODEL GRID

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

0–175

175–350

350–525

525–700

700–875

875–1,050

1,050–1,225

1,225–1,400

UCCU THICKNESS,

IN METERS

LCCU—THRUSTS THICKNESS,

IN METERS

Mvs THICKNESS,

IN METERS

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 25. Definition of hydraulic-conductivity parameters and unit thickness for 

Mesozoic volcaniclastic and sedimentary rocks (Mvs), upper clastic confining 

unit (UCCU), and lower clastic confining unit (LCCU) thrust-fault units.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 59

Table 14. Hierarchical structure of hydrogeologic unit parameters and major events guiding parameter delineation

First order 

parameters1

Second order 

parameters2

Third order

parameters3

Fourth order

parameters4

Fifth order

parameters5

Sixth order

parameters5

K1 Confining units – 

crystalline and 

clastic rocks

K11 Crystalline 

rocks

K111 Precambrian 

granites and metamorphic rocks 

(pCgm)

Zones to determine 

pCgm role in 

Death Valley large 

hydraulic gradient

Zones to determine 

pCgm role at major 

discharge areas

K112 TertiaryJurassic intrusives 

(TJi)

Zone to test extent of 

TJi in Greenwater 

Range and role 

in discharge at 

Franklin Lake

K12 Clastic rocks K121 Upper clastic 

confining unit 

(UCCU)

K122 Lower clastic 

confining units 

(LCCU, 

LCCU_GP, 

LCCU_T1, 

LCCU_T2)

K1221 Lower clastic 

confining unit 

(LCCU) - 

Depth less than 

300 meters below 

land surface

Zones based on large 

hydraulic gradient 

areas and regions 

with similar structural deformation

Zones based on 

isolated sections 

and areas potentially impacting 

flow-system 

discharge

K1222 Lower clastic 

confining unit 

(LCCU) - 

Depth greater than 

300 meters below 

land surface

Zones based on large 

hydraulic gradient 

areas and regions 

with similar structural deformation

Zones based on 

isolated sections 

and areas potentially impacting 

flow-system 

discharge

K1223 Lower clastic 

confining unit 

thrusts 

(LCCU_GP, 

LCCU_T1, 

LCCU_T2)

K123 Mesozoic 

volcaniclastic and 

sedimentary rocks 

(Mvs, Mvs_KS, 

Mvs_LC)

K2 Carbonate rocks K21 Lower 

carbonate aquifer 

(LCA, LCA_GP, 

LCA_LC, 

LCA_T1, 

LCA_T2)

K211 Lower 

carbonate aquifer 

(LCA)

Major structural 

zones defined by 

Carr (1990) and 

Faunt (1997).

Refinement of structural zones based 

on hydrologic data 

and more detailed 

geologic data

Zones to test uncertainty in hydrogeologic framework 

model and identify 

areas critical to 

flow-system 

discharge

K212 Lower 

carbonate aquifer 

thrusts (LCA_GP, 

LCA_LC, 

LCA_T1, 

LCA_T2)

K2121 Lower 

carbonate aquifer 

T1 thrust 

(LCA_T1)

K2122 Lower 

carbonate aquifer 

T2 thrust 

(LCA_T2)

K2123 Lower 

carbonate aquifer 

Gass Peak thrust 

(LCA_GP)
60 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

First order 

parameters1

Second order 

parameters2

Third order

parameters3

Fourth order

parameters4

Fifth order

parameters5

Sixth order

parameters5

K2124 Lower 

carbonate aquifer 

Lee Canyon thrust 

(LCA_LC)

K22 Upper carbonate 

aquifer (UCA)

K3 Volcanic rocks K31 Undifferentiated 

volcanic rocks 

outside of Nevada 

Test Site (VU)

Zones based on major 

volcanic regions

Zones based on 

volcanic centers

K32 Southern 

Nevada Test 

Site/Yucca Mountain volcanic rocks

K321 Volcanic 

aquifer (VA)

K322 Volcanic 

confining unit 

(VCU)

K33 Timber Mountain caldera 

complex/Pahute 

Mesa volcanic 

rocks

K331 Belted Range 

aquifer (TBA)

Vertical anisotropy 

parameter added

K332 Basal 

confining unit 

(TBCU) 

(nonwelded tuff)

Vertical anisotropy 

parameter added

K333 Basal aquifer 

(TBQ) (welded 

tuff)

Vertical anisotropy 

parameter added

K334 Bullfrog 

confining unit 

(TCB)

Vertical anisotropy 

parameter added

K335 Paintbrush/Calico Hills 

tuff cone (TC)

Vertical anisotropy 

parameter added

K336 Timber Mountain aquifer (TMA)Vertical anisotropy 

parameter added

K4 Basin-fill units K41 QuaternaryTertiary valley-fill 

alluvium (QTal)

Zones for basins with 

similar depositional environments

Zones for major 

basins

Zones to identify 

areas critical to 

flow system 

discharge

K42 QuaternaryTertiary playa 

deposits (QTp)

Zones for playas with 

similar depositional environments – Death 

Valley, Amargosa 

Valley, all others

Zones for major 

playas within 

Death Valley

K43 Tertiary sediments/Death 

Valley 

section(TSDVS)

Zones inside and 

outside Death 

Valley

Zones based on 

discharge areas 

(northern and 

central zones 

within Death 

Valley)

Zones to test importance of TSDVS 

outside of Death 

Valley

1 Parameters subdivided by major rock types.

2 Parameters subdivided by major hydrogeologic units.

3 Parameters subdivided by major hydrogeologic units and zones with similar characteristics.

4 Parameters subdivided by zones with similar characteristics, depth greater or less than 300 meters, thrust-faulted hydrogeologic units, and vertical 

anisotropy. 5

 Parameters subdivided by zones with similar characteristics.

Table 14. Hierarchical structure of hydrogeologic unit parameters and major events guiding parameter delineation—Continued
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 61

Table 15. Description of lower clastic confining unit (LCCU) horizontal hydraulic-conductivity parameters

Parameter name Description

K1221LOCU1 and 

K1221LOCL1

This zone includes LCCU rocks in the Bare Mountain area and to a lesser extent in the Sarcobatus Flats 

area. This zone was delineated to differentiate Bare Mountain LCCU rocks from rocks in the Grapevine Mountains and Bullfrog Hills. LCCU rocks in the Bare Mountain area could potentially have 

different hydraulic properties because Bare Mountain has undergone detachment faulting and Bullfrog Hills has undergone a degree of hydrothermal alteration.

K1221LOCU2 and 

K1221LOCL2

This zone includes LCCU rocks in the Penoyer and Emigrant Valley areas. This zone was delineated to 

evaluate whether or not the LCCU provides an effective barrier to flow out of Penoyer and Emigrant 

Valleys into Tikaboo and Indian Springs Valleys.

K1221LOCU3 and 

K1221LOCL3

This zone includes LCCU rocks in the northwest Walker Lane from the Slate Ridge and Lida Valley 

area to Pahute Mesa. The geometry and extent of these rocks in the subsurface north of Pahute Mesa 

is highly interpretive.

K1221LOCU4 and 

K1221LOCL4

This zone includes LCCU rocks in the Inyo-Mono structural zone. The geometry and extent of these 

rocks are highly interpretive under Death Valley and less so where they outcrop in the Panamint 

Range.

K1221LOCU5 and 

K1221LOCL5

This zone includes thick sections of LCCU rocks in the northwest and northeast part of the Spring 

Mountains where they have likely undergone deformation due to thrust faulting.

K1221LOCU6 and 

K1221LOCL6

This zone includes a small area of LCCU rocks in the vicinity of the Desert Range. Rocks in this area 

are interpreted as being affected by the structural deformation occurring in the Pahranagat shear 

zone.

K1221LOCU7 and 

K1221LOCL7

This zone includes a thick section of LCCU rocks interpreted as extending beneath Pahrump Valley. 

Rocks in this zone are believed to be affected by extensional faulting that caused the development of 

Pahrump Valley.

K1221LOCU8 and 

K1221LOCL8

This zone includes a relatively thick section of LCCU rocks interpreted as occurring in the subsurface 

immediately west of Death Valley Junction. The geometry, extent, and hydraulic nature of these 

rocks are interpreted from geologic structure of the southern Funeral Mountains and the Furnace 

Creek Fault zone.

K1221LOCU9 and 

K1221LOCL9

This zone includes relatively shallow LCCU rocks in the Shoshone/Tecopa area. The geometry, extent, 

and hydraulic nature of these rocks are interpreted from surface mapping.

K1221LOCU10 and 

K1221LOCL10

This zone includes a very thick sections of LCCU rocks interpreted to occur in the vicinity of Desert 

Dry Lake and the Sheep Range where they have likely undergone deformation due to thrust faulting 

in the Sheep Range and shear faulting associated with the Las Vegas Valley shear zone.

K1221LOCU11 and 

K1221LOCL11

This zone includes LCCU rocks in the Jackass Flats and Amargosa Farms areas. The geometry, extent, 

and hydraulic nature of these rocks are interpreted from their apparent association with the Rock 

Valley fault.

K1221LOCU12 and 

K1221LOCL12

This zone includes a thick section of LCCU rocks in the Bullfrog Hills and the Grapevine Mountains. 

This zone was delineated to evaluate the uncertainty of the geometry, extent, and hydraulic nature of 

these rocks that apparently provide a very effective barrier to flow from Sarcobatus Flat to the Amargosa Valley and from the Amargosa Valley to Death Valley.
62 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Table 16. Description of Precambrian granites and metamorphic rocks (pCgm) horizontal hydraulic-conductivity parameters

Parameter name Description

K1111PCGM1 This zone includes pCgm rocks in the Bare Mountain Bullfrog Hills area. The pCgm rocks in this area 

are interpreted as providing a barrier to flow between the Sarcobatus Flat/Oasis Valley area and the 

northwestern arm of the Amargosa Valley. They are differentiated from other pCgm rocks in the region 

because they are exposed core complexes of detachment faulting and have potentially been altered by 

hydrothermal processes in the Oasis Valley area.

K1112PCGM2 This zone includes pCgm rocks in the northern part of the model domain. The pCgm rocks in this area 

are interpreted as being very deep and relatively intact.

K1113PCGM3 This zone includes pCgm rocks beneath Death Valley and the Panamint Range. The complexity of 

geologic structures and the lack of subsurface data result in highly interpretive geometry and extent for 

rocks in this zone.

K1114PCGM4 This zone includes pCgm rocks in the Spring Mountains and Pahrump Valley area. The pCgm rocks in 

this area are interpreted as being quite deep. This zone was delineated so that the hydraulic conductivity of pCgm rocks could be changed to be more representative of LCCU rocks that are stratigraphically above the pCgm rocks.

K1115PCGM5 This zone includes pCgm rocks in the Tecopa/Shoshone area. The lack of subsurface data in this zone 

suggests that the hydrogeologic framework model is highly interpretive for this unit. This zone was 

delineative so that the hydraulic conductivity of pCgm rocks could be changed to be more representative of LCCU rocks that are stratigraphically above the pCgm rocks.

K1116PCGM6 This zone includes pCgm rocks in the Black Mountains and the Amargosa Range. The geology of this 

region is extremely complex and highly metamorphosed. This zone was delineated so the relative 

uncertainty of the hydrogeologic framework model in this area could be evaluated.

K1117PCGM7 This zone includes pCgm rocks in the interpreted NE/SW-trending structural zone extending from the 

Black Mountain Caldera to the Bonnie Claire area (Faunt, 1994; Carr, 1984). This zone was delineated 

to evaluate the conceptual model of more transmissive crystalline rocks associated with this feature.

K1118PCGM8 This zone includes pCgm rocks in the Grapevine Mountains. The geology of this region is extremely 

complex and the geometry and extent of these units are highly interpretive. This zone was delineated 

so the relative uncertainty of the hydrogeologic framework model in this area could be evaluated.

Table 17. Description of Tertiary-Jurassic intrusives (TJi) horizontal hydraulic-conductivity parameters

Parameter name Description

K1121TJIU1 and 

K1122TJIL1

This zone includes all Tertiary- through Jurassic-aged granitic intrusive rocks present in most of the 

model domain exclusive of the Greenwater Range.

K1121TJIU2 and 

K1122TJIL2

This zone includes Tertiary- through Jurassic-aged granitic intrusive rocks present in the Greenwater 

Range. These rocks were differentiated from granitic rocks in other parts of the model domain 

because the presence of small-K granitic rocks resulted in too much ground-water discharge at 

Franklin Lake playa.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 63

36°

37°

38°

117° 116°

NEVADA TEST

SITE BOUNDARY

LCA_T1 ZONES

WITH PARAMETER

NAME

K2121LCAT1

LCA_T2 ZONES

WITH PARAMETER

NAME

K2122LCAT2

LCA_GP ZONES

WITH PARAMETER

NAME

K2123LCAGP

LCA_LC ZONES

WITH PARAMETER

NAME

K2124LCALC

UCA ZONES

WITH PARAMETER

NAME

K2CARBVUCA

K2CARBVUCA

K2121LCAT1

K2122LCAT2

K2123LCAGP

K2124LCALC

EXPLANATION

MODEL GRID

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

0–75 

75–150

150–225

225–300

300–375

375–450

450–525

525–600

LCA_T1 THICKNESS,

IN METERS

LCA_T2 THICKNESS,

IN METERS

LCA_GP THICKNESS,

IN METERS

LCA_LC THICKNESS,

IN METERS

UCA THICKNESS,

IN METERS

0 40

0 80 KILOMETERS 40

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

0–125

125–250

250–375

375–500

500–625

625–750

750–875

875–1,000

Figure 26. Definition of hydraulic-conductivity parameters and unit thickness for upper carbonate 

aquifer (UCA) and for lower carbonate aquifer (LCA) thrust-fault units.
64 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K211hZon16

K211hZon15

K211hZon11

K211hZon13

K211hZon12

K211hZon14

K211hZon19

K211hZon17

K211hZon10

K211hZone3

K211hZone2

K211hZone7

K211hZone8

K211hZone1

K211hZone9

K211hZone6

K211hZone5

K211hZone4

K211hZon18

36°

37°

38°

117° 116°

LCA THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

LCA ZONES

WITH PARAMETER

NAME 0 - 250

250 - 500

500 - 750

750 - 1,000

1,000 - 1,250

1,250 - 1,500

1,500 - 1,750

1,750 - 2,000

K211hZone8

MODEL GRID

BOUNDARY

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 27. Definition of lower carbonate aquifer (LCA) hydraulic-conductivity zone 

parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 65

unit still had a high CSS value, it was subdivided into 

spatial zones defining different K parameters (fig. 27). 

These zones were initially defined based on structuralphysiographic subsections identified by Carr (1990) 

and Faunt (1994). Although delineating these zones 

helped improve model fit and the simulation of 

regional potentiometric features, they were too 

coarsely, or too regionally, defined to help improve 

subregional features. Therefore, an iterative approach 

was used to refine and subdivide the structural-physiographic subsections, so that they better reflected the 

hydrogeologic conditions being simulated. The HFM 

provided the basis for modifications of the original 

structural-physiographic subsections. Zones were 

defined and adjusted to be consistent with the HFM 

and observed water levels. Small hydraulic gradient 

areas with relatively uniform geology and consistent 

water levels were interpreted to have related hydrogeologic characteristics. Attempts were made to keep 

these types of areas in one K parameter.

On a subregional scale, simulating the large 

hydraulic gradient from Pahute Mesa into Yucca Flat 

required additional modification of the structuralphysiographic subsection zones. For example, the 

boundary of the original structural zone divided the 

head observations representing the small hydraulic 

gradient areas of Yucca Flat into two zones. The northwestern boundary of the Spotted Range–Mine Mountain structural zone (parameter K211hzon11) was 

modified to coincide with the CP thrust west of Yucca 

Flat (fig. 27). This minor modification allowed heads 

to be better simulated in the western Yucca Flat and 

Yucca Mountain areas. A zone was also created in the 

area north of Yucca Mountain and west of Yucca Flat 

(parameter K211hzon15) to determine if the LCA was 

important in defining the large hydraulic gradient in 

this area (fig. 27). The LCA in this zone is juxtaposed 

against the Timber Mountain and Pahute Mesa 

volcanic units. Ultimately, an HFB was delineated in 

the model domain along the trend of the UCCU to 

provide a more effective barrier between Pahute Mesa 

and western Yucca Flat. This HFB (HFB_Eleana) 

effectively represents the more local-scale barrier 

effects of the UCCU that are minimized when the 

HFM is resampled to the flow-model grid (fig. 9).

In some cases, additional hydrogeologic detail 

was required to better simulate flow out of regional 

discharges areas. The hydrogeologic representation of 

the area from Ash Meadows to Franklin Lake Playa, 

for example, is a good illustration. Initially, one highsensitivity LCA K parameter (parameter K211hzone5) 

was defined for this area (fig. 27). During a series of 

parameter sensitivity evaluations, this parameter was 

subdivided, ultimately, into three parameters 

(K211hzone5, K211hzon18, and K211hzon19), and it 

was discovered that the LCA rocks in the central part 

of Ash Meadows (parameter K211hzon18) had the 

highest sensitivity and had the most influence on 

hydraulic heads and flows in this area (fig. 27).

The Las Vegas Valley shear zone (LVVSZ) is 

another area where increasingly complex hydrogeologic conceptual models were used to improve model 

fit. LCA rocks in areas immediately north and east of 

the LVVSZ possess a distinct oroflexural bending 

associated with stresses at the fault zone. The 

hydraulic nature of LCA rocks in this area is believed 

to be affected by this structural condition. In fact, these 

areas were interpreted to have preferential flow directions along this structural fabric. To resolve high sensitivities for LCA K parameters in this area, additional 

LCA zone parameters (K211hzone9 and K211hzon10) 

were added near the shear zone (fig. 27). Although 

better model fit in this area resulted, the model 

currently does not accurately simulate observed conditions in this area. This may result from an inaccurate 

representation of the hydrogeologic complexity of the 

area in the current HFM. A summary of how lower 

carbonate aquifer parameters were subdivided is 

presented in table 14. A description of LCA parameters is presented in table 18.

Volcanic Rocks

The volcanic rock group consists of the undifferentiated volcanic (VU), volcanic aquifer (VA), 

volcanic confining unit (VCU), and the Timber Mountain/Pahute Mesa hydrogeologic units (TBA, TBCU, 

TBQ, TCB, TC, and TMA). Although these units 

occur throughout the modeled area, they most 

commonly occur in the central and northern parts of 

the domain in the vicinity of the NTS where they have 

been extensively studied (fig. 28 through fig. 36).

Initially, all of the volcanic units were grouped 

into one K parameter that was calculated as being the 

second most sensitive K parameter. The first step in 

evaluating these units was to subdivide them into three 

parameters consisting of Timber Mountain/Pahute 

Mesa volcanic units, Yucca Mountain volcanic units, 

and undifferentiated volcanic rocks (VU). Although 

the spatial extent of the Timber Mountain/Pahute 
66 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Table 18. Description of lower carbonate aquifer (LCA) horizontal hydraulic-conductivity parameters

Parameter name Description

K211hzone1 This zone includes LCA rocks in the SE Walker Lane structural zone southwest of the Pahrump-State Line 

Fault in the area around Shoshone and Tecopa, Calif. LCA rocks in this zone occur in fairly discontinuous, but relatively thick sections.

K211hzone2 This zone includes LCA rocks in the northwest part of the model domain in the Walker Lane structural 

zone. LCA in this zone is concentrated in an area extending north of Oasis Valley and northern Sarcobatus Flat to Stonewall Mountain and northeast to Gold Flat and extreme western Pahute Mesa. Because 

these LCA rocks occur only in the subsurface, the geometry, extent, and hydraulic nature are highly 

interpretive.

K211hzone3 This zone includes a thick section of LCA rocks in the Basin and Range structural-physiographic subsection of the study area. The LCA rocks in this area are slightly extended and interpreted as being fairly 

transmissive.

K211hzone4 This zone includes LCA rocks that are interpreted as having similar attributes as those in K211hzone3. 

However, these LCA rocks have potentially larger hydraulic conductivities associated with the northeast-southwest trending faulting in the Pahranagat shear zone. 

K211hzone5 This zone includes LCA rocks around and beneath Amargosa Flats and was delineated because of 

extremely high parameter sensitivities for LCA rocks in the Ash Meadows area.

K211hzone6 This zone includes LCA rocks in the Spring Mountains. It was delineated to resolve issues of high parameter sensitivity and large hydraulic gradients observed on the northeast side of the Spring Mountains.

K211hzone7 This zone includes LCA rocks in the Grapevine Mountains in the vicinity of Bullfrog Hills. This zone was 

delineated to test the hypothesis of smaller hydraulic conductivity associated with hydrothermal alteration in LCA rocks near Oasis Valley.

K211hzone8 This zone includes LCA rocks in the Inyo-Mono structural-physiographic subsection of the study area. 

This zone delineates LCA rocks that occur mostly in the Panamint Range and in the subsurface beneath 

Death Valley. Very little is known about the hydraulic nature of these rocks.

K211hzone9 This zone includes LCA rocks in an area of distinct oroflexural bending immediately north of the Las 

Vegas Valley fault. The hydraulic nature of LCA rocks in this area is believed to be affected by this 

structural condition.

K211hzon10 This zone includes LCA rocks immediately north of the Las Vegas flexural zone described in 

K211hzone9. This zone was originally delineated because of its somewhat different structural character 

from K211hzone9.

K211hzon11 This zone includes very thick, highly fractured and faulted LCA rocks mostly in the Spotted Range–

Mine Mountain structural zone. The hydraulic nature of LCA rocks in this area is believed to be affected 

by the extreme northeast-southwest faulting and fracturing.

K211hzon12 This zone includes LCA rocks at the southern end of the Funeral Mountains. The geometry and extent of 

LCA rocks in this zone are highly interpretive in the subsurface.

K211hzon13 This zone includes LCA rocks west and northwest of Death Valley Junction along Furnace Creek Wash. 

This zone was delineated to evaluate the sensitivity of LCA rocks that could provide a ground-water 

pathway from the Amargosa Valley to the Furnace Creek discharge.

K211hzon14 This zone includes a thick section of LCA rocks interpreted as extending beneath Pahrump Valley. Rocks 

in this zone are believed to be affected by extensional faulting that caused the development of Pahrump 

Valley.

K211hzon15 This zone includes a relatively thick section of LCA north and west of the CP thrust. Rocks in this zone 

were delineated to account for the uncertainty in the geometry and extent of the LCA and the potential 

interfingering with rocks having similar hydraulic characteristics of the UCCU.

K211hzon16 This zone includes a relatively thick, isolated section of LCA rocks interpreted as occurring beneath 

Kawich Valley and northern Cactus Flats.

K211hzon17 This zone includes LCA rocks in the Cottonwood Mountains and Last Chance Range. The geometry, 

extent, and hydraulic nature of these rocks in the subsurface are highly uncertain.

K211hzon18 This zone includes LCA rocks in the immediate vicinity of the Ash Meadows spring line. This zone was 

delineated to account for the highly fractured nature of LCA rocks that control a large component of 

ground-water discharge in the model.

K211hzon19 This zone includes LCA rocks in the Franklin Lake playa area. This zone was delineated to differentiate 

these LCA rocks from those in the immediate vicinity of Ash Meadows.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 67

Mesa and Yucca Mountain volcanic units is relatively 

small, they had a high sensitivity that probably is due 

to the large number of observation boreholes penetrating these units. To improve model fit, K parameters 

were created for both of the Yucca Mountain hydrogeologic units (VA and VCU). Timber Mountain/Pahute Mesa units (TBA, TBCU, TBQ, TCB, TC, 

and TMA) were also subdivided into individual 

parameters. As a result of adding these parameters, 

model fit in the NTS area was improved; most of the 

improvement could be attributed to the TMA and VA 

parameters, which appeared to have the most impact 

on controlling hydraulic heads and improving model 

fit. The TC, TBCU, and TBQ Timber Mountain/Pahute Mesa hydrogeologic units were further 

subdivided into zone parameters to isolate those rocks 

located in the Claim Canyon caldera area. These zone 

parameters were developed to allow for a smaller K in 

the volcanic units of the Claim Canyon caldera, thus 

providing for a more effective barrier between Pahute 

Mesa and Yucca Mountain. Descriptions of the TC, 

TBCU, and TBQ units that were subdivided into zone 

parameters are provided in table 19, table 20, and table 

21. The spatial distribution and definition of all 

Timber Mountain/Pahute Mesa hydrogeologic units 

are presented in figs. 28 through 33.

The differentiation of the VA and VCU parameters was initially intended to isolate units of similar 

hydrologic characteristics. In fact, the Yucca Mountain 

volcanic units (VA and VCU) were intended to represent the aquifers (VA) and confining units (VCU) in 

the NTS area; however, in the final calibrated model 

configuration the VA and VCU units were further 

subdivided into zone parameters (figs. 34 and 35) 

allowing for differing K values in different parts of 

Yucca Flat and Yucca Mountain. This result illustrates 

the high degree of spatial variability in the hydraulic 

properties of these units from the Yucca Flat/Rainier 

Mesa area to the Yucca Mountain area. In fact, 

geologic formations that are aquifers in Yucca Flat 

commonly grade into confining units at Yucca Mountain. Laczniak and others (1996, p. 15 and table 1) 

describe the hydrogeologic reasoning for this variation, stating that the hydrologic properties of these 

volcanic deposits are governed chiefly by the mode of 

eruption and cooling, by the extent of fracturing, and 

by the degree of secondary alteration. These characteristics typically vary within a given hydrogeologic unit. 

The final parameter configuration of the VA and VCU 

units appears to support this reasoning. Descriptions of 

the VA and VCU parameters are provided in tables 22 

and 23.

Increasingly detailed conceptualizations of the 

VU unit also were required to improve model fit in 

various parts of the model domain. For example, water 

levels in the northern part of the model domain 

appeared to be influencing the sensitivity of the VU 

parameter. Therefore, a zone array was used to subdivide the VU hydrogeologic unit based on volcanic 

centers in the region (fig. 36). Because hydrologic data 

were insufficient to delineate all of these volcanic 

centers, several were grouped together in the southern 

and eastern parts of the model domain. The final calibrated model configuration included eight VU zone 

parameters, the most sensitive of which was 

K315VU_NC located in Cactus and Gold Flats 

(fig. 36).

The accurate simulation of the large hydraulic 

gradient in the Bullfrog Hills area required increasingly complex representations of the hydrogeologic 

conceptual model in the vicinity of Oasis Valley. 

These conceptual models were required to improve 

model fit for both hydraulic heads and groundwater discharges. Originally, discharge at Oasis Valley 

was smaller than expected, and water levels immediately south of the Bullfrog Hills were too low. To 

improve model fit, an additional zone parameter 

(K318VU_BFH) for the VU hydrogeologic unit was 

created for the Bullfrog Hills region (fig. 36). In 

addition, an HFB was delineated along the trend of the 

Bullfrog Hills to represent a geometrically complex 

detachment fault that is interpreted to impede groundwater flow from Sarcobatus Flat to the northwestern 

Amargosa Valley (fig. 9). Although this conceptual 

alternative improved model fit to both hydraulic heads 

and ground-water discharges, the final calibrated 

model configuration still does not accurately simulate 

observed conditions. Perhaps an improved representation of the complex 3D hydrogeology in the vicinity of 

the Bullfrog Hills is required to better simulate 

ground-water conditions in this part of the model 

domain.

The final calibrated model configuration 

contains numerous parameters representing the hydrogeologic units in the volcanic rocks of the Death 

Valley region. Although many of the parameters do not 

have high sensitivities, their definition was warranted 

to evaluate the importance of various hydrogeologic 

units in the flow model. A summary of how volcanic
68 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

TBA THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

MODEL GRID

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

TBA ZONES

WITH PARAMETER

NAME

K331VOLTBA

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

K331VOLTBA

36°

37°

38°

117° 116°

Figure 28. Definition of Belted Range aquifer (TBA) hydraulic-conductivity zone 

parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 69

K3322TBC

K3322TBC

36°

37°

38°

117° 116°

K3322TBC2

K3321TBCCC

TBCU THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

TBCU ZONES

WITH PARAMETER

NAME 0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K3322TBC

MODEL GRID

BOUNDARY

EXPLANATION

0 40

0 40 80 MILES

80 KILOMETERS Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 29. Definition of Timber Mountain/Pahute Mesa basal confining unit (TBCU) 

hydraulic-conductivity zone parameters and unit thickness.
70 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K3332TBQ

K3331TBQCC

K3332TBQ

36°

37°

38°

117° 116°

TBQ THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

TBQ ZONES

WITH PARAMETER

NAME 0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K3332TBQ

MODEL GRID

BOUNDARY

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 30. Definition of Timber Mountain/Pahute Mesa basal aquifer (TBQ) 

hydraulic-conductivity zone parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 71

36°

37°

38°

117° 116°

TCB THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

MODEL GRID

BOUNDARY

TCB ZONES

WITH PARAMETER

NAME

K334VOLTCB

K334VOLTCB

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 31. Definition of Bullfrog confining unit (TCB) hydraulic-conductivity zone 

parameters and unit thickness.
72 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K3352TC

K3352TC

K3351TCCC

36°

37°

38°

117° 116°

TC THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

TC ZONES

WITH PARAMETER

NAME 0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K3352TC

MODEL GRID

BOUNDARY

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 32. Definition of Paintbrush/Calico Hills tuff cone (TC) hydraulic-conductivity 

zone parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 73

36°

37°

38°

117° 116°

TMA THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

MODEL GRID

BOUNDARY

TMA ZONES

WITH PARAMETER

NAME

K336VOLTMA

K336VOLTMA

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 33. Definition of Timber Mountain aquifer (TMA) hydraulic-conductivity zone 

parameters and unit thickness.
74 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K3212VASKM

K3213VAYF

K3211VAYM

36°

37°

38°

117° 116°

VA THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

MODEL GRID

BOUNDARY

VA ZONES

WITH PARAMETER

NAME

K3212VASKM

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 34. Definition of volcanic aquifer (VA) hydraulic-conductivity zone parameters 

and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 75

K3221VCUYM

K3222VCUYF

36°

37°

38°

117° 116°

VCU THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

MODEL GRID

BOUNDARY

VCU ZONES

WITH PARAMETER

NAME

K3221VCUYM

EXPLANATION

0 40

0 40 80 KILOMETERS

80 MILES

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon

Figure 35. Definition of volcanic confining unit (VCU) hydraulic-conductivity zone 

parameters and unit thickness.
76 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K311VU_SE

K311VU_SE

K313VU_YFF

K316VU_NE

K315VU_NC

K314VU_SAR K317VU_STN

K312VU_YM

K318VU_BFH

36°

37°

38°

117° 116°

VU THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750 –2,000

MODEL GRID

BOUNDARY

VU ZONES

WITH PARAMETER

NAME

K311VU_SE

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

Figure 36. Definition of undifferentiated volcanic rocks (VU) hydraulic-conductivity 

zone parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 77

Table 19. Description of Paintbrush/Calico Hills tuff cone (TC) horizontal hydraulic-conductivity parameters

Parameter name Description

K3351TCCC This zone includes the TC rocks in the Claim Canyon caldera area. This zone was delineated to differentiate these TC rocks that are interpreted as having smaller K resulting from hydrothermal alteration.

K3352TC This zone includes the TC rocks in the Timber Mountain area.

Table 20. Description of Timber Mountain/Pahute Mesa basal confining unit (TBCU) horizontal hydraulic-conductivity 

parameters

Parameter name Description

K3321TBCCC This zone includes the TBCU rocks in the Claim Canyon caldera area. This zone was delineated to differentiate these TBCU rocks that are interpreted as having smaller K resulting from hydrothermal alteration.

K3322TBC This zone includes the TBCU rocks in the Timber Mountain area.

K3322TBC2 This zone includes the TBCU rocks in the eastern Pahute Mesa area. This zone was delineated to differentiate these TBCU rocks that are interpreted as having larger K than other rocks in the Timber Mountain 

caldera.

Table 21. Description of Timber Mountain/Pahute Mesa basal aquifer (TBQ) horizontal hydraulic-conductivity parameters

Parameter name Description

K3331TBQCC This zone includes the TBQ rocks in the Claim Canyon caldera area. This zone was delineated to differentiate these TBQ rocks that are interpreted as having smaller K resulting from hydrothermal alteration.

K3332TBQ This zone includes the TBQ rocks in the Timber Mountain area.

Table 22. Description of volcanic aquifer (VA) horizontal hydraulic-conductivity parameters

Parameter name Description

K3211VAYM This zone includes the VA rocks in the Yucca Mountain area. This zone was delineated to differentiate the 

variable K of these VA rocks. The variability is recognizable as a result of the large concentration of 

hydraulic-head observations in this unit.

K3212VASKM This zone includes the VA rocks in the eastern Jackass Flats area. This zone was delineated to differentiate 

the variable K of these VA rocks. The variability is recognizable as a result of the large concentration of 

hydraulic-head observations in this unit.

K3213VAYF This zone includes the VA rocks in the Yucca Flat area. This zone was delineated to differentiate the variable K of these VA rocks. The variability is recognizable as a result of the large concentration of 

hydraulic-head observations in this unit.

Table 23. Description of volcanic confining unit (VCU) horizontal hydraulic-conductivity parameters

Parameter name Description

K3221VCUYM This zone includes the VCU rocks in the Yucca Mountain area. This zone was delineated to differentiate 

these VCU rocks as having hydraulic properties more representative of ash-flow tuffs in the Yucca 

Mountain area.

K3222VCUYF This zone includes the VCU rocks in the Yucca Flat and eastern Jackass Flats area. This zone was delineated to differentiate these VCU rocks as having hydraulic properties more representative of welded 

tuffs in the Yucca Flat and eastern Jackass Flats area.
78 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

parameters were subdivided is presented in table 14. A 

description of VU parameters is presented in table 24. 

Basin-Fill Units

The basin-fill rock group consists of the Quaternary-Tertiary valley-fill alluvium (QTal), QuaternaryTertiary playa deposits (QTp), and Tertiary sediments/Death Valley section (TSDVS) hydrogeologic 

units. Regionally, these units are relatively thin and 

discontinuous between basins; they dominate local 

ground-water flow patterns. The QTal is the most 

widely distributed basin-fill unit, and it is present in all 

basins in the region. The QTp, which has a smaller 

areal extent than the QTal, only occurs in playa areas, 

and is not present in all basins. The TSDVS is present 

predominantly in Death Valley but occurs locally in 

the central and southern parts of the model domain. 

Initially these units were grouped into one K parameter, and the sensitivity was less than the carbonate and 

volcanic rock parameters. Parameters were subsequently created for the individual units, and some 

improvement in model fit was achieved.

Because the depositional conditions vary in each 

valley, zone parameters were defined to evaluate the 

impact of this variability on the hydrologic characteristics of basin-fill units throughout the region (figs. 37, 

38, and 39). For example, QTp zone parameters were 

created to group playas with similar depositional environments (fig. 37). Discharging playas similar to the 

Death Valley saltpan contain large amounts of salts 

whereas playas similar to Yucca Flat contain large 

amounts of fine-grained clays typical of a dry playa. 

Zone parameters also were used to determine the 

importance of TSDVS units in controlling groundwater discharge to various parts of Death Valley 

(fig. 38). In many cases, the QTp and TSDVS units 

were used to limit the flow of ground water to 

discharge areas. 

Basin-fill parameters predominantly influenced 

water levels and flows near discharge areas. For 

example, the relatively thick sequence of QTal near 

Bennett Spring in Pahrump Valley controlled the 

direction of ground-water flow paths in the basin fill 

(fig. 39). Flow out of observations G-STEWRT-P and 

G-STEWRT-V in Stewart Valley and observations 

G-PAH-BENT and G-PAH-MANS in Pahrump Valley 

were influenced by the hydraulic conductivity of the 

QTal parameter (K415VALPAH) in this area. Similarly, a thick sequence of QTal also exists near 

Amargosa Farms. The hydraulic conductivity of this 

QTal parameter (K413VALAMV) influences 

hydraulic heads and ground-water discharges near 

Ash Meadows and Franklin Lake playa (fig. 18). A

Table 24. Description of undifferentiated volcanic rocks (VU) horizontal hydraulic-conductivity parameters

Parameter name Description

K311VU_SE This zone includes undifferentiated VU rocks in the southern and eastern parts of the model domain.

K312VU_YM This zone includes VU rocks in the immediate vicinity of Yucca Mountain and Crater Flat. VU rocks in 

this zone were delineated because they exhibit dominantly north-south faulting and fracturing.

K313VU_YFF This zone includes VU rocks in the immediate vicinity of Yucca and Frenchmen Flats. VU rocks in this 

zone were delineated because they exhibit dominantly northeast-southwest faulting and fracturing and 

possess a different degree of alteration than VU rocks in the Yucca Mountain area.

K314VU_SAR This zone includes VU rocks in the Sarcobatus Flat area. VU rocks in this zone were delineated because 

they exhibit various degrees of hydrothermal alteration.

K315VU_NC This zone includes VU rocks in the north and central part of the model domain in areas coincident with 

Kawich Valley and Cactus and Gold Flats. This zone was delineated to include VU rocks associated 

with the Cathedral Ridge and Kawich Ranges calderas.

K316VU_NE This zone includes VU rocks in the northeast part of the model domain. This zone was delineated to 

include a very thick, north-south-trending trough of volcanic rocks extending south from the Quinn 

Canyon Range caldera.

K317VU_STN This zone includes VU rocks in the vicinity of Stonewall Mountain and the Cactus Range, which are 

physiographic features resulting from calderas.

K318VU_BFH This zone includes VU rocks in the Bullfrog Hills area, immediately west of Oasis Valley. This zone was 

differentiated from the K314VU_SAR rocks because the apparent high degree of hydrothermal alteration in this area results in extremely small hydraulic-conductivity rocks.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 79

K4213SoutP

K4212CentP

K4211MesqP

K422AmargP K426FrankP

K423QTPlay

K425TecopP

K424PahruP

36°

37°

38°

117° 116°

QTp THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

MODEL GRID

BOUNDARY

QTp ZONES

WITH PARAMETER

NAME 0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K4213SoutP

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

Figure 37. Definition of Quaternary-Tertiary playa deposits (QTp) hydraulicconductivity zone parameters and unit thickness.
80 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

K431_TSeds

K431_TSeds

K4311TSUAM

K4321TSDVN

K4322TSDVC

36°

37°

38°

117° 116°

TSDVS THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

MODEL GRID

BOUNDARY

TSDVS ZONES

WITH PARAMETER

NAME

K4322TSDVC

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

Figure 38. Definition of Tertiary sediments/Death Valley section (TSDVS) 

hydraulic-conductivity zone parameters and unit thickness.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 81

K415VALPAH

K419VALTEC

K414VALDVC

K414VALDVN

K416VALNOR

K411VALCJF

K412VALLVV

K416VALNOR

K418VALPEN

K417VALCGF

K413VALAMV

36°

37°

38°

117° 116°

QTal THICKNESS, IN METERS

NEVADA

TEST SITE

BOUNDARY

QTal ZONES

WITH PARAMETER

NAME 0–250

250–500

500–750

750–1,000

1,000–1,250

1,250–1,500

1,500–1,750

1,750–2,000

K419VALTEC

MODEL GRID

BOUNDARY

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

Figure 39. Definition of Quaternary-Tertiary valley-fill alluvium (QTal) hydraulicconductivity zone parameters and unit thickness.
82 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

summary of how basin-fill parameters were subdivided is presented in table 14. Descriptions of 

QTal, QTp, and TSDVS parameters are presented in 

tables 25, 26, and 27.

Vertical Anisotropy

Vertical anisotropy parameters were initially 

defined for the major rock types. During the calibration process additional vertical anisotropy parameters 

were added for the UCA, LCA, QTp, QTal, TSDVS, 

and Timber Mountain/Pahute Mesa volcanic units. 

The sensitivity for these vertical anisotropy parameters 

was consistently low, and these parameters did not 

impact the simulation of heads and flows. Wide ranges 

of values were used for the vertical anisotropy parameters in attempts to better understand how they affected 

the flow system. The limited number of deep head 

observations and the regional model scale probably 

contributed to the low sensitivity of these parameters. 

Table 28 provides a brief description, parameter 

values, and CSS for the vertical anisotropy parameters.

Horizontal Flow Barriers

Many hydrogeologic features were simulated as 

horizontal flow barriers and evaluated (fig. 9 and table 

6) to determine their sensitivity in the flow model. 

Ranges of values were used to define these HFB 

parameters in an effort to better understand how they 

affected the flow system. Many of these parameters 

had no apparent effect on the simulation of heads and 

flows and were subsequently assigned parameter

Table 25. Description of Quaternary-Tertiary valley-fill alluvium (QTal) horizontal hydraulic-conductivity parameters

Parameter name Description

K411VALCJF This zone includes the basin-fill deposits of Crater and Jackass Flats and Rock Valley. Basin-fill materials in these valleys are dominated by volcanic alluvium derived from Yucca Mountain, Skull Mountain, and surrounding areas. Some amounts of carbonate are derived from Bare Mountain and the 

Striped Hills areas. Locally the basin fill is known to be significantly interbedded with the tuff units. 

Also, in many areas the basin fill includes lava flows.

K412VALLVV This zone includes the basin-fill units in and along the Las Vegas Valley shear zone. The zone is known 

to contain fine-grained materials associated with playas and late Tertiary and early Quaternary 

ground-water discharge areas. The zone is also likely to contain materials associated with the Las 

Vegas Valley fault that would potentially result in a mixture of basin-fill materials.

K413VALAMV This zone includes the basin fill of the Amargosa Valley. It is differentiated from the other basin-fill 

materials because of its known high variability of unconsolidated and consolidated content, including 

fluvial, alluvial, and lacustrine deposits, heterogeneous debris flow and fan deposits, and volcanic 

tuffs, lava flows, and tuffaceous sediments.

K414VALDVN This zone includes the basin fill of northern Death Valley, which contains a tremendous variety of materials derived from carbonate, clastic, granitic, volcanic, fluvial, and lacustrine sedimentary sources. It 

is highly variable, extremely high-energy colluvial material, interbedded with low-energy playa 

and eolian materials at Mesquite Flat.

K414VALDVC This zone includes the basin fill of central Death Valley, which contains a tremendous variety of materials derived from carbonate, clastic, granitic, volcanic, fluvial, and lacustrine sedimentary sources. It 

is highly variable, extremely high-energy colluvial material, interbedded with low-energy playa materials at the edge of the Death Valley saltpan.

K415VALPAH This zone includes the basin fill of Pahrump Valley, which is dominated by alluvial material derived 

from carbonate sources. In some areas, the basin fill contains locally high concentrations of materials 

derived from the Stirling quartzite that occurs in the Nopah Range.

K416VALNOR This zone includes the basin fill of the northern part of the Death Valley region. In the eastern part of the 

zone, basin-fill materials are dominantly derived from carbonate rocks with minor amounts of clastic 

rocks. In the western part of the zone the alluvium is dominated by volcanic-derived alluvial materials 

and may also have interbeds of tuff and lava flows.

K417VALCGF This zone is coincident with Cactus and Gold Flats. While somewhat similar to the other valleys in the 

western part of K416VALNOR zone, basin fill in this zone is noted for having somewhat noticeably 

higher amounts of silt.

K418VALPEN This zone is coincident with the basin-fill units of Penoyer Valley. The zone was differentiated from 

K416VALNOR because of the difficulty maintaining hydraulic heads near the Penoyer playa. This 

may suggest that these basin-fill materials contain more fine-grained materials similar to those in 

Cactus and Gold Flats.

K419VALTEC This zone includes basin-fill material in the Tecopa/Shoshone area. These basin-fill units contain 

significant amounts of lacustrine materials associated with pluvial Lake Tecopa.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 83

Table 26. Description of Quaternary-Tertiary playa deposits (QTp) horizontal hydraulic-conductivity parameters

Parameter name Description

K4211MesqP This zone includes the Quaternary playa materials occupying Mesquite Flat. This playa at the extreme 

north end of Death Valley includes a significant amount of fine-grained material, but also has a 

considerably high content of eolian sand and silt.

K4212CentP This zone includes Cottonball, Middle, and Badwater Basins on the Death Valley saltpan. The playa is 

dominantly silt and clay, containing extremely large amounts of salt.

K4213SoutP This zone contains playa materials at the extreme south end of Death Valley. These units are not as 

heavily laden with salts as the area to the north. The playa materials are high in silt and clay. This area 

is different from the Mesquite Flat playa because it does not contain as much sand as the Mesquite 

playa area.

K422AmargP This zone includes the playas of the Amargosa Valley including Amargosa Flat (Peter’s Playa) and 

Alkali Flat (Franklin Lake). These materials are either present-day or past ground-water discharge 

areas and contain large quantities of clay, salt and(or) spring carbonates. They can be differentiated 

from the playas in Death Valley because they do not contain the extremely high quantities of salt as 

those deposits; however, the Amargosa Valley ground-water-discharging playas contain considerably 

different fine-grained materials than the dry lake-bed playas of the remainder of the model area.

K423QTPlay This zone includes the dominantly dry playas of the Death Valley region. These playas are characterized 

by the high quantities of fine-grained silt and clay that they contain.

K424PahruP This zone includes the wet, ground-water-discharging playas of Pahrump Valley. These playas represent 

present-day discharging playas and are interpreted as having large quantities of clay, silt, and salt, and 

some spring deposits.

K425TecopP This zone includes the lake beds of pluvial Lake Tecopa. These playa units are differentiated from other 

past- and present-day playa/lake beds because they contain large quantities of water-laid ash-fall 

materials.

K426FrankP This zone includes playa units in the Franklin Well area of the Amargosa Valley. It is differentiated from 

other playa units in the Amargosa Valley because it contains a large amount of paludal and lacustrine 

deposits.

Table 27. Description of Tertiary sediments/Death Valley section (TSDVS) horizontal hydraulic-conductivity parameters

Parameter name Description

K431_Tseds This zone includes the Tertiary sedimentary rocks that occur dominantly in the Amargosa Valley and to 

the east of the Nevada Test Site. These hydrogeologic units include mostly lithified, or consolidated, 

alluvial fan deposits. In some cases they may include lacustrine limestone deposits and water-laid 

tuffaceous sediments.

K4311TSUAM This zone includes the Tertiary sedimentary units in the northwestern part of the Amargosa Valley immediately south of the Bullfrog Hills and west of Bare Mountain. These units are interpreted as 

containing larger quantities of clastic-derived, lithified fanglomerate material.

K4321TSDVN This zone includes the Tertiary sedimentary rocks in northern Death Valley in the vicinity of Mesquite 

Flat, Grapevine Springs, and Ubehebe Crater. These typically low-permeability rocks are quite variable and contain alluvial, fluvial, lacustrine, and tuffaceous sedimentary materials.

K4322TSDVC This zone includes the Tertiary sedimentary rocks in central Death Valley in the vicinity of Furnace 

Creek Wash. The dominant formations of this hydrogeologic unit include the Artist Drive Formation, 

the Furnace Creek Formation, and the Funeral Formation. These typically low-permeability rocks are 

quite variable and contain alluvial, fluvial, lacustrine, and tuffaceous sedimentary materials.
84 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

values of 1.0, which effectively removed them as flow 

barriers. Other HFB parameters were defined and 

assigned values to better represent changes in 

hydraulic conductivity (table 29).

Observation Data Review and Reweighting

During the model-calibration process, 

hydraulic-head and ground-water-discharge observations and corresponding weights were evaluated to 

determine if they appropriately constrained the flow 

model. In some cases, these model constraints, 

which were originally thought to be representative of 

predevelopment regional ground-water conditions, 

were modified.

Modifications to hydraulic-head observations 

involved either removing hydraulic-head observations 

or increasing standard deviations used to calculate 

observation weights. Hydraulic-head observations 

were removed when reinspection of the data resulted 

in a required change to the UseFlag (see “HydraulicHead Observations” section). In many cases, the 

authors concluded that some weights were unrealistically large (particularly those with standard deviations 

between 0 and 5 m), particularly when considering the 

inherent model error associated with a regional-scale 

simulation with grid cells of 1,500 m on the side. In 

some cases, the initial standard deviations (fig. 20) 

were determined to be unrealistic and were increased, 

particularly in large hydraulic gradient areas where 

Table 28. Vertical anisotropy parameter descriptions, values, and composite scaled sensitivities

Parameter

name Description

Parameter 

value

(Kh/Kv)

Composite scaled

sensitivity

K1_VANI Vertical anisotropy for pCgm, TJi, UCCU, and LCCU 1 1.79

K2CARBVUCA Vertical anisotropy for UCA 3 7.8610–5

K2CARBVLCT Vertical anisotropy for LCA thrust units 4.53 1.5710–2

K2CARBVLCA Vertical anisotropy for LCA 4 0.193

K3_VOLVANI Vertical anisotropy for VU, VA, and VCU 0.960 4.4110–2

K31TBAVANI Vertical anisotropy for TBA 1 4.5910–3

K32TBCVANI Vertical anisotropy for TBCU 1 5.7710–3

K33TBQVANI Vertical anisotropy for TBQ 1 1.8110–3

K34TCBVANI Vertical anisotropy for TCB 1 4.3010–3

K35TCVANI Vertical anisotropy for TC 1 1.2310–2

K36TMAVANI Vertical anisotropy for TMA 1 5.5710–2

K4VANIQTAL Vertical anisotropy for QTal 1 5.7110–2

K4VANIQTP Vertical anisotropy for QTp 1 7.8110–2

K4VANITSDV Vertical anisotropy for TSDVS 1 0.229

model error would be apparent. Throughout the model, 

standard deviations were increased to a minimum of 

3 m. The distribution of hydraulic-head standard deviations in the calibrated model more appropriately 

reflects the potential for model error, and most standard 

deviations are in the 3- to 10-m range (fig. 20).

Modifications also were made to ground-waterdischarge observation coefficients of variation during 

the calibration process. Ground-water discharge observations, however, were not modified. The approach 

used to determine coefficients of variation may not 

have adequately considered all sources of observation 

error. Model error, discharge-estimation methods, and 

magnitude of discharge rate were considered during the 

calibration process and, where necessary, revised coefficients of variation were modified to reflect (1) the 

cumulative error, (2) the relative observation importance, and (3) the confidence in the observation.

Because of the regional scale of the flow model, 

some hydrologic and geologic features were generalized. For example, many discharge areas represent individual springs that are significantly smaller in area than 

the simulated 1,500-m by 1,500-m grid. Small-scale 

features such as localized variations in hydraulic-head 

gradient, fault and fracture geometry, and abrupt 

lithology changes influence ground-water discharge 

rates. It was not possible to explicitly represent these 

types of features in the regional-scale model, and errors 

in simulation resulted.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 85

Similarly, discharge areas with flow less than 

1,000 m3

/d were difficult to simulate. These discharge 

areas are relatively minor considering the overall volumetric budget and model scale. In some cases, a small 

volumetric discharge from a discrete spring may be the 

most accurately measured in the system; however, 

model error could be equivalent to or larger than the 

flow magnitude. To address this issue, coefficients of 

variation of 50 or 100 percent were assigned to flow 

observations that were less than 1,000 m3

/d. A 

100-percent error for these small-volume discharge 

areas was considered acceptable for a regional-scale 

model.

In some cases, individual springs such as Travertine, Texas, and Nevares Springs were simulated. 

Measurement or estimation of these springs also has 

associated error. For example, a discharge measurement at a discrete, single-orifice spring with a defined 

channel can be measured within 3 to 5 percent of the 

actual flow (Rantz, 1982). Many springs in the region, 

however, have diffuse, bifurcating discharges that are 

more difficult to measure and have measurement 

errors of 10 percent or greater. Small discharge seeps, 

also common in the DVRFS (for example, Grapevine 

Springs), are even more difficult to quantify and are 

often estimated with errors of at least 50 percent. 

Model Evaluation

After calibration, the DVRFS model was evaluated to assess the likely accuracy of simulated results. 

An advantage of calibrating the DVRFS model using 

nonlinear regression is that there exists a substantial 

methodology for model evaluation that facilitates a 

better understanding of model strengths and weaknesses. A protocol exists to evaluate the likely accuracy of simulated hydraulic heads and ground-water 

discharge, estimated and not estimated parameter 

values and associated sensitivities and confidence 

intervals, and other measures of parameter and prediction uncertainty.

Evaluation of Simulated Hydraulic Head and 

Ground-Water Discharge

The values matched by the regression (the 

elements of vector y of equation 1) initially included 

732 measured hydraulic-head and 46 measured 

ground-water-discharge observations. During calibration, 73 hydraulic-head observations were removed 

from the data set because of obvious errors revealed 

during closer evaluation and 7 head observations were 

added from recently drilled boreholes; thus, 666 head 

observations were matched in the final regression. 

Matching each of the 46 measured ground-water 

discharges during calibration was difficult. However, 

matching these 46 groups of ground-water discharges 

was more appropriate than attempting to match 

discharge at each of the 903 independent model cells 

that make up the ground-water discharge groups. Ultimately, 46 ground-water discharge observations, along 

with 666 hydraulic-head observations discussed 

above, were used as observations in the regression.

Table 29. Horizontal flow barrier parameter values 

and composite scaled sensitivities

Parameter

name 

Conductance, 

in meters 

squared per day

Composite 

scaled

sensitivity

HFB_CDVFCF 1 1.3510–6

HFB_DeathV 1 8.5910–6

HFB_IndSpr 2.6710–6 0.158

HFB_KeaneW 1 5.6610–7

HFB_LVVSZ 1.0010–7 0.125

HFB_NDVFCF 1 2.2210–6

HFB_Pahute 1 8.7510–6

HFBAmargos 1.4510–6 6.6110–2

HFBAshMead 1.0010–4 7.0310–2

HFBBareMtn 1.0010–3 2.4010–7

HFBBareMtF 1.0010–5 0.536

HFBEagleMt 1.0010–5 1.7510–3

HFBCraterF 1 2.5610–5

HFBFurnace 1 1.2310–5

HFBGravity 1.0010–6 8.5410–2

HFBHwy95 1.0010–5 0.156

HFBSaratog 1.0010–6 6.8710–3

HFBPahrump 1.0010–5 0.227

HFBStewart 1.0010–3 4.0710–2

HFBRestSpr 1 6.2610–5

HFBSolitar 1.0010–9 4.3410–4

HFBTecopa 1 9.0610–6

HFBTecPass 1 5.4810–7

HFB_Eleana 1.0010–9 4.5010–2

HFB_NCFYM 1 4.5710–5

HFBBulfrog 1.0010–6 0.378
86 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

Evaluation of Model Fit

Unweighted and weighted residuals (defined 

after equation 1) are important indicators of model fit 

and, depending somewhat on data quality, model accuracy. Consideration of unweighted residuals is intuitively appealing because the values have the dimensions of the observations and indicate, for example, 

that a hydraulic head is matched to within 10 m. 

Unweighted residuals can, however, be misleading 

because observations are measured with different 

accuracy.

Weighted residuals demonstrate model fit relative to what is expected in the calibration based on 

the precision, or noise, of the data. They are less 

intuitively appealing because they are dimensionless 

quantities that equal the number of standard deviations 

or coefficients of variation needed to equal the 

unweighted residual.

Unweighted hydraulic-head residuals tended to 

be larger in areas with moderate to large hydraulic 

gradients than in areas with flat gradients, so these 

three types of areas are discussed separately. In areas 

of relatively flat hydraulic gradients, the largest 

unweighted residuals have absolute values less than 

20 m and are commonly less than 10 m. In areas of 

moderate hydraulic gradients, the differences between 

simulated and observed heads are sometimes larger (as 

large as 50 m); however, they are commonly less than 

20 m. In areas of large hydraulic gradients, the difference between simulated and observed heads can 

exceed 100 m (although that occurrence is rare); 

however, the simulated gradients are similar to the 

gradients evident from the data. The match is good 

considering the over 2,000-m difference in hydraulic 

head across the model domain and the size of the grid 

relative to the width of some of the steep hydraulic 

gradients.

Matching ground-water discharges was difficult 

but provided important information to the calibration. 

Indeed, it would not have been possible to support the 

complex representation of the system produced 

through the calibration without the ground-water 

discharge data. The sum of all simulated ground-water 

discharge is 286,800 m3

/d; the sum of estimated, or 

observed, ground-water discharge is 295,500 m3/d 

(table 30). As a result, simulated total ground-water 

discharge is within about 3 percent of observed.

When weighted as described above, S(b) for 

heads equals 6,178, for flows it is 395, and the total is 

6,573. The standard error of the regression equals 3.1, 

which indicates that overall model fit is consistent 

with head standard deviations that are 3.1 times the 

assigned values. For flow, overall model fit is consistent with 3.1 times the assigned coefficients of variation of 10 to 100 percent. Thus, effective model fit is 

between 31 and 310 percent.

Spatial Distribution of Unweighted and 

Weighted Residuals

Comparison of simulated hydraulic heads with 

the potentiometric surface map developed by 

D'Agnese and others (1998) indicates that the DVRFS 

model results depict major features of the head distribution well. In general, areas of flat and large 

hydraulic gradients are appropriately located. A more 

detailed evaluation of simulated hydraulic heads can 

be obtained through inspection of maps of residuals. 

Unweighted residuals for hydraulic heads were 

plotted to show their spatial distribution (fig. 40). 

Good fit to observed hydraulic heads (residuals with 

absolute values less than 10 m) generally occurs in 

areas of flat hydraulic gradients; moderate fit to 

observed heads (residuals with absolute values of 10 to 

20 m) occurs in the remainder of the flat hydraulic 

gradient areas. Poorer fit to observed heads (residuals 

with absolute values of greater than 20 m) appears to 

occur in large hydraulic gradient areas. Poorest fit to 

observed hydraulic heads occurs in the vicinity of 

Indian Springs, western Yucca Flat, and the southern 

Bullfrog Hills. Most of these inaccuracies can be 

attributed to (1) insufficient representation of the 

hydrogeology in the HFM, (2) misinterpretation of 

water levels, and (3) model error associated with grid 

cell size.

Weighted head residuals are plotted in figure 41. 

Patterns in the spatial distribution of weighted residuals indicate a nonrandom distribution and suggests 

that model error exists. Simulated heads in northwestern Cactus Flat, southern Railroad Valley, and 

Desert Valley are consistently higher than observed; 

however, all of these areas are close to model boundaries, and existing data are of poorer quality than in the 

remainder of the model domain. In the northwestern 

part of the Amargosa Valley, simulated heads are 

lower than observed nearer to the Bullfrog Hills and 

higher than observed farther south, suggesting some 

bias. For the remainder of the model, in areas of 

concentrated head observations there appear to be 
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 87

residuals that are both greater than and less than 

expected, suggesting a random distribution of 

weighted head residuals.

Ground-water discharge residuals are plotted in 

figure 42 along with the range of probable error associated with the observation. Residuals are calculated 

as observed minus simulated, and ground-water 

discharge has a negative sign convention. As simulated, the model appears to have a fairly random distribution of residuals with a slight bias toward being 

negative, indicating that simulated flows in these areas 

are less than observed flows. The largest unweighted 

ground-water discharge residuals occur at Ash 

Meadows (northern and central area), Sarcobatus Flat 

(northeastern area), Death Valley (Badwater and 

Cottonball Basins, Furnace Creek fan, and western 

alluvial fans), Penoyer Valley, and Pahrump Valley 

(near Manse Spring). The two major discharge areas 

that would contribute the largest volumetric error to 

the model are Ash Meadows and Death Valley. Ash 

Meadows (central area) is also one of the largest 

discharging sites in the DVRFS flow model; therefore, 

moderate percentages of error in the simulated 

discharge could result in comparatively large volumetric errors. While Death Valley is of concern, the 

ground-water discharge observations used in the 

DVRFS model are preliminary and should not be 

considered authoritative values of discharge from the 

Death Valley saltpan.

The plot of ground-water discharge residuals 

gives a better representation of how well the model 

matched ground-water discharge observations. This 

plot shows the acceptable range of error in the simulated ground-water discharge. In other words, simulated discharges within this range are acceptable 

because they are within the observation error. In most 

cases, simulated small-volume ground-water discharge 

values are within, or quite close to, the acceptable 

range. Some large-volume ground-water discharge 

areas at Ash Meadows, Oasis Valley, Tecopa, 

Pahrump, and Death Valley, however, are not 

discharging expected volumes (fig. 42) and, in one 

case at Death Valley (Cottonball Basin), there is a 

discharge of 9 times the expected volumetric error. 

The discharges for each individual discharge observation, however, appear to be randomly distributed 

within the larger, regional ground-water discharge 

area. Total ground-water discharge from these regional 

ground-water discharge areas appears to adequately 

represent natural conditions in each major discharge 

Table 30. Water budget for the simulated values in the 

calibrated model compared with estimates from this study 

[Values rounded to the nearest 100 m3/d; m3/d, cubic meters per day; 

~, approximate; <, less than]

Water budget component Simulated 

(m3/d)

Estimated

(m3/d)

FLOW IN

Constant Head 49,500 ~23,400

Mud Lake 23,000 3,400

Cactus Flat 25,500 Unknown

Pahranagat Lakes 1,000 ~20,000

Recharge 266,800 <969,200

TOTAL FLOW IN 316,300 <992,600

FLOW OUT

Constant Head 29,700 ~ –23,400

Pahranagat Lakes 22,400 ~ –20,000

Salt Creek 800 Unknown

Mud Lake 6,500 –3,400

Discharge 286,800 295,500

Ash Meadows

(G-AM-*)

60,400 60,800

Oasis Valley

(G-OV-*)

20,400 19,400

Tecopa basin

(G-TC-*)

12,900 21,300

Shoshone basin

(G-SHOSH-*)

10,000 7,100

Sarcobatus Flat

(G-SARCO-*)

47,200 43,900

Franklin Well

(G-FRANKWEL)

1,000 1,200

Franklin Lake

(G-FRNKLK-*)

6,200 3,400

Chicago Valley

(G-CHIGAGOV)

3,600 1,500

Indian and Corn Creek 

Springs

(G-INDIANSP, 

G-CORNCREK)

4,400 2,900

Grapevine Springs

(G-GRAPE-*)

4,400 3,500

Death Valley

(G-DV-*)

77,200 92,200

Penoyer Valley

(G-PENOYERV)

2,100 12,800

Stewart and Pahrump 

Valleys

(G-STEWRT-*, 

G-PAH-*)

37,000 25,500

TOTAL FLOW OUT 316,500 ~272,100

FLOW IN – FLOW OUT –200 --
88 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

36°

37°

38°

117° 116°

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

0

0

0

100

900

300

500

800

1,000

1,100

1,200 1,300

1,400

1,400

1,500

1,600

700

200

900

100

200

300

400

500 600

700

800

800

900

900

1,000

1,000

1,300

1,500

1,600

1,100

1,200

1,200

1,400

1,100

1,000

1,100

1,200

1,400

1,300

NEVADA TEST SITE

BOUNDARY

MODEL GRID

BOUNDARY

GREATER THAN 100

50–100

35–50

20–35

10–20

–10 – 10

–20 – –10

–35 – –20

–50 – –35

–100 – –50

LESS THAN –100

SIMULATED HYDRAULICHEAD CONTOUR—Contour

interval is 100 meters. Datum

is sea level

DRAIN CELLS HYDRAULIC-HEAD RESIDUALS, IN METERS

100

Figure 40. Hydraulic-head residuals (observed minus simulated) and simulated 

potentiometric surface for model layer 1. 
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 89

36°

37°

38°

117° 116°

EXPLANATION

Universal Transverse Mercator projection,

Zone 11. Shaded-relief base from 1:250,000-scale

Digital Elevation Model; sun illumination from

northwest at 30 degrees above horizon 0 40

0 40 80 KILOMETERS

80 MILES

0

0

0

100

900

300

500

800

1,000

1,100

1,200 1,300

1,400

1,400

1,500

1,600

400

600

700

200

900 100

200

300

400

500

600

700

800

800

900

900

1,000

1,000

1,300

1,500

1,600

1,100

1,200

1,200

1,400

1,100

1,000

1,100

1,200

1,400

1,300

NEVADA TEST SITE

BOUNDARY

SIMULATED 

HYDRAULIC-HEAD

CONTOUR—Contour

interval is 100 meters.

Datum is sea level

MODEL GRID

BOUNDARY

DRAIN CELLS WEIGHTED RESIDUALS,

IN STANDARD DEVIATIONS

GREATER THAN 6

4 – 6

2 – 4

1 – 2

-1 – 1

-2 – -1

-4 – -2

-6 – -4

LESS THAN -6

100

Figure 41. Hydraulic-head weighted residuals (observed minus simulated) and 

simulated potentiometric surface for model layer 1. 
90 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

area (fig. 43). Discharge estimates range from 

1,200 m3/d in the Franklin Well area to 

92,200 m3/d at Death Valley. Differences between 

these estimates and those presented in other reports are 

discussed in Laczniak and others (2001). Observed 

and simulated ground-water discharge and weighted 

residuals for ground-water discharge are presented in 

figs. 44 and 45.

Distribution of Weighted Residuals Relative to 

Weighted Simulated Values

To evaluate model results for systematic model 

error or errors in assumptions concerning observations 

and weights, weighted residuals are plotted against 

weighted simulated values for head and ground-waterdischarge observations. Ideally, weighted residuals 

will vary randomly about zero regardless of the simulated value.

 Figure 46 shows that most of the weighted 

residuals for hydraulic heads in the DVRFS model 

vary randomly about a value of zero. Four values are 

greater than +9.3, which is three times the regression 

standard error of 3.1; eleven values are less than –9.3. 

For normally distributed values only 3 in 1,000, on 

average, would be so different from the expected 

value. While this distribution is slightly biased by 

several large negative values, the distribution is still 

largely random. In previous modeling investigations, 

the large negative values were attributed to wells 

occurring in areas of potentially perched water levels 

or in areas where the model could not represent large 

hydraulic gradients. The same observations are 

apparent in the current DVRFS model.

The weighted residuals for ground-water 

discharges shown in figure 46 appear to vary randomly 

about a value of zero. None of the weighted residuals 

for ground-water discharges are greater than 9.3 or 

less than –9.3, indicating that all the flow residuals are 

within three times the regression standard error.

Normality of Weighted Residuals and Model Linearity

The normality of the weighted residuals and 

model linearity are important to the use of measures of 

parameter and prediction uncertainty, such as linear 

confidence intervals. Specifically, the weighted residuals need to be normally distributed and the model 

needs to be effectively linear for the calculated linear 

confidence intervals on estimated parameters and

Figure 42. Ground-water discharge residuals (observed minus simulated) with expected observed discharge variation.

–16,000

–14,000

–12,000

–10,000

–8,000

–6,000

–4,000

–2,000

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

0

DISCHARGE RESIDUAL, IN CUBIC 

METERS PER DAY 

HIGH EXPECTED

RESIDUAL

LOW EXPECTED

G-AM-NORTHG-AM-UPDRNG-AM-CENTERG-AM-SOUTHG-AM-AMFLTG-OV-CARSLG-OV-COFFRG-OV-SPRDLG-OV-OASISG-OV-BEATYG-TC-TECOPG-TC-AMCANG-TC-SPERYG-TC-CHRNCG-TC-RESTSG-SHOSH-NG-SHOSH-SG-STEWRT-VG-STEWRT-PG-SARCO-NEG-SARCO-SWG-SARCO-CHG-FRANKWELG-FRNKLK-NG-FRNKLK-SG-FRNKLK-EG-CHICAGOVG-INDIANSPG-GRAPE-SPG-GRAPE-SCG-DV-SARATG-DV-CONFIG-DV-MORMNG-DV-BADWTG-DV-MIDDLG-DV-FRNFNG-DV-COTTNG-DV-MESQUG-DV-WESTFG-DV-TRVRTG-DV-TEXASG-DV-NEVAR

G-PENOYERV

G-PAH-MANSG-PAH-BENTG-CORNCREK

OBSERVATION NAME
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 91

DISCHARGE VARIATION

 (Laczniak and others, 2001, table 21)

SIMULATED DISCHARGE

OBSERVED DISCHARGE

MAJOR DISCHARGE AREAS

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

DISCHARGE, IN CUBIC METERS PER DAY 

Ash Meadows

Oasis Valley

Tecopa basin

Shoshone basin

Sarcobatus Flat

Franklin Well

Franklin Lake

Chicago Valley

Indian and Corn

Creek Springs

Grapevine Springs

Death Valley

Penoyer Valley

Stewart and 

Pahrump 

Valleys

SIMULATED DISCHARGE

OBSERVED DISCHARGE

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

DISCHARGE, IN CUBIC METERS PER DAY 

G-AM-NORTHG-AM-UPDRNG-AM-CENTERG-AM-SOUTHG-AM-AMFLTG-OV-CARSLG-OV-COFFRG-OV-SPRDLG-OV-OASISG-OV-BEATYG-TC-TECOPG-TC-AMCANG-TC-SPERYG-TC-CHRNCG-TC-RESTSG-SHOSH-NG-SHOSH-SG-STEWRT-VG-STEWRT-PG-SARCO-NEG-SARCO-SWG-SARCO-CHG-FRANKWELG-FRNKLK-NG-FRNKLK-SG-FRNKLK-EG-CHICAGOVG-INDIANSPG-GRAPE-SPG-GRAPE-SCG-DV-SARATG-DV-CONFIG-DV-MORMNG-DV-BADWTG-DV-MIDDLG-DV-FRNFNG-DV-COTTNG-DV-MESQUG-DV-WESTFG-DV-TRVRTG-DV-TEXASG-DV-NEVAR

G-PENOYERV

G-PAH-MANSG-PAH-BENTG-CORNCREK

OBSERVATION NAME

Figure 43. Simulated and observed ground-water discharge for major discharge areas.

Figure 44. Observed and simulated ground-water discharge.
92 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

G-AM-NORTH

G-AM-UPDRN

G-AM-CENTER

G-AM-SOUTH

G-AM-AMFLT

G-AM-CARSL

G-OV-COFFR

G-OV-OASIS

G-OV-SPRDL

G-TC-TECOP

G-TC-AMCAN

G-OV-BEATY

G-TC-SPERY

G-TC-CHRNC

G-TC-RESTS

G-SHOSH-N

G-SHOSH-S

G-STEWRT-P

G-STEWRT-V

G-SARCO-SW

G-SARCO-CH

G-SARCO-NE

G-FRANKWEL

G-FRNKLK-S

G-FRNKLK-E

G-FRNKLK-N

G-CHICAGOV

G-INDIANSP

G-GRAPE-SP

G-GRAPE-SC

G-DV-SARAT

G-DV-CONFI

G-DV-MORMN

G-DV-BADWT

G-DV-MIDDL

G-DV-FRNFN

G-DV-COTTN

G-DV-MESQU

G-DV-WESTF

G-DV-TRVRT

G-DV-TEXAS

G-PENOYERV

G-DV-NEVAR

G-PAH-BENT

G-CORNCREK

G-PAH-MANS

OBSERVATION NAME

WEIGHTED DISCHARGE RESIDUAL, IN STANDARD DEVIATIONS –2–4–6

–8

86420

10

Figure 45. Ground-water discharge weighted residuals (observed minus simulated).
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 93

predicted heads and flows to accurately represent 

simulation uncertainty. In this report, confidence intervals are presented only on estimated parameter values.

The normal probability graph of the weighted 

residuals of the final calibrated model is shown in 

figure 47. The points would be expected to fall along a 

straight line if the weighted residuals were both independent and normally distributed. Clearly, the points 

do not fall along a straight line. One possibility is that 

the residuals are normally distributed, but they are 

correlated instead of being independent. Correlations 

are derived from the fitting of the regression.

The source of correlation can be investigated 

using the graphical procedures described by Cooley 

and Naff (1990). Normally distributed random 

numbers generated to be consistent with the regression-derived correlations are called correlated normal 

numbers and are shown in figure 48. Most of the curvilinearity in figure 48 cannot be attributed to regression-derived correlations. This analysis suggests that 

the weighted residuals are not normally distributed.

Model linearity was tested using a statistic 

referred to as the modified Beale's measure (Cooley 

and Naff, 1990). The modified Beale's measure calculated for the DVRFS model equals 0.44, which is 

between the critical values of 0.048 and 0.54. If 

Beale's measure is less than 0.048, the model is effectively linear. If Beale's measure is greater than 0.54, 

the model is highly nonlinear. Thus, the final calibrated model tends toward being nonlinear. 

The lack of normality of the weighted residuals 

and the degree of nonlinearity of the DVRFS model 

indicate that linear confidence intervals may be inaccurate. It can be concluded from previous work by 

Christensen and Cooley (1996) that linear confidence 

intervals often can be used as rough indicators of 

simulation uncertainty, even in the presence of some 

model nonlinearity. The nonnormal weighted residuals 

indicate a greater degree of potential error in the linear 

confidence intervals. Despite this problem, linear 

confidence intervals are used in this work as rough 

indicators of the uncertainty in estimated parameter 

values.

HEAD OBSERVATION, USEFLAG 

 IS "STEADY STATE 

 (REGIONAL SCALE)"

HEAD OBSERVATION, USEFLAG

 IS "INSUFFICIENT DATA"

GROUND-WATER DISCHARGE 

 OBSERVATION

–15.5

–12.4

–9.3

–6.2

–3.1

3.1

6.2

9.3

12.4

15.5

0

–50 0 50 100 150 200 250 300 350 400 450 500 550 600 650

WEIGHTED RESIDUAL

WEIGHTED SIMULATED EQUIVALENT

Figure 46. Weighted residuals relative to weighted simulated equivalent.
94 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

–4

–3

–2

–1

0

1

2

3

4

–16 –14 –12 –10 –8 –6 –4 –2 0 2 4 6 8 10 12

WEIGHTED RESIDUAL

STANDARD NORMALLY DISTRIBUTED NUMBER

HEAD OBSERVATION, USEFLAG IS

"STEADY STATE (REGIONAL SCALE)"

HEAD OBSERVATION, USEFLAG IS

"INSUFFICIENT DATA"

GROUND-WATER DISCHARGE OBSERVATION

–4

–3

–2

–1

0

1

2

3

4

–10 –5 0 5 10 15

WEIGHTED RESIDUAL

CORRELATED NORMAL NUMBER

HEAD OBSERVATION, USEFLAG IS

"STEADY STATE (REGIONAL SCALE)"

HEAD OBSERVATION, USEFLAG IS

"INSUFFICIENT DATA"

GROUND-WATER DISCHARGE OBSERVATION

Figure 47. Normal probability plot of weighted residuals.

Figure 48. Normal probability plot of weighted residual against correlated normal number.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 95

Evaluation of Estimated Parameter Values and 

Sensitivities

Composite scaled sensitivities were used during 

calibration to decide what parameters to include and 

exclude from the estimation process. Parameters with 

relatively high CSS values often were included in the 

estimation process, while parameters with relatively 

low CSS values were not included. In some cases, a 

parameter may have had a high enough sensitivity to 

be easily estimated by the regression, but was correlated (as determined from parameter correlation coefficients) with another parameter of higher sensitivity. 

In these cases, the parameter of lower sensitivity typically was left unestimated. The set of parameters estimated by regression in the DVRFS model includes the 

11 most sensitive parameters having less than 85 percent correlation.

These parameters are interpreted as representing 

the most important system characteristics, as indicated 

by evaluating composite scaled sensitivities. This analysis helps to ensure that the measures of prediction 

uncertainty calculated using the model will reflect 

most of the uncertainty in the system because all 

measures of prediction uncertainty presently available 

mostly propagate the uncertainty of the estimated 

parameter values. Uncertainty in other aspects of the 

model is not propagated into the uncertainty measures 

as thoroughly.

If a model represents a physical system 

adequately, and the observations used in the regression 

(hydraulic heads and ground-water discharges for the 

DVRFS model) provide substantial information about 

the parameters being estimated, it is reasonable to 

think that the parameter values that produce the best 

match between the observed and simulated heads and 

discharges would be realistic values. Thus, model 

error would be indicated by unreasonable estimates of 

parameters for which the data provide substantial 

information.

A measure of the amount of information 

provided by the observations for any parameter is the 

CSS value and the linear confidence interval on the 

parameter. Generally, a parameter with a large CSS 

value will have a small confidence interval relative to a 

parameter with a smaller CSS value. If an estimated 

parameter value is unreasonable and the data provide 

enough information that the linear 95-percent confidence interval on the parameter estimate also excludes 

reasonable parameter values, the problem is less likely 

to be lack of data or insensitivity and more likely to be 

model error or misinterpreted hydraulic-conductivity 

data. 

 Figures 49 through 55 show CSS values, estimated and unestimated parameter values, modelcalculated 95-percent linear confidence intervals, and 

95-percent linear confidence intervals of the geometric 

mean gathered as part of a related study (Belcher and 

others, 2001). The 95-percent confidence intervals for 

the particular units were calculated assuming a lognormal distribution at ±2 standard deviations from the 

geometric mean (Helsel and Hirsch, 1992, p. 73–74). 

In most cases, the estimated parameter values and 

related model-calculated 95-percent linear confidence 

intervals are within the range of K values determined 

by Belcher and others (2001).

No prior information was included in the sumof-squares objective function to restrict the estimation 

process; only the model design and the observation 

data influenced parameter estimation. Estimation of 

the most important parameters without prior information has the advantage of allowing a more direct test of 

the model using the observation data (the hydraulic 

heads and ground-water discharges). In this approach, 

the available information on reasonable parameter 

values is used to evaluate the estimated parameter 

values. For the DVRFS model, this evaluation revealed 

very little indication of model error. 

The confidence intervals on the parameter estimates shown in figures 49 through 55 may seem unrealistically small, but this is largely because they 

represent the confidence interval for the mean 

hydraulic-conductivity value of the defined hydrogeologic unit or zone. As pointed out by Hill (1998), 

confidence intervals on mean values are rapidly 

reduced from the entire range of the population as data 

are applied to the estimation of the mean. The validity 

of the idea that the hydrogeologic units have uniform 

“mean” or “effective” values is, of course, a basic 

hypothesis of the modeling approach used in this 

work. The ability of a model, developed using this 

approach, to reproduce the measured hydraulic heads 

and ground-water discharges as well as is done by the 

DVRFS model suggests that the approach is likely to 

be valid for this system.

Partly because of model nonlinearity, the CSS 

values change somewhat as the parameter values 

change. As a result, the evaluation of CSS values was 

repeated frequently. The final values changed somewhat but were still quite similar to initial values and
96 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and CaliforniaK1111PCGM1

K1112PCGM2

B.

K1113PCGM3

K1114PCGM4

K1116PCGM6

K1118PCGM8

PARAMETER NAME

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

K1115PCGM5

K1117PCGM7

K1121TJIU1

K1122TJIL1

K1121TJIU2

K1122TJIL2

EXPLANATION

LOW 

HIGH

EXPECTED 95-PERCENT CONFIDENCE INTERVAL

 (Belcher and others, 2001) 

PARAMETER VALUE

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

A.

0.0068 0.0012 0 0

0.1276

0.0230

0.0657

0.0004

0.0094 0.0017 0.0065

COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

0.3162

101

100

10–1

10–2

10–3

10–4

10–5

10–6

10–7

10–8

Figure 49. Parameters defining hydraulic conductivity for Precambrian granites and metamorphic rocks 

(pCgm) and Tertiary-Jurassic intrusives (TJi): A, Composite scaled sensitivity; B, Hydraulic conductivity 

assigned for each parameter and expected parameter-value range.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 97

0.0225

0.2855

0.0217 0.0006

0.0766

0.0001

0.0919

0.0380

0.1621

0.0007

0.2226

1.9503

0.4701

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

ESTIMATED

A.

K1222LOCL6

K1222LOCL7K1222LOCL8K1222LOCL9

K1222LCL10

PARAMETER NAME

K1212UPCLO

K1222LOCL1K1222LOCL2

K1222LOCL3

K1222LOCL4K1222LOCL5

K1222LCL11K1222LCL12

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

B.

EXPLANATION

LOW EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

UPPER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS (MODEL-CALCULATED)

HIGH EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

LOWER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS (MODEL-CALCULATED)

PARAMETER VALUE

10–1

10–2

10–3

10–4

10–5

10–6

10–7

10–8

Figure 50. Parameters defining hydraulic conductivity for the upper clastic confining unit (UCCU) and lower clastic confining 

unit (LCCU), hydrogeologic units greater than 300 meters below land surface: A, Composite scaled sensitivity; B, Hydraulic 

conductivity assigned or estimated for each parameter, expected parameter-value range, and linear 95-percent confidence 

interval for estimated parameters.
98 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

A.

0

0.05

0.10

0.15

0.20

0.25

0.30

0.0007

0.2838

0.0001 0.0019 0.0004

0.1143

0.1552

0.0269

0.1715

0.0007 0

0.0173

0

B.

K1211UPCUP

K1221LOCU1

K1221LOCU2K1221LOCU3

K1221LOCU4K1221LOCU5K1221LOCU6K1221LOCU7K1221LOCU8K1221LOCU9

K1221LCU10K1221LCU11

K1221LCU12

PARAMETER NAME

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

EXPLANATION

LOW 

HIGH

EXPECTED 95-PERCENT CONFIDENCE INTERVAL

 (Belcher and others, 2001) 

PARAMETER VALUE

100

10–1

10–2

10–3

10–4

10–5

10–6

10–7

10–8

Figure 51. Parameters defining hydraulic conductivity for the upper clastic confining unit (UCCU) and lower 

clastic confining unit (LCCU), hydrogeologic units less than 300 meters below land surface: A, Composite 

scaled sensitivity; B, Hydraulic conductivity assigned for each parameter and expected parameter-value 

range.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 99

COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

ESTIMATED

A.

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.2555

0.2379

0.3472

0.1503

0.3385

0.1033

0.1924 0.1771

0.6918

0.7801

0.9123

1.2786

0.7734

1.1692

0.0843

0.0605

0.2106

0.2633

0.1415

0.0047

B.K211hzone1

K211hzone2K211hzone3

K211hzone4

K211hzone5K211hzone6K211hzone7

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

K211hzone8K211hzone9K211hzon10

K211hzon11

K211hzon12

K211hzon13

PARAMETER NAME

K211hzon14

K211hzon15K211hzon16K211hzon17

K211hzon18

K211hzon19

K22_CARBS

EXPLANATION

LOW EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

UPPER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS 

 (MODEL-CALCULATED)

HIGH EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

LOWER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS

 (MODEL-CALCULATED)

PARAMETER VALUE

101

102

100

10–1

10–2

10–3

10–4

Figure 52. Parameters defining hydraulic conductivity for the lower carbonate aquifer (LCA) and the upper 

carbonate aquifer (UCA): A, Composite scaled sensitivity; B, Hydraulic conductivity assigned or estimated for 

each parameter, expected parameter-value range, and linear 95-percent confidence interval for estimated 

parameters.
100 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

A.

0

0.5

1.0

1.5

2.0

2.5

COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

ESTIMATED

0.9489

0.3564

0.5004

0.5110

0.05140.0062 0.0020

0.1187

0.2947

0.00390.0057

0.0698

0.0008

0.2560

0.0040

1.9905

1.1382

0.4087

1.2294

0.4099

0

0.0885

0.0589

B.K311VU_SE

K312VU_YM

K313VU_YFF

K314VU_SAR

K315VU_NC

K316VU_NE

K317VU_STN

K318VU_BFH

K3211VAYM

K3212VASKM

K3213VAYF

K3221VCUYM

K3222VCUYF

K331VOLTBA

K3321TBCCC

K3322TBC

K3322TBC2K3331TBQCCK3332TBQK334VOLTCBK3351TCCCK3352TCK336VOLTMA

PARAMETER NAME

HYDRAULIC CONDUCTIVITY,

IN METERS PER DAY

EXPLANATION

LOW EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

UPPER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS (MODEL- 

 CALCULATED)

HIGH EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

LOWER 95-PERCENT CONFIDENCE INTERVAL 

 FOR ESTIMATED PARAMETERS (MODEL- 

 CALCULATED)

PARAMETER VALUE

101

102

100

10–1

10–2

10–3

10–4

10–5

Figure 53. Parameters defining hydraulic conductivity for undifferentiated volcanic rocks (VU), volcanic aquifer 

(VA), volcanic confining unit (VCU), and Timber Mountain/Pahute Mesa volcanic rocks (TBA, TBCU, TBQ, TCB, 

TC, TMA): A, Composite scaled sensitivity; B. Hydraulic conductivity assigned or estimated for each parameter, 

expected parameter-value range, and linear 95-percent confidence interval for estimated parameters.
NUMERICAL MODEL OF REGIONAL GROUND-WATER FLOW 101

0.0166

0.0961

0.17240.1827 0.1189

0.03220.0012

0.09800.0685

0.0768

0.0159

0.0659

0.16040.1272

0.2256

0.3841

0.0919

0.3911

0.0577

0.3169

1.7653

0.6428

A.

B.

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

COMPOSITE SCALED SENSITIVITY

NOT ESTIMATED

ESTIMATED

K411VALCJFK412VALLVVK413VALAMVK414VALDVNK414VALDVCK415VALPAHK416VALNORK417VALCGFK418VALPENK419VALTECK4211MesqPK4212CentPK4213SoutPK422AmargPK423QTPlayK424PahruPK425TecopPK426FrankPK431_TSedsK4311TSUAMK4321TSDVNK4322TSDVC

PARAMETER NAME

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

EXPLANATION

LOW EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

UPPER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS 

 (MODEL-CALCULATED)

HIGH EXPECTED 95-PERCENT CONFIDENCE

 INTERVAL (Belcher and others, 2001)

LOWER 95-PERCENT CONFIDENCE INTERVAL

 FOR ESTIMATED PARAMETERS 

 (MODEL-CALCULATED)

PARAMETER VALUE

101

102

100

10–1

10–2

10–3

Figure 54. Parameters defining hydraulic conductivity for basin fill: Quaternary-Tertiary valley-fill alluvium (QTal), 

Quaternary-Tertiary playa deposits (QTp), and Tertiary sediments/Death Valley section (TSDVS): A, Composite 

scaled sensitivity; B, Hydraulic conductivity assigned or estimated for each parameter, expected parameter-value 

range, and linear 95-percent confidence interval for estimated parameters.
102 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

0

0.1

0.2

0.3

0.4

0.5

A.

COMPOSITE SCALED SENSITIVITY

0.0631

0.0046

0.1701

0.0177

0.0297

0.3968

NOT ESTIMATED

HYDRAULIC CONDUCTIVITY, IN 

METERS PER DAY

B.

K1223LOCTH K123MESCLA K2121LCAT1 K2122LCAT2 K2123LCAGP K2124LCALC

PARAMETER NAME

EXPLANATION

LOW 

HIGH

EXPECTED 95-PERCENT CONFIDENCE INTERVAL

 (Belcher and others, 2001) 

PARAMETER VALUE

10–5

10–4

10–3

10–2

10–1

100

101

10–6

10–7

Figure 55. Parameters defining hydraulic conductivity for lower clastic confining unit (LCCU) thrusts, Mesozoic 

volcaniclastic and sedimentary rocks (Mvs), and lower carbonate aquifer (LCA) thrusts: A, Composite scaled 

sensitivity; B, Hydraulic conductivity assigned for each parameter and expected parameter-value range.
MODEL LIMITATIONS AND POTENTIAL FUTURE IMPROVEMENTS 103

generally indicate that the parameters being estimated 

were the most important parameters. Exceptions occur 

for parameters that were correlated with parameters 

with larger CSS values and for parameters that mostly 

influence model fit to a single observation.

Evaluation of Simulated Water Budgets

The large areal extent of the DVRFS and the 

uncertainties associated with boundary conditions 

precludes a comprehensive and accurate assessment of 

all ground-water inflows and outflows. As a result, it is 

difficult to compare the simulated volumetric budget 

to conceptual flow-system budget estimates. A discussion of the simulated water budget, however, is still 

warranted.

A water budget for the Death Valley regional 

ground-water flow system that is based on simulated 

and estimated values for various inflow and outflow 

components is presented in table 30. The results are 

important because they indicate that despite significant 

strides in reducing the likely uncertainty in flux out of 

the flow system at various ground-water discharge 

points within the flow system, the high uncertainty in 

recharge makes it difficult to determine what part of 

the model has the most error with regard to flux. 

Significant uncertainties still exist with regard to 

recharge and flow-system boundary conditions.

Summary of Model Evaluation

The results presented in this section suggest that 

the DVRFS model reproduces the measured hydraulic 

heads and estimated water-budget components reasonably accurately, but with noted levels of uncertainty. In 

addition, the estimated parameter values are reasonable.

Because the weighted residuals are not entirely 

random, some model error may be indicated. This is 

mostly related to the occurrence of (1) large positive 

weighted residuals for some hydraulic-head observations located dominantly in large hydraulic gradient 

areas, (2) large weighted residuals for ground-water 

discharges at Ash Meadows, and (3) large, positive 

weighted residuals for ground-water discharges at 

Oasis Valley. In addition, weighted residuals are not 

normally distributed. These results, combined with the 

previously discussed observation that every model 

update considered thus far significantly improved the 

model fit, suggests that additional calibration and 

reduction in conceptual model uncertainty may significantly improve model accuracy. This analysis 

suggests that the DVRFS model is a reasonable representation of the physical system, but evidence of 

model error exists.

MODEL LIMITATIONS AND POTENTIAL 

FUTURE IMPROVEMENTS

Despite the model error and the above-described 

inadequacies, the DVRFS model contains the most upto-date representation of hydrogeologic data for the 

Death Valley region. The inherent limitations result 

from uncertainty in three basic aspects of the model, 

including inadequacies in (1) hydraulic-head and 

ground-water discharge observations, (2) representation of geologic complexity in the hydrogeologic 

framework model, and (3) representation of the 

ground-water system in the flow model. These limitations are discussed below and means for improvement 

are offered.

Observation Limitations

Hydraulic-head and ground-water discharge 

observations constrain model calibration through the 

parameter estimation process; therefore, uncertainty in 

these observations results in uncertainty in flow model 

results. Although head observations were thoroughly 

analyzed prior to and throughout the calibration 

process, there was still uncertainty regarding (1) the 

quality of the observation data, (2) appropriateness of 

the hydrogeologic interpretations, and (3) the way in 

which the observation is represented in the flow 

model.

Quality of Observations

The sparse distribution and high concentration, 

or clustering, of hydraulic-head observations limit the 

flow model because it results in the overemphasis of 

many observations in isolated areas, thus biasing those 

parts of the model. Outside the Yucca Mountain, NTS, 

Amargosa Farms, and Pahrump Valley areas waterlevel data are sparse, both spatially and temporally. 

High clustering of observations is noted in some areas 

of the Amargosa Desert, Yucca Mountain, Pahute 

Mesa, Yucca Flat, and Pahrump. A method of better 
104 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

distributing weights for these situations needs to be 

imposed.

Some hydraulic-head observations used to 

constrain the model are affected by pumping and, thus, 

introduce bias. Many of the observations in agricultural areas are, in fact, pumping wells. Because many 

of the wells in the Amargosa and Pahrump Valleys 

were drilled at the start of, or after, ground-water 

development, it is difficult to determine which, if any, 

of these observations likely represent predevelopment 

conditions. A transient simulation of the flow system 

would better represent ground-water conditions in 

these areas.

Interpretation of Observations

It is difficult to determine whether head observations represent regional or perched conditions. Field 

testing is often not sufficient to conclusively distinguish between regional and perched conditions. The 

data necessary to unequivocally determine the presence of perched water are rarely, if ever, available. 

Because large simulated hydraulic-head residuals in 

recharge areas often suggest the possibility of perched 

water, the hydraulic-head observation is either 

removed or the observation weight decreased. Fewer 

observations, or observations with lower weights, 

result in higher model uncertainty. Further evaluation 

of potentially perched ground-water levels in many 

parts of the model domain would be required to reduce 

this uncertainty.

Large hydraulic gradient areas also are difficult 

to interpret, and the lack of water levels to clearly 

define these features only exacerbates the situation. 

Hydraulic-head observations defining large hydraulic 

gradients also typically are associated with perched or 

localized water. Removing these uncertainties would 

require additional field work and well testing.

Accurate ground-water discharge estimates for 

evapotranspiration areas have not been available in the 

DVRFS until very recently. In fact, spatially distributed ground-water discharge observations for the 

region are only available beginning in 1995 (Laczniak 

and others, 1996). Prior to 1995, Malmberg (1967) 

was the only investigator to show the spatial distribution of ET by phreatophytes in Pahrump Valley. The 

map developed by Malmberg depicted phreatophyte 

occurrence and corresponding ET for water years 1959 

through 1961. This lack of data limits how well these 

discharge areas and related areas can be simulated 

because the predevelopment steady-state assumption 

suggests that these rates accurately describe groundwater discharge at that time. Clearly this is not the case 

because modern-day phreatophyte zones that are used 

to estimate ET rates have been impacted by pumping. 

These data deficiencies are not recoverable and the 

related uncertainty will be present in any steady-state 

model of the flow system. A transient model may be 

better suited to utilize these data because the observations may be used at an appropriately corresponding 

time step.

Representation of Observations

Simulating small discharge volumes 

(<700 m3/d) was difficult in the DVRFS model. This 

limitation may be resolved either by creating a higher 

resolution model with smaller grid cells or by 

removing the small-volume observations from the 

model.

The elevation assigned to defined drains in the 

DVRFS model also affected the model’s ability to 

accurately simulate ground-water conditions. The 

elevation of drains in ET areas was set to the lowest 

corresponding land-surface elevation based on a 

1:24,000-scale digital elevation model. This method 

may not accurately approximate the extinction depth 

for all discharge areas, particularly in areas with 

highly variable root depth and discontinuous areas of 

capillary fringe. Penoyer Valley is an example of a 

discharge area that may have a zone of fairly extensive 

capillary fringe. Observed hydraulic heads occurring 

in flow model cells with drains are lower than the 

drain elevation, which is set to land surface. The result 

is that water will not discharge at the Penoyer Valley 

drain if the hydraulic heads are simulated accurately.

Incised drainages and other focused discharge 

areas are difficult to accurately simulate at a grid resolution of 1,500 m. In many cases, the hydraulic 

conductivity of the hydrogeologic units present at the 

land surface regulates the simulated discharge. In situations where this construct does not regulate flow, a 

consistent method for using drain conductance needs 

to be employed.

An alternative method of representing groundwater discharge using the ET or Stream Packages of 

MODFLOW-2000 may reduce these model representation inadequacies.
MODEL LIMITATIONS AND POTENTIAL FUTURE IMPROVEMENTS 105

Hydrogeologic Framework Limitations

Accurate simulation of many of the important 

flow-system characteristics depends on an accurate 

understanding and representation of the hydrogeologic 

conceptual model. Limitations exist in the groundwater flow model because of the difficulties inherent 

in the interpretation and representation of the complex 

geometry and spatial variability of hydrogeologic 

materials and structures in a hydrogeologic framework 

and ground-water flow model.

Complex Geometry

Geometric complexity of hydrogeologic materials and structures are apparent throughout the model 

domain. One notable example is the Las Vegas Valley 

shear zone. Simulation of hydraulic heads in the area 

are limited by the current understanding of faultsystem geometry and the appropriateness of the representation in the hydrogeologic framework and groundwater flow model.

Similarly, the large hydraulic gradient that 

extends from the Groom Range through the Belted and 

Eleana Ranges to Yucca Mountain and the Bullfrog 

Hills is inadequately simulated because of limitations 

in hydrogeologic data that define the complex geometries that cause the feature; however, the feature is also 

inadequately simulated because of simplifications 

inherent in HFM and ground-water flow model 

construction and discretization.

Complex Spatial Variability

As with complex hydrogeologic geometries, 

spatial variability of material properties of the hydrogeologic units and structures also is underrepresented 

in the DVRFS model as a result of data limitations and 

simplifications resulting from HFM and flow-model 

construction and discretization. The assumption of 

homogeneity within a given hydrogeologic unit in the 

HFM or K-zone in the ground-water flow model limits 

the simulation by removing the potential effects of 

variability in grain-size distribution, degree of 

welding, and fracture density and orientation. A 

particularly important area where this impacts the 

simulation is in the vicinity of Oasis Valley and the 

Bullfrog Hills where the observed effects of hydrothermal alteration are incompletely characterized by 

data and inadequately represented in the HFM and 

ground-water flow model. In fact, many of the inadequacies of the simulation within the SWNVF are in 

part due to the underrepresentation of local-scale 

hydrogeologic complexities in the regional-scale HFM 

and ground-water flow models.

Flow Model Limitations

Two basic limitations of the DVRFS model are 

inherent in construction of the numerical flow model. 

These are (1) representation of the physical framework, and (2) representation of the hydrologic conditions.

Physical Framework Representation

Discretization and abstraction of the physical 

hydrogeologic framework impose limitations on all 

components of the HFM and numerical ground-water 

flow. While the 1,500-m resolution is appropriate to 

represent regional-scale conditions, higher resolution 

would improve simulation accuracy particularly in 

areas of geologic complexity. The larger grid cells tend 

to generalize important local-scale complexities that 

have an impact on regional hydrologic conditions. 

This situation is particularly prevalent in large 

hydraulic gradient areas where sharp geologic contacts 

or local-scale fault characteristics can influence 

regional hydraulic heads and ground-water discharges. 

Ultimately, smaller grid cells will be required or more 

local-scale models constructed for critical areas in the 

DVRFS.

Hydrologic Conditions Representation

The hydrologic conditions that, perhaps, most 

influence the DVRFS simulation are the representation 

of external and internal boundary conditions.

Limitations in external boundary-condition definition are the result of both incomplete understanding 

of natural conditions and poor representation of the 

natural conditions in the numerical flow model. 

Because very little data exist in the areas defined as 

lateral flow-system boundaries, the boundaries are 

highly uncertain. Also, defining these boundaries in a 

steady-state flow model is effectively limited to either 

a no-flow or a constant-head boundary. Both types of 

boundary definitions impose significant constraints on 

model results. Reducing these limitations through 

improved characterization of system boundaries in the 
106 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

field or definition of the boundaries in the flow model 

would reduce model uncertainty.

Perhaps the most significant limitation in 

internal boundary conditions exists in the representation of ground-water recharge. The limitation is threefold, including (1) inaccurate estimation of net 

infiltration due to data-input limitations to the net 

infiltration model, (2) the assumption that net infiltration results in net regional recharge, and (3) the apparently small K of hydrogeologic units at the water table 

limits the amount of recharge that can enter the 

regional flow system.

As described above, the INFIL model developed 

for this study clearly overestimates net infiltration in 

many parts of the flow-model domain. The limitations 

of the net infiltration model may be resolved by 

improving the data inputs associated with root depth in 

bedrock areas, bedrock permeability, soil moisture 

properties, and precipitation distribution.

Using net infiltration as the numerical flowmodel input for recharge assumes that all infiltrating 

water that passes the root zone ultimately crosses the 

water table. This assumption ignores the possibility 

that infiltrating water could be intercepted and diverted 

by a zone of lower permeability in the unsaturated 

zone. It also ignores the possibility of deep evaporation of water in the unsaturated zone. Despite the 

attempt of the authors to minimize this limitation, the 

current results still require improvement. This limitation may be resolved by developing a means to 

account for deep, unsaturated-zone processes that may 

reduce or redistribute infiltrating water.

SUMMARY

In the early 1990’s, two numerical models of the 

Death Valley regional ground-water flow system were 

developed by DOE. In general, the two models are 

based upon the same basic hydrogeologic data set. In 

1998, the DOE requested that the USGS develop and 

maintain a ground-water flow model of the Death 

Valley regional ground-water flow system in support 

of DOE programs at the Nevada Test Site. The purpose 

of developing this “second-generation” regional model 

was to enhance the knowledge and understanding of 

the Death Valley regional ground-water flow system as 

new information and tools are developed. The USGS 

also was encouraged by DOE to cooperate to the 

fullest extent with other Federal, State and local entities in the region to take advantage of the benefits of 

their knowledge and expertise.

The short-term objective of the DVRFS project 

was to develop a steady-state representation of the 

predevelopment conditions of the ground-water flow 

system utilizing the two geologic interpretations used 

to develop the previous numerical models. The longterm objective of the DVRFS project was to construct 

and calibrate a transient model that simulates the 

ground-water conditions of the study area over the 

historical record utilizing a newly interpreted hydrogeologic conceptual model. This report describes the 

result of the predevelopment steady-state model 

construction and calibration.

The DVRFS is situated within the southern 

Great Basin, a subprovince of the Basin and Range 

physiographic province and encompasses approximately 100,000 km2

 bounded by lat. 35oN. and 

38o

15'N. and by long. 115oW. and 118oW. The arid 

landscape is dominated by isolated mountain ranges 

rising abruptly from broad, alluvium-filled desert 

basins. Altitudes range from 86 m below sea level in 

Death Valley to 3,600 m above sea level in the Spring 

Mountains. Altitude differences between valleys and 

adjoining mountains exceed 1,500 m; such abrupt 

changes may cause relatively large gradients in the 

potentiometric surface.

Climatic conditions in the DVRFS are 

controlled by both altitude and latitude. The northern 

part of the region forms part of the Great Basin Desert, 

which is characterized by warm, dry summers and 

cold, dry winters. The southern part of the region 

forms part of the Mojave Desert, which is characterized by hot, dry summers and warm, dry winters. The 

central region around the NTS has been called the 

Transition Desert, which represents a mixing of the 

two climates. Precipitation in the region is influenced 

by two distinct storm patterns affecting the desert 

climate—one in the winter, the other in summer. 

Winter precipitation (dominantly snow) tends to be of 

low intensity and long duration and covers large areas. 

In contrast, most summer rains result from localized 

convective thunderstorms of high intensity and short 

duration.

The soils and vegetation of the Death Valley 

region are controlled to a significant degree by 

climatic, geomorphic, and hydrologic factors. These 

relations are highly variable and complex. Soils in the 

DVRFS typically follow a pattern of lithosols on the 

mountains, medium- to coarse-textured soils on allu-
SUMMARY 107

vial fans and terraces, and fine-grained, alluvial soils 

in the valley floors. In general, the soils of the mountains and hills are shallow and exhibit a coarse texture 

with little moisture-holding capacity. Vegetation distributions in the region are influenced by many factors. 

Water availability and temperature exhibit latitudinal 

and altitudinal controls that are dominantly climate 

related. Thus, vegetation communities in the region 

demonstrate both topographic and geographic 

patterns. Mixing of the cold, northern Great Basin 

Desert climate with the warm, southern Mojave Desert 

climate results in a heterogeneous distribution of plant 

associations rather than distinct homogeneous associations.

The DVRFS has a long and active geologic 

history, including intermittent marine and nonmarine 

sedimentation, large-scale compressive deformation, 

plutonism, volcanism, and extensional tectonics. 

Consequently, diverse rock types, ages, and deformational structures are often juxtaposed to each other. As 

a result, subsurface conditions are variable and 

complex. Knowledge of the geologic diversity beneath 

the alluvial basins is indirect in most of the region.

The DVRFS consists of Proterozoic- and 

Cambrian-aged clastic and crystalline rocks; Paleozoic-aged clastic and carbonate rocks; clastic and 

intrusive rocks of Mesozoic age; varied fluvial, 

paludal, and playa sedimentary deposits of Pliocene 

age; volcanic rocks and alluvium; and Tertiary alluvium and colluvium and eolian deposits of Quaternary 

age. The region has been altered by several major 

structural events. Structural and tectonic features of 

the study area reveal a long, complex tectonic evolution.

Ground-water flow in the DVRFS generally can 

be described as dominated by interbasinal flow and 

may be conceptualized as having two main components: a series of relatively shallow and localized flow 

paths that are superimposed on deeper, regional flow 

paths. A significant component of the regional groundwater flow is through a thick Paleozoic carbonate rock 

sequence. Throughout the regional flow system, 

ground-water flow is probably controlled by extensive 

and prevalent structural features that result from 

regional faulting and fracturing.

The conceptual model of the DVRFS used for 

this study is adapted from the two previous groundwater modeling studies. A description of flow-system 

characteristics includes a discussion of system boundaries, hydrogeologic framework and structures, and 

the source, occurrence, and movement of ground 

water. Estimates of ground-water system influx 

through natural infiltration and discharge by evapotranspiration also were updated for this investigation.

The upper boundary surface of the Death Valley 

ground-water flow system is the water table. Under 

natural, or predevelopment, conditions, ground water 

moves into the flow system across this boundary as 

regional ground-water recharge or out of the system 

across this boundary at natural ground-water discharge 

sites, such as springs and wet playas. The lower 

surface is an arbitrary depth where ground-water flow 

is dominantly horizontal and moves with such small 

velocities that the volumes of water involved do not 

significantly affect regional flow estimates. The lateral 

boundaries of the ground-water flow system may be 

either no-flow or flow boundaries. No-flow conditions 

exist where ground-water movement across the 

boundary is prevented by physical barriers or divergence of ground-water flow paths resulting in a 

ground-water divide. Lateral flow boundaries exist 

where ground-water potentiometric gradients permit 

flow across a boundary through fractures or higher 

permeability zones. Lateral flow boundaries (underflow) include sections of the delineated boundary 

where ground water is interpreted to flow into the 

system at Mud Lake and Salt Creek or out of the 

system at Pahranagat Lakes.

The 3D digital hydrogeologic framework model 

developed for the DVRFS contains elements of both of 

the HFMS used in the previous investigations. The 

hydrogeologic framework defines the physical geometry and composition of the surface and subsurface 

materials through which ground water flows. The 

HFM defines the fundamental nature of the groundwater flow system and includes a definition both of the 

hydrogeologic units and the hydrogeologic structures 

in the basin.

The major hydrogeologic units from oldest to 

youngest are: the lower clastic confining unit (LCCU), 

the lower carbonate aquifer (LCA), the upper clastic 

(Eleana) confining unit (UCCU), the upper carbonate 

aquifer (UCA), the volcanic aquifers (VA) and 

volcanic confining units (VCU), and the alluvial 

aquifer (QTal). The LCCU forms the basement and is 

generally present beneath the other units except in 

caldera complexes. The LCA is the most extensive and 

transmissive unit in the region. It is present in most of 

the area, although it does not control ground-water 

flow within the caldera complexes. The UCCU is 
108 A Three-Dimensional Numerical Model of Predevelopment Conditions in the Death Valley Regional Ground-Water Flow System, 

Nevada and California

present in the north-central part of the NTS. It typically impedes flow between the overlying UCA and 

underlying LCA, and is associated with many of the 

large hydraulic gradients in and around the NTS. The 

VA and VCU form a stacked series of alternating aquifers and confining units in and around the SWNVF. 

The VA are moderately transmissive and are saturated 

in the western sections of the NTS. Because of the 

volume of ground water withdrawn from the QTal, it 

forms a discontinuous, albeit important, aquifer in the 

region. 

Hydrogeologic structures likewise influence 

ground-water flow. Faults in the Death Valley region 

may affect ground-water flow by acting as relative 

barriers, conduits, and combined conduit/barriers, or 

by offsetting strata of different hydrologic characteristics. A subset of the mapped faults in the DVRFS was 

chosen to be incorporated as relative barriers in the 

flow model. These faults were chosen based on their 

orientation and length. Faults longer than 5 km and 

generally perpendicular to the ground-water flow 

direction were selected to be incorporated as relative 

barriers.

As dictated by project scope, very little reinterpretation and refinement were made where these two 

framework models disagree; therefore, limitations in 

the hydrogeologic representation of the flow system 

exist. Despite limitations, the HFM provides the best 

representation to date of the hydrogeologic units and 

structures that control regional ground-water flow and 

serves as an important information source used to 

construct and calibrate the predevelopment steadystate flow model.

This study includes a detailed description of the 

source, occurrence, and movement of ground water in 

the DVRFS which is adapted from D'Agnese and 

others (1997, p. 62–69) to incorporate elements of the 

UGTA regional model study (U.S. Department of 

Energy, 1997). The ground-water flow in the DVRFS 

is most easily described in terms of three major subregional flow systems. These subdivisions are for 

descriptive purposes only and do not define discrete, 

independent flow systems. Each subregion name 

reflects the part of Death Valley into which it 

discharges. Because the Central Death Valley subregion is so large, it is divided into three ground-water 

basins. The subregions, or the ground-water basins, 

are further divided into sections. Flow in each section 

is defined by an interpreted dominant flow path.

A complex array of mechanisms accounts for 

flow into, through, and out of the regional groundwater flow system. Natural discharges from the 

regional ground-water flow system occur by ET and 

springs. In this study, ET rates were adapted from a 

related investigation that developed maps of ET areas 

and computed rates from micrometeorological data 

collected within the local area over a multiyear period. 

In some cases, historical spring flow records were 

used to derive ground-water discharge rates for 

isolated regional springs.

For this investigation, a process-based, numerical model was developed to estimate net infiltration. 

This result can be used to represent recharge by 

assuming that water that infiltrates past the “root zone” 

ultimately becomes ground-water recharge. Estimates 

of net infiltration based on this water-balance 

approach, however, suggests that recharge in the 

DVRFS exceeds the estimated ground-water discharge 

for the basin by almost a factor of four. The net infiltration model contains some assumptions and limitations with regard to soil moisture, soil thickness, ET 

rates, bedrock permeability, and precipitation distributions that may result in these higher than expected 

values. Despite this limitation, the net infiltration 

model does provide a good starting point for estimating regional ground-water recharge in the groundwater flow model.

The DVRFS was simulated using a 3D steadystate model that incorporated a nonlinear least-squares 

regression technique to estimate aquifer parameters. 

The numerical modeling program MODFLOW-2000 

was used to construct a finite-difference model 

consisting of 194 rows, 160 columns, and 15 layers. 

The grid cells were oriented north-south and were of 

uniform size, with side dimensions of 1,500 m. The 

model grid encompasses about 70,000 km2 and the 

layers span thicknesses of 50 to 300 m.

The required model-parameter values were 

supplied by discretization of the 3D HFM and digital 

representations of the remaining conceptual model 

components. The 3D simulation and corresponding 

sensitivity analysis supported the hypothesis of interactions between a relatively shallow local and subregional flow system and a deeper, dominant regional 

system controlled by the carbonate aquifer.

During calibration of the model, techniques 

available in MODFLOW-2000 allowed for estimation 

of a series of parameters that provided a best fit to 

observed hydraulic heads and outflows. Numerous 
REFERENCES CITED 109

conceptual models were evaluated to test the validity 

of various interpretations about the flow system. 

Conceptual model evaluations focused on testing 

hypotheses concerning (1) the location and type of 

flow-system boundaries, (2) the definition of recharge 

areas, and (3) variations in interpretation of the hydrogeologic framework. For each hypothesis tested, a new 

set of parameters was estimated using MODFLOW2000, and the resulting new simulated hydraulic heads 

and ground-water discharges were compared to 

observed values. Only those conceptual model 

changes contributing to a significant improvement in 

model fit, as indicated by a reduction in the sum of 

squared errors, were retained in the final optimized 

model.

The final calibrated model was evaluated to 

assess the likely accuracy of simulated results by 

comparing measured and expected quantities with 

simulated values. Good fit to observed hydraulic heads 

(residuals with absolute values less than 10 m) generally occurs in areas of flat hydraulic gradients; 

moderate fit to observed heads (residuals with absolute 

values of 10 to 20 m) occurs in the remainder of the 

flat hydraulic gradient areas. Poorer fit to observed 

heads (residuals with absolute values of greater than 

20 m) appears to occur in large hydraulic gradient 

areas. Poorest fit to observed hydraulic heads occurs in 

the vicinity of Indian Springs, western Yucca Flat, and 

the southern Bullfrog Hills. Most of these inaccuracies 

can be attributed to (1) insufficient representation of 

the hydrogeology in the HFM, (2) misinterpretation of 

water levels, and (3) model error associated with grid 

cell size.

Ground-water discharge residuals are fairly 

random, with as many areas where simulated flows are 

less than observed flows as areas where simulated 

flows are greater. The largest unweighted groundwater discharge residuals were at Ash Meadows 

(central area), Sarcobatus Flat (northeastern area), 

Death Valley (Badwater and Cottonball Basins, 

Furnace Creek fan, and western alluvial fans), Penoyer 

Valley, and Pahrump Valley (near Manse Spring). The 

two major discharge areas that would contribute the 

largest volumetric error to the model are Ash 

Meadows and Death Valley. Ash Meadows (central 

area) is also one of the largest discharging sites in the 

DVRFS flow model; therefore, moderate percentages 

of error in the simulated discharge could result in 

comparatively large volumetric errors. While Death 

Valley is of concern, the ground-water discharge 

observations used in the DVRFS model are preliminary and should not be considered authoritative values 

of discharge from the Death Valley saltpan.

Estimated parameters were evaluated to determine if reasonable values were estimated by the 

regression. All estimated parameter values are within 

the range of expected values. Overall evaluation of the 

model indicates that although the model is clearly an 

improvement on previous representations of the flow 

system, there is an indication of important uncertainties and model error.

Despite the model error and described inadequacies, the current DVRFS model contains the most upto-date representation of hydrogeologic data for the 

Death Valley region. The inherent limitations result 

from uncertainty in three basic aspects of the model, 

including inadequacies in (1) the flow-model observation quality, interpretation, and representation; (2) the 

interpretation and representation of the complex 

geometry and spatial variability of hydrogeologic 

materials and structures in the hydrogeologic framework and ground-water flow model, and (3) the detail 

of representation of the physical framework and the 

hydrologic conditions in the flow model. These limitations are discussed and means for improvement are 

offered.

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