Prepared in cooperation with the

U.S. DEPARTMENT OF THE INTERIOR, 

NATIONAL PARK SERVICE and

INYO COUNTY, CALIFORNIA

Estimated Ground-Water Discharge by

Evapotranspiration from Death Valley, 

California, 1997–2001

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4254
(Back of Cover)
Estimated Ground-Water Discharge by

Evapotranspiration from Death Valley, 

California, 1997–2001

By Guy A. DeMeo, Randell J. Laczniak, Robert A. Boyd, 

J. LaRue Smith, and Walter E. Nylund

U.S. GEOLOGICAL SURVEY

Carson City, Nevada

2003

Prepared in cooperation with the

U.S. DEPARTMENT OF THE INTERIOR, 

NATIONAL PARK SERVICE and

INYO COUNTY, CALIFORNIA

Water-Resources Investigations Report 03-4254
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

Any use of trade, product, or firm names in this publication is for descriptive

purposes only and does not imply endorsement by the U.S. Government

For additional information Copies of this report can be

contact: purchased from:

District Chief U.S. Geological Survey 

U.S. Geological Survey Information Services

333 West Nye Lane, Room 203 Building 810

Carson City, NV 89706–0866 Box 25286, Federal Center

Denver, CO 80225–0286

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

http://nevada.usgs.gov
CONTENTS III

CONTENTS

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

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

Purpose and Scope ..................................................................................................................................................... 2

Previous Investigations............................................................................................................................................... 2

Acknowledgments...................................................................................................................................................... 4

Description and Setting.............................................................................................................................................. 4

Climate............................................................................................................................................................ 4

Vegetation ....................................................................................................................................................... 4

Hydrology....................................................................................................................................................... 6

Evapotranspiration Units ....................................................................................................................................................... 6

Evapotranspiration Rates ....................................................................................................................................................... 8

Site Selection and Instrumentation............................................................................................................................. 10

Energy-Budget Methods ............................................................................................................................................ 13

Bowen Ratio Method...................................................................................................................................... 13

Eddy Correlation Method ............................................................................................................................... 15

Micrometeorological Data ......................................................................................................................................... 15

Daily and Annual Evapotranspiration........................................................................................................................ 15

Comparison of Bowen Ratio and Eddy Correlation Methods........................................................................ 18

Estimates of Annual Evapotranspiration ............................................................................................................................... 19

Estimates of Annual Ground-Water Discharge.......................................................................................................... 23

Limitations ................................................................................................................................................................. 25

Summary................................................................................................................................................................................ 25

References Cited ................................................................................................................................................................... 26

FIGURES

1. Map showing Death Valley ground-water flow system, California and Nevada............................................................... 3

2. Map showing location of valley floor and other major physiographic features of Death Valley ..................................... 5

3. Graphs showing temporal modified soil-adjusted vegetation indices (MSAVI) clusters of 

signatures used to delineate vegetation in Death Valley ........................................................................................ 7

4. Map showing distribution of evapotranspiration units and location of instrumented sites in Death Valley...................... 9

5. Photographs of evapotranspiration sites at (A) salt-encrusted playa, (B) bare-soil playa, (C) pickleweed, 

(D) Eagle Borax, (E) Mesquite Flat, and (F) Mesquite Flat mixed grasses, Death Valley, California, 

August 1998–October 2001 ................................................................................................................................... 11

6. Schematic of typical micrometeorological data-collection site for computation of evapotranspiration 

by (A) Bowen ratio method and (B) eddy correlation method, Death Valley, California, 1997–2001 .................. 12

7. Schematic showing components of the surface-energy budget ......................................................................................... 14

8. Graphs showing 20-minute averaged micrometeorological data collected from the Mesquite Flat 

mixed-grasses site, Death Valley, California, July 29–August 2, 2000 ................................................................. 16

9. Graph showing examples of (A) energy-budget fluxes and (B) daily evapotranspiration computed 

with micrometeorological data collected from the Mesquite Flat mixed-grasses site, Death 

Valley, California, July 29–August 5, 2000 ........................................................................................................... 17

10. Graph showing example of daily evapotranspiration and smoothed fit for data collected at the 

Mesquite Flat mixed-grasses site, Death Valley, California, July 2000–October 2001......................................... 18

11. Graph showing comparison of (A) measured annual precipitation and (B) calculated daily 

evapotranspiration from micrometeorological data-collection sites, Death Valley, 

California, 1997–2001 ........................................................................................................................................... 19

12. Correlation plot of daily evapotranspiration of Bowen ratio method and eddy correlation method, 

at Mesquite Flat mixed-grasses site, Death Valley, California, July 2000–October 2001 ..................................... 20
IV Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001

13. Graph showing comparison of (A) annual precipitation and (B) daily evapotranspiration at the 

salt-encrusted playa micrometeorological data-collection site, Death Valley, California, 

January 1999–July 2001......................................................................................................................................... 22

14. Photograph of salt-crystal deposits on a rain gage following the evaporation of surface flooding near the 

salt-encrusted playa micrometeorological data-collection site, Death Valley, California, April 2001 .................. 23

15. Graph showing relation of vegetation density and ground-water evapotranspiration for vegetated 

evapotranspiration units, Death Valley, California, August 1997–October 2001 .................................................. 24

TABLES

1. Evapotranspiration units determined from temporal analysis of modified soil-adjusted vegetation indices 

(MSAVI), Death Valley, California ........................................................................................................................ 8

2. Location, depth of peizometer, and period of data collection for instrumented sites used to collect 

micrometeorological data, Death Valley, California, 1997–2001 .......................................................................... 10

3. Rates of annual evapotranspiration, measured precipitation, and annual ground-water 

evapotranspiration at data-collection sites in Death Valley, California, 1997–2001.............................................. 20

4. Annual ground-water discharge rate and total annual ground-water discharge for 

evapotranspiration units, Death Valley, California, 1997–2001............................................................................. 21

CONVERSION FACTORS AND VERTICAL DATUM

Temperature: 

Temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) by using the formula °F = (1.8 × °C) + 32. Temperature in 

degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) by using the formula °C = 0.556(°F-32). Temperature in degrees kelvin (°K) 

can be converted to degrees Fahrenheit (°F) by using the formula °K = (°F+459.67)/1.8. 

Sea Level: In this report, sea level refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived from 

a general adjustment of the first-order leveling networks of the United States and Canada.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C).

NOTE: English units are used throughout this report, except in instances where a measurement has no english-unit equivalent.

Multiply By To obtain

inch (in.)

foot (ft)

mile (mi)

Length

25.4

0.3048

1.609

millimeter

meter

kilometer

acre

square mile (mi2)

Area

0.4047

2.590

square hectometer

square kilometers 

acre-foot (acre-ft)

Volume

0.001233 cubic hectometer

Watt per square foot (W/ft2

)

Energy Flux

10.76 Watt per square meter

pounds per square inch (lb/in2)

Pressure

6.895 kilopascal 

cubic foot per second (ft3/s)

foot per second (ft/s)

Velocity or Rate

0.0283

0.3048

cubic meter per second 

meter per second 

acre-foot per year (acre-ft/yr)

gallons per minute (gal/min)

Volumetric Rate

0.001233

0.0631

cubic hectometer per year

liter per second
CONTENTS V

Symbols and Acronym

Cp Specific heat of air at a constant pressure

DVRFS Death Valley regional ground-water flow system

E Rate of water-vapor flux

el,u Vapor pressure at lower or upper reference point

ET Evapotranspiration

ETgw Component of evapotranspiration from ground water

G Soil-heat flux

GPS Global Positioning System

H Sensible-heat flux

kh Turbulent transfer coefficient of heat in air

kv Turbulent transfer coefficient of vapor

L Soil-adjustment factor

MSAVI Modified soil-adjusted vegetation index

NPS National Park Service

NWI National Wetland inventory

NWS National Weather Service

P Ambient air (barometric) pressure

p Annual local precipitation

Rn Net radiation

Sw Ground- or surface-water accumulation resulting from flooding or precipitation

T Air temperature

T’ Instantaneous deviation of air temperature from the mean 

Tl,u Air temperature at lower or upper reference point

THPs Temperature-humidity probes

TM Thematic mapper

USGS U.S. Geological Survey

w’ Instantaneous deviation of vertical wind speed from the mean

zl,u Lower and upper reference heights of air temperature and vapor pressure

β Bowen ratio

ε Ratio of molecular weight of water vapor to dry air

γ Psychrometric constant

λ Latent heat of vaporization for water

λE Latent-heat flux

ρv’ Instantaneous deviation of water vapor from the mean

ρa Density of air

ρw Density of water
(This page intentionally left blank.)
ABSTRACT 1

ABSTRACT

The U.S. Geological Survey, in cooperation 

with the National Park Service and Inyo County, 

Calif., collected field data from 1997 through 2001 

to accurately estimate the amount of annual 

ground-water discharge by evapotranspiration 

(ET) from the floor of Death Valley, California. 

Multispectral satellite-imagery and National 

Wetlands Inventory data are used to delineate 

evaporative ground-water discharge areas on the 

Death Valley floor. These areas are divided into 

five general units where ground-water discharge 

from ET is considered to be significant. Based 

upon similarities in soil type, soil moisture, vegetation type, and vegetation density; the ET units 

are salt-encrusted playa (21,287 acres), bare-soil 

playa (75,922 acres), low-density vegetation 

(6,625 acres), moderate-density vegetation (5,019 

acres), and high-density vegetation (1,522 acres). 

Annual ET was computed for ET units with 

micrometeorological data which were continuously measured at six instrumented sites. Total ET 

was determined at sites that were chosen for their 

soil- and vegetated-surface conditions, which 

include salt-encrusted playa (extensive salt encrustation) 0.17 feet per year, bare-soil playa (silt and 

salt encrustation) 0.21 feet per year, pickleweed 

(pickleweed plants, low-density vegetation) 0.60 

feet per year, Eagle Borax (arrowweed plants and 

salt grass, moderate-density vegetation) 1.99 feet 

per year, Mesquite Flat (mesquite trees, highdensity vegetation) 2.86 feet per year, and 

Mesquite Flat mixed grasses (mixed meadow 

grasses, high-density vegetation) 3.90 feet per 

year. 

Precipitation, flooding, and ground-water 

discharge satisfy ET demand in Death Valley. 

Ground-water discharge is estimated by deducting 

local precipitation and flooding from cumulative 

ET estimates.

Discharge rates from ET units were not 

estimated directly because the range of vegetation 

units far exceeded the five specific vegetation units 

that were measured. The rate of annual groundwater discharge by ET for each ET unit was determined by fitting the annual ground-water ET for 

each site with the variability in vegetation density 

in each ET unit. The ET rate representing the 

midpoint of each ET unit was used as the representative value. The rate of annual ground-water 

ET for the playa sites did not require scaling in 

this manner. Annual ground-water discharge by 

ET was determined for all five ET units: saltencrusted playa (0.13 foot), bare-soil playa (0.15 

foot), low-density vegetation (1.0 foot), moderatedensity vegetation (2.0 feet), and high-density 

vegetation (3.0 feet), and an area of vegetation 

or bare soil not contributing to ground-water 

discharge unclassified (0.0 foot).

The total ground-water discharge from ET 

for the Death Valley floor is about 35,000 acre-feet 

and was computed by summing the products of the 

area of each ET unit multiplied by a corresponding 

ET rate for each unit. 

Estimated Ground-Water Discharge by Evapotranspiration 

from Death Valley, California, 1997–2001 

By Guy A. DeMeo, Randell J. Laczniak, Robert A. Boyd, J. LaRue Smith, and Walter E. Nylund
2 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

INTRODUCTION

The climate in Death Valley during summer 

months is one of the hottest and most arid in the world. 

The lowest land-surface elevation in the Western Hemisphere is in Death Valley, and much of the valley floor 

and areas of extreme temperature occur at elevations 

below mean sea level. Despite this harsh climate, 

numerous plant and animal species survive within 

Death Valley. Many of these plant and animal species 

are unique to the area, and depend on habitat provided 

by spring- and ground-water discharge. 

Death Valley is a major natural discharge area of 

the Death Valley regional ground-water flow system 

(DVRFS), a regionally extensive closed hydrologic 

basin in southern Nevada and eastern California. The 

flow system, as delineated by Harrill and others (1988), 

encompasses an area of about 15,800 mi2 (fig. 1). 

Ground-water discharge in Death Valley occurs at a 

series of springs and seeps along the east side of the 

valley and on the valley floor, as regional subsurface 

underflow, and within relatively large areas of the 

valley floor by evaporative discharge. Evaporative discharge occurs as vapor flux through bare soil, transpiration from phreatophytic vegetation, and evaporation 

from open-water surfaces. For the purposes of this 

report, the combined processes of evaporative discharge is referred to as evapotranspiration (ET). 

Individual components of the ground-water 

budget in Death Valley are not accurately known and 

are difficult to quantify. Spring discharge often is 

poorly channeled and difficult to directly measure, 

especially in densely vegetated areas. Ground-water 

data typically are too sparse to accurately estimate 

regional subsurface flow between basins. Groundwater discharge by ET from Death Valley previously 

has not been estimated with collection of direct field 

measurements and observations. However, ET from 

selected areas of Death Valley has been estimated using 

indirect techniques, such as collecting pan ET rates and 

applying coefficients (Hunt and others, 1966) and by 

extrapolating ET rates determined for similar vegetation types in other areas of the southwestern United 

States (D’Agnese and others, 1997; Prudic and others, 

1995).

 The U.S. Geological Survey (USGS), in cooperation with the National Park Service (NPS) and Inyo 

County, California, from 1997 through 2001, estimated 

ground-water discharge from Death Valley by ET using 

direct field measurements, observations, and satelliteimagery data. This study applied techniques developed 

and refined during numerous studies of ET in Nevada 

and California, such as Walker and Eakin (1963), Duell 

(1990), Nichols (1991), Carman (1993), Nichols and 

others (1997), Laczniak and others (1999), Berger and 

others (2001), and Reiner and others (2002). More 

accurate estimates of ground-water discharge by ET 

will improve overall understanding of the DVRFS, 

provide information to develop a water budget for the 

basin, and help NPS managers evaluate the effects of 

future ground-water withdrawals within the DVRFS on 

the natural resources of Death Valley. 

Purpose and Scope

The purpose of this report is to document results 

of a study to estimate evaporative ground-water discharge from Death Valley. The main focus of the study 

was the valley floor where shallow ground water either 

sustains phreatophytic plant communities or evaporates 

through bare soils. Localized areas of ET near a series 

of major springs along the eastern margin of Death 

Valley were not addressed in this study. 

Analyses of satellite-imagery data and results 

from the National Wetlands Inventory (NWI; Cowardin 

and LaRoe, 1979) were applied to delineate the valley 

floor into areas of similar vegetation and soil types. ET 

rates were computed using micrometeorological data 

collected from instrumented sites within representative 

areas of similar vegetation and soil types. Finally, 

delineated areas and associated ET rates were used to 

estimate annual evaporative ground-water discharge 

from Death Valley.

Previous Investigations

Mendenhall (1909) is one of the earliest publications on water resources and hydrogeology in the Death 

Valley region. Subsequent publications addressing 

water resources and hydrogeology include Thompson 

(1929), Winograd (1962, 1971), Hunt and others 

(1966), Winograd and Thordarson (1975), Bedinger 

and others (1989), Prudic and others (1995), and 

Steinkampf and Werrell (2001). Studies addressing 

evapotranspiration within the region include Czarnicki 

(1990), Duell (1990), Nichols and others (1997), 

Laczniak and others (1999), Laczniak and others 

(2001), and Reiner and others (2002). 
INTRODUCTION 3

Inyo County

San Bernadino County

Clark County

Lincoln County

Clark County

Nye County

Esmeralda County

Nye County

Tecopa

Tonopah

Pahrump

Beatty

Las Vegas

Nevada Test Nevada Test

Site

Nevada Test

Site

118 ̊ 117 ̊ 116 ̊ 115 ̊

Kawich

Range

Kawich Range

Pahrump ValleyPahrump Valley

Spring

Mountains

Spring

Mountains

Panamint Range

Death

Valley

Death

Valley

Shoshone

36 ̊

37 ̊

38 ̊

35 ̊

Panamint Range

Ama rg osa RiverAma rg osa River

CALIFORNIA

NEVADA

Base from U.S. Geological Survey digital data, 1:100,000,

1978–89: Universal Transverse Mercator Projection Zone 11.

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

Death Valley Regional Flow System boundary—From Harrill and others (1988)

Sarcobatus Flat

Oasis Valley

Franklin Well area Stewart Valley

Ash Meadows

Franklin Lake

Chicago Valley

Shoshone area

Tecopa/California Valley area

Death Valley (not evaluated

 as part of study)

Ground-Water Discharge Area

EXPLANATION

0 40 MILES

40 KILOMETERS

20

0 20

Amargosa Desert

Sheep

Range

Sheep Rang

Am

e

argosa Desert

Mountain M

Yucca

ountain

Yucca

Mountain

Amargosa

Range

Amargosa

Range

Franklin Franklin 

Well 

area

Franklin 

Well 

area

Cottonwood Mountains Cottonwood Mountains Cottonwood Mountains

NEVADA

CALIFORNIA

Map

Area

Figure 1. Death Valley ground-water flow system, California and Nevada.
4 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

Acknowledgments 

The NPS-Water Resources Division and Inyo 

County, California, contributed cooperative funding for 

this study. The authors express their appreciation to 

Richard Anderson, Mel Essington, Doug Threloft, 

and other NPS employees at Death Valley National 

Park who provided assistance with permits needed to 

access environmentally sensitive areas within the park, 

logistics and site support for collecting field data and 

observations, and insight and information about natural 

resources in Death Valley. The authors also extend their 

appreciation to William D. Nichols formerly of the 

U.S. Geological Survey for his time and valuable 

technical assistance in this work. 

Description and Setting

Death Valley National Park is in southeastern 

California and adjacent parts of southern Nevada 

(fig. 2). The park includes all of Death Valley, adjacent 

mountainous areas, and several other smaller neighboring valleys. 

Death Valley is within the southern Great Basin 

region of the Basin and Range physiographic province 

of the western United States. Topography in the region 

is characterized by numerous linear, north-south trending, elongate valleys that are separated by fault-block 

mountain ranges. The topography is the result of an 

extensive period of tectonic activity and crustal extension that is still active. The valleys are filled with 

relatively thick sequences of Cenozoic alluvium eroded 

from adjacent mountain ranges and evaporite deposits. 

Valleys are characterized by alluvial fans sloping to 

relatively flat floors; elevation of individual valley 

floors ranges from 282 ft below sea level to about 5,000 

ft above sea level. Basement rocks beneath the valleys 

consist of thick sequences of Proterozoic and Paleozoic 

carbonate, clastic, and volcanic rocks. Blocks of basement rock uplifted along vertically displaced faults 

form mountain ranges adjacent to the valleys. Higher 

peaks in these mountain ranges crest at 8,000 to greater 

than 11,000 ft above sea level (Hunt and others, 1966). 

Death Valley is a 156-mile long structural trough 

that generally trends north-south to northwest-southeast and is bounded by the Panamint Range and 

Cottonwood Mountains to the west, the Amargosa 

Range mountains to the east (fig. 1), and the Owlshead 

Mountains to the south (fig. 2). 

The floor of Death Valley consists of three areas 

(fig. 2): Mesquite Flat, the saltpan, and an area south of 

the saltpan along the Amargosa River channel. The 

saltpan is a saline playa-lake flat characterized by areas 

of salt-encrusted deposits and saliniferous soils. The 

saltpan is subdivided into areas from north to south 

named Cottonball Basin, Middle Basin, and Badwater 

Basin (Hunt and others, 1966). Most of the floor of 

Death Valley lies below sea level; the lowest elevation 

in the Western Hemisphere (282 ft below sea level) is 

in Badwater Basin in the central part of Death Valley. 

Telescope Peak in the Panamint Range is the highest 

elevation (11,049 ft above sea level) near the Death 

Valley National Park. Mesquite Flat is an area northwest of the saltpan characterized by areas of phreatophytic vegetation supported by shallow ground water. 

Climate

Death Valley is one of the hottest and most arid 

places in the Western Hemisphere with temperatures 

commonly exceeding 120°F during summer months. 

The greatest temperature ever recorded by the National 

Weather Service (NWS) in the United States was 134°F 

(Desert Research Institute, 2003); which occurred in 

Death Valley on July 10, 1913. Soil-surface temperatures as high as 150°F were recorded during this study 

in Death Valley. Precipitation in the Death Valley area 

is erratic varying with time, location, and altitude. The 

mean annual rainfall at Furnace Creek Ranch is 2.26 in. 

(NWS station ID 042319, 1961–2001). In higher parts 

of adjacent mountain ranges, mean annual precipitation 

has exceeded 10 in. Precipitation tends to be greatest in 

winter and least in summer, generally increases with 

altitude, and tends to vary temporally with wet and dry 

cycles such as El Niño and La Niña. Relative humidity 

can be less than 5 percent during summer months. 

Winds in Death Valley frequently are gusty and typically blow from the southeast during the afternoons and 

from the northwest in the early mornings.

 Vegetation

Large areas of Death Valley are covered by Lower 

Sonoran zone vegetation and bare soils (Hunt, 1966). 

Lower Sonoran zone vegetation primarily consists 

of creosotebush shrubs (Larrea tridentate), although 

other xerophytes such as desert holly (Atriplex hymenelytra), cattle spinach (Atriplex polycarpa), burroweed 

(Franseria dumosa), and incienso (Encelia farinose)
INTRODUCTION 5

Base from U.S. Geological Survey digital data, 1:100,000,

1978–89: Universal Transverse Mercator Projection Zone 11.

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

Area

of

map

Funeral Moutains Funeral Moutains

Black 

Mountains Mountains

Panamint Panamint

Range

Grapevine Grapevine

Mountains Mountains

Owlshead wlshead

Mountains ountains

Death Valley eath Valley

Telescope

Peak

Telescope

Peak

Death Valley eath Valley

Junction unction

Valley

Amargosa margosa

Stovepipe

Wells

Stovepipe

Wells

117 ̊00' 116 ̊30'

36 ̊00'

36 ̊30'

VALLEY NATIONAL PARKVALLEY NATIONAL PARK

Valley

Floor

Valley

Floor

Boundary of

Death Valley

discharge area

Boundary of

Death Valley

discharge area

118 ̊ 116 ̊

36 ̊

38 ̊

40 ̊

NEVADA

CALIFORNIA

ARIZONA

DEATH VALLEY

NATIONAL PARK

Nevada

California California

DEATH

Badwater Basin Badwater Basin

Middle

Basin

Cottonball Cottonball

Basin

Mesquite Flat Mesquite Flat

Salt Creek Salt Creek

Amargosa River Amargosa River

Travertine

Springs

Narvares

Springs

Texas

Springs

Travertine

Springs

Narvares

Springs

Texas

Springs

Cottonwood Mountains Cottonwood Mountains Cottonwood Mountains

373

127

127

190

0

0 10 20 30 KILOMETERS

10 20 30 MILES

Furnace Creek Ranch Furnace Creek Ranch urnace Creek Ranch

Figure 2. Location of valley floor and other major physiographic features of Death Valley.
6 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

commonly occur. Density of the xerophytes ranges 

from just a few per acre to a maximum of several 

hundred per acre (Hunt and others, 1966). Bare soils 

occur in saltpan and playa areas where vegetation does 

not grow because of excessive salinity or in areas of 

well-developed desert pavement such as on parts of 

alluvial fans along valley margins. Limited areas of 

phreatophytic vegetation occur within spring-discharge 

areas and other areas of shallow ground water where 

the water is not too saline. Types of phreatophytic 

vegetation include mesquite trees (Prosopis), pickleweed (Allenrolfea), arrowweed (Pluchea sericea), 

salt and bunch grasses (Atriplex canescens, Nassella 

pulchra), assorted marsh grasses, and other desert 

shrubs. Although limited in total acreage, these areas 

provide important habitat for numerous wildlife species. Higher altitude areas of Death Valley National 

Park, such as the Panamint Range and the Grapevine 

and Cottonwood Mountains (fig. 2), support upper 

Sonoran and Arctic/Alpine vegetation zones.

Hydrology

Death Valley is a terminal discharge area for 

regional ground-water flow; as a result, ground water 

occurs at relatively shallow depths in many parts of the 

valley. In addition to regional interbasin ground-water 

flow, water enters Death Valley from precipitation that 

infiltrates in bordering mountains and valley margin 

areas, intermittent surface-water runoff along ephemeral channels into the valley during infrequent rainstorms, and water discharged at valley margin springs 

and seeps that infiltrates and flows to the valley floor. 

Springs and seeps in Death Valley occur where water is 

forced to the surface by low-permeability structural 

deposits (such as near Salt Creek; fig. 2) or where water 

ponds in sand and gravel deposits adjacent to lowpermeability silt deposits (such as marginal areas along 

the saltpan). Regional ground water also discharges at 

a series of springs above the eastern margin of the 

valley and tends to occur along high-angle faults. 

Ground water beneath the saltpan is hypersaline; 

specific-conductance field measurements of shallow 

ground water exceed 200,000 μS/cm. Specific conductance of ground water in other areas can vary, exceeding 20,000 μS/cm near Salt Creek.

Water discharged by springs and seeps is either 

diverted to supply water for park operations, is used by 

riparian vegetation, or tends to rapidly infiltrate or 

evaporate. Discharge at most of the springs tends to 

diffuse and does not follow an established channel. 

Springs above the eastern margin of the valley typically 

discharge greater than 0.5 ft3/s (Steinkampf and 

Werrell, 2001). Springs in other areas typically discharge less than 6.0 x 10-3 ft3

/s. The Amargosa River 

(fig. 1) is an intermittent stream that drains about 5,546 

mi2 to the east of Death Valley and terminates at the 

southern end of the Death Valley saltpan (Badwater 

Basin; fig. 2). Intermittent flow occurs in the lower part 

of the Amargosa River after some rainfall events causing surface water to occasionally be discharged onto 

the Death Valley saltpan. Salt Creek drains Mesquite 

Flat and mountainous areas in the northern part of 

Death Valley, north of the saltpan. The lower reach of 

Salt Creek flows during most of the year, typically 

discharging about 1.0 ft3/s.

EVAPOTRANSPIRATION UNITS

ET units are areas of similar vegetation density, 

soil type, and moisture content. ET rates have been 

shown to correlate well with these vegetation and soil 

characteristics (Ustin, 1992; Laczniak and others, 

1999; Nichols, 2001; and Reiner and others, 2002). 

Five ET units were delineated within the discharge area 

to minimize the variability of evapotranspiration rates 

within an area of interest and to facilitate the estimation 

of total evaporative discharge from the floor of Death 

Valley. Evaporative discharge from each ET unit was 

calculated as the product of the unit’s area and average 

evapotranspiration rate. Total evaporative discharge 

from the valley floor was computed by summing the 

evaporative discharge computed for each of the units.

Three ET units dominated by phreatophytic 

vegetation were mapped using satellite (Thematic 

Mapper) imagery. These units were mapped at a 

resolution of about 100 x 100 ft—the pixel size of 

Thematic Mapper (TM) imagery. The average surface 

reflectance (percent reflectance) for the six spectral 

bands within the visible to infrared range [0.4–2.4 μm 

(micrometer)] of a TM image was estimated by correcting the raw spectral data for scene illumination, atmospheric conditions, viewing geometry, and instrument 

response characteristics (Lillesand and Kiefer, 1987). 

The procedure used to map vegetation-dominated 

ET units was TM bands 3 (red wavelength) and 4 (near 

infrared wavelength). The use of selected bands to map 

vegetation is commonly referred to as a vegetation 

index (Elvidge and Zhikang, 1995). The specific index 
EVAPOTRANSPIRATION UNITS 7

0

0.05

0.10

0.15

0.25

0.25

3/94 3/94 8/94 8/94 6/95 6/95 9/95 9/95 4/96 4/96 8/96 8/96

3/94 3/94 8/94 8/94 6/95 6/95 9/95 9/95 4/96 4/96 8/96 8/96

ET Unit 1

High-Density Vegetation

ET Unit 2

Moderate-Density Vegetation

ET Unit 3

Low-Density Vegetation

ET Unit 0

Unclasified (No Vegetation)

MODIFIED SOIL-ADJUSTED VEGETATION INDEX (MSAVI)

DATE, in month/year DATE, in month/year

0

0.05

0.10

0.15

0.25

0.25

used for this study is known as the modified soiladjusted vegetation index (MSAVI; Qi and others, 

1994). Soil-adjusted indices were developed to 

minimize soil influences within the measured spectra 

and are more sensitive to spatial changes in vegetation 

(Heute, 1988).

Vegetation-dominated ET units were delineated 

using multiple MSAVI values computed from six TM 

scenes imaged over 3 years (1994–96). The time series 

constructed from these six MSAVI values is referred 

to as a temporal signature (fig. 3). These temporal 

signatures are inclusive of vegetation changes caused 

by seasonal and longer-term climatic variations. This 

approach differs from that used by Laczniak and others 

(1999, 2001) and Reiner and others (2002) in other 

major discharge areas within the DVRFS. Their 

approach delineated vegetation and soil covers using 

spectral signatures derived from the six TM bands in 

the visible and near-infrared regions of the electromagnetic spectrum. A total of 275 temporal signatures 

were defined on the basis of changes in the temporal 

MSAVI values. Nineteen of the 275 signatures were 

determined by analyzing MSAVI values for vegetation 

near sites equipped with instrumentation to measure 

ET rates. The remaining 256 signatures were computed 

using an algorithm (ERDAS®, Inc., 1997) that determines a given number of unique signatures from the 

range of MSAVI values within an image (specific time) 

and throughout the images (entire time period). One 

signature was assigned to each pixel (fig. 3) using a 

matching algorithm. This procedure was applied only 

to that part of the imagery within the area of probable 

ground-water discharge as delineated by Harrill and 

others (1988).

Figure 3. Temporal modified soil-adjusted vegetation indices (MSAVI) clusters of signatures used to delineate vegetation in 

Death Valley.
8 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

Vegetation-dominated ET units were defined by 

grouping similar temporal signatures into three clusters 

of similar vegetation density. Individually and collectively the three resulting ET units (low-, moderate-, and 

high-density vegetation; fig. 4) have limited areal 

extent constituting only about 13,100 acres or about 12 

percent of the land cover within the ground-water 

discharge area (table 1).

Two other ET units were mapped to represent 

soil-dominated land covers within the discharge 

area. These units were delineated on the basis of field 

reconnaissance information and wetland-unit extents 

reported in the NWI (U.S. Fish and Wildlife Service, 

1996). The two soil-dominated ET units are bare and 

salt encrusted (fig. 4, table 1). Areas of low-density 

vegetation identified within these soil-dominated units 

were checked and validated by field visitation. Areas 

falsely identified through remote sensing were eliminated and assigned to the appropriate soil unit. 

Together these two units account for about 97,000 acres 

or about 88 percent of the land cover within the groundwater discharge area. 

EVAPOTRANSPIRATION RATES

ET is a process by which water, at or near the 

earth’s surface, evaporates or is transpired by vegetation and is thereby transferred to the atmosphere. As 

ET occurs, water changes state from a liquid to a gas by 

consuming energy from solar and terrestrial radiation, 

turbulent kinetic energy, and from heat stored in soil 

and atmosphere. The relation between changing energy 

consumption and corresponding water loss is the basis 

for energy-budget methods used to estimate ET rates. 

Energy-budget methods were applied in this study 

using micrometeorological and soil data collected from 

representative vegetation groups in Death Valley to 

estimate daily and annual ET rates. 

Table 1. Evapotranspiration units determined from temporal analysis of modified soil-adjusted vegetation indices (MSAVI), 

Death Valley, California

[Symbol: --, not applicable. Abbreviations: ET, evapotranspiration; ft, feet]

1Vegetation cover descriptor: low density is from greater than 5 to less than 15 percent; moderate density is 15–50 percent; and high density is greater than 50 percent. Soil-moisture descriptor presented in relative terms. Sources for depth-towater information are U.S. Geological Survey National Water Information System at web site <http://waterdata.usgs.gov/ 

nwis> (last retrieved August 2003), and depth-to-water measurements made during the study.

ET-unit 

identifier

ET-unit

number

ET-unit

area,

in acres

General description of ET unit1

UCL 0 -- Unclassified (UCL) area with no substantial ET from any ground-water sources; water 

table typically greater than 20 ft below land surface; soil very dry.

HDV 1 1,522 Area of high-density vegetation (HDV), primarily marsh and meadow grasses, and 

mesquites; perennially flooded; water table typically ranges from near land surface to 

about 20 ft below land surface; soil wet to dry.

MDV 2 5,019 Area dominated by moderate-density vegetation (MDV), primarily salt and bunch 

grasses, arrowweed, mesquite, minor pickleweed; water table typically ranges from 

about 2 to 20 ft below land surface; soil moist to dry.

LDV 3 6,625 Area dominated by low-density vegetation (LDV), primarily salt grass, pickleweed, and 

shrub mesquite; water table typically ranges from about 5 to 20 ft below land surface; 

soil damp to dry.

BSP 5 75,922 Area of playa dominated by bare-soil playa (BSP), primarily silt; some salt encrustation; 

water table typically ranges from a near land surface to about 10 ft below land surface; 

water table declines during summer months; occasionally flooded; soil damp to dry.

SEP 6 21,287 Area of playa dominated by salt-encrusted playa (SEP); occasionally flooded, water 

table typically near land surface to about 5 ft below land surface; salt wet to dry.
EVAPOTRANSPIRATION RATES 9

EXPLANATION

Playa

Vegetation

Bare soil (76,000 acres)

Salt encrusted (21,000 acres)

Low density (LDV; 6,600 acres)

Moderate density (MDV; 5,000 acres)

High density (HDV; 1,500 acres)

Evapotranspiration (ET) units—Text in

parenthesis gives total acrage of unit. See

table 1 for detailed unit description. Shaded

relief (gray) identifies areas of no substantial ground-water ET.

Boundary of Death

Valley discharge area

Instrumented sites

Base from U.S. Geological Survey digital data, 1:100,000,

1978–89: Universal Transverse Mercator Projection Zone 11.

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

0 10 20 MILES

0 10 20 KILOMETERS

DEATH VALLEY NATIONAL PARK DEATH VALLEY NATIONAL PARK

Nevada

California California

Owlshead wlshead

Mountains ountains

Black 

M

Black 

ountains Mountains

Panam

ni t Panamint

Range

Telescope

Peak

Telescope

Peak

Salt-encrusted playa alt-encrusted playa

S. Badwater

(Bare-soil playa)

S. Badwater

Eagle Borax (Bare-soil playa)

(MDV)

Eagle Borax

(MDV)

Funeral MoutainsFuneral Moutains

Mesquite Flat (HDV) esquite Flat (HDV)

Mixed Grasses (HDV) ixed Grasses (HDV)

Pickleweed

(LDV)

Pickleweed

(LDV)

117°00' 116°30'

36°00'

36°30'

Furnace

Creek Ranch

Furnace

Creek Ranch

373

127

190

Figure 4. Distribution of evapotranspiration units and location of instrumented sites in Death Valley.
10 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

A number of methods have been developed to 

estimate ET using the relation between water loss and 

energy consumption. In this study the Bowen ratio 

method (Bowen, 1926) was applied to estimate ET 

rates in Death Valley because of its robustness, reliability, and accuracy. At one site ET rates also were estimated using the eddy correlation method (Stull, 1988) 

to compare with the Bowen ratio method as a check to 

see if similar results could be achieved with two different methodologies.

Site Selection and Instrumentation

Sites were selected for data collection based on 

field reconnaissance and preliminary ET-unit classification. Instrumented platforms (ET sites) were 

installed at six locations (fig. 4, table 2): salt-playa 

site (extensive salt encrustation), bare-soil playa site 

(silt and salt encrustation), pickleweed site (pickleweed 

plants, low-density vegetation), Eagle-Borax site 

(arrowweed plants and salt grass, moderate-density 

vegetation), Mesquite Flat site (mesquite trees, highdensity vegetation), and Mesquite Flat mixed grass site 

(mixed meadow grasses, high-density vegetation). An 

ET site was not established for the open-water unit 

because its area (only 2 acres) was insignificant in comparison to other ET units. Sites were not installed in 

locations outside the valley floor (fig. 2). For purposes 

of this study, the valley floor is considered the principal 

discharge area for Death Valley. A photograph of each 

ET site is shown in figure 5. Data on site coordinates 

and measurement period-of-record are listed in table 2. 

The distance between instrument sensors and the 

upwind edge (upwind fetch) of the environment under 

study also was an important parameter for site selection. A proper fetch ensures that sampled heat and 

vapor fluxes are representative of the environment of 

interest. To achieve a proper fetch, sites were selected 

where the distance to the edge of the environment was 

at least 100 times the height of the highest air temperature-humidity sensor (about 800 ft; Campbell, 1977). 

All ET sites were equipped with instruments to collect 

micrometeorological data for energy-budget calculations using the Bowen ratio method. These instruments 

could either directly measure energy-budget components or be used to compute these components from 

direct measurements. A schematic of instrumentation 

used to collect data for the Bowen ratio and eddy correlation methods is shown in figure 6. 

Table 2. Location, depth of piezometer, and period of data collection for instrumented sites used to collect 

micrometeorologic data, Death Valley, California, 1997–2001

[Abbreviations: ET, evapotranspiration. ET units description: SEP, salt-encrusted playa; BSP, bare-soil playa; LDV, low-density

vegetation, MDV, moderate-density vegetation; HDV, high-density vegetation. See table 1 for description of ET units]

1 Site has two piezometers for water-level measurements at multiple depths below land surface.

ET-site name ET unit Latitude Longitude

Depth of 

piezometer, 

in feet

Period of data

collection

Salt playa1 SEP 36°13′40′′ 116°47′07′′ 13.8/3.9 06/98 – 07/01

Bare-soil playa1 BSP 36°12′52′′ 116°46′29′′ 10.8/5.6 08/97 – 12/99

Pickleweed LDV 36°17′12′′ 116°53′12′′ 14.0 03/98 – 08/00

Eagle Borax MDV 36°11′46′′ 116°52′04′′ 9.3 01/00 – 09/01

Mesquite Flat HDV 36°44′45′′ 117°08′11′′ 14.2 01/99 – 09/01

Mesquite Flat mixed grasses HDV 36°44′19′′ 117°08′44′′ 9.2 03/00 – 09/01
EVAPOTRANSPIRATION RATES 11

A B

C D

E F

A B

C D

E F

Figure 5. Evapotranspiration sites at (A) salt-encrusted playa, (B) bare-soil playa, (C) pickleweed, (D) Eagle Borax, (E) Mesquite 

Flat, and (F) Mesquite Flat mixed grasses, Death Valley, California, August 1998–October 2001.
12 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001

3.28 ft

4.92 ft

0.16 ft 

Net

Radiometer

(net radiation)

Temperature

Humidity

Probe

(upper air

temperature and 

vapor pressure)

Temperature

Humidity

Probe

(lower air

temperature and

vapor pressure)

Water Well

Submersible Pressure Sensor

(water level)

Soil-Temperature Probes

(average soil temperature) Soil-Moisture Probe

(soil moisture)

Soil-Heat Flux Plate

WATER TABLE WATER (soil-heat flux) TABLE

Anemometer

(wind

speed)

Anemometer

(wind

speed)

1.64 ft

Infrared 

temperature

sensors

(surface & plant

temperature)

*

*

* Optional instrumentation not used for for estimating ET

Automatic exchange mechanism

0.33 ft

4.92ft

3-D sonic anemometer

Krypton hygrometer

Fine wire thermocouple

Land Surface

A

B

Figure 6. Schematic of typical micrometeorological data-collection site for computation of evapotranspitation by (A) Bowen ratio method and (B) eddy correlation 

method, Death Valley, California, 1997–2001.
EVAPOTRANSPIRATION RATES 13

Site instruments included:

• A net radiometer to measure incoming and 

reflected short- and long-wave radiation,

• Two air temperature-humidity probes (THPs) 

at reference heights of 4.92 and 8.2 ft above 

average plant canopy height,

• An anemometer to measure wind speed at the 

same height as upper THP,

• Two soil heat-flux (SHF) plates to measure the 

rate of change of heat stored in the soil or water 

profile at 0.16 ft below land surface,

• Subsurface soil-temperature probes (evenly 

spaced between land surface and SHF),

• A soil-moisture probe angled between land 

surface and 0.16 ft below land surface,

• A submersible pressure sensor to continually 

measure water levels in a shallow piezometer, 

and

• A volumetric precipitation gage. 

The THPs were calibrated by the manufacturer 

to specifically operate within the range of extremely 

low relative humidity and high air-temperature values 

common to Death Valley. An automatic exchange 

mechanism was used to swap the THPs between 

reference heights every 10 minutes to remove any air 

temperature and relative humidity bias that exists 

between the two probes (Fritschen and Simpson, 1989; 

Fritschen and Fritschen, 1993). Four of the six sites 

also were equipped with two infrared temperature 

sensors to measure soil-surface and plant-canopy 

temperatures and a second anemometer at the height 

of the lower THP (fig. 6A). 

The Mesquite Flat mixed-grasses site was additionally equipped with instruments (fig. 6B) to collect 

data needed to compute sensible- and latent-heat flux 

using the eddy correlation method: 

• A krypton hygrometer to measure water-vapor 

density and

• A three-dimensional sonic anemometer to 

measure vertical wind speed and air temperature. 

Energy-Budget Methods

Bowen Ratio Method

The exchange of energy at the earth’s surface 

during the ET process can be described as a balance 

between incoming and outgoing energy fluxes (fig. 7). 

The balance for an arid environment such as Death 

Valley can be mathematically expressed using an 

energy-budget equation of the form

(1)

where is net radiation, 

 is sensible-heat flux (energy per second per 

area),

 is soil-heat flux (energy per second per area),

 is latent-heat flux (energy per second per 

area),

 is latent heat of vaporization for water 

(energy per mass), and

 is rate of water-vapor flux (mass per time per 

area). 

Net radiation is the principal source of energy at 

the surface of the earth and is the algebraic sum of 

incoming and outgoing long- and short-wave radiation. 

Soil-heat flux is the energy stored in the soil near the 

earth’s surface. Net radiation and soil-heat flux can be 

directly computed using field data. The difference 

between net radiation and soil-heat flux is the energy 

available for sensible- and latent-heat flux. 

Sensible-heat flux is the energy used to heat air at 

the earth’s surface and is proportional to the product of 

the temperature gradient and the turbulent-transfer 

coefficient for heat. This flux can be expressed as

(2)

where is density of air (mass per volume),

 is specific heat of air at a constant pressure 

(energy per mass per temperature),

 is turbulent transfer coefficient of sensible 

heat (area per time),

 is lower (l) or upper (u) reference point 

temperature of air, and

 is lower (l) or upper (u) height at which 

reference point temperature of air is 

measured (length).

Rn = H G+ + λE

Rn

H

G

λE

λ

E

H ρaCpkh Tl Tu ( ) – zl zu = ( ) ⁄ ( ) –

ρa

Cp

kh

Tl u,

zl u,
14 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

Net

Radiation Latent-Heat

Flux

Sensible-Heat

Flux

Long- and Short-Wave

Radiation

Soil-Heat

Flux

Incoming Reflected/

Emitted

Rn = H + G + λE

λ

Land

Surface

Rn

Latent-heat flux is the energy used for ET. In a 

flux-gradient format, it is proportional to the product of 

the vapor-pressure gradient and the turbulent-transfer 

coefficient of vapor and can be expressed as

(3)

where is ratio of molecular weight of water vapor to 

dry air (dimensionless),

 is turbulent transfer coefficient of vapor (area 

per time),

 is ambient air (barometric) pressure (force per 

area),

 is lower (l) or upper (u) reference point vapor 

pressure (force per area), and

 is lower (l) or upper (u) height at which reference point vapor pressure is measured 

(length).

Equations 2 and 3 cannot be directly solved 

because the turbulent-transfer coefficients of heat 

and vapor are not known. Bowen (1926) developed an 

indirect method for solving the energy-budget equation 

by assuming the turbulent-transfer coefficients were 

equal and expressing the ratio of sensible-heat flux to 

latent-heat flux as

(4)

where is the ratio and referred to 

as the psychrometric constant. The psychrometric 

constant is a function of air pressure and temperature 

and is almost constant for a given altitude (Fritschen 

and Gay, 1979). The ratio of sensible-heat flux to 

latent-heat flux as expressed in equation 4 is referred 

to as the Bowen ratio . When the is substituted 

into equation 1, latent-heat flux can be re-expressed as

. (5)

λE λρaεkv ( ) ⁄ P el eu ( ) ( ) – zl zu = ⁄ ( ) –

ε

kv

P

el u,

zl u,

H ⁄ γ ( ) λE Tl Tu ( ) – el eu = ⁄ ( ) –

γ PCp ( ) λε ⁄ ( )

( ) β β

λE R = ( )β n – G ⁄ ( ) + 1

Figure 7. Schematic showing components of the surface-energy budget.
EVAPOTRANSPIRATION RATES 15

ET is the mass flux of water into the atmosphere 

and can be calculated with latent-heat flux as

(6)

where is the rate of evapotranspiration (length per 

time) and

is the density of water.

Eddy Correlation Method

Rapidly ascending and descending turbulent 

currents of air exchanging heat and water vapor 

between the land surface and the atmosphere are called 

eddies. The eddy correlation method determines H and 

 by calculating a covariance between fluctuations in 

vertical wind speed, air temperature, and water-vapor 

density. Vertical wind-speed, air-temperature, and 

water-vapor density data are sampled at high frequencies and used to calculate H and . Unlike the Bowen 

ratio, H and can be determined independently of 

or . Using the eddy correlation method, H is proportional to the covariance between the vertical wind 

speed and instantaneous air temperature measured at a 

point and can be expressed as 

(7)

where is the instantaneous deviation of vertical 

wind speed from the mean (length per time) and is 

the instantaneous deviation of air temperature from the 

mean (degrees kelvin).

Using this same approach, is proportional to 

the covariance between the instantaneous vertical wind 

speed and water vapor density at a point and can be 

expressed as

 (8)

where is the deviation of water-vapor density from 

the mean (mass per volume). ET is calculated for the 

eddy correlation method by substitution of from 

equation 8 and into equation 6. 

Micrometeorological Data 

Micrometeorological data required for solving 

the energy budget by the Bowen ratio or eddy correlation methods were collected at each ET site for a period 

of 1 year or more. A minimum period of 1 year was 

required to evaluate and document seasonal fluctuations in ET rates and compute an annual ET value. 

Additional years of data were acquired to better assess 

annual changes in ET that may result from climatic 

variations, such as differences between dry and wet 

years. Micrometeorological data were collected for this 

study from August 1997 to October 2001 (table 2). 

Data collected for the Bowen ratio calculations 

were sampled at either 10- or 30-second intervals. For 

eddy correlation calculations, the krypton hygrometer 

and the three-dimensional sonic anemometer both 

sampled data at a 0.1-second interval. Bowen ratio and 

eddy correlation data were collected and stored for 

final use as 20-minute averages that were then used to 

calculate 20-minute average ET rates. An example of 

micrometeorological data needed to compute the 

Bowen ratio is shown in figure 8 for a 5-day period at 

the Mesquite Flat mixed-grasses site.

Daily and Annual Evapotranspiration

Daily ET rates were determined by summing 

20-minute average ET rates computed from measured 

micrometeorological data and calculated energy fluxes. 

This process includes (1) determining individual 

components of the energy budget (net radiation, latentheat flux, sensible-heat flux, and soil-heat flux; fig. 9A) 

from measured micrometeorological data, (2) using 

equation 6 to determine 20-minute ET rates from 

20-minute latent-heat flux, and (3) determining daily 

ET by summing 20-minute ET values for a 24-hour 

period (fig. 9B). Energy-budget fluxes typically follow 

a diurnal pattern, obtaining their maximum values during daylight hours when incoming solar radiation is at 

its peak. How energy is partitioned between these 

components is dependant on the site environment. For 

example, evaporation generally is the dominant process 

at sites with dense vegetation or moist soils. This environment typically causes latent-heat flux to be greater 

than sensible-heat flux. At sites with a relatively high 

percentage of exposed bare soil or dormant vegetation, 

heating of the land surface generally is the dominant 

process and this environment typically causes sensibleand soil-heat fluxes to exceed latent-heat flux. 

Due to periodic equipment failures collected data 

were sometimes not usable for ET calculation. These 

data gaps ranged from 12 to 63 consecutive days and 

collectively up to 110 days during 1 year of data collection at the Mesquite Flat site. Daily ET values are

ET λE λρw = ⁄ ( )

ET

ρw

λE

λE

λE

Rn G

H ρaCp = w'T'

w'

T'

λE

λE λw'ρv = '

ρv'

λE
16 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001

211 212 213 214 215 216

JULIAN DAY, REFERENCED TO JANUARY 1, 2000

0

10

20

30

40

50

60

70

80

About 9.84 feet above land surface

-2

0

2

4

6

8

10

12

14

16

Flux plate 1 (about 2 inches below land surface)

Flux plate 2 (about 2 inches below land surface)

80

85

90

95

100

105

SUBSURFACE SOIL TEMPERATURE Average temperature between land surface 2 inches below

SOIL-HEAT FLUX

NET RADIATION

NET RADIATION, IN

WATTS PER SQUARE FOOT

SOIL-HEAT FLUX, IN

WATTS PER SQUARE FOOT

SUBSURFACE SOIL

TEMPERATURE, IN

DEGREES FAHRENHEIT

0

5

10

15

20

25

30

35

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

60

80

100

120

140

160

TEMPERATURE,

IN DEGREES FAHRENHEIT

VAPOR PRESSURE,

IN POUNDS PER SQUARE INCH

WIND SPEED,

IN FEET PER SECOND

AIR TEMPERATURE

VAPOR PRESSURE

WIND SPEED

Upper height (about 8.20 feet above land surface)

Lower height (about 4.92 feet above land surface)

Upper height (about 8.20 feet above land surface)

Lower height (about 4.92 feet above land surface)

(about 9.84 feet above land surface)

Figure 8. Twenty-minute averaged micrometeorological data collected from the Mesquite Flat mixedgrasses site, Death Valley, California, July 29–August 2, 2000.
EVAPOTRANSPIRATION RATES 17

211 212 213 214 215

JULIAN DAY, REFERENCED TO JANUARY 1, 2000

0

0.1

0.2

0.3

0.4

-20

0

20

40

60

80

Net radiation

Latent-heat flux

Soil-heat flux

Sensible-heat flux

20-MINUTE AVERAGED ENERGY-BUDGET FLUXES

DAILY TOTALED EVAPOTRANSPIRATION

FLUX DENSITY,

IN WATTS PER SQUARE FOOT

DAILY EVAPOTRANSPIRATION,

IN INCHES

A

B

summed to compute a cumulative ET data set. Gaps in 

the ET data set were interpolated using Simpson’s rule 

of numerical integration to maintain continuity in the 

data set and obtain an annual ET rate. A smoothing of 

the ET data set was done to reduce variability in daily 

ET that typically is caused by changes in daily weather 

and associated fluctuations in micrometeorological 

parameters (fig. 10). 

Temporal fluctuations in ET rates are closely 

related to seasonal conditions such as solar radiation, 

precipitation, and plant vigor whereas spatial differences in annual ET rates are related to plant density, 

availability of water, and soil-surface conditions. Daily 

ET rates at vegetated sites typically are highest in the 

late spring to early fall because net radiation is at its 

highest value (fig. 11). Conversely, daily ET rates are 

lowest from the late fall to early spring months because 

net radiation is at its lowest values. Similar seasonal 

fluctuations in daily ET occurred at sites where the 

environment could not support vegetation, such as the 

salt and bare-soil playa sites, even though daily ET 

rates at these sites were significantly less than rates at 

the vegetated sites. 

Annual ET rates for areas of Death Valley span 

a wide range of values that reflect the diversity of the 

different environments (fig. 11B). Encrusted-salt and 

bare-soil surfaces that are relatively impermeable and 

not able to support vegetation typically have very low 

ET rates. For these non-vegetated sites, total ET ranged 

from 0.17 ft/yr for the salt playa to 0.21 ft/yr for the

Figure 9. Examples of (A) energy-budget fluxes and (B) daily evapotranspiration computed with micrometeorological data 

collected from the Mesquite Flat mixed-grasses site, Death Valley, California, July 29–August 5, 2000.
18 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

0

0.01

0.02

0.03

DAILY EVAPOTRANSPIRATION, IN FEET

Bowen ratio raw data [54.4 inches]

Bowen ratio polynomial fit [54.2 inches] No data

No data

200 300 400 500 600 700

JULIAN DAY, REFERENCED TO JANUARY 1, 2000

bare-soil playa (table 3). In other environments where 

soils can sustain vegetation, ET increases as plant 

density increases. For these vegetated sites, total ET 

ranged from 0.60 ft/yr for pickleweed to 3.90 ft/yr 

Mesquite Flat mixed grasses. 

Longer-term micrometeorological data indicate 

that annual total ET in Death Valley varied as a function 

of annual precipitation (fig. 11B); however, the modest 

amounts of annual precipitation in Death Valley are not 

sufficient to sustain local vegetation and account for a 

very small amount of the measured ET. Therefore 

native phreatophytic vegetation must survive on local 

ground water which is the source for the majority of the 

measured estimated ET.

Comparison of Bowen Ratio and Eddy Correlation 

Methods

Daily ET rates were estimated for vegetation at 

the Mesquite Flat mixed-grasses site using the eddy 

correlation method to compare estimates made with the 

Bowen ratio method. An independent check of Bowen 

ratio based ET rates was necessary because of the 

uncertainty associated with applying energy-budget 

methods in the extreme climate of Death Valley. 

Micrometeorological data were collected for the 

Bowen ratio method from March 2000 to September 

2001 and for the eddy correlation method from July 

2000 to October 2001.

From March 2000 to September 2001, daily ET 

values for the Mesquite Flat mixed-grasses site were 

estimated using equation 6 for Bowen ratio and eddy 

correlation methods (fig. 12). Using the Bowen ratio 

method, the minimum daily ET was 0.0 in. on January 

7, 2001, and the maximum daily ET was 0.30 in. on 

June 30, 2001. The average daily ET computed with the 

Bowen ratio method was 0.10 in., and the total ET for 

the period was 53.4 in. In comparison, using the eddy 

correlation method, the minimum daily ET was 0.0 in. 

on January 9, 2001, and the maximum daily ET was 

0.30 in. on June 22, 2001. Using the eddy correlation 

method, the average daily ET was 0.10 in., and the total 

ET for the period was 55.4 in. Relatively small discrepancies between Bowen ratio and eddy correlation 

Figure 10. Example of daily evapotranspiration and smoothed fit for data collected at the Mesquite 

Flat mixed-grasses site, Death Valley, California, July 2000–October 2001.
ESTIMATES OF ANNUAL EVAPOTRANSPIRATION 19

0 365 730 1,095 1,460 1,825

JULIAN DAY, REFERENCED TO JANUARY 1, 1997

0

0.005

0.010

0.015

0.020

0.025

0.030

DAILY EVAPOTRANSPIRATION, IN FEET

S. Badwater (bare-soil) site [0.21 feet per year]

Salt-encrusted playa site [0.17 feet per year]

Pickleweed site [0.60 feet per year]

Eagle-Borax site [2.00 feet per year]

Mesquite Flat site [2.90 feet per year]

Mesquite Flat mixed-grasses site [3.90 feet per year]

0

0.1

0.2

0.3

0.4

0.5

1997 1998 1999

YEAR

2000 2001

A

B

0.141 foot

(1.69 inches)*

 0.355 foot

(4.26 inches)

0.050 foot

(0.60 inch)

0.047 foot

(0.56 inch)

0.282 foot

(3.38 inches)*

ANNUAL PRECIPITATION,

IN FEET

estimates of ET are owing largely to errors associated 

with interpolating ET for data gaps caused by shortterm Bowen ratio instrument failure.

ESTIMATES OF ANNUAL 

EVAPOTRANSPIRATION

Estimates of annual ground-water discharge for 

each ET unit were computed by multiplying acreage 

of the ET unit by its estimated annual ground-water 

evapotranspiration rate (table 4). This rate was determined for each site by correcting estimates of annual 

ET for the effects of precipitation or surface-water 

inflow to the valley floor. Although shallow ground 

water is the primary source for ET in Death Valley, 

local precipitation and flooding of surface-water runoff 

from areas adjacent to the valley must be accounted for 

to accurately quantify the amount of ground water 

discharged by ET. The amount of ground water used for 

ET annually can be estimated by subtracting local 

precipitation and surface-water inflow components as 

expressed by

Figure 11. Comparison of (A) measured annual precipitation (number above bar is total annual precipitation; asterisk 

indicates an incomplete year of data) and (B) calculated daily evapotranspiration from micrometeorological datacollection sites (bracketed number is estimated annual evapotranspiration for that site), Death Valley, California, 

1997–2001.
20 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001

0 0.005 0.010 0.015 0.020 0.025 0.030

DAILY EVAPOTRANSPIRATION ESTIMATED USING EDDY CORRELATION, IN FEET

DAILY EVAPOTRANSPIRATION ESTIMATED USING BOWEN RATIO, IN FEET

r

2

 = 0.99 1:1 ratio line

0

0.005

0.010

0.015

0.020

0.025

0.030

Table 3. Rates of annual evapotranspiration, measured precipitation, and annual ground-water evapotranspiration 

at data collection sites in Death Valley, California, 1997–2001

[Abbreviations: ET, evapotranspiration. ET-unit site description: BSP, bare-soil playa; SEP, salt-encrusted playa; LDV, low-density vegetation; MDV, 

moderate-density vegetation; HDV, high-density vegetation; ET, annual ET rate, annual surface-water and ground-water discharge 

by ET; ETgw, annual ground-water discharge. Description of ET units listed in table 1]

ET-site name ET-unit site 

Annual ET 

rate, in feet 

per year

Annual

precipitation, 

in feet per 

year

Annual ground-water

evapotranspiration rate, 

in feet per year

Salt-encrusted playa SEP 0.17 1

0.39 2

1 Data are for years when flooding of the playa did not occur. 2 Data are for years when flooding of the playa did occur.

0.041

0.282

0.131

-1.40 3

3 Data are for years when flooding of the playa did occur including the quantity of surface-water accumulation resulting from flooding 

of overland flow not shown in table.

Bare-soil playa BSP 0.211

0.372

0.061

0.352

0.151

0.012

Pickleweed LDV 0.60 0.06 0.54

Eagle Borax MDV 2.00 0.23 1.80

Mesquite Flat HDV 2.90 0.16 2.70

Mesquite Flat mixed grasses HDV 3.90 0.25 3.60

Figure 12. Correlation plot of daily evapotranspiration of Bowen ratio method (Y-axis), and eddy correlation 

method (X-axis), at Mesquite Flat mixed-grasses site, Death Valley, California, July 2000–October 2001.
ESTIMATES OF ANNUAL EVAPOTRANSPIRATION 21

. (9)

where is annual ground-water evapotranspiration (length),

 is total estimated annual evapotranspiration (length),

is the quantity of ground- and surfacewater accumulation resulting from 

flooding of overland flow (length), and

 is annual local precipitation (length).

Although it was not possible to precisely determine Sw, 

approximations were made based on indirect evidence 

of flooding. For years when flooding did not occur, Sw

could be set equal to zero, reducing equation 9 to

. (10)

Table 4. Annual ground-water discharge rate and total

annual ground-water discharge for evapotranspiration 

units, Death Valley, California, 1997-2001

[Abbreviations: ET, evapotranspiration; ETgw, annual ground-water 

discharge by ET. ET-unit site description: SEP, salt-encrusted playa; BSP, 

bare-soil playa; LDV, low-density vegetation; MDV, moderate-density 

vegetation; HDV, high-density vegetation. Description of ET units are 

listed in table 1]

Flooding of the Amargosa River (fig. 1) from 

intense storms made significant contributions to the 

total ET on the salt and bare-soil playas in 1998 and on 

the salt playa in 2001. For example, extensive rains in 

1998 resulted in flooding of part of the bare-soil playa. 

No visible evidence of stagnant-flood water existed at 

the bare-soil playa site but it was apparent that significant over-land flow had occurred due to the heavy 

rains. As a result, total annual ET for 1998 was relatively high (0.36 ft as compared to 0.21 ft in 1999). 

By applying equation 10, the ETgw component was 

determined to be relatively low (0.012 ft).

Heavy rains totaling 0.28 ft were measured 

between January 1 and July 23, 2001 (204-day period), 

on the salt playa and lead to extensive flooding of the 

site (fig. 13). The maximum surface-water depth at the 

salt-playa site was about 0.57 ft as estimated by the 

height of salt-crystal deposits on a rain gage (fig. 14). 

Shallow wells at the site had been removed prior to this 

wet period, but increases in shallow-water levels resulting from these storms were estimated to be about 1.3 ft. 

Total ET during this period was estimated at 0.39 ft. 

Water accumulated from local precipitation and surface-water inflow is assumed to account for the 

increase in ground- and surface-water levels and 

totaled 1.84 ft. A total ETgw value of –1.40 ft was calculated using equation 10 and these data. These data 

demonstrate that precipitation and surface water 

exceed total ET for the year resulting in ETgw being a 

negative value. Because all ET was derived from 

sources other than ground water, ETgw was neglected 

for this ET unit in 2001.

The results indicate that (1) the volume of available surface water at the salt playa in 2001 exceeded 

total evaporative discharge and (2) that some surface 

water may have infiltrated into the shallow aquifer. 

Excess available surface water may infiltrate on the salt 

and bare-soil playas or at the margins of the playas, particularly in principal areas of surface-water inflow. In 

these areas, such as along the Amargosa River and Salt 

Creek (fig. 2), the permeability of shallow sediment 

horizons likely is greater than other areas of the playa. 

However, the uncertainty of estimating available water 

and potential infiltration on the playa using equation 9 

is relatively high. For example, equation 9 does not 

account for changes in aquifer storage due to rising or 

falling ground-water levels. Moreover, the accuracy of 

estimating ungaged surface-water inflow to the salt and 

bare-soil playas is relatively low. 

ET-unit 

identifier

(see table 1)

ET-unit 

area, 

in acres

Annual 

groundwater 

discharge

rate 1, in 

feet per

year

1 Data are for years when flooding of the playa did not 

occur.

Annual 

groundwater

discharge1, 

in acre-feet

BSP 76,000 0.15 11,400

SEP 21,000 0.13 2,730

LDV 6,600 1.0 6,600

MDV 5,000 2.0 10,000

HDV 1,500 3.0 4,500

Total

annual

groundwater 

discharge, 

in acre-feet

34,800

ET ET S p gw = − w −

ETgw

ET

Sw

p

ETgw = ET − p
22 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

Long-term flooding conditions (stagnant surface 

water) were never observed at any of the vegetated ET 

unit sites. Therefore, annual ground-water discharge at 

pickleweed, Eagle-Borax, Mesquite Flat, and Mesquite 

Flat mixed-grasses sites is determined by assuming Sw

in equation 9 is equal to zero. Using equation 10, 

annual ETgw at these sites was estimated by subtracting 

mean annual precipitation (table 3) from annual ET. 

Ground-water discharge for each site is not representative of the entire unit but only of the location 

where an instrument is placed because each vegetated 

ET unit spans a spectrum of densities. A plot of groundwater discharge versus relative vegetation cover was 

constructed to obtain representative rates for each unit. 

The value at the midpoint of each vegetated ET unit 

was used as the representative ground-water discharge 

rate for that unit (fig. 15). 

The final estimate of ETgw represents annual 

ETgw at the corresponding midpoints of low-, moderate-, and high-density ET units and equaled 1.0 ft, 2.0 

ft, and 3.0 ft, respectively (table 4). Total ground-water 

ET from the floor of Death Valley was determined by 

extrapolating ETgw rates for low-, moderate-, and highdensity vegetation using satellite imagery. Therefore, 

estimates of ETgw computed for each ET unit using 

equation 10 were adjusted to reflect an average ETgw

rate for each vegetation-density group. 

Figure 13. Comparison of (A) annual precipitation and (B) daily evapotranspiration at the salt-encrusted playa 

micrometeorological data-collection site, Death Valley, California, January 1999–July 2001.

EVAPOTRANSPIRATION

1999 (0.17 foot)

2000 (0.18 foot)

2001 (0.39 foot)

0.28 foot

(3.38 inches)

0.05 foot

(0.56 inch)

2000

YEAR

2001

0.05 foot

(0.60 inch)

1999

A

B

ANNUAL PRECIPITATION,

IN FEET

DAILY EVAPOTRANSPIRATION, IN FEET

0

0.1

0.2

0.3

0.4

0

0.005

0.010

0.015

0.020
ESTIMATES OF ANNUAL EVAPOTRANSPIRATION 23

Estimates of Annual Ground-Water Discharge

Annual ground-water discharge from the valley 

floor of Death Valley was estimated in this study using 

data sets of annual ET. All shallow ground-water 

discharge from the valley floor is assumed to transpire 

or evaporate from the five delineated ET unit types. 

Shallow ground-water levels of the valley floor are 

maintained by periodic infiltration of rainfall and 

surface flooding from the Amargosa River and Salt 

Creek (fig. 2), by infiltration of discharge from numerous springs and seeps adjacent to valley floor deposits, 

and possibly by ground-water underflow from mountains surrounding Death Valley or from the regional 

carbonate aquifer. Estimates of annual discharge in this 

report do not include ET from vegetation supported by 

higher-altitude springs adjacent to the valley floor on 

the west sides of the Funeral Mountains and Grapevine 

Mountains (fig. 2).

Estimates of annual ground-water discharge 

are based on data sets from 1997 through 2001 and 

represent a very short-term climatic interval. Recorded 

precipitation ranged from less than 0.08 ft/yr (1 in/yr) 

to greater than 0.25 ft/yr (3 in/yr). During years of 

higher precipitation when flooding was observed on the

Figure 14. Salt-crystal deposits on a rain gage following the evaporation of surface flooding near the salt-encrusted playa 

micrometeorological data-collection site, Death Valley, California, April 2001.
24 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

salt and bare-soil playas ground-water discharge from 

these ET units was considered to be negligible because 

precipitation and inflow exceeded total ET. For this reason, two estimates of annual ground-water discharge 

were determined for Death Valley; one representative 

of a year when flooding did occur on the playas and the 

other representative of a year when flooding did not 

occur. These discharge values were then averaged 

together based on a periodic flood frequency to establish what the annual ground-water discharge would be 

over time. 

Results for years of drier-than-average precipitation conditions (table 3), when flooding on the salt and 

bare-soil playas did not occur, annual ground-water 

discharge from the floor of Death Valley was estimated 

to be about 35,000 acre-ft. Annual ground-water discharge for years of wetter-than-average precipitation 

conditions, when flooding did occur at the salt and 

bare-soil playas, was estimated to be less than zero, 

indicating that the volume of available surface water 

exceeded total evapotranspiration. During the wetterthan-average period used to calculate annual discharge, 

ETgw increased slightly (100 to 500 acre-ft) at the 

vegetated sites. Significant differences in estimated 

annual discharge between drier- and wetter-than-average years in Death Valley is due primarily to flooding 

on the playas that contribute most, if not all, water used 

for ET in these ET units. 

Annual ground-water discharge from the valley 

floor, representative of long-term climatic conditions, 

was estimated by a weighted-average method using a 

maximum of 35,000 acre-ft (representing years without 

playa flooding) and a minimum of 22,000 acre-ft 

(representing years with playa flooding). Assuming 

reasonable, but arbitrarily selected, flood frequencies 

of three-floods in a 20-year period, two-floods in a 

10-year period, and one-flood in a 10-year period, 

annual ground-water discharge was estimated at about 

33,000 acre-ft. This result shows that although flooding 

of the salt and bare-soil playas significantly decreases 

the annual ground-water discharge these periodic 

events have little impact on the ground-water discharge 

over time.

Figure 15. Relation of vegetation density and ground-water evapotranspiration for vegetated 

evapotranspiration units, Death Valley, California, August 1997–October 2001.

0

1

2

3

4

ANNUAL GROUND-WATER EVAPOTRANSPIRATION

LOW-DENSITY

VEGETATION (LDV)

MODERATE-DENSITY

VEGETATION (MDV)

HIGH-DENSITY

VEGETATION (HDV)

PICKLEWEED

EAGLE BORAX

MESQUITE FLAT

MESQUITE FLAT

MIXED GRASSES

(3.6 feet per year)

(0.5 feet per year)

(1.8 feet per year)

(2.7 feet per year)

LDV

MDV

HDV

(3.0 feet per year)

(2.0 feet per year)

(1.0 feet per year)

RELATIVE VEGETATION COVER, IN PERCENT

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
SUMMARY 25

Ground-water discharge estimates from a reconnaissance study (Hunt and others, 1966) and a regional 

ground-water flow model (Prudic and others, 1995) 

were compared to results from this study. Although 

the areas used in these studies differ slightly from the 

current study, comparisons are useful for presenting a 

range of possible discharge values and a general understanding of corresponding methods and limitations.

In a study by Hunt and others (1966), annual 

ground-water discharge was estimated at about 12,900 

acre-ft, for an area from Cottonball Basin south to 

Badwater Basin (fig. 2), based on data collected during 

winter months between 1957–60. This estimate 

accounts for discharge from seeps, springs, and vegetated areas along the perimeter of the salt and bare-soil 

playas and from adjacent mudflat areas on the playas 

(about 8,800 acre-ft/yr). Hunt’s estimate also accounts 

for discharge from Travertine Springs complex, and 

Texas and Narvares Springs on the west sides of the 

Funeral and Grapevine Mountains (fig. 2) along the 

eastern margin of the valley (about 4,100 acre-ft/yr). 

This estimate does not include discharge by groundwater evaporation from much of the salt-encrusted and 

bare-soil playas or from Mesquite Flat (fig. 2). For the 

current study, annual ground-water discharge from the 

valley floor between Cottonball Basin and Badwater 

Basin was estimated at 24,000 acre-ft. Not including 

discharge from springs along the eastern margin of the 

valley, the estimate of discharge by Hunt and others 

(1966) is about two and a half times less than estimated 

ground-water discharge for the current study. Differences between these discharge estimates are largely 

due to differences in the ET rates applied and the 

acreage of application. 

Limitations

Underlying limitations and assumptions in 

the methods and techniques used in this study may 

affect the accuracy of estimated total annual groundwater discharge from the floor of Death Valley. One 

limitation in applying the Bowen ratio method is the 

advection of air from ET units other than the one being 

sampled (Ohmura, 1982). Uncertainties resulting from 

difficulties in the Bowen ratio likely were small 

because (1) sites were carefully located within areas 

of relatively large fetch and (2) instrument bias was 

minimized by using an automatic exchange mechanism 

to alternate the positions of the THPs between reference heights twice in a data collection period. 

ET rates computed with data collected from 

six sites over a relatively short period of time were 

assumed to adequately represent the hydrologic and 

meteorological conditions in Death Valley. These rates 

vary spatially from changes related to soil cover and 

vegetation type, and density as well as temporally from 

changes in vegetation density and vigor. These changes 

in land-surface cover are responses to cycles in climate. 

To account for the variations in land-surface cover 

satellite-imagery data for a 3-year period were applied 

so that wet-and-dry conditions in Death Valley were 

used in the final analysis. These satellite data, however, 

were from years other than when field data were 

collected creating some uncertainty as to the actual 

acreages of the ET units.

Runoff from storms will periodically flood areas 

of the valley floor. However, the magnitude and frequency of the flooding are unable to be quantified 

because drainages from valley margins are not gaged. 

SUMMARY

The USGS, in cooperation with the NPS and Inyo 

County, California, conducted a study from 1997 

through 2001 to improve estimates of the amount of 

ground water naturally discharged by ET from the floor 

of Death Valley. Relatively large amounts of ground 

water are discharged by vapor flux through areas of 

salt-encrusted and bare-soil playas on the valley 

floor and by transpiration from areas of phreatophytic 

vegetation primarily along the western margin of the 

valley, in Mesquite Flat, and adjacent to reaches of the 

Amargosa River.

Satellite-imagery data and results from the NWI 

were used to identify areas with significant rates of ET 

on the valley floor and to delineate these areas into 

zones of similar vegetation and soil characteristics 

(known as ET units). Five unique ET units were identified: salt-encrusted playa (21,287 acres), bare-soil 

playa (75,922 acres), low-density vegetation (6,525 

acres), moderate-density vegetation (5,019 acres), and 

high-density vegetation (1,522 acres). A sixth ET unit, 

open water, had been considered for the final results, 

however, with a total area of only 2 acres annual ET 

is estimated to be only 17 acre-ft. This estimate is less 
26 Estimated Ground-Water Discharge by Evapotranspiration from Death Valley, California, 1997–2001 

than two thousandths of the total ground-water discharge for the entire study area and is not included 

within the final result.

Representative ET rates for salt-encrusted playa, 

bare-soil playa, low-density vegetation, moderatedensity vegetation, and high-density vegetation were 

computed by collecting micrometeorological data at 

instrumented sites within these ET units and solving 

an energy-budget equation using the Bowen ratio 

method. Daily ET rates were determined at each site by 

summing 20-minute averaged values computed with 

micrometeorological data sampled at intervals between 

10 and 30 seconds. Annual ET rates for ET units ranged 

between 0.17 ft for the salt playa and 3.89 ft for dense 

vegetation. 

The amount of annual ground-water discharge 

by ET from the floor of Death Valley as estimated in 

this study is 35,000 acre-ft and was computed by multiplying the area of each ET unit by its corresponding 

annual ET rate adjusted for annual rainfall. 

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