Scientific Investigations Report 2006-5252

U.S. Department of the Interior

U.S. Geological Survey

Evaporation from Lake Mead, Arizona and Nevada, 1997–99 

Prepared in cooperation with the Bureau of Reclamation
Cover: Photograph of Lake Mead, Arizona-Nevada, looking east from Boulder Basin. 

(Photograph taken by Craig L. Westenburg, U.S. Geolological Survey, March 31, 1998)
Evaporation from Lake Mead, Arizona

and Nevada, 1997–99

By Craig L. Westenburg, Guy A. DeMeo, and Daron J. Tanko

Prepared in cooperation with the Bureau of Reclamation

Scientific Investigations Report 2006-5252

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Westenburg, C.L., DeMeo G.A., and Tanko, D.J., 2006, Evaporation from Lake Mead, Arizona and Nevada, 1997–99: 

U.S. Geological Survey Scientific Investigations Report 2006-5252, 24 p.
iii

Contents

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

Introduction.....................................................................................................................................................1

Description of Study Area ...................................................................................................................1

Background............................................................................................................................................3

Purpose and Scope ..............................................................................................................................3

Estimating Evaporation from Lake Mead...................................................................................................4

Energy-Budget Method .......................................................................................................................7

Meteorological Data.............................................................................................................................8

Evaporation Rates...............................................................................................................................13

Daily Rates ..................................................................................................................................13

Monthly Rates.............................................................................................................................15

Annual Rates...............................................................................................................................17

Previously Reported Evaporation Rates ..................................................................................................18

Alternative Methods of Estimating Lake Mead Evaporation................................................................20

Summary........................................................................................................................................................22

Acknowledgments.......................................................................................................................................22

References Cited..........................................................................................................................................22

Figures

Figure 1. Map showing location of study area and evaporation stations, Lake Mead,

Arizona and Nevada ........................................................................ 2

Figure 2. Photographs of four floating platforms on Lake Mead ................................. 4

Figure 3. Graph showing Lake Mead daily water-level elevation and date of station

installation, February 1997–December 1999 ............................................. 5

Figure 4. Schematic of instrument configuration on floating platforms on Lake Mead,

Arizona and Nevada, and of energy fluxes at water surface........................... 6

Figure 5. Graphs showing air temperature, water temperature, and relative humidity

collected at four floating instrumented platforms on Lake Mead, Arizona and

Nevada, 1999 ................................................................................. 11

Figure 6. Graphs showing air temperature, water temperature, and relative humidity

collected at the Sentinel Island evaporation station, Nevada, 1997–99............... 12

Figure 7. Graph showing wind speed and direction at four floating instrumented

platforms on Lake Mead, Arizona and Nevada, 1999.................................... 13

Figure 8. Graphs showing daily evaporation rates at four floating instrumented

platforms at Lake Mead, Arizona and Nevada, 1999 .................................... 14

Figure 9. Graphs showing total and average monthly evaporation at three floating

instrumented platforms on Lake Mead, Arizona and Nevada, 1997–99 ............... 16
iv

Figure 10. Graphs showing comparison of total monthly evaporation at four evaporation

stations with average monthly evaporation at all stations and three

open-water evaporation stations with average monthly evaporation at

open-water stations, April 1998–October 1999 .......................................... 17

Figure 11. Graph showing annual Lake Mead evaporation, 1953–94 .............................. 18

Figure 12. Box plot showing regression of monthly average net radiation at the Water

Barge Cove station and total monthly evaporation at Sentinel Island station

for: January through May 1997–99 and June through December 1997–99 ......... 20

Figure 13. Graph showing total monthly evaporation (average of open-water stations)

and total monthly evaporation computed from monthly average net radiation

at the Water Barge Cove station ......................................................... 21

Figure 14. Graph showing comparison of total monthly evaporation (average of

open-water stations) and monthly evaporation computed from monthly mean

net radiation at the Water Barge Cove station .......................................... 21

Tables

Table 1. Average maximum and average minimum air temperature and average annual

precipitation at Las Vegas and Overton, Nevada, 1961–90 ........................... 2

Table 2. Site information for four floating instrumented platforms used to collect data to

compute evaporation from Lake Mead, Arizona and Nevada, 1997–99............... 5

Table 3. Description of data, collected at four floating instrumented platforms, used to

compute evaporation from Lake Mead, Arizona and Nevada, 1997–99............... 6

Table 4. Number of months with complete data, collected at four floating instrumented

platforms, used to compute evaporation from Lake Mead, Arizona and

Nevada, 1997–99 ........................................................................... 7

Table 5. Monthly mean meteorological and water-temperature data measured at four

floating instrumented platforms on Lake Mead, Arizona and Nevada, 1997–99 ... 9

Table 6. Total and average monthly evaporation from Lake Mead at four floating

instrumented platforms, Arizona and Nevada, 1997–99................................. 15

Table 7. Average monthly evaporation and computed monthly evaporative loss from Lake

Mead, Arizona and Nevada, July 1997–December 1999 .............................. 17

Table 8. Total monthly, total annual, and average monthly Lake Mead evaporation, 

1953–95........................................................................................ 19

Figures—Continued
Conversion Factors, Datums, and Acronyms and Symbols

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2) 

acre 0.004047 square kilometer (km2)

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

Flow rate

inch per day (in/day) 25.4 millimeter per day

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3/yr)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

miles per hour (mi/hr) 0.447 meters per second

Knot 1.609 kilometer per day

Energy flux

Watts per square foot (W/ft2) 10.7643 Watts per square meter 

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

Bar 100 kilopascal (kPa) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8
vi

Datums

Vertical coordinate information is referenced to the U.S. Geological Survey datum, adjustment of 

1912, locally known as “Power House Datum.” Add 0.55 foot to convert to datum of 1929, leveling 

of 1935. Add 0.43 foot to convert to datum of 1929, leveling of 1940. Add 0.40 foot to convert to 

datum of 1929, leveling of 1948. Add 0.03 foot to convert to datum of 1929, leveling of 1963. No 

elevations have been converted to datum of 1929. Datum of 1929 is known as National Geodetic 

Vertical Datum of 1929 (NVGD of 1929) and was formerly called “Sea-Level Datum of 1929.”

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Acronyms and Symbols

Meaning

γ empirical constant (equation 2)

ρ density of evaporated water (equations 1, 3)

γc psychrometric constant (equation 3)

E evaporation rate (equations 1, 3)

e0 vapor pressure of saturated air at temperature of the water surface (equations 2, 3)

ea vapor pressure of the air above the water surface (equations 2, 3) 

L latent heat of vaporization (equations 1, 3)

P atmospheric pressure (equation 2)

Qθ change in energy stored in a body of water (equations 1, 3)

Qv net energy advected into a body of water (equation 1)

Qb net energy lost by the body of water through the exchange of long wave radiation

 between the atmosphere and the body of water (equation 1)

Qn net radiation (equation 3)

Qr reflected solar radiation (equation 1)

Qs solar radiation incident to the water surface (equation 1)

R Bowen ratio (equations 1, 2)

Rn monthly net radiation at Water Barge Cove

T0 temperature of water (equations 2, 3)

Ta temperature of the air (equations 2, 3)

USGS United States Geological Survey
Abstract

Lake Mead is one of a series of large Colorado River 

reservoirs operated and maintained by the Bureau of 

Reclamation. The Colorado River system of reservoirs 

and diversions is an important source of water for millions 

of people in seven Western States and Mexico. The U.S. 

Geological Survey, in cooperation with the Bureau of 

Reclamation, conducted a study from 1997 to 1999 to 

estimate evaporation from Lake Mead. For this study, 

micrometeorological and hydrologic data were collected 

continually from instrumented platforms deployed at four 

locations on the lake, open-water areas of Boulder Basin, 

Virgin Basin, and Overton Arm and a protected cove in 

Boulder Basin. Data collected at the platforms were used 

to estimate Lake Mead evaporation by solving an energybudget equation. The average annual evaporation rate at 

open-water stations from January 1998 to December 1999 was 

7.5 feet. Because the spatial variation of monthly and annual 

evaporation rates was minimal for the open-water stations, a 

single open-water station in Boulder Basin would provide data 

that are adequate to estimate evaporation from Lake Mead.

Introduction

Description of Study Area

Lake Mead is the largest reservoir by volume in the 

United States and was formed when Hoover Dam was 

completed in 1935 (fig. 1). It took until July 1941 for water 

to fill Lake Mead, which has a maximum surface elevation 

of 1,229 ft, a maximum surface area of 162,700 acres, and a 

maximum available capacity of 27,377,000 acre-ft (Bureau 

of Reclamation, 1967). At the maximum elevation of 1,229 ft 

Lake Mead extends 65.9 mi upstream of Hoover Dam and 

has a maximum width of 9.3 mi (LaBounty and Horn, 1997, 

p. 95). The average lake elevation from 1942 (first complete 

calendar year of full pool) to 1995, based on monthly end-ofmonth elevations, is 1,169.9 ft (retrieved at URL: http://www.

usbr.gov/lc/riverops.html), which corresponds to a lake surface 

area of 125,600 acres.

The drainage area of Lake Mead at Hoover Dam 

is about 171,700 mi2 (Tadayon and others, 2000, p. 90). 

Ninety-seven percent of the inflow into Lake Mead is from 

the Colorado River (13.12 million acre-ft/yr, Tadayon and 

others, 2000, p. 80) with the remaining 3 percent from the 

combined flow of Las Vegas Wash (148,000 acre-ft/yr; Jones 

and others, 2000, p. 94), Muddy River (11,700 acre-ft/yr; 

Jones and others, 2000, p. 55), Virgin River (176,000 acreft/yr; Jones and others, 2000, p. 55), and ephemeral streams. 

The average annual release from Hoover Dam from 1935 to 

1999 was about 10.1 million acre-ft (Tadayon and others, 

2000, p. 99). Flow of the Colorado River at Diamond Creek 

(130 mi upstream of Hoover Dam) in calendar year 1999 was 

12.69 million acre-ft, while the release from Hoover Dam 

was 11.04 million acre-ft (Tadayon and others, 2001, p. 110 

and 128). Retention time for Lake Mead averages 3.9 years, 

depending on release and flow patterns.

Lake Mead is in an environment with a warm, arid 

climate. From 1961 to 1990, the average maximum air 

temperature was 105.9oF for Las Vegas, Nev., and 107.8oF 

for Overton, Nev. (fig. 1), whereas the average minimum air 

temperature was 33.6oF and 28.0oF, respectively (table 1). 

Average annual precipitation (1961–90) was 4.13 in. for 

Las Vegas and 3.31 in. for Overton (table 1). This warm, 

arid environment is conducive for high rates of evaporation. 

Sparsely vegetated, gentle to moderately sloping alluvial fans 

and steep, barren, rocky cliffs surround the lake. Generally, 

the adjacent hills rise to low or moderate height above the lake 

surface. The vast majority of lake is exposed to winds from the 

southwest to southeast.

Evaporation from Lake Mead, Arizona 

and Nevada, 1997–99

By Craig L. Westenburg, Guy A. DeMeo, and Daron J. Tanko
Figure 1. Location of study area and evaporation stations, Lake Mead, Arizona and Nevada.

D

C B

A

NEVADA

ARIZONA

NEVADA

ARIZONA

0 5

0 5 10

10

20 KILOMETERS

20 MILES

B Data-collection site and letter—

See table 2 for station names and 

location data

EXPLANATION

RIVER

RIVER

COLORADO

COLORADO

Virgin Boulder Basin

Basin

Echo

Bay Overton

Arm

LAKE

MEAD

Las

Vegas

Bay

Water

Barge

Cove

Callville

Bay

Virgin River

Muddy

River

Las Vegas

Wash

Base from U.S. Geological Survey digital data 1:100,000 scale, 1987: 

Universal Transverse Mercator Projection, Zone 11

Las

Vegas

Overton

CALIFORNIA

NEVADA UTAH

ARIZONA

Glen

Canyon

Dam Las

Vegas

Study

area

Colorado

River

Lake

Mead

35°30'

36°00'

36°30'

115°00' 114°30' 114°00'

Hoover

Dam

Table 1. Average maximum and average minimum air temperature and average annual precipitation at Las Vegas and Overton, 

Nevada, 1961–90.

[Data for Las Vegas and Overton obtained from Desert Research Institute at www.wrcc.dri.edu. The period 1961–90 was selected because it coincides with 

official National Climatic Data Center records for Las Vegas. The 1961–90 data for Overton are unofficial]

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

National Weather Service Office, McCarran Airport, Las Vegas, Nevada

Air temperature, in degrees Fahrenheit

Average maximum 57.3 63.3 68.8 77.5 87.8 100.3 105.9 103.2 94.7 82.1 67.4 57.5 80.5

Average minimum 33.6 36.8 43.8 50.7 60.2 69.4 76.2 74.2 66.2 54.3 42.6 33.9 53.7

Precipitation, in inches

Average 0.48 0.48 0.42 0.21 0.28 0.12 0.35 0.49 0.28 0.21 0.43 0.38 4.13

Overton, Nevada

Air temperature, in degrees Fahrenheit

Average maximum 62.0 66.0 72.2 81.8 91.0 100.5 107.8 105.4 97.8 87.7 69.8 60.7 83.8

Average minimum 28.0 32.5 38.0 45.9 53.3 61.6 69.6 69.7 59.1 49.7 37.7 30.5 48.1

Precipitation, in inches

Average 0.20 0.19 0.29 0.36 0.14 0.10 0.11 0.21 0.45 0.16 0.61 0.49 3.31

 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Background

Lake Mead is one of a series of large Colorado River 

reservoirs operated and maintained by the Bureau of 

Reclamation. The Colorado River system of reservoirs and 

diversions is an important source of water for millions of 

people in seven Western States and Mexico. The series of 

reservoirs along the Colorado River ensures a reliable supply 

of water for municipal and agricultural uses; allows for the 

generation of hydroelectric power; reduces the occurrence 

of uncontrolled floods within the basin; provides ecological 

habitat for numerous aquatic, riparian, and avian species; and 

provides a source of water-based recreation for millions of 

people.

The demand for Colorado River water has increased 

greatly as the population of the Southwestern United States 

has grown and as the maintenance and the development of 

ecological habitats has become a priority. Recent (2000) 

estimates of demand for Colorado River water in the Lower 

Colorado River Basin (parts of Utah, Arizona, Nevada, and 

California) are for irrigation of 2.7 million acres of land 

and municipal supply for 18 million people (Paul Matuska, 

Bureau of Reclamation, Lower Colorado River Regional 

Office, written commun., 2000). In contrast, Stanley (1960, 

p. 84) estimated that in 1949 Colorado River water was used 

to irrigate 500,000 acres of crops in Arizona and California 

and 300,000 acres of crops in Mexico and was used as 

municipal supply for 3.5 million people in California. Bureau 

of Reclamation (2000, p. 1) reported that in 1999 a total 

10.95 million acre-ft of Colorado River water was used by 

the lower basin States (7.98 million acre-ft) and Mexico 

(2.97 million acre-ft).

Accurate estimates of water-budget components of a 

reservoir system are required to manage increased demands 

for available water supplies. Budget components such as 

withdrawals, diversions, return flows, change in storage 

contents, and tributary inflows usually can be quantified 

with direct measurements. Natural losses from the reservoir 

system, such as open-water evaporation, seepage, and 

evapotranspiration from riparian vegetation, are quantified 

using methods that are more indirect. These components 

typically are estimated with a variety of methods such as 

empirically derived relations developed with data collected 

at other locations, calculations using data and results from 

regional studies, statistically distributing water-budget 

residuals to selected components, or applying reasonable 

assumptions about the hydrodynamics of a reservoir system.

Open-water evaporation is a significant loss of water 

from Lake Mead. From 1953 to 1994, evaporation of 

water from Lake Mead was estimated to be 6.4 ft/yr, or 

about 791,000 acre-ft/yr (based on an average surface area 

of 125,600 acres). From 1955 to August 1995, the U.S. 

Geological Survey (USGS) computed Lake Mead evaporation 

using a mass-transfer method. Data required for using this 

method are wind speed and water temperature, collected at a 

floating platform in Boulder Basin, and air temperature and 

vapor pressure, collected at an airport in Las Vegas, Nev., and 

reported by the National Weather Service (Harbeck, 1958a). 

In September 1995, the Bureau of Reclamation removed the 

floating platform from the lake and the wind-speed and watertemperature data required for the evaporation computation 

were no longer available. Alternative data sources (such as 

National Park Service data) were investigated, but due to 

the empirical nature of the method, were determined to be 

inadequate for estimating evaporation rates.

As a result, the Bureau of Reclamation proposed a 

cooperative study with the USGS to develop a method 

to estimate Lake Mead evaporation rates and to evaluate 

previously reported rates. The method developed by the USGS 

would be transferable to other large open-water areas managed 

by the Bureau of Reclamation or other agencies in the arid 

Southwest.

Purpose and Scope

This report presents the results of a study to refine 

estimates of evaporation from Lake Mead. Previously, 

estimated evaporation from Lake Mead (1955–95) may not 

be representative because of the methods used, and because 

some data used in the mass-transfer method were collected 

in the Las Vegas Valley rather than at the surface of the 

lake. In this report, previously used methods of computing 

evaporation are described, evaluated, and compared to those 

used in the current study for data collected for 1997–99, and 

to alternative methods of computing evaporation using modern 

(2005) equipment and techniques. Additionally, selected data 

collected during previous and current studies are summarized, 

including monthly and annual Lake Mead evaporation rates for 

1952–95 and 1997–99.

To better refine estimates of evaporation from Lake 

Mead, atmospheric and water properties were collected from 

1997 to 1999 at four floating platforms, which were installed 

in water of varying depth and fetch to sample differing 

environmental conditions at the lake. Near-continual data 

collected at Overton Arm, Boulder Basin, Virgin Basin, and 

near Callville Bay (fig. 1) were used with an energy-budget 

method to determine daily, monthly, and annual evaporation 

rates for 1997–99.

Introduction 
Estimating Evaporation from 

Lake Mead

Lake Mead evaporation was computed using 

micrometeorological data collected at four floating 

instrumented platforms deployed in Water Barge Cove near 

Callville Bay, in Boulder Basin near Sentinel Island, in the 

Virgin Basin, and in the Overton Arm near Echo Bay (fig. 1). 

Platform station identifier, name, and date of installation are 

listed in table 2.

The platform in Water Barge Cove (site A) was in a 

relatively shallow cove protected from prevailing winds 

(figs. 1 and 2). The platforms in Boulder Basin near 

Sentinel Island (site B), Virgin Basin (site C), and Overton 

Arm (site D) were in different areas of the lake that are 

representative of open-water conditions in each basin. The 

depth of water at the four platforms fluctuated during the study 

due to water-level changes of Lake Mead. Lake elevation 

ranged from less than 1,196 ft in February 1997 to almost 

1,216 ft in September 1998 (fig. 3).

A. Water Barge Cove B. Sentinel Island

C. Virgin Basin D. Overton Arm

06/09/98

05/26/98

05/26/98

03/02/99

Figure 2. Four floating platforms on Lake Mead. Photographs taken by Craig L. Westenburg, U.S. Geological Survey.

 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Table 2. Site information for four floating instrumented platforms used to collect data to compute evaporation from Lake Mead, 

Arizona and Nevada, 1997–99.

[Symbol: –, not applicable]

Site Station name

Location Installation Removal

Latitude Longitude Date Water depth 

(feet) Date Water depth(feet)

A Water Barge Cove 36o07'40'' 114o43'36'' 01-31-97 35 02-09-00 50

B Sentinel Island 36o03'15'' 114o45'05'' 06-23-97 336 – –

C Virgin Basin 36o05'54'' 114o27'45'' 04-09-98 81 01-27-00 85

D Overton Arm 36o18'31'' 114o22'40'' 03-01-99 65 05-05-00 61

F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1997 1998 1999

WATER-LEVEL ELEVATION, IN FEET

1,220

1,215

1,210

1,205

1,195

1,200

Sentinel Island barge installed 

at water-level elevation of 

1,201.54 feet

Water Barge Cove station 

installed at water-level 

elevation of 1,195.68 feet

Virgin Basin barge 

installed at 

water-level elevation 

of 1,213.87 feet

Overton Arm barge installed 

at water-level elevation of 

1,213.22 feet

Figure 3. Lake Mead daily water-level elevation and date of station installation, February 1997–December 1999.

Each floating platform was equipped with instruments 

to measure and record meteorological data and water 

temperature. Air temperature and relative humidity were 

measured with a temperature-humidity probe (THP), wind 

speed and direction were measured with an anemometer 

and wind monitor, net radiation was measured with a net 

radiometer, and water temperature was measured at various 

depths with temperature probes (fig. 4, table 3). A two-point 

mooring system was used to secure each barge to prevent 

drifting and maintain the directional aspect of the wind 

monitor and net radiometer. The number of months with 

complete data, collected at four floating platforms for 

1997–99, used to compute evaporation from Lake Mead is 

listed in table 4.

Estimating Evaporation from Lake Mead 
Battery

box

Anchor

line

Anchor

line

Net radiometer

Instrument

shelter

Temperaturehumidity probe

Wind

monitor

Fish

Net

radiation

Watertemperature

probes

Water surface

Aluminum catamaran barge

Anemometer

Weight

Latent

heat flux

Sensible

heat flux

Water

heat flux

Navigation

beacon

Qe

Qθ

Qh

Qn

Figure 4. Instrument configuration on floating platforms on Lake Mead, Arizona and Nevada, and of energy fluxes at water surface.

Table 3. Description of data, collected at four floating instrumented platforms, used to compute evaporation from Lake Mead, Arizona 

and Nevada, 1997–99.

[Symbol: –, no remark]

Type of measurement Instrument used Associated 

energy-budget term Remarks

Net radiation Net radiometer Net radiation –

Air temperature Temperature-humidity probe Bowen ratio Measured 6.6 feet above the water surface.

Relative humidity Temperature-humidity probe Bowen ratio Measured 6.6 feet above the water surface.

Water-surface temperature Temperature probe Bowen ratio, latent heat 

of vaporization

–

Water temperature below 

the surface

Temperature probe Change in energy stored 

in water

Measured 3.3, 6.6, and 9.9 feet below surface or 6.6, 

13.2, and 19.8 feet below water surface.

Wind speed Anemometer None Measured 6.6 feet above the water surface.

Wind direction Wind monitor None Measured 13.2 feet above the water surface, except 

33.0 feet at Sentinel Island.

 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Table 4. Number of months with complete data, collected at four floating instrumented platforms, used to compute evaporation from 

Lake Mead, Arizona and Nevada, 1997–99.

[Distances are height above water surface, except water temperature, which is depth below water surface. Symbol: –, no data]

Parameter

Water Barge Cove Sentinel Island Virgin Basin Overton Arm

1997 1998 1999 1997 1998 1999 1998 1999 1999

Air temperature

 6.6 feet 9 12 12 6 12 10 8 12 10

13.2 feet 9 12 – – – – – – –

33.0 feet – – – 6 12 10 – – –

Relative humidity

 6.6 feet 9 12 12 6 12 10 8 12 10

13.2 feet 9 12 – – – – – – –

33.0 feet – – – 6 12 10 – – –

Wind speed

 6.6 feet 9 12 12 5 12 10 8 12 9

13.2 feet 9 12 – – – – – – –

33.0 feet – – – 6 12 10 – – –

Wind direction – 9 12 – 10 10 – 9 10

Net radiation 9 12 12 6 12 10 8 12 10

Water temperature below surface 9 12 12 6 12 10 8 12 10

 3.3 feet 9 12 – – – – – – –

 6.6 feet – – – 6 12 10 8 12 10

 9.9 feet 9 12 – – – – – – –

13.2 feet 9 12 12 6 12 10 8 12 10

19.8 feet – – – 6 12 10 8 12 10

Energy-Budget Method

The energy-budget method was used to measure 

evaporation from Lake Mead. This method also was used to 

quantify evaporation from western reservoirs in the 1950s 

(Anderson, 1954), and has been used by the USGS on a 

few long-term lake studies (Sturrock and Rosenberry, 1992; 

Rosenberry and others, 1993; Swancar and others, 2000). 

The energy-budget method is the most accurate method for 

measuring lake evaporation (Winter, 1981). Use of the energybudget method requires a large amount of data collection, 

but the effort is important because accurate measurements 

of lake evaporation are rare. An energy budget is similar to a 

water budget in that the change in stored energy is equal to the 

fluxes in and out of the system. Energy drives the process of 

evaporation because the water at the surface of the lake must 

absorb a certain amount of energy (latent heat of vaporization; 

approximately 580 calories per gram) before the water will 

evaporate. After some derivation, the following equation is 

used to calculate evaporation from the lake (Anderson, 1954):

E = (Qs – Qr – Qb + Qv – Qθ)/[ρL(R + 1)], (1)

where

E is evaporation rate,

Qs is the solar radiation incident to the water surface,

Qr is the reflected solar radiation,

Qb is the net energy lost by the body of water through 

the exchange of long-wave radiation between the 

atmosphere and the body of water,

Qv is the net energy advected into the body of water,

Qθ is the change in energy stored in the body of water,

ρ is the density of evaporated water,

L is the latent heat of vaporization, and

R is the Bowen ratio.

Bowen (1926) expressed the ratio as:

R = γ[T0 – Ta)/(e0 – ea)]P, (2)

where

γ is an empirical constant,

T0 is the temperature of the water surface,

Ta is the temperature of the air,

e0 is the vapor pressure of saturated air at the temperature 

(T0),

ea is the vapor pressure of the air above the water 

surface, and

P is atmospheric pressure.

Estimating Evaporation from Lake Mead   
Net radiation (Qn) was measured at Lake Mead 

evaporation platforms and replaces three energy terms (Qs, Qr, 

and Qb) in equation 1. Net advected energy (Qv) is disregarded 

based on the assumption that advected energy is negligible 

during a 20-minute evaporation period.

Thus, after modifying equation 1 by substituting for 

net radiation, removing the net advected energy term, and 

replacing R with equation 2, evaporation can be estimated 

from measured meteorological and hydrological parameters:

E = (Qn – Qθ)/[ρLγc[(T0 – Ta)/(e0 – ea)] + 1], (3)

where γc, is a psychrometric constant, a product of γ and P

(Laczniak and others, 1999, p. 20–21).

Meteorological Data

Meteorological and water temperature data were used to 

compute evaporation rates for Lake Mead. Measurements were 

made every 10 or 30 seconds and were averaged for 20-minute 

periods. Some of the missing or incorrect 20-minute data 

were estimated or computed to maximize the amount of data 

available for the evaporation computation. Where data were 

missing for short periods, they were estimated from trends of 

the data before and after the periods of missing or incorrect 

data. Where data were missing or incorrect for longer periods, 

they were computed from other available data at that station, 

data from another station, or data for another year. Typically, 

a regression was developed with two sets of measurements for 

a different period, but for similar environmental conditions, 

using a complete set of data. The regression was then used to 

estimate the missing or incorrect data from a complete set of 

data.

Monthly values of meteorological data and water 

temperatures were computed from 20-minute averaged 

data collected at each station for 1997, 1998, and 1999 

(table 5). Average daily air temperature was about the same 

at all four platforms (fig. 5), whereas relative humidity and 

water temperature were similar at the open-water platforms. 

Generally, water temperature was higher and relative humidity 

was lower at the sheltered cove platform (Water Barge Cove) 

than at the open-water platforms.

Daily air temperature, water temperature, and relative 

humidity at the Sentinel Island station were compared for 

1997, 1998, and 1999 (fig. 6). Air temperature varies from 

year to year, but the seasonal pattern is consistent. The 

maximum daily air temperature occurs about mid-July, with 

some daily average air temperatures exceeding 95oF, and the 

minimum daily air temperature occurs from late December 

through February. Water temperature did not vary much from 

year to year. Maximum daily water temperature occurs in late 

July to early August and the minimum daily water temperature 

occurs in late February. Relative humidity fluctuated from day 

to day and differed greatly from year to year (fig. 6). However, 

there is a seasonal pattern of high relative humidity in January 

that gradually decreases to a low at the end of June, followed 

by a gradual increase to higher relative humidity in December.

Daily average wind-speed and direction data for 1999 

are plotted in figure 7. Points are plotted a distance from the 

origin relative to the wind speed and in the direction from 

the origin that represents wind direction (measured with zero 

degrees (0o) corresponding to a north wind). Thus, a daily 

average wind speed of 5 mi/hr with a direction of 120o would 

be plotted on the circle representing 5 mi/hr and in the lower 

right quadrant. Daily average wind speeds of less than 1 mi/hr 

were not plotted to avoid a dense cluster of points at the origin. 

Wind direction was predominately from the southeast to the 

southwest, occasionally from the northwest, and rarely from 

the northeast. Most daily wind speeds were less than 

10 mi/hr. The Virgin Basin location experienced more daily 

wind greater than 10 mi/hr than other locations.

   Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Table 5. Monthly mean meteorological and water-temperature data measured at four floating instrumented platforms on Lake Mead, 

Arizona and Nevada, 1997–99.

[Symbol: –, no data]

Site Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1997

Air temperature, in degrees Fahrenheit

Water Barge Cove – 54.3 62.6 66.2 90.32 83.12 86.9 88.9 81.3 68.4 58.5 48.6

Sentinel Island – – – – – – 90.1 90.7 81.9 76.8 59.7 50.4

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

Water temperature at water surface, in degrees Fahrenheit

Water Barge Cove – 55.9 59.5 62.1 68.5 78.1 81.9 84.4 82.6 74.1 66.2 59.9

Sentinel Island – – – – – – 79.7 82.6 81.1 79.9 65.8 57.2

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

Water temperature, in degrees Fahrenheit, measured at 6.6 feet below the water surface

Water Barge Cove – 55.9 58.6 61.3 75.0 77.0 80.6 83.1 81.5 73.4 65.8 59.7

Sentinel Island – – – – – – 79.0 81.7 80.4 72.9 65.5 59.5

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

Relative humidity, in percent

Water Barge Cove – 40 34.9 33.6 25.0 29.6 31.5 37.9 47.9 37.5 47.9 43.0

Sentinel Island – – – – – – 33.8 38.5 50.1 44.0 50.1 45.7

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

Wind speed, in miles per hour, measured at 6.6 feet above the water surface

Water Barge Cove – 6.02 5.12 6.91 6.35 7.52 5.48 6.38 4.54 5.37 3.53 4.41

Sentinel Island – – – – – – 5.75 – 7.16 7.63 6.69 7.83

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

1998

Air temperature, in degrees Fahrenheit

Water Barge Cove 49.8 50.5 56.3 61.2 69.4 79.0 89.1 89.8 80.6 67.8 56.7 49.1

Sentinel Island 51.1 51.3 56.8 61.2 69.4 78.8 89.1 89.8 81.0 68.7 58.3 50.9

Virgin Basin – – – 61.9 68.4 77.2 87.1 87.8 78.8 66.7 56.1 48.4

Overton Arm – – – – – – – – – – – –

Water temperature at water surface, in degrees Fahrenheit

Water Barge Cove 56.7 55.8 57.9 61.0 68.4 75.0 84.2 85.6 82.2 72.7 65.3 59.5

Sentinel Island 55.8 54.7 56.3 59.4 65.1 72.5 81.0 83.3 81.0 71.6 65.0 59.4

Virgin Basin – – – 59.0 63.3 70.5 80.6 82.9 79.9 70.2 63.9 57.9

Overton Arm – – – – – – – – – – – –

Water temperature, in degrees Fahrenheit, measured at 6.6 feet below the water surface

Water Barge Cove 56.7 56.1 58.3 61.2 68.3 74.7 83.3 84.7 81.7 72.3 65.3 59.7

Sentinel Island 56.1 55.0 56.3 59.4 64.8 72.0 81.1 83.5 81.0 71.6 65.0 59.4

Virgin Basin – – – 59.4 64.0 71.2 81.3 83.5 80.6 70.9 64.4 58.6

Overton Arm – – – – – – – – – – – –

Relative humidity, in percent

Water Barge Cove 55.3 58.0 50.3 43.3 36.2 33.1 37.7 36.4 41.9 38.5 47.4 41.8

Sentinel Island 57.0 58.6 50.3 43.3 37.0 34.3 39.2 38.3 44.0 41.7 47.6 45.6

Virgin Basin – – – 44.2 39.5 36.6 43.6 43.2 48.8 46.3 52.8 51.3

Overton Arm – – – – – – – – – – – –

Wind speed, in miles per hour, measured at 6.6 feet above the water surface

Water Barge Cove 4.38 7.65 5.91 6.85 7.14 6.00 6.17 6.40 6.13 5.10 4.88 6.60

Sentinel Island 6.58 9.57 8.37 8.26 9.17 7.87 7.67 7.36 6.89 7.94 7.47 9.44

Virgin Basin – – – 8.19 9.87 9.15 8.03 8.05 8.43 7.87 7.70 9.04

Overton Arm – – – – – – – – – – – –

Estimating Evaporation from Lake Mead   
Table 5. Monthly mean meteorological and water-temperature data measured at four floating platforms on Lake Mead, Arizona and 

Nevada, 1997–99.—Continued

[Symbol: –, no data]

Site Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1999

Air temperature, in degrees Fahrenheit

Water Barge Cove 51.1 52.3 59.5 59.5 72.9 82.9 86.2 87.3 81.7 70.3 59.4 50.9

Sentinel Island 52.2 53.2 59.9 59.5 72.7 82.9 86.4 87.3 82.2 72.9 – –

Virgin Basin 49.6 51.4 58.1 58.5 70.7 80.6 84.7 85.6 79.7 68.9 58.1 58.2

Overton Arm – – 58.1 58.8 71.1 80.8 84.9 86.5 80.6 68.5 57.6 49.6

Water temperature at water surface, in degrees Fahrenheit

Water Barge Cove 56.1 55.4 58.1 59.9 68.2 77.2 81.1 82.4 80.4 73.6 66.6 59.4

Sentinel Island 55.8 54.9 55.9 58.8 66.2 72.9 79.3 81.5 79.2 72.3 – –

Virgin Basin 55.0 54.3 55.9 58.1 65.1 71.8 79.7 81.7 79.2 72.3 65.7 59.2

Overton Arm – – 56.7 59.7 66.6 75.4 81.9 83.5 81.9 75.0 68.0 60.8

Water temperature, in degrees Fahrenheit, measured at surface and 6.6 feet below water surface

Water Barge Cove 56.3 55.8 57.9 59.5 68.2 77.2 82.2 83.5 81.9 75.0 68.0 60.8

Sentinel Island 55.9 54.9 55.9 58.6 66.0 73.2 79.7 81.7 80.4 75.2 – –

Virgin Basin 55.6 54.9 56.7 58.8 65.8 72.5 80.6 82.0 79.7 72.5 65.8 58.8

Overton Arm – – 56.7 59.0 65.8 75.0 81.5 82.9 80.1 71.2 64.9 58.1

Relative humidity, in percent

Water Barge Cove 44.5 43.8 38.3 39.9 31.1 29.0 41.0 35.0 36.5 33.4 40.3 37.4

Sentinel Island 48.4 47.7 39.9 42.8 35.3 28.8 42.7 37.3 39.7 39.0 – –

Virgin Basin 53.3 52.1 44.2 46.6 38.8 32.8 48.3 42.5 45.8 41.7 48.5 46.7

Overton Arm – – 47.1 48.4 39.9 38.4 50.7 42.5 43.2 40.9 47.8 44.5

Wind speed, in miles per hour, measured at 6.6 feet above the water surface

Water Barge Cove 5.03 5.06 6.42 6.64 7.40 5.91 6.76 6.40 5.15 4.18 3.74 4.77

Sentinel Island 8.05 7.32 8.64 9.19 7.61 9.22 8.55 7.38 8.41 9.02 – –

Virgin Basin 7.20 7.58 9.11 9.73 8.14 9.75 8.34 7.76 8.21 7.96 6.67 9.26

Overton Arm – – 9.24 9.62 8.19 9.37 8.43 7.65 6.47 5.62 8.32 –

10 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
nv16-0046_fig05

40

50

60

70

80

90

100

DAILY AIR TEMPERATURE, IN DEGREES FAHRENHEIT 40

50

60

70

80

90

100

DAILY WATER TE

MPERATURE, IN DEGREES FAHRENHEIT

0

20

40

60

80

100

DAILY RELATIVE HU

MIDITY, IN PERCENT

JANUARY FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER

1999

Water Barge Cove

Sentinel Island

Virgin Basin

Overton Arm

Air temperature

Water temperature

Relative humidity

Figure 5. Air temperature, water temperature, and relative humidity collected at four floating instrumented platforms on Lake 

Mead, Arizona and Nevada, 1999. See figure 1 and table 2 for platform locations and site information.

Estimating Evaporation from Lake Mead 11
nv16-0046_fig06

30

40

50

60

70

80

90

100

DAILY AIR TEMPERATURE, IN DEGREES FAHRENHEIT 30

40

50

60

70

80

90

100

DAILY WATER TE

MPERATURE, IN DEGREES FAHRENHEIT

0

20

40

60

80

100

DAILY RELATIVE HU

MIDITY, IN PERCENT

JANUARY FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER

1997

1998

1999

Air temperature

Water temperature

Relative humidity

Figure 6. Air temperature, water temperature, and relative humidity collected at the Sentinel Island evaporation station, Nevada, 

1997–99.

12  Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Evaporation Rates

Evaporation rates were computed at 20-minute intervals 

to evaluate diurnal fluctuations of lake evaporation. The 20-

minute period evaporation rates also were used to identify 

periods of poor or missing energy-budget data. Daily 

evaporation rates are the sum of 20-minute periods, monthly 

rates are the sum of daily evaporation and annual rates are the 

sum of monthly evaporation.

Daily Rates

Daily evaporation rates at the four evaporation stations 

were compared for calendar year 1999 (fig. 8) and showed 

similar daily fluctuations. However, the magnitude of 

fluctuations in daily evaporation was greater at Virgin Basin 

and Sentinel Island than at Overton Arm or Water Barge Cove. 

Daily evaporation was greatest from late June through early 

July and was least from mid-December through late January.

nv16-0046_fig07

5

10

15

20

0°

90°

180°

270° 0 5 10 15 20

Water Barge Cove

Sentinel Island

Virgin Basin

Overton Arm

Station name

EXPLANATION

Figure 7. Wind speed and direction at four floating instrumented platforms on Lake Mead, Arizona and Nevada, 1999.

Estimating Evaporation from Lake Mead 13
nv16-0046_fig08

JANUARY FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

DAILY EVAPORATION, IN INCHES 

1999

Water Barge Cove 

Sentinel Island

Virgin Basin

Overton Arm 

Figure 8. Daily evaporation rates at four floating instrumented platforms at Lake Mead, Arizona and Nevada, 1999.

14  Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Monthly Rates

To evaluate the temporal variation in monthly evaporation 

at each station, monthly evaporation rates for each station 

were averaged and compared to the total evaporation rate for 

each month of data collection. Temporal data were available 

for Water Barge Cove, Sentinel Island, and Virgin Basin 

stations (table 6); however, monthly data were insufficient 

for the Overton Arm station to compute average monthly 

evaporation. Total monthly evaporation rates at three stations, 

with some exceptions, generally were within 10 percent of 

average monthly rates; consequently, annual variation in 

monthly evaporation typically was minimal between 1997 and 

1999. Some months, however, exhibited significant differences 

between average and total evaporation. These differences were 

as great as 31 percent at Water Barge Cove (February 1997; 

fig. 9A), 14 percent at Sentinel Island (January 1998 and 1999; 

fig. 9B), and 22 percent at Virgin Basin (December 1998 and 

1999; fig. 9C). Some of the difference in monthly evaporation 

rates from year to year may be due to errors in meteorological 

and water-temperature data collected at each station, but yearto-year differences for most months likely are due to actual 

differences in evaporation.

To evaluate the spatial variation in evaporation, total 

monthly evaporation rates at all four stations were averaged 

for every month of data collection and the average was 

compared to the total monthly evaporation rate at each station 

(fig. 10A). For each station, total monthly evaporation rates 

compared well to average monthly rates with correlation 

coefficients of 0.96 or higher. However, total monthly 

evaporation at Water Barge Cove generally was less than total 

evaporation rates at the open-water stations when rates were 

less than 6.5 in. Total monthly evaporation rates for all three 

open-water stations were nearly equal and compared well to 

average evaporation rates (fig. 10B); correlation coefficients 

for Sentinel Island and Overton Arm were 0.98, and for Virgin 

Basin was 0.96. This evaluation suggests that the spatial 

variation in evaporation is minimal for open-water areas of 

Lake Mead.

The monthly volume of evaporated water was computed 

using the average monthly open-water evaporation rate, in 

feet, and the average monthly surface area of Lake Mead for 

July 1997 through December 1999, in acres, (calculated from 

water elevation data at URL: <http://www.usbr.gov/lc/riverops.

html> and the Reclamation area capacity tables, Bureau 

of Reclamation, 1967). The volume of water evaporated in 

1 month ranged from 46,000 acre-ft in February 1998 to 

126,000 acre-ft in July 1998.

Table 6. Total and average monthly evaporation from Lake Mead at four floating instrumented platforms, Arizona and Nevada, 1997–99.

[Evaporation data, in inches. Symbol: –, data not measured or computed]

Site Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Total monthly evaporation, 1997

Water Barge Cove – 5.4 9.1 7.9 – 10.8 10.8 9.2 8.3 7.6 4.1 2.6

Sentinel Island – – – – – – 9.1 9.1 8.8 8.7 6.4 4.6

Virgin Basin – – – – – – – – – – – –

Overton Arm – – – – – – – – – – – –

Total monthly evaporation, 1998

Water Barge Cove 2.9 3.2 6.6 8.2 8.9 10.2 10.1 8.8 8.5 6.7 3.7 2.6

Sentinel Island 3.7 3.6 7.1 7.4 9.9 9.4 10.0 8.9 8.9 8.5 7.3 6.0

Virgin Basin – – – 8.6 9.7 9.1 9.9 8.7 9.7 7.1 5.9 4.1

Overton Arm – – – – – – – – – – – –

Average evaporation, 1998

All available sites 3.3 3.4 6.9 8.0 9.5 9.6 10.0 8.8 9.0 7.4 5.6 4.2

Open-water sites 3.7 3.6 7.1 8.0 9.8 9.2 9.9 8.8 9.3 7.8 6.6 5.0

Total monthly evaporation, 1999

Water Barge Cove 3.0 3.8 7.4 6.8 9.2 10.5 9.4 8.7 9.4 8.0 4.3 3.1

Sentinel Island 5.0 4.4 8.1 6.9 9.0 9.2 8.4 8.6 9.9 8.8 6.7 –

Virgin Basin 4.0 5.3 7.9 7.1 9.6 9.3 8.8 8.7 9.1 8.0 7.3 6.5

Overton Arm – – 7.8 6.9 9.5 9.5 9.4 8.6 9.5 8.7 7.0 4.1

Average evaporation, 1999

All available sites 4.0 4.5 7.8 6.9 9.3 9.6 9.0 8.6 9.5 8.4 6.3 4.5

Open-water sites 4.5 4.8 8.0 7.0 9.4 9.4 8.9 8.6 9.5 8.5 7.0 5.3

Estimating Evaporation from Lake Mead 15
nv16-0046_fig09

JANUARY FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER

MONTHLY EVAPORATION, IN INCHES

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

1997

1998

1999

Average

Data collection year

A. Water Barge Cove, 1997–99

B. Sentinel Island, 1997–99

C. Virgin Basin, 1998–99

Figure 9. Total and average monthly evaporation at three floating instrumented platforms on Lake Mead, Arizona and Nevada, 

1997–99.

16  Evaporation from Lake Mead, Arizona and Nevada, 1997–99
nv16-0046_fig10

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0 2 4 6 8 10 12

EVAPORATION, IN INCHES

AVERAGE MONTHLY EVAPORATION OF OPEN-WATER SITES, IN INCHES

Water Barge Cove R = 0.980

Sentinel Island R = 0.963

Virgin Basin R = 0.961

Overton Arm R = 0.985

One-to-one line

Sentinel Island R = 0.983

Virgin Basin R = 0.962

Overton Arm R = 0.985

One-to-one line

B

A

Figure 10. Comparison of total monthly evaporation at (A) 

four evaporation stations with average monthly evaporation 

at all stations and (B) three open-water evaporation stations 

with average monthly evaporation at open-water stations, April 

1998–October 1999. R is correlation coefficient.

Annual Rates

The average monthly rates for the Lake Mead 

open-water evaporation stations were computed for 

1998 and 1999. For open-water stations, the sum of the 

average monthly rates for 1998 was 88.9 in. (7.4 ft) and 

the sum for 1999 was 90.7 in. (7.6 ft; table 7). For these 

2 years, the average annual Lake Mead evaporation 

rate was 89.8 in. (7.5 ft). Monthly evaporation rates 

were available for only the Sentinel Island station for 

January–March 1998, and those rates are used instead of 

an average rate.

Evaporation rates at the Sentinel Island station are 

generally representative of evaporation of the lake as a 

whole. For April 1998 through November 1999, the total 

evaporation at the Sentinel Island station was 161.3 in., 

whereas, the total of average monthly evaporation for 

the open-water stations was 159.9 in. The difference of 

1.4 in. is less than 1 percent of the total average openwater evaporation.

The annual volume of water evaporated from Lake 

Mead exceeded 1.1 million acre-ft in 1998 and 1999 

(table 7), which probably is higher than a long-term 

average annual evaporation due to higher-than-normal 

lake elevations and corresponding larger-than-normal 

surface area for the period. For example, the average 

surface area of Lake Mead was 125,000 acres from 1942 

to 1995 and the computed average annual evaporation 

rate was 7.5 ft from 1997 to 1998, which would equal a 

long-term average annual volume of 937,500 acre-ft of 

evaporated water.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

Evaporation, in inches

1997 – – – – – – 9.1 9.1 8.8 8.7 6.4 4.6 –

1998 3.7 3.6 7.1 8.0 9.8 9.2 9.9 8.8 9.3 7.8 6.6 5.0 88.9

1999 4.5 4.8 8.0 7.0 9.4 9.4 8.9 8.6 9.5 8.5 7.0 5.3 90.7

 Average 4.1 4.2 7.5 7.6 9.6 9.3 9.3 8.8 9.2 8.3 6.7 5.0 189.8

Evaporation, in acre-feet

1997 – – – – – – 112,000 112,000 108,000 109,000 81,000 59,000 –

1998 48,000 46,000 91,000 102,000 125,000 117,000 126,000 113,000 119,000 101,000 85,000 64,000 1,137,000

1999 57,000 61,000 101,000 88,000 117,000 116,000 110,000 108,000 120,000 108,000 89,000 67,000 1,142,000

1Average annual evaporation rate is only for 1998–99.

Table 7. Average monthly evaporation and computed monthly evaporative loss from Lake Mead, Arizona and Nevada, July 1997–

December 1999.

[Evaporation rate, in inches, are average rates for available open-water sites (table 6). Volume of water was computed by multiplying average open-water 

evaporation rates, converted to feet, by lake-surface area, in acres. Lake-surface area derived from area-capacity tables and mean daily elevation. For 

October 1997, monthly evaporation at Water Barge Cove was used. Symbol: –, data not computed]

Estimating Evaporation from Lake Mead 17
Previously Reported Evaporation Rates

The reported average annual evaporation of 6.4 ft from 

1953 through 1994 (fig. 11, table 8) is 15 percent less than 

the estimated average annual evaporation of 7.5 ft from 1997 

to 1999. The USGS reported monthly evaporation rates 

for Lake Mead from October 1953–September 1995 (U.S. 

Geological Survey, 1961–64, 1965–75, 1976–82; Hendricks, 

1964; Frisbie and others, 1983–85; Pupacko and others, 1988, 

1989a, 1989b, 1990; Bostic and others, 1991; Garcia and 

others, 1992; Hess and others, 1993; Emett and others, 1994; 

Clary and others, 1995; Bauer and others, 1996). Harbeck 

(1958b, table 13, p. 37) reported monthly evaporation rates for 

March 1952 to September 1953. G.E. Koberg (U.S. Geological 

Survey, written commun., 1964) revised the reported monthly 

evaporation rates for October 1952 to September 1962; 

however, those revised rates were never published. Koberg 

did not revise the reported monthly rates for March 1952 

to September 1952. Previously reported evaporation rates 

were not entered in a database or summarized in any single 

publication; these rates were available only from the various 

publications and written communications.

The average annual evaporation, average monthly 

evaporation, and standard deviation of monthly rates were 

compared for two periods of previously estimated Lake Mead 

evaporation, 1953–73 and 1974–94. Evaporation rates were 

available for 12 months of each year during those periods and 

each period consists of 21 years (table 8). The average annual 

evaporation for 1953–73 is 6.7 ft; whereas, the average annual 

evaporation for 1974–94 is 6.0 ft. Compared to 1974–94, the 

mean monthly evaporation during 1953–73 was greater for 

every month except November.

The standard deviation (variability) of monthly 

evaporation rates for 1953–73 was less than for 1974–94 

for every month except May. Only the annual rates of 5.9 ft 

in 1965 and 7.4 ft in 1956 differed from the average annual 

rate of 6.7 ft for 1953–73 by greater than 10 percent. In 

contrast, annual evaporation was more variable for 1974–94 

with rates for 9 years that differed by more than 10 percent 

from the average annual rate of 6.0 ft. The greater variability 

probably was related to estimated monthly wind speed due to 

mechanical problems with the anemometer at Lake Mead.

1953 1954 1955 195619571958195919601961196219631964196519661967196819691970197119721973197419751976197719781979198019811982198319841985198619871988198919901991199219931994

30

40

50

60

70

80

90

100

ANNUAL EVAPORATION, IN INCHES

1953 to 1994 equals 76.3 inches 

1953 to 1973 equals 80.6 inches

1974 to 1994 equals 72.0 inches

Average annual evaporation for selected 

Figure 11. Annual Lake Mead evaporation, 1953–94.

18 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Table 8. Total monthly, total annual, and average monthly Lake Mead evaporation, 1953–95.

[Historical references for this table are U.S. Geological Survey, 1961–64, 1965–75, 1976–82; Hendricks, 1964; Frisbie and others, 1983–85; Pupacko and others, 

1988, 1989a, 1989b, 1990; Bostic and others, 1991; Garcia and others, 1992; Hess and others, 1993; Emett and others, 1994; Clary and others, 1995; and Bauer 

and others, 1996. Symbol: –, no data available]

Total evaporation, in inches

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1953 4.1 4.4 4.7 6.2 8.6 8.0 7.6 10.6 8.8 7.3 6.5 6.9 83.7

1954 4.3 3.6 5.9 5.5 8.3 8.3 9.8 9.6 8.4 8.2 5.5 6.1 83.5

1955 5.0 4.2 5.4 7.0 7.3 7.9 9.3 8.1 10.5 7.3 7.6 4.6 84.2

1956 4.1 4.8 5.2 5.9 7.9 9.5 9.3 11.5 7.8 9.1 7.9 5.8 88.8

1957 3.8 2.6 4.8 6.0 6.2 8.3 8.9 10.7 7.6 7.7 8.2 5.7 80.5

1958 4.9 2.7 4.9 4.0 8.1 9.5 10.5 9.6 9.5 8.6 6.5 4.3 83.1

1959 4.2 3.8 5.1 5.1 8.1 8.4 9.6 9.5 9.8 8.0 6.1 5.7 83.4

1960 3.6 4.6 4.6 6.0 7.0 9.0 9.4 11.9 7.1 7.3 5.8 5.6 81.9

1961 3.5 3.7 5.0 5.0 8.2 9.1 9.8 9.0 9.9 8.2 6.3 4.7 82.4

1962 4.5 3.2 6.0 4.2 8.0 8.0 10.2 11.0 8.8 9.2 5.4 4.6 83.1

1963 5.5 3.9 4.7 6.4 7.2 8.0 10.6 10.2 7.5 6.6 7.0 5.4 83.0

1964 5.2 4.8 5.0 6.0 4.6 7.4 10.2 9.3 8.7 6.3 7.1 4.9 79.5

1965 3.4 3.8 4.0 4.2 6.3 7.1 7.9 9.7 9.4 5.2 5.0 4.9 70.9

1966 4.6 3.6 4.1 6.4 6.8 8.0 10.1 10.0 7.4 7.2 5.5 5.2 78.9

1967 4.5 3.5 5.5 6.2 5.6 6.7 7.8 7.6 9.0 8.4 5.1 7.0 76.9

1968 4.0 2.3 4.3 5.8 6.8 7.5 7.0 7.5 8.1 7.7 7.6 6.9 75.5

1969 3.4 3.2 4.3 6.1 5.8 8.5 6.9 8.8 7.8 9.2 5.6 5.6 75.2

1970 4.2 3.1 4.6 5.9 6.8 6.4 7.5 7.3 10.1 9.5 6.8 5.6 77.8

1971 4.3 4.5 5.0 6.3 6.2 7.5 7.8 8.2 10.6 7.7 6.0 6.8 80.9

1972 4.2 2.9 4.2 6.2 7.4 7.5 9.4 8.3 7.9 7.3 5.8 6.8 77.9

1973 4.5 2.2 3.3 4.3 5.7 8.1 11.1 10.5 10.0 8.6 7.5 5.2 81.0

1974 3.6 4.3 4.3 5.6 7.4 7.1 9.4 7.4 8.6 8.9 5.6 7.4 79.6

1975 5.7 6.3 5.6 5.7 6.6 9.2 10.8 12.6 6.9 11.5 9.1 4.8 94.8

1976 3.8 3.6 5.1 5.7 5.2 9.7 8.5 10.6 7.4 7.5 6.0 6.0 79.1

1977 4.1 3.4 5.7 4.4 6.1 6.0 9.4 9.6 9.7 6.5 6.4 4.2 75.5

1978 3.7 3.7 3.4 4.3 6.2 7.9 7.7 10.5 8.6 6.7 5.6 6.8 75.1

1979 4.6 3.3 3.3 4.5 6.5 8.0 8.3 9.0 7.1 9.3 8.1 4.5 76.5

1980 5.1 3.5 4.1 4.0 6.7 7.2 9.4 9.1 6.2 7.9 6.0 3.5 72.7

1981 3.5 2.9 3.2 5.0 4.1 6.9 9.6 6.9 7.7 7.9 5.1 4.9 67.7

1982 3.8 2.3 4.6 3.5 6.5 8.3 6.3 5.8 7.2 6.6 6.4 4.9 66.2

1983 2.4 2.4 3.6 5.6 7.2 8.5 10.3 7.3 5.6 6.5 8.7 3.7 71.8

1984 3.0 3.7 4.4 4.5 5.9 6.5 7.1 6.2 3.8 9.5 5.5 4.9 65.0

1985 3.6 3.7 4.4 3.3 4.8 7.0 7.4 8.4 6.7 5.5 6.4 5.0 66.2

1986 2.8 3.0 3.4 5.6 6.1 5.9 8.0 7.1 9.8 5.9 5.1 6.1 68.8

1987 3.4 3.1 3.8 3.1 5.5 6.6 8.8 7.5 6.8 5.1 3.6 5.6 62.9

1988 3.3 2.1 2.8 3.4 4.7 5.1 6.2 6.2 6.3 4.8 6.8 4.6 56.3

1989 3.5 3.1 4.2 4.5 5.2 4.9 7.6 9.8 4.8 5.2 3.5 5.5 61.8

1990 3.8 3.8 2.7 4.2 7.5 6.8 7.9 8.0 8.4 5.8 7.0 5.8 71.7

1991 2.8 2.0 4.2 2.5 5.8 9.5 8.4 11.2 9.6 9.0 6.9 5.5 77.4

1992 4.1 2.7 2.6 6.0 8.1 5.3 4.7 4.0 3.8 4.2 4.4 2.8 52.7

1993 1.6 3.0 2.4 4.7 8.6 5.1 12.9 9.8 9.4 8.1 9.8 4.7 80.1

1994 4.1 7.3 3.5 5.4 5.4 8.8 9.1 12.7 8.5 8.9 11.2 4.6 89.5

1995 3.2 2.4 2.5 2.6 3.0 8.9 9.3 12.5 – – – –

Average monthly

1953–95 3.9 3.5 4.3 5.0 6.5 7.6 8.8 9.1 8.0 7.5 6.5 5.3 76.0

1953–73 4.3 3.6 4.8 5.7 7.0 8.0 9.1 9.5 8.8 7.8 6.4 5.6 80.6

1974–94 3.6 3.5 3.9 4.5 6.2 7.2 8.5 8.6 7.3 7.2 6.5 5.0 72.0

Standard deviation

1953–73 0.6 0.8 0.6 0.9 1.1 0.8 1.3 1.3 1.1 1.1 1.0 0.8

1974–94 .9 1.3 .9 1.0 1.1 1.5 1.8 2.3 1.8 1.9 2.0 1.1

Previously Reported Evaporation Rates 19
Alternative Methods of Estimating Lake 

Mead Evaporation

Two alternative methods were examined for estimating 

Lake Mead evaporation: (1) continuing operation of a 

complete open-water energy-budget station and (2) estimating 

evaporation with daily net radiation measured at a readily 

accessible location on Lake Mead.

Based on results of the current study, operation of one 

complete energy-budget station located in an open-water area 

of the lake would provide an accurate estimate of evaporation 

for all of Lake Mead. For example, monthly evaporation for 

the open-water station at Sentinel Island in Boulder Basin is 

within 5 percent of the estimated total monthly evaporation 

of all open-water areas of Lake Mead. This method requires 

standard measurements of water temperature at and below 

the water surface, air temperature, relative humidity, and net 

radiation. Also, because equipment on an open-water barge 

is exposed to extreme weather and water conditions, routine 

maintenance of station equipment is necessary.

A regression of average monthly evaporation at Sentinel 

Island (table 6) and net radiation at the Water Barge Cove 

station was developed for 29 months (1997–99) to determine 

the relation between net radiation at a station that is accessible 

(Water Barge Cove) and evaporation at an open-water station 

(Sentinel Island). The regression was developed for two 

periods of the year: January to May when heat storage in 

the lake is decreasing (fig. 12A), and (2) June to December 

when heat storage in the lake is increasing (fig. 12B). Data 

were plotted with net radiation at Water Barge Cove as the 

independent variable and Sentinel Island evaporation as the 

dependent variable. The coefficient was 0.820 for January 

to May and 0.850 for June to December, indicating that 

measurements of monthly average net radiation at Water Barge 

Cove, or a site with similar environmental conditions on the 

lake, may be used to approximate lake evaporation.

The accuracy of the net radiation alternative method was 

evaluated by applying empirical equations derived for each 

heat-storage period to determine total monthly evaporation 

and by comparing these estimates to the average monthly 

evaporation for open-water stations. Monthly average net 

radiation at Water Barge Cove was used to compute monthly 

evaporation from July 1997 through December 1999 

(30 months) using empirical equations representing the best-fit 

lines (fig. 12):

E = Rn *0.3093 + 3.1663, (4)

(applied from January through May) and

E = 1.1845 * Ln(Rn) + 5.7387, (5)

(applied from January through May), where Rn is monthly net 

radiation at Water Barge Cove, in Watts per square foot, and E 

is monthly evaporation, in inches.

AVERAGE MONTHLY NET RADIATION (Rn) AT WATER BARGE COVE,

IN WATTS PER SQUARE FOOT

0 5 10 15 20 25

0

2

4

6

8

10

12

TOTAL MONTHLY EVAPORATION (E) AT SENTINEL ISLAND, IN INCHES

0

2

4

6

8

10

12

E = 0.3093(Rn) + 3.1663

R2 = 0.8201

E = 1.1845Ln(Rn) + 5.7387

R2 = 0.8497

A. January through May 1997–99

B. June through December 1997–99

Figure 12. Regression of monthly average net radiation at 

the Water Barge Cove station and total monthly evaporation 

at Sentinel Island station for: (A) January through May 1997–

99 and (B) June through December 1997–99. R2 is coefficient 

of determination.

20 Evaporation from Lake Mead, Arizona and Nevada, 1997–99
Computed monthly evaporation rates were compared 

to average monthly evaporation at the open-water stations 

(fig. 13). The minimum and maximum monthly openwater evaporation rates were 3.6 in. (February 1998) and 

9.9 in. (July 1998), respectively. Whereas, the minimum 

and maximum computed monthly evaporation rates from 

Water Barge Cove net radiation were 3.8 in. (January 1999) 

and 9.4 in. (June 1999), respectively. Of the 30 monthly 

evaporation rates compared, four monthly rates differed 

from open-water evaporation rates by more than 10 percent 

(fig. 14). The largest difference in evaporation rates was 

37.5 percent for February 1998, and the average difference 

was 1 percent.

Monthly evaporation rates were summed to compare 

annual evaporation for open-water stations and computed 

evaporation using empirical equations. Total open-water 

evaporation was 7.4 ft compared to a computed evaporation 

rate of 7.5 ft for 1998 (a 1.4 percent difference). Additionally, 

open-water evaporation was 7.6 ft compared to a computed 

rate of 7.4 ft for 1999 (a 2.6 percent difference).

Figure 14. Comparison of total monthly evaporation (average 

of open-water stations) and monthly evaporation computed 

from monthly mean net radiation at the Water Barge Cove 

station.

nv16-0046_fig13

0

2

4

6

8

10

MONTHLY EVAPORATION, IN INCHES

J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1997 1998 1999

Average of open-water stations

Computed from Water Barge Cove net radiation

Figure 13. Total monthly evaporation (average of open-water stations) and total monthly evaporation computed from monthly 

average net radiation at the Water Barge Cove station.

2

3

4

5

6

7

8

9

10

11

2 3 4 5 6 7 8 9 10 11

MONTHLY EVAPORATION COMPUTED FROM WATER BARGE COVE NET RADIATION,

IN WATTS PER SQUARE FOOT 

MONTHLY OPEN-WATER EVAPORATION, IN INCHES

10-percent confidence band

One-to-one line

Alternative Methods of Estimating Lake Mead Evaporation  21
Summary

Air temperature, relative humidity, wind speed and 

direction, and water temperature were measured from 1997 

to 1998 at four floating instrumented platforms on Lake 

Mead. One of four floating platforms was in a shallowwater sheltered area of Boulder Basin (Water Barge Cove); 

the remaining three platforms were in open-water areas of 

Boulder Basin, Virgin Basin, and Overton Arm. Data were 

averaged in 20-minute intervals and these averages were used 

with the energy-budget equation to compute estimates of lake 

evaporation.

Generally, micrometeorological and water-temperature 

data were similar at the open-water locations. Relative 

humidity and wind speed were lower, and water temperature 

was higher, at the sheltered cove platform (Water Barge Cove) 

than at the open-water platforms. Air temperature was about 

the same at all locations.

Monthly evaporation rates were evaluated for temporal 

variations at each station and spatial variations between 

stations. At each station, monthly evaporation rates did not 

significantly vary from year to year for the period of data 

collection. Moreover, spatial variation in monthly evaporation 

rates was minimal for the open-water stations. However, 

monthly evaporation at Water Barge Cove generally was 

less than monthly evaporation at other stations when the 

monthly evaporation rate was less than 6.5 in. Estimated 

monthly evaporation rates were summed for each year of data 

collection to determine annual evaporation rates. The average 

annual evaporation rate for Lake Mead from January 1998 to 

December 1999 was 7.5 ft.

The estimated annual evaporation of 7.5 ft for the period 

1997–99 is about 15 percent greater than previously reported 

annual rates of about 6.4 ft for 1953–94, which was estimated 

using a mass-transfer equation that was derived initially during 

a 1952–53 Lake Mead water-loss study. Reported annual 

evaporation rates from 1953 to 1994 fluctuated from 6 to 

7.5 ft/yr prior to 1975; after 1975, annual evaporation rates 

fluctuated from 4.4 to 7.9 ft/yr.

During the 1997–99 study, monthly average netradiation data for a shallow-water site (Water Barge Cove) 

was regressed with monthly total evaporation data for an 

open-water site (Sentinel Island) to evaluate an alternative 

method of estimating lake evaporation. Empirical equations 

derived from this regression were used to compute monthly 

evaporation rates, and these rates were compared to average 

monthly evaporation rates at open-water stations. Only 4 of 

the 30 computed monthly evaporation rates differed from 

monthly open-water rates by more than 10 percent, indicating 

that monthly net radiation at Water Barge Cove, or a site with 

similar environmental conditions on the lake, may be used to 

approximate lake evaporation. Monthly evaporation rates were 

summed to compare annual evaporation for open-water sites 

and computed evaporation using empirical equations. Total 

open-water evaporation was 7.4 ft compared to a computed 

evaporation rate of 7.5 ft for 1998 (a 1.4 percent difference). 

Additionally, open-water evaporation was 7.6 ft compared to a 

computed rate of 7.4 ft for 1999 (a 2.6 percent difference).

Acknowledgments

Many agencies cooperated with the USGS during this 

evaporation study. The Bureau of Reclamation provided the 

USGS with a boat and a barge for meteorological equipment. 

The National Park Service provided temporary use permits, 

two barges, and space on a National Park Service pumping 

barge for meteorological equipment. They also provided 

historical Lake Mead weather data, manpower and equipment 

for anchoring barges, and security for the barges and 

equipment. The Las Vegas National Weather Service provided 

Las Vegas weather data.

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24  Evaporation from Lake Mead, Arizona and Nevada, 1997–99
For more information concerning the research in this report, contact the 

Director, Nevada Water Science Center

U.S. Geological Survey, 2730 N. Deer Run Road

Carson City, Nevada 89701

http://nv.water.usgs.gov
SIR 2006-5252 Westenburg and others Evaporation from Lake Mead, Arizona and Nevada, 1997–99