Environmental Engineering Reference
In-Depth Information
In arid regions such as the Hoover Dam on the Arizona/Nevada border there is
another reason for Lake Mead's low water levels: evaporation. This is a major issue
and concern of the USGS and others. If water totals are at a premium to begin with
(and they are), the evaporation rate of the remaining water is important and must
be closely monitored. As a case in point, consider the following USGS 1997-1999
report on the impact of evaporation on Lake Mead.
e
stImatIng
e
vaporatIon
from
L
ake
m
ead
*
Lake Mead is one of a series of large Colorado River reservoirs operated and main-
tained 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 energy-budget equation. The average annual evapo-
ration rate at open-water stations from January 1998 to December 1999 was 7.5 ft.
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 pro-
vide data that are adequate to estimate evaporation from Lake Mead.
Lake Mead is the largest reservoir by volume in the United States and was formed
when Hoover Dam was completed in 1935. It took until July 1941 for water to fill
Lake Mead, which has a maximum surface elevation of 1229 ft, a maximum sur-
face 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 1229 ft, Lake Mead
extends 65.9 miles upstream of Hoover Dam and has a maximum width of 9.3 miles
(LaBounty and Horn, 1997). The average lake elevation from 1942 (first complete
calendar year of full pool) to 1995, based on monthly end-of-month elevations, is
1169.9 ft, which corresponds to a lake surface area of 125,600 acres.
The drainage area of Lake Mead at Hoover Dam is about 171,700 m
2
(Tadayon et
al., 2000). Ninety-seven percent of the inflow into Lake Mead is from the Colorado
River (13.12 million acre-ft/yr (Tadayon et al., 2000), with the remaining 2% com-
ing from the combined flow of the Las Vegas Wash (148,000 acre-ft/yr) and the
Virgin River (176,000 acre-ft/yr) (Jones et al., 2000), as well as ephemeral streams.
The average annual release from Hoover Dam from 1935 to 1999 was about 10.1
million acre-ft (Tadayon et al., 2000). Flow of the Colorado River at Diamond
Creek (130 miles upstream of Hoover Dam) in calendar year 1999 was 12.69 mil-
lion acre-ft, while the release from Hoover Dam was 11.04 million acre-ft (Tadayon
et al., 2001). Retention time for Lake Mead averages 3.9 years, depending on release
and flow patterns.
*
Adapted from Westennberg, C.I. et al.,
Estimating Evaporation from Lake Mead (1997-99)
, USGS
Scientific Investigation Report 2006-5252, U.S. Geological Survey, Washington, DC, 2006.
