Environmental Engineering Reference
In-Depth Information
Lake Mead is in an environment with a warm, arid climate. From 1961 to 1990,
the average maximum air temperature was 105.9°F for Las Vegas, Nevada, and
107.8°F for Overton, Nevada, whereas the average minimum air temperatures were
33.6°F and 28.0°F, respectively. Average annual precipitation (1961-1990) was 4.13
in. for Las Vegas and 3.31 in. for Overton. This warm, arid environment is conducive
to 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 moderated height above the lake surface. The vast majority of lake is
exposed to winds from the southwest to southeast.
Computation Methods
Lake Mead evaporation was computed using micrometeorological data collected at
four floating instrumented platforms deployed in Water Barge Cove near Callville
Bay, Boulder Basin near Sentinel Island, the Virgin Basin, and in the Overton Arm
near Echo Bay. The platform in Water Barge Cove was in a relatively shallow cove
protected from prevailing winds. The platforms in Boulder Basin near Sentinel
Island, Virgin Basin, and Overton Arm 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 1196 ft in February 1997 to almost 1216 ft in
September 1998. 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. A two-point mooring system was used to secure
each barge to prevent drifting and maintain the directional aspect of the wind moni-
tor and net radiometer.
Energy-Budget Method to Measure Evaporation
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
and has been used by the USGS on a few long-term lake studies (Rosenberry et al.,
1993; Sturrock and Rosenberry, 1992). The energy-budget method is the most accu-
rate method for measuring lake evaporation (Winter, 1981). Use of the energy-budget
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
DID YOU KNOW?
The amount of heat necessary to change 1 kilogram of a liquid into a gas is
called the
latent heat of vaporization
. When this point is reached, the entire
mass of substance is in the gas state. The temperature of the substance at which
this change from liquid to gas occurs is known as the
boiling point.
