Scientific Investigations Report 2006-5252
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5252
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 shallow-water 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 net-radiation 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).
U.S.
Department of the Interior | U.S. Geological
Survey
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