Scientific Investigations Report 2006-5252
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5252
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.
Computed monthly evaporation rates were compared to average monthly evaporation at the open-water stations (fig. 13). The minimum and maximum monthly open-water 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).
U.S.
Department of the Interior | U.S. Geological
Survey
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