Nevada Water Science Center


Evapotranspiration Studies in Nevada

Contact Information

Mike Moreo
Hydrologist
Phone: (702) 564-4625
Email:

 

Mailing Address
USGS
Nevada Water Science Center
160 N. Stephanie St.
Henderson, NV 89074

 

Nevada Water Science Center
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Lake Mead Sentinel Island

Site identification: 360314114450501

Study contact: Craig Westenburg

ET Data

Annual ET reported (in inches): 90.0

Study period: 6/23/97 - 6/23/98

Days recorded: 365 from 6/23/97 to 6/23/98

 

Measurement method: Bowen ratio

Data: Download from NWISWeb

Graph of ET data

Location information (varies with lake elevation)

Latitude,
in decimal degrees
Longitude,
in decimal degrees
Altitude
36.0541666667 -114.751388889 1202

 

Photo of ET site

 

ET Source Area

Open water

 

Study Information

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.

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).

 

Study Publication: SIR 2006-5252

 

 

 

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