Lake Mead Evaporation Rates
USGS and the Bureau of Reclamation are cooperating on a multi-phase study to improve 24-Month Study (24MS) model projections by improving monthly estimates of evaporation from Lake Mead and other lower Colorado River Basin reservoirs. The 24MS is an operational planning tool used by Reclamation to project future Colorado River reservoir volumes and potential dam operations based on current and forecasted hydrologic conditions, and operational policies and guidelines. Monthly evaporation from Lake Mead is a water-budget term currently used in the 24MS. Evaporation rates were measured at Lake Mead for 3 years (March 2010 through February 2013) for phase 1 of the study. Phase 2 will continue evaporation monitoring at Lake Mead for 3 additional years and begin 3 years of evaporation monitoring at another lower Colorado River Basin reservoir, Lake Mohave. Eddy covariance (EC) is the primary measurement method. EC is a state-of-the-art method that has long been used for estimating evapotranspiration, but has not often been used to estimate lake evaporation. This study is ongoing: however, the first in a series of reports has been published documenting the first 2 years of evaporation measurements.
Abstract from Scientific Investigations Report 2013-5229
Evaporation from Lake Mead was measured using the eddy-covariance method for the 2-year period starting March 2010 and ending February 2012. When corrected for energy imbalances, annual eddy-covariance evaporation was 2,074 and 1,881 millimeters (81.65 and 74.07 inches), within the range of previous estimates. There was a 9-percent decrease in the evaporation rate and a 10-percent increase in the lake surface area during the second year of the study compared to the first. These offsetting factors resulted in a nearly identical 720 million cubic meters (584,000 acre feet) evaporation volume for both years. Monthly evaporation rates were best correlated with wind speed, vapor pressure difference, and atmospheric stability. Differences between individual monthly evaporation and mean monthly evaporation were as much as 20 percent. Net radiation provided most of the energy available for evaporative processes; however, advected heat from the Colorado River was an important energy source during the second year of the study. Peak evaporation lagged peak net radiation by 2 months because a larger proportion of the net radiation that reaches the lake goes to heating up the water column during the spring and summer months. As most of this stored energy is released, higher evaporation rates are sustained during fall months even though net radiation declines. The release of stored heat also fueled nighttime evaporation, which accounted for 37 percent of total evaporation. The annual energy-balance ratio was 0.90 on average and varied only 0.01 between the 2 years, thus implying that 90 percent of estimated available energy was accounted for by turbulent energy measured using the eddy-covariance method. More than 90 percent of the turbulent-flux source area represented the open-water surface, and 94 percent of 30-minute turbulent-flux measurements originated from wind directions where the fetch ranged from 2,000 to 16,000 meters. Evaporation uncertainties were estimated to be 5 to 7 percent.
Site identification: 360500114465601
Measurement method: Eddy covariance
Data: Download from NWISWeb
Study period: 3/01/10- 2/28/11
Annual ET reported (in inches): 81.65
Days recorded: 365 from 3/01/10 to 2/28/11
Study period: 3/01/11- 2/29/12
Annual ET reported (in inches): 74.07
Days recorded: 366 from 3/01/11 to 2/29/12
Study period: 3/01/12- 2/28/13
Annual ET reported (in inches): 74.34
Days recorded: 365 from 3/01/12 to 2/28/13
Location information (varies with lake elevation)
in decimal degrees
in decimal degrees
Turbulent fluxes measured by eddy-covariance sensors at site EC-2.
Location: Lakes Mead and Mohave, southern Nevada
Start Date: 2010
End Date: 2017
Cooperators: Bureau of Reclamation
USGS Nevada Water Science Center
160 N. Stephanie St.
Henderson, NV 89074
phone: (702) 564-4625
EC: Eddy Covariance
24MS: 24-month study