Evaporation from Lake Mead and Lake Mohave, Lower Colorado River Basin, Nevada and Arizona
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The Bureau of Reclamation currently operates a model that projects future Colorado River reservoir volumes and potential dam operations based on current and forecasted hydrologic conditions, and operational policies and guidelines (the 24-Month Study). Each month a water budget is developed and Colorado River reservoir volumes and releases are projected for the next 24-month period. Reservoir evaporation is a water-budget term currently used in the model that is based on poorly documented modifications to original USGS estimates that resulted from pioneering reconnaissance efforts in the 1940’s and 1950’s. The USGS Nevada Water Science Center and Reclamation are cooperating on a multi-phase study to improve 24-Month Study model projections by improving monthly estimates of evaporation from Lake Mead and Lake Mohave. The continuous data needed to compute monthly evaporation are being collected from floating-platform and land-based measurement stations located at each reservoir.
Evaporation measurements began in March 2010 at Lake Mead and May 2013 at Lake Mohave. Data collection/analysis methods and monthly evaporation results for Lake Mead through February 2012 were documented in a USGS Scientific Investigations Report. Monthly evaporation and associated datasets for both reservoirs through April 2015 were published in a USGS Data Release. A final report featuring 9 years of data from Lake Mead and 6 years of data from Lake Mohave will be written following the end of data collection in April 2019.
The continuous data needed to compute monthly evaporation are being collected from a floating platform and a land-based eddy covariance station located at each reservoir. Water-temperature profiles and net radiation are measured from the floating platforms. Eddy covariance relies on high-frequency measurements of water-vapor density and wind-velocity vectors by fast-response sensors. Eddies are turbulent airflow caused by wind, surface roughness, and convective heat flow in the atmospheric surface layer. Eddies transfer energy and mass between land and water surfaces and the atmosphere through a process referred to as turbulent exchange. Eddy covariance provides the most direct measure of turbulent exchange currently available. Fluxes of water vapor and heat can be measured directly without the application of empirical constants by finding the covariance between these scalars and vertical wind speed. Evaporation (positive latent-heat flux) occurs when water vapor in upward moving eddies is greater than in downward moving eddies. Likewise, sensible-heat flux is positive (from the surface to the atmosphere) when upward moving eddies are warmer than downward moving eddies. Unlike most other methods, eddy covariance is particularly well-suited to measuring evaporation from Lakes Mead and Mohave because there is no reliance on difficult-to-measure energy- and water-budget components. Methods used to compute evaporation and estimate uncertainty will follow those described by Moreo and Swancar.
RESERVOIR EVAPORATION RESEARCH
Preliminary results of the current study are making a significant contribution to the body of reservoir evaporation research.
Annual evaporation from Lake Mead (6.01 ft) was about 12 percent greater than evaporation from Lake Mohave (5.36 ft) during the two-year period from May 2013 to April 2015.
Less evaporation at Lake Mohave compared to Lake Mead primarily is due to the difference between inflowing water temperature from Hoover Dam (cooler) and outflowing water temperature from Davis Dam (warmer). This temperature difference results in a persistent loss of energy (negative net advection) meaning that less energy is available to drive evaporation at Mohave than at Mead. The mean annual evaporation rate measured at Mead (6.19 ft, n=5) is less than the summed monthly coefficients used by the 24-Month Study model (6.50 ft), and at Mohave the mean annual evaporation rate (5.36 ft, n=2) is considerably less than the summed monthly coefficients (7.31 ft).
The annual evaporation volume computed for 2014 was 27,000 acre-feet less than estimated for the 24MS model at Lake Mead and 53,000 acre-feet less at Lake Mohave.
Differences between current study monthly evaporation and 24-Month Study coefficients range from minor to substantial with the largest differences occurring during summer and fall at Lake Mead and winter through summer at Lake Mohave (figs. 1 and 2). A lag in current study evaporation relative to the seasonal solar pattern is evident at both reservoirs. Heat storage during spring and early summer (increasing water temperatures) result in a decrease in available energy and less evaporation than estimated for the 24-Month Study coefficients. The subsequent release of that stored energy during fall and winter (decreasing water temperatures) increases the available energy which results in greater evaporation than estimated for the 24-Month Study coefficients. This seasonal lag explains most of the differences observed between current study monthly evaporation and 24-Month Study coefficients at Lake Mead. This seasonal trend also is evident at Lake Mohave, but the large monthly differences here are attributed mainly to the lack of historical evaporation measurements.
CURRENT PHASE OF STUDY
The current phase of the study is collecting 3 additional years of evaporation measurements at Lake Mead and Lake Mohave. Year-to-year differences between monthly evaporation rates and mean monthly evaporation rates are as much as 20 percent, underscoring the need for additional measurements to develop more robust long-term monthly means. The longer-term datasets will be used to better quantify the influence of seasonal or annual changes in climate on mean-monthly evaporation rates so that modeling projections are more representative of mean-annual climatic conditions. This phase of the study will begin to address the temporal limitations associated with current Lake Mead and Lake Mohave monthly evaporation rates and how evaporation variability is/might be projected into the future.