Evaporation from Lake Mead and Lake Mohave, Lower Colorado River Basin, Nevada and Arizona
Since 2010, the U.S. Geological Survey (USGS) Nevada Water Science Center (NVWSC) has worked with with the Bureau of Reclamation to update evaporation estimates for Lake Mead.
The NVWSC calculates evaporation using a combined approach that includes both eddy-covariance and energy balance methods. Eddy-covariance directly measures turbulent fluxes by taking rapid measurements (10 times per second) of water vapor flux and three-dimensional wind vectors. This data helps estimate latent heat and sensible heat flux. The energy balance method estimates the available energy in the system using existing water temperature data, reservoir volume, and measured meteorological data. The methods for calculating evaporation and estimate uncertainty follow the guidelines set by Moreo and Swancar (2012) and Earp and Moreo (2021).
The NVWSC operates a real-time eddy covariance monitoring station on Lake Mead. This station collects continuous energy flux and meteorological data, transmitting 30-minute values to the publicly available online database, USGS Water Data for the Nation. Evaporation measurements at Lake Mead began in March 2010 and are currently ongoing. Measurements at Lake Mohave were made from May 2013 to May 2019. All continuous data collected from Lake Mead (Site Number: 360500114465601) and Lake Mohave (Site Number: 352129114363501) are available for download.
RESULTS
- Evaporation estimates made by the USGS were compared to the evaporation coefficients used in Bureau of Reclamation’s 24-Month Study (24MS), a forecasting tool used to project Colorado River system conditions. The average annual evaporation at Lake Mead was 1,896 millimeters
(mm; 6.22 feet), while average annual evaporation at Lake Mohave is 1,718 mm (5.64 ft).
- Evaporation at Lake Mohave is less than at Lake Mead primarily 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 less energy is available to drive evaporation at Mohave than at Mead.
- A seasonal lag in evaporation relative to the solar pattern is evident at both reservoirs. During spring and early summer, increasing solar radiation mainly warms the lake water, capturing energy that could drive evaporation. This results in less evaporation than expected during the early summer months. The subsequent release of this stored energy in fall and winter increases the energy available for evaporation, leading to greater evaporation rates in early fall.
Since 2010, the U.S. Geological Survey (USGS) Nevada Water Science Center (NVWSC) has worked with with the Bureau of Reclamation to update evaporation estimates for Lake Mead.
The NVWSC calculates evaporation using a combined approach that includes both eddy-covariance and energy balance methods. Eddy-covariance directly measures turbulent fluxes by taking rapid measurements (10 times per second) of water vapor flux and three-dimensional wind vectors. This data helps estimate latent heat and sensible heat flux. The energy balance method estimates the available energy in the system using existing water temperature data, reservoir volume, and measured meteorological data. The methods for calculating evaporation and estimate uncertainty follow the guidelines set by Moreo and Swancar (2012) and Earp and Moreo (2021).
The NVWSC operates a real-time eddy covariance monitoring station on Lake Mead. This station collects continuous energy flux and meteorological data, transmitting 30-minute values to the publicly available online database, USGS Water Data for the Nation. Evaporation measurements at Lake Mead began in March 2010 and are currently ongoing. Measurements at Lake Mohave were made from May 2013 to May 2019. All continuous data collected from Lake Mead (Site Number: 360500114465601) and Lake Mohave (Site Number: 352129114363501) are available for download.
RESULTS
- Evaporation estimates made by the USGS were compared to the evaporation coefficients used in Bureau of Reclamation’s 24-Month Study (24MS), a forecasting tool used to project Colorado River system conditions. The average annual evaporation at Lake Mead was 1,896 millimeters
(mm; 6.22 feet), while average annual evaporation at Lake Mohave is 1,718 mm (5.64 ft).
- Evaporation at Lake Mohave is less than at Lake Mead primarily 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 less energy is available to drive evaporation at Mohave than at Mead.
- A seasonal lag in evaporation relative to the solar pattern is evident at both reservoirs. During spring and early summer, increasing solar radiation mainly warms the lake water, capturing energy that could drive evaporation. This results in less evaporation than expected during the early summer months. The subsequent release of this stored energy in fall and winter increases the energy available for evaporation, leading to greater evaporation rates in early fall.