Improved reservoir evaporation data informs water supply management

Improved reservoir evaporation data informs water supply management Posted on October 9, 2024

Water supply planning can be compared to putting together a puzzle-even a single missing piece can change the whole picture. To help solve the water availability puzzle, the Texas Water Development Board (TWDB) is partnering with several agencies to improve and expand estimates of reservoir evaporation to more accurately plan for the future.

Reservoir evaporation, especially in dry years, can be the predominant driver of a system's water supply depletion. In 2011, the most intense single year of drought on record in Texas, the amount of water lost from Texas reservoirs through evaporation was higher than municipal water use. Knowing how much water is being lost to evaporation is critical to effective reservoir management and preparing for future drought conditions.

History of reservoir evaporation data in Texas

Since the 1960s, the TWDB has compiled monthly evaporation rates for the state with data collected by cooperators and the National Weather Service. This data has been provided to the regional water planning groups since 1997 for use in the Texas Water Availability Models (WAMs), which are used by the Texas Commission on Environmental Quality to evaluate water rights applications.

Historically, reservoir evaporation data have been compiled using water level changes at a Class A evaporation pan located adjacent to a reservoir and an adjustment factor (known as pan-to-lake coefficients). While this technology has been used for more than a century, there are known deficiencies in the data because it does not reflect actual conditions within the reservoir - such as how warm the water is, how dry the air over the water is, how windy it is, and how far wind travels over water. These factors impact how much water evaporates from a reservoir.

"If that data is incorrect, then we have either over-allocated or we are being too conservative in our estimates of what water is available in the future," said Dr. Nelun Fernando, Manager of the TWDB's Water Availability Program.

Developing a new dataset

Last year, the TWDB partnered with Texas A&M University, the Desert Research Institute, the U.S. Army Corps of Engineers - Fort Worth District, and the Lower Colorado River Authority (LCRA) to release a new dataset called the Reservoir-specific Daily Evaporation Dataset for Texas, which provides daily, reservoir-specific evaporation rates for 188 major water supply reservoirs in Texas dating back to 1980. The new dataset incorporates meteorological factors, like wind and air temperature, that aren't accounted for in the pan evaporation data. It is based on the Daily Lake Evaporation Model developed by Texas A&M University (Zhao et al., 2024).

The LCRA, which manages the Highland Lakes in Central Texas, is comparing the historical data collected from evaporation pans to the new reservoir-specific, daily evaporation dataset. The new dataset shows that evaporation rates are higher in the fall than the pan evaporation method estimated and that the annual average evaporation rate has increased in Texas since 1981. In 2023, evaporation was the biggest water user from the Highland Lakes.

"The information we're gathering is really critical, and the ability to forecast, understand, and learn from it will make us better when developing the Water Management Plan in the future," Phil Wilson, General Manager at LCRA, said at the organization's Water Operations Committee meeting in August.

Validating the dataset

With grant funding from the U.S. Bureau of Reclamation, real-time data is also being collected from state-of-the-art monitoring stations that sit on the water in several reservoirs across Texas. The stations include buoys on Red Bluff Reservoir, Lake Meridith, Lake Buchanan, Choke Canyon Reservoir, and a Collison Floating Evaporation Pan on Twin Buttes Reservoir. This data will be used to validate the reservoir-specific, daily evaporation dataset for Texas reservoirs to give stakeholders higher confidence in the dataset.

With grant funding from the LCRA, an additional Collison Floating Evaporation Pan will soon be installed on Lake Buchanan, enabling LCRA to compare data collected from this station to the buoy station data and the computed evaporation data from the reservoir-specific, daily evaporation dataset. With additional funding from the U.S. Bureau of Reclamation, a buoy station will also soon be installed on Sam Rayburn Reservoir.

"All of the work we have done is part of a concerted effort to better understand exactly how much water we have in the state and know how much will be needed in the future," said Dr. Fernando.

The TWDB will hold a stakeholder briefing on reservoir evaporation monitoring, estimation, and data validation efforts on November 4, 2024, from 1:00 p.m.-4:00 p.m. The meeting is open to the public through Microsoft Teams.

Zhao, B., Huntington, J., Pearson, C., Zhao, G., Ott, T., Zhu, J., Weinberg, A., Holman, K.D., Zhang, S., Anderson, R., Strickler, M., Cotter, J., Fernando, N., Nowak, K., and Gao, H., 2024. Developing a general Daily Lake Evaporation Model and demonstrating its application in the state of Texas. Water Resources Research, 60(3), p.e2023WR036181.

This article is posted in Water Planning/ Technology/ Weather/ Water Supply/ Water Data.

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