Enhancing Reservoir Evaporative Loss Estimates: a Multi-Pronged Approach to Monitor Surface Water Evaporation in Texas

Evaporative water loss from reservoirs in Texas can exceed total municipal water use during extreme drought years, such as 2011. Accurate estimates of reservoir evaporation are important for determining water availability, which has far-reaching implications for not only the municipal, agricultural, and industrial sectors of Texas but also for national security, given the water-reliant petroleum mining and production facilities located in Texas that accounted for ~40 percent of total U.S. crude oil production in 2018. The existing operational practice, primarily using Class A evaporation pans, for observing or estimating surface water evaporation in Texas, has large uncertainty and many areas of Texas do not have evaporation observations. Such gaps in observations result in large uncertainties in water availability estimates, which in turn have implications for water rights permitting, water planning, and drought contingency planning. Increasing evaporation observations and improving the current operational practice of using pan evaporation rates to infer reservoir evaporation rates will help the state improve the evaporation dataset used for water permitting and drought planning.

We present an approach to enhance surface water evaporation monitoring in Texas using state-of-the-art technology. The multi-pronged approach, led by the Texas Water Development Board (TWDB), is a collaborative effort between various water stakeholders in Texas, including the U.S. Army Corps of Engineers (South West Division – Fort Worth District), river authorities, and a number of reservoir owners. The project builds upon on-going evaporation monitoring research that TWDB is conducting at Lake Limestone, in the Brazos River basin. The approach will enable us to improve existing information on how pan evaporation rates can be adjusted to represent true lake evaporation, ensure more areas of the state are covered by the evaporation observation network, and enable us to provide more accurate estimates of reservoir evaporation loss. The improved reservoir evaporation dataset will be used to provide daily estimates of evaporative water loss at all monitored reservoirs in Texas via the reservoirs page on Water Data for Texas (https://waterdatafortexas.org/reservoirs/statewide).