Direct Measurement of Lake Evaporation—A Texas Experiment

Direct measurement of lake evaporation – A Texas experiment

Andrew Weinberg, Michael Vielleux, Yujuin Yang

Evaporation is an important component of the hydrological cycle and for water planning in Texas. Great effort has been made to estimate the evaporation from 188 major lakes in Texas. In the past, that estimation has solely relied on the evaporation data collected from on-shore evaporation pans. Lake evaporation is estimated from pan evaporation using pan-to-lake coefficients from the National Weather Services. The pan-to-lake coefficients are estimated from the ratio of pan evaporation and the estimated free water surface evaporation, which is determined either by meteorological methods or by detailed water budget analysis.

Two issues may affect the accuracy of lake surface evaporation estimates made using the traditional on-shore pan measurements. First, the pan evaporation is not always measured under the same environmental conditions as the lake; water body and water surface temperatures and climate conditions (rain, winds, solar, etc.) vary between the lake and on-shore measurement locations. Second, the free water surface evaporation itself is an approximation of lake surface evaporation under real-world conditions. To add another complexity, the pan data can be affected by wild animal drinking water from the pan.

In 2018, Texas Water Development Board, together with the Brazos River Authority, launched a pilot evaporation monitoring project on Lake Limestone to simultaneously collect data from an on-shore Class A pan, an off-shore floating Class A pan, on-shore and off-shore meteorological stations, and an off-shore eddy covariance system. The purposes of this pilot project are (1) to collect or estimate lake evaporation data under true water body and water surface conditions and (2) to ground-truth the pan-to-lake coefficients. Class A pans on- and off-shore will directly measure the evaporation rate, the weather stations will collect climate data for estimating free-water evaporation using the Penmen Montieth equation, and the eddy covariance system will collect water vapor flux data.

This paper discusses the design of the measurement systems that TWDB staff built for the floating pan and eddy covariance device, the data collection, processing, and storage systems, and the installation and maintenance of the system on Lake Limestone. We will publish the data and analysis of the data collected in this effort once the data collection and data analysis are completed.