Better data, better water planning: Deploying a Collison floating evaporation pan on Lake Buchanan

Better data, better water planning: Deploying a Collison floating evaporation pan on Lake Buchanan Posted on June 23, 2026

Evaporation is a silent tax on Texas reservoirs. It doesn't make headlines, but it still reduces our water supply year-round, day and night. Most water planners estimate evaporation losses from reservoirs using land-based pans. These estimates are useful, but when it comes to water supply planning, the Texas Water Development Board (TWDB) is always looking for higher-accuracy and higher-resolution data.

And accurate tracking of reservoir evaporative losses is increasingly important as our growing state frequently experiences severe droughts. Standard methods for estimating evaporation on reservoirs can be 20 to 40 percent different from actual evaporation rates. Reducing this uncertainty is critical to giving water-resource managers a better understanding of current and future water supplies and enable better water supply management decisions.

Enter the Collison floating evaporation pan-monitoring technology that the TWDB, in partnership with the Lower Colorado River Authority (LCRA), is deploying on Lake Buchanan. Lake Buchanan was built by the LCRA to help manage flooding along the Colorado River, act as municipal water supply, generate hydropower, and came online in 1939. It was the first of the Texas Highland Lakes to be formed, and with over 22,000 acres of surface water, Lake Buchanan is also the largest of the Highland Lakes.

How the Collison floating evaporation pan works

A Collison floating pan floats directly on the reservoir and consists of a shallow, cylindrical metal vessel, typically around four feet in diameter. It sits inside a larger floating ring or raft that keeps it level as the lake rises and falls. Water from the reservoir is pumped into the pan continuously, maintaining the same water level inside and out.

Traditionally, evaporation data comes from Class A evaporation pans located on land near the reservoir, though evaporation measured from such pans is usually significantly higher than the amount of evaporation from a reservoir. Why? Because a Class A pan heats and cools differently than the large body of water it's next to. Land pans typically see less wind fetch and more radiative interference from ground heat. That's why water planners apply monthly reduction factors to pan evaporation estimates. The factors, however, were developed using historical data and are not based on current measurements of conditions at the reservoir location-such as wind speed and direction, water and air temperature, sunlight and humidity.

The secret to the Collison floating evaporation pan's accuracy involves coupling the greater reservoir with the water in the pan. Because the pan water and lake water share the same temperature and wave action, you eliminate the "land bias" present in regular evaporation pans-no hot soil underneath, no sheltered wind shadow.

The Collison pan's specific innovation is its patented self-trimming buoyancy and a wave-dampening design. This design is particularly elegant because it self-levels, reduces slosh, and maintains a consistent fetch regardless of reservoir elevation. Combine that with a precision gauge that measures water loss at daily or sub-daily intervals and you get a more precise evaporation rate measurement.

The Collison pan systems are built and installed by Agua del Sol Consultants in New Mexico, and the design is refined with each new system that is built and deployed on reservoirs in the southwestern United States-the pan deployed on Lake Buchanan is the fourth version of this technology.

Why evaporation data is critical to water planning

It's critical to track evaporation from reservoirs because that loss is one of the biggest water "uses" in Texas. How big? In 2011, evaporative loss from Texas reservoirs was greater than the total municipal water demand in the state for that year. Generally, evaporation data can also be useful to reservoir operators in their day-to-day operations because it not only provides the daily water balance of the reservoir but also allows them to calculate the actual inflows of water.

To keep track of evaporative losses, the TWDB gets daily estimates from each of the major water supply reservoirs across the state. This includes data from Lake Buchanan, which was a new model-based reservoir evaporation dataset released in 2023. The data collected from this new Collison floating evaporation pan will be used for quality control and validation of TWDB reservoir-specific data sets-which is required by the Texas Commission on Environmental Quality if the new model-based dataset is to be incorporated in the water availability models used for water rights permitting and long range water planning. In turn, these data sets improve the TWDB water availability models used by regional water planning groups across the state.

According to Dr. Nelun Fernando, Manager of the Water Availability and TexMesonet Departments at the TWDB, "there are two main input parameters from hydrometeorology that go into calculating water availability in the surface water availability models-naturalized flow and net evaporation. Before the Collison pan, evaporation rate was determined using land-based (Class A) evaporation pans, the measurements from which have much uncertainty. That's why we (the TWDB) have invested heavily in recent years to improve the accuracy and resolution of evaporation data through new technology like the Collison system."

No silver bullet, just better information

The Collison floating evaporation pan on Lake Buchanan won't create a single new drop of water, but it will give us a clearer understanding of a major water supply and ensure that water planners are working with the most accurate data available-and better water planning means a more secure water future for Texas.

This article is posted in Water Planning / Technology / Water Supply / Reservoirs .