Up, Up and Away

Up, Up and Away

Like a thief in the heat, evaporation is one of the Brazos River Authority’s biggest water clients.

It doesn’t matter if it’s in a lake, stream, river, puddle or a glass of water sitting out on your porch — evaporation affects our water supply, especially during the summer months as the summer heat can produce higher evaporation rates than other times of the year.

Bryan Schilder,Possum...
Possum Kingdom Lake photo courtesy of Bryan Schilder

Heat triggers the water cycle process known as evaporation, which causes water to form into a gas. The United States Geological Survey defines evaporation as “the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor.” 

“When we get to the middle of summer, that’s when we have our highest evaporative losses,” BRA Water Services Manager Aaron Abel said. “For the larger reservoirs, like Possum Kingdom Lake, we can lose hundreds of acre-feet of water in one day to evaporation. If we have prolonged periods of higher temperatures, lower humidities, and higher winds we’ll see higher evaporative losses.” 

Evaporation depends on several factors other than heat, however. Wind speed, temperature, surface area of the waterbody and relative humidity all impact how much water evaporates. With all these factors combined, evaporation is one of the reasons why none of the Brazos River Authority’s reservoirs are considered a “constant level lake.” 

A constant level lake is one that is artificially managed to remain at a specific level by using another source of water to replenish losses due to evaporation or water use. While several factors can affect a lake’s level, some within human control, evaporation is a constant, natural part of the water cycle that uses nearly twice the amount of the BRA’s annual water use.

Photo courtesy of Laird Rixford

In 2020, the total water use from the BRA system of reservoirs was 288,968 acre-feet. In comparison, the total evaporation loss from the system was measured at 540,244 acre-feet, nearly twice the amount of water used by cities, industry, agriculture and mining combined. One acre-foot is equivalent to over 325,000 gallons of water.

As temperatures rise, increased demand by residents, agriculture, cities, industry, power plants and others on water stored can also draw down a lake’s level. This impact is made worse during periods of extended drought when water entering reservoirs is lowest. 

However, the seasonal outlook for this summer shows ENSO-neutral conditions, which calls for Texas having equal chances of above, near, or below-normal temperatures for July through September, with West Texas likely receiving above-average temperatures. The seasonal outlook also shows that, except for West Texas, most of the state should receive above-normal rainfall during these months as well. With equal chances of above and below-average temperatures, along with forecasted above-normal rainfall, the effects of evaporation may be mitigated this summer.

Even when there is plenty of water stored in the BRA’s water supply system, evaporation still occurs. 

“Another thing that many people may not consider is that higher lake levels mean that the water will cover more surface area, which means the sun has more water to act on,” Abel said. “That results in the possibility for more evaporative losses as well.” 

Evaporation cannot be prevented, but there are some ways that you can alleviate its effects during your everyday chores. When not in use, place a pool cover over your pool to reduce evaporation loss. When watering your plants during the summer, try watering when the sun is down, as it can greatly reduce the amount of water that is lost to evaporation. The prime time to water is during the very early morning hours before the sun rises.

Though it can be frustrating for those who live near Texas lakes or turn to them for recreation, fluctuating lake levels are a sign that reservoirs are successfully supplying water to thirsty Texans during times of need and that the water cycle is still turning its wheels. 

For information on the Brazos River Authority’s water accounting summary, click here.