When water seemingly vanishes

When water seemingly vanishes

Remember in elementary school when the teacher first explained the water cycle? An image included arrows pointing from the sky to the ground, and then, from the ground to the sky. 

Maybe that was your first time learning that evaporation was the act of water changing from a liquid to a gas or vapor. And maybe that was the last time you put much thought into the matter. 


For many Brazos River Authority employees, monitoring the effects of evaporation is a daily practice. Those effects are highest during the summer months as temperatures soar across the Lone Star State.

Several things affect the amount of evaporation that occurs in a body of water, including wind speed, temperature, surface area of the waterbody and relative humidity. High evaporation rates during summer months combined with higher-than-normal water usage by those in the Brazos River basin, causes the amount of water supply stored in our lakes to drop, resulting in lower lake levels. 

Without rain to replenish the water supply reservoirs, conservation and tracking are vital to ensuring everyone has the water they need when they need it.

Evaporation plays such a big role for BRA hydrologists because it’s the biggest “user” of water throughout the Brazos River basin.

In 2019, 458,071 acre-feet of water was lost due to evaporation. Comparatively, 264,454 acre-feet of water was used by cities, industries, agriculture, and mining combined.

An acre-foot is the amount of water needed to cover one acre, 43,560 square feet, with one foot of water, or 325,851 gallons of water.

These numbers account for water across the entire BRA water supply system. The system includes the three BRA reservoirs, Lakes Possum Kingdom, Granbury and Limestone, and eight US Army Corps of Engineers reservoirs under contract with the federal government to store water, including Lakes Aquilla, Belton, Georgetown, Granger, Proctor, Somerville, Whitney and Stillhouse Hollow Lake.


The drought of the 1950s taught Texans a hard lesson that not having water stored when it is needed can be disastrous. The state had not seen dry conditions like the 1950s until 2011, and the drought continued to early 2015.

In 2011, those in the Brazos River basin used about 488,000 acre-feet of water for the entire year. The evaporation rate for the months of June through August alone in that year was 220,000 area feet.

Water conservation and smart usage are important year-round as Texas never knows when the next drought will start or when the skyrocketing temperatures of summers last longer than usual.

The total amount of acre-feet evaporation took each year was:

•    2019: 458,071
•    2018: 470,120
•    2017: 494,061
•    2016: 535,326
•    2015: 479,294
•    2014: 411,633

For more information on evaporation and why lake levels fluctuate, go here.