Supplying water to people throughout the Brazos River basin is the role of the Brazos River Authority’s system of reservoirs, and the steady use of that water leads to lower water levels in our reservoirs. Water from BRA reservoirs is used by cities, industries, energy companies, agricultural producers, and mining operations; but, another major factor that affects water levels, especially during hot, dry and windy months, is simple evaporation. 

As part of the water cycle, evaporation is the process where water is transformed from liquid to vapor.  Evaporation has a significant impact on the Brazos River basin and is therefore closely monitored at both Brazos River Authority and Army Corps of Engineer reservoirs throughout the Brazos basin.

“In the summer, on a hot and windy day, the evaporation rate can be up to half an inch of water per day or more,” said Aaron Abel, senior water resources planner for the BRA. But, the evaporation process is active and affecting water levels year-round.

Evaporation routinely has a greater impact on reservoirs than everyday water supply use.  In 2016, total water use by BRA customers for municipal, industrial, agricultural and mining use was 226,348 acre-feet.

In contrast, evaporation took the equivalent of 535,326 acre-feet of water from basin reservoirs.

Although conditions have greatly improved in the Brazos River basin after the recent five-year drought ended, evaporation still has a significant impact.

“At Possum Kingdom (Lake) recently, we’ve had an evaporation rate of over 400 acre-feet (of water) per day,” Abel said. “For PK, that’s not an uncommon amount.”

Fortunately, this past summer water levels in system reservoirs have not fallen like during the drought years of 2011-2015, because precipitation has been greater over the last two years, helping to replenish basin reservoirs.

During the worst of the drought years, 2011, Abel noted that customers 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.

“That’s something that makes you say ‘wow!” Abel said. The evaporation rate for those three months was almost half of what the total water use rate was for the whole year.”

The United States Geological Survey or USGS notes that surface water is by far the largest contributor to water vapor in the atmosphere.

“Studies have shown that the oceans, seas, lakes and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration (the release of water from plant leaves),” according to the USGS.

The hotter the temperature, the more evaporation occurs. That explains why in the record drought from 2011-2015, water levels in the Brazos River basin reservoirs fell below what had been standard before and what continues to be standard after that time.

The Science Learning Hub notes that four main factors determine evaporation levels:

  • The size of a water body. “The greater the surface area, the higher the evaporation rate.”
  • Heat energy. The more heat, the faster water molecules vibrate. This results in water molecules near the surface being transformed into vapor and moving from the surface of the earth into the atmosphere.
  • Atmospheric pressure. Low-pressure conditions allow water molecules to rise into the atmosphere more easily. High pressure means the molecules are pushed down and are less likely to escape into the atmosphere.
  • Air movement or wind. Strong winds end up lifting water molecules from the earth’s surface, leading to increased levels of evaporation.

 “In the severe drought year of 2011, evaporation losses ranged from nearly three feet per year in east Texas to almost seven feet per year in west central and north-central Texas,” according to the Texas Living Water Project.  “Yes, that means that a lake in these more arid regions of Texas would have dropped nearly seven feet just due to evaporation, without any human water supply withdrawals.”

The drought of 2011 to early 2015 was followed by flooding in 2015 and 2016. Fortunately, conditions have been more balanced in 2017, but evaporation plays a significant role in water planning.

“The Texas Water Development Board has a repository of water data, and a lot of our modeling of water availability is based on that,” Abel said. “The evaporation rate is a key factor in accounting for our water resources.”

Fortunately, that rate is something the BRA closely monitors in helping to secure water resources for the future.

To learn more about evaporation go here.  To learn more about the effects evaporation has on the water resources available in the Brazos River Authority system, go here