Evaporation often consumes twice as much water as the total used by cities, industry, agriculture and mining concerns from the Brazos River Authority system of water supply reservoirs.
The fact is often staggering to those hearing it for the first time.
The loss of more than 470,000 acre-feet of water in 2018 was due to evaporation, compared to the roughly 284,000 acre-feet of water used by municipalities, industrial, irrigation, and mining combined, said BRA Water Services Manager Aaron Abel. Similarly, in 2017 the total water use from the BRA system of reservoirs was 252,987 acre-feet, while total evaporation from the system was measured at 494,061 acre-feet.
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.
Evaporation, the process of water changing from a liquid to a gas, is due to more than just the heat.
While increasing Texas summer temperatures can be a major cause to the amount of evaporation occurring across the reservoirs, high winds contribute to the decrease in water levels caused by evaporation, Able said. The bigger the size of the surface of the body of water also means the larger the amount of evaporation to occur.
“Most people are surprised when we tell them our biggest user of water is evaporation,” Abel said.
The topic often arises when the BRA is asked why one of our reservoirs does is not kept at a constant level.
But there are few “constant level lakes,” in Texas and none within the BRA System. A constant level lake is artificially managed to remain at a specific level via using another source of water. The BRA reservoirs were built for water supply. Since the reservoirs are used as a water supply source, the water levels will fluctuate based on water needs and climate conditions.
The Brazos River Authority water supply reservoirs includes three BRA reservoirs, Lakes Possum Kingdom, Granbury and Limestone and eight US Army Corps of Engineers reservoirs under contract to store water including Lakes Aquilla, Belton, Georgetown, Granger, Proctor, Somerville, Whitney and Stillhouse Hollow Lake.
Interestingly enough, on a humid day, not much evaporation occurs, Abel said.
If it’s raining outside, it is difficult for more water to enter an already saturated environment, he said.
Evaporation is a constant, natural part of the water cycle.
Essentially, the water cycle is the motion of the water from the ground to the atmosphere and back again. Of the many processes involved in the hydrologic cycle, the most important are evaporation, transpiration, condensation, precipitation and runoff, according to the National Weather Service.
For more on the amounts of water supply consumed by evaporation and use from the BRA System of reservoirs, see the Brazos River Authority’s water accounting summary here.