The impact of evaporation is more than meets the eye

In the summertime, it’s common to feel the heat beating down on you while outside, but have ever noticed that when people water their yard on a hot afternoon, it dissipates quickly? That’s because heat affects the process known as evaporation causing water to form into a gas regardless if it’s a lake, stream, river or a puddle of water.

The United States Geological Survey describes 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.”

Though it may seem elementary, understanding the water cycle is valuable to everyone living in Texas, as the summer heat can produce higher evaporation rates than any other time of year. That’s why most cities recommend watering your lawn in the morning before 10 a.m. when evaporation isn’t as high. Water availability is crucial for evaporation to occur, meaning a waterbody will have a higher evaporation rate than dry soil.

What factors influence the rate of evaporation? According to BRA Water Services Manager Aaron Abel, it’s not just heat. “Evaporation depends on a number of factors including wind speed, temperature, surface area of the waterbody and relative humidity.”

According to Abel, in 2017 the total water use from the BRA system of reservoirs was 252,987 acre-feet. In comparison, total evaporation from the system was measured at 494,061 acre-feet, nearly twice the amount of water used by cities, industry, agriculture and mining combined.

Knowing that evaporation nearly doubles total water usage, here are some other eye-opening facts you may not know:

• Only 10 percent of evaporated water from oceans that falls as precipitation transfers onto land.
• If humidity is 100 percent, no more water can evaporate due to the air already being loaded with water.
• Heavy storms can temporarily stop evaporation due to high air pressure pressing on bodies of water.
• Average annual lake evaporation rates across the BRA reservoir system range from 4 feet to 5 feet with rates as high as 6 feet per year in extreme drought years.
• Daily evaporation rates during very hot and dry periods can exceed ½ inch per day in some reservoirs within the BRA water supply system.
• A water molecule can spend up to 10 days in the air following evaporation.
• Evaporation is a major contributor to water loss for swimming pools in the summer.

It’s important to recognize how evaporation plays its role in the water cycle and affects fluctuating water levels at the BRA reservoirs. For more information on the water cycle, click here. For information on the Brazos River Authority’s water accounting summary, click here.