Thanks to recent rainfall, the City of Abilene has successfully scalped 1 billion gallons of water from the Clear Fork of the Brazos River to bolster its water supply in Lake Fort Phantom Hill.
For perspective, a typical Olympic-sized swimming pool contains about a million gallons of water, said Rodney Taylor, Director of Water Utilities.
"So when you’re thinking of a billion (gallons), you’re thinking of 1,000 swimming pools," he said.
The city’s largest elevated storage tower holds 2 million gallons, while Abilene’s average daily consumption is about 22 million gallons per day.
"So, in a scenario where you pump over a billion gallons, you’re capturing well over a month’s supply of water," Taylor said.
The extra supply will help the city meet its municipal and industrial needs. Currently, the lake is just shy of 79% full, thanks to a combination of several recent diversion opportunities and the natural effects of rainfall.
Scalping involves using large pumps to divert and capture water from a flowing river to an adjacent lake at a higher elevation.
Water diversion is not an everyday occurrence and relies on specific parameters, including river flow, rainfall, and overall reservoir capacity, aligning perfectly, Taylor said. Other factors, including the cost of operating the pumps that make the process possible, must also be taken into account.
The City has a permit with the Texas Commission on Environmental Quality, allowing it to divert water using a gated structure built across the river’s channel.
"When stream conditions are right, we can close gates and ‘impound’ water – we can stack it up behind the gates," Taylor said.
This process is necessary to create a pool of water that allows the City’s pumps to operate.
Under ideal conditions, the City can transfer up to 30,000 acre-feet of water annually, which is a little less than half of the lake’s capacity, Taylor said. An acre-foot is about 326,000 gallons of water.
"And then, we could take out about 25,000 acre-feet, our water right permit for yield out of the lake, for municipal and industrial purposes," Taylor said.
Typically, around 600 acre-feet of water must pass through a downstream measuring station within a 24-hour period before the gates can be closed and water impounded. Higher flow rates allow for diversion to begin sooner.
The City’s original pump station on West Lake Road consists of three 50 million gallon per day capacity pumps. Five newer pumps offer an additional capacity of 100 million gallons per day. Each pump can operate independently.
"Depending on the river flow, we can operate a combination of pumps to extract the available water," Taylor said. "Our pump capacity is designed to be just within the limits of the quantity that we can pump at any given time."
The City has other water diversion opportunities, Taylor noted.
Dead Man Creek, for example, can at times spill over into a channel excavated many years ago that also flows into the lake, no pumps required.
Even in the best-case scenario, one shouldn't expect to stand on the lake’s shores and see it fill because of scalping, Taylor said.
Water would still need to be released even if all the pumps are engaged "because the river has more capacity than the city has authorized capacity to pump," he said.
Evaporation also plays a role in how much scalped water eventually makes it to taps in Abilene and surrounding areas. The hot and dry climate of West Texas, combined with high evaporation rates and wind exposure, leads to significant water loss.
"On a hot day, even when we’re pumping a lot of water out of the lake, we’re losing more water to evaporation than we are from our maximum pumping rate,” he said. “Molecules are just flying off the surface of that water."
The city loses approximately 58-60 inches of water per year due to evaporation alone, Taylor said.
It’s also possible, Taylor said, that an abundance of rain could cause the lake itself to spill over, meaning some previously scalped water could be lost.
"But it’s still water that we would not have otherwise had access to" – and if conditions are right, a great way to supplement the City’s supply, he said.