Drought impacting more Brazos River reservoirs

Drought impacting more Brazos River reservoirs

We may officially have entered fall, but continued high temperatures, water use, and evaporation rates, combined with the lack of sufficient rain, equates to a basin desperately hoping for rain.

For the past several months, low flows in the Brazos River have prompted the need to release water from several Brazos River Authority System reservoirs to meet water supply needs in the lower Brazos River basin. But as the supply in these reservoirs decline, hydrologists are challenged by balancing local needs with areas lacking adequate supply.

The next step is to look north, toward Possum Kingdom Lake and Lake Granbury. The two reservoirs, each currently storing more than 85 percent of their water supply capacity, are next in the line of reservoirs due to release water for downstream water supply needs if drought conditions don't significantly improve.

Where it started

All surface water in Texas is owned by the state and held in trust for use by its' citizens. The state tasked the BRA with managing a specific amount of that water and making it available to municipal, industrial, mining, irrigation, and agricultural interests for beneficial use throughout the Brazos River basin through water supply contracts.

There are 11 reservoirs in the Brazos River Authority water supply system that work together to meet the water needs of the basin. These lakes are spread across a watershed that declines in elevation from where the Brazos River begins in the Upper basin to the Gulf of Mexico in the lower basin, declining at about three feet to one-half foot per mile as the river flows downstream.  

The concept behind a water supply reservoir is to capture and store water during wet times for use, especially during periods of drought, when rainfall and river flows are down. 

Though many consider the reservoir located near their home as belonging to their city, county or region, the water stored in the 11 reservoirs permitted to the Brazos River Authority function as a "system" or a series of linked storage facilities with the capability of supplying water both locally and to other areas within the system via release into a river or tributary or in some cases via pipeline.   

Each occurrence of drought is unique. Due to the current drought, decreased streamflow along the mainstem of the Brazos River below the reservoirs has caused natural flows within the river to drop to a point that required releases of water stored in the BRA reservoir system.  Since June 6, the BRA has been releasing water from several system reservoirs to meet needs in along the lower Brazos River basin, including lakes Somerville, Whitney, Limestone and, most recently, Lake Belton, BRA Senior Hydrologist Chris Higgins explained. 

The basin did experience several significant rain events in August that generated enough runoff to pause water supply releases for about three weeks. 

But the drought was not dampened enough, and as streamflow ran low, those who needed the water couldn't access it, and water supply releases from reservoirs resumed Sept. 14.

Understanding the process

During times of drought, BRA hydrologists analyze, in part, the system as a whole, considering local water supply needs, available storage in each reservoir, constraints in water rights, and multiple other factors to determine which reservoir has enough water to allow a stored water release to occur, said Aaron Abel, BRA water services manager. He explained that the goal is not to empty a lake and then move to the next one. Instead, hydrologists work to use the water stored in the lakes as evenly and efficiently as possible to balance the drought impacts across the basin.

Also, water released from reservoirs closer to the area needing additional water supply will lose less water during transport – whether that's from evaporation, water seeping into the river bed, or vegetation absorbing the water, Higgins said. Water released from Possum Kingdom Lake, for instance, can take up to 21 days to reach the lower portion of the Brazos River basin – leaving a lot of time for water loss. Possum Kingdom Lake is also the reservoir furthest north, meaning there's no other lake to refill it.

Higgins said that at this point in the drought, the BRA must use resources from other parts of the basin to operate the water supply system effectively and efficiently as possible. Last week, the release from Lake Somerville was stopped and replaced with a release from Lake Belton. 

The current plan is to release about 18,000 acre-feet of water from Lake Belton through mid-October before the release is changed to another reservoir within the basin, Higgins said. That may equate to a 3-foot drop in the lake. Without the downstream release, the lake was predicted to have dropped about 1.2 feet during that period between September 22nd and mid-October, Higgins said.

"If dry conditions persist and water supply releases are still needed once we're finished with Lake Belton, we'll transfer our release to the mainstem system (Lakes Possum Kingdom and Granbury) in the upper basin," Higgins said. "Of course, all of this is contingent on a number of factors including any weather variations we may experience over the next several weeks."

The BRA water supply system could still be in drought conditions come spring, so it's important to focus on all water conservation efforts for all reservoirs within the system: Possum Kingdom, Granbury, Limestone, Whitney, Aquilla, Belton, Stillhouse Hollow, Georgetown, Granger, Somerville, and Proctor.

"There are an infinite number of ways that the system could be managed," Higgins said. "However, we manage the system in the most efficient manner possible to ensure we meet the long-term water supply needs in the basin while balancing the stresses on the system cause by continued drought conditions.  With all that said, our ability to effectively manage such a complex system of reservoirs in part comes from decades of combined knowledge passed down from the professionals that came before us." 

There are only two ways to fill up a reservoir – it either rains right on top of the lake, or water enters from runoff or releases upstream.

October is typically one of the two wettest months for the area, so Mother Nature could turn on her healing powers and send floodwaters, throwing out all release plans.

Coming up

Water supply releases have not been made from the Upper Basin mainstem system since 2011. Based on current forecasts, Higgins said that water supply needs in the lower basin could require a release from Possum Kingdom Lake that would draw the lake down by about 2 feet during October. The water supply release would account for almost 1 foot of the decline, and the remaining foot would be a result of meeting local water supply needs and evaporation rates during that time as well as maintaining the balance between Possum Kingdom and Granbury in accordance with the BRA PK-Granbury-Whitney Water Management Study finalized in 2011. The levels in Lake Granbury will also be affected by downstream water supply needs. With PK's forecast decline of two feet, Lake Granbury would drop about 1.4 feet, with about 0.6 feet of that decline attributed to the water supply release and 0.8 feet to local use and evaporation. 

When the weather is kind to us and the lakes are full, they are great spots to go and have fun, enjoying recreation. But recreation has always been a secondary benefit to the reservoirs' intended purpose. 

It wasn't until 1941 – merely 81 years ago – that these lakes existed at all. Each human-made reservoir was constructed with a goal, to either serve as a water supply source, to help with flood control, or to cool the power plants that allow us to turn on the lights each day, 

Because of that, the water levels of the 11 reservoirs within the Brazos River Authority water supply system will always fluctuate as their supplies are used.

In the future, the proposed Allens Creek Reservoir will help alleviate some of the stress on the upper basin reservoirs during drought by meeting the needs of the lower basin. The BRA recently purchased full ownership of the long-planned reservoir, which will provide an estimated 100,000 acre-feet of additional water supply for the basin. The reservoir is planned to be "off-channel," meaning it will be built near the Brazos River on Allens Creek, a tributary of the Brazos, near the cities of Sealy and Wallis in Austin County.

The BRA has a Drought Contingency Plan that is designed to work, with efforts by other state and local agencies, to manage water during times of scarcity. To view the BRA's Drought Contingency Plan, go here

And if you'd like to watch our hydrologists speak more about the drought, go to our YouTube page here.