Additional reservoirs to begin making downstream releases for water supply

DeCordova Bend Dam, June 2022
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A lack of sufficient rain combined with continued use of water supply, equates to a basin desperately hoping for rain and lakes continuing to dip.

For the past several months, the Brazos River hasn’t had much streamflow. So, to meet water supply needs in the lower basin, releases are being made from several Brazos River Authority reservoirs. As the supply in these reservoirs declines, BRA hydrologists are challenged to balance local needs with areas lacking adequate supply.

Since June 6, the BRA has been releasing water from several system reservoirs including Lakes Somerville, Limestone, Whitney and Belton to meet downstream water supply needs. Possum Kingdom Lake and Lake Granbury – which are in the northern portion of the basin – are each currently storing more than 85 percent of their water supply capacity. They are now next in the line 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 BRA water supply system that together work to meet the basin’s water needs. The concept behind a water supply reservoir is to capture and store water when it’s raining for use when rainfall and river flows are down. This becomes increasingly important during droughts.

Some people think their nearby lake belongs to their city or county. However, water stored in these 11 reservoirs is permitted to the Brazos River Authority to function as a "system" or series of linked storage facilities. These reservoirs can then supply water locally and to other areas within the system via release into a river or tributary, or in some cases, via pipeline.

After the above-normal rainfall in late August to early September, some locations within the basin and in Texas have not seen measurable rainfall in over 30 days, said Aaron Abel, BRA water services manager.

Overall, the BRA water supply system is approximately 73 percent full, and the system continues to operate as intended. The reservoirs throughout the system captured the needed water during the wetter times, primarily last year, to be used during this dry year and has successfully met the needs of our customers throughout the basin.

Due to the current drought, decreased streamflow along the mainstem of the Brazos River below the reservoirs has caused natural flows to drop to a point that requires releases of water stored in the BRA reservoir system.

But didn’t it rain in August? There were some significant rain events that generated runoff, allowing the BRA to pause water supply releases for about three weeks.

But then the drought picked up where it left off. 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

BRA hydrologists analyze 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, Abel said.

The goal is not to empty a lake and then move to the next one, Abel said. Instead, hydrologists work to use the stored water as evenly and efficiently as possible to balance drought impacts across the basin.

Hydrologists also take water loss into consideration. Reservoirs closer to the area needing additional water supply will lose less water during transport – whether that's from evaporation, water seeping into the riverbed, or vegetation absorbing the water, BRA Senior Hydrologist Chris Higgins explained.

For instance, water released from Possum Kingdom Lake can take up to 21 days to reach the lower part of the basin. Three weeks is a lot of time for water loss. PK Lake is also the reservoir furthest north, meaning there's no other lake to refill it.

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, Higgins said. Recently, 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, 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, we'll transfer our release to the mainstem system (Lakes Possum Kingdom, Whitney 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 water conservation efforts for all reservoirs within the system: Possum Kingdom, Granbury, Limestone, Whitney, Aquilla, Belton, Stillhouse Hollow, Georgetown, Granger, Somerville, and Proctor.

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 feet of the decline, and the remaining foot would be a result of meeting local water supply needs, evaporation rates during that time and maintaining the balance between Possum Kingdom and Granbury in accordance with the BRA PK-Granbury-Whitney Water Management Study finalized in 2011.

This water will need to travel downstream, enter Lake Granbury, and then be released from Lake Granbury to continue traveling downstream.

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.

If you visited a lake during the drought of 2010-2014, you know our current situation is not as dire as during that period. PK and Lake Granbury, though low, still have more than 85 percent of water stored behind their dams.

Managing a system

When the weather is kind to us and the lakes are full, they are great for enjoying recreation. But recreation has always been a secondary benefit to its intended purpose.

It was only 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, help with flood control, or 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 lake would provide an estimated 100,000 acre-feet of additional water supply. 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, in Austin County.

"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 caused 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.

“The BRA has continued to coordinate closely with the Texas Commission on Environmental Quality (TCEQ) Brazos Watermaster Program and our customers to ensure that the needs of the basin are satisfied through the release of stored water from the BRA water supply system,” Abel said.

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.