Water -- it seems like such a simple thing – a few molecules clumped together making a clear liquid that can be found
just about anywhere on Earth. But when it comes to fresh water, there is only so much to go around to meet the needs of the world’s growing population.
Such is the case in the Brazos River basin, where communities, lakeside homeowners, industry, farmers, ranchers and
weekend recreationists all rely on a steady supply of this limited resource.
Balancing those needs is an important part of the Brazos River Authority’s mission. While it is not possible to make
sure everyone has an unlimited supply of water all the time, we strive to find ways to make sure the available water is shared fairly among those who need it.
A prime example of this effort was the completion of a Lake Management Study recently conducted by the Authority.
Three reservoirs located in the upper region of the Brazos River basin were included in the study; lakes Possum Kingdom (PK), Granbury and Whitney.
Lakes PK and Granbury are Authority-owned and operated, serving as water supply reservoirs. Lake Whitney is a flood
control reservoir owned and operated by the US Army Corps of Engineers.The Authority contracts with the Corps for a relatively small amount of storage
in Lake Whitney for water supply.
The study focused on the sometimes competing demands between Lake Granbury and Possum Kingdom for water
supply and recreation.
The study has its roots in 2007. That year, the BRA stopped
producing hydroelectric power at Possum Kingdom Lake’s Morris Sheppard Dam. Up to that point, water from Possum Kingdom was released downstream regularly to Lake Granbury
as part of the electric generation process.
Hydropower had been generated since before Lake Granbury was built in 1969. As a result, lakeside homeowners and
others who enjoy recreation in the area began to rely on the reservoir being near full most of the time.
As a short term solution, the Authority began a 1:1 drawdown ratio between the two lakes. This meant water was
released from PK to ensure Lake Granbury’s lake level was never lower than PK’s.
Unfortunately, over the following two years a drought caused the levels of both lakes to drop. About
the same time, Luminant Energy announced plans to build two more reactors at its Comanche Peak Nuclear Power Plant, which would divert additional water from Lake Granbury.
Lakefront homeowners and others who use the lake regularly began to be concerned about lower lake levels’ impact on their access to the lake and on property values.
As a result, the Authority hired the engineering firm Halff Associates in 2010 to assist with a study
to find the fairest possible way to distribute the water between the two lakes while continuing to meet water supply needs. Over the next several months, crews surveyed both lakes, taking note of the elevation above
lake bottom for about 3,500 docks, ramps and marinas at Lake Granbury and more than 1,600 at Possum Kingdom.
It soon became clear that the “equal” 1:1 drawdown plan would not impact the lakes equally. Possum Kingdom is a much deeper
lake than Granbury. If the levels were down 5 feet at both lakes, roughly about 15 percent of the docks and other features would be unusable at PK, compared to more than 50
percent at Granbury.
The study considered several ways of balancing the water needs for now and into the future, accounting for the possibility
of drought and additional water needs to expand Comanche Peak. After all the factors were weighed, it appears the best way to ensure that both lakes are equally affected by
dropping water levels is to regulate the lakes based on zones of lake depths.
Under this method, when Possum Kingdom’s lake level is above 992 feet mean sea level (msl), the levels between
PK and Granbury will be maintained at a 1.75:1 ratio. That means for every foot Granbury goes down, water will be released from PK until the drawdown ratio of 1.75:1
is achieved. Below that level, which historic models shows happens about 10 percent of the time, the ratio will vary between 1:1 and 1.75:1 depending on water use levels.
The study concluded that with the zonal method, accounting for additional demand for Comanche Peak, Granbury would be
nearly a half-foot higher on average than under equal drawdown and between 1.5 and 2.5 feet higher than equal drawdown during drought. At the same time, Possum Kingdom would
be more than 1.4 feet higher on average than historic levels.
As a result of the study, Brazos River Authority staff proposed adoption of the zonal method to balance lake levels.
In early 2011, officials held meetings in Granbury and Graford to present the proposal and to hear public feedback. On April 18, staff presented the proposal and public
comment to the Authority’s Board of Directors, which voted unaminously to move forward with the zonal method.
With a finite resource such as water, completely meeting everyone’s needs is rarely, if ever, possible. However, with this
new plan in place, the BRA will be able to more fairly serve the interests of all who rely on the water at both lakes into the foreseeable future.
For more information about the study, please click