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 among those who utilize it.
A prime example of this effort was the completion of a Lake Management Study conducted by the BRA just prior to the significant drought that occurred between 2011 and 2015.
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 BRA-owned and operated, serving as water supply reservoirs. Lake Whitney is a multi-purpose
(flood control, hydropower, and water supply) reservoir owned and operated by the US Army Corps of Engineers.The BRA
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 from Possum Kingdom Lake many decades 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 BRA began a plan for an “equal” 1:1 drawdown ratio between the two lakes. This meant water was
released from Possum Kingdom Lake to ensure Lake Granbury’s lake level was never lower than PK’s.
Unfortunately, over the following two years drought conditions caused the levels of both lakes to drop.
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 BRA hired the engineering firm Halff Associates in 2010 to assist with a study
to find a balanced approach 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 Lake.
It soon became clear that the “equal” 1:1 drawdown plan would not impact the lakes equally.
If the levels were down 5 feet at both lakes, roughly about 15 percent of the docks and other
features would be unusable at Possum Kingdom Lake, compared to more than 50 percent at Lake Granbury.
The study considered several ways of balancing the water needs for now and into the future.
After all the factors were weighed, the best way to ensure that both lakes are equally affected
during drought conditions was a 1.75:1 ratio. That means releases would be made as needed from
Possum Kingdom Lake so that for every 1 foot of drawdown experienced at Lake Granbury, a
corresponding 1.75 feet of drawdown would be experienced at Possum Kingdom Lake.
If Possum Kingdom Lake were to drop more than 7 feet below full, which historic models
show happens less than 10 percent of the time, the ratio would vary between 1:1 and 1.75:1
depending on water use levels.
The study concluded that with this method, Lake Granbury would be about 0.3 feet higher on average than historical conditions and between about 0.75 feet and 1.7 feet higher
than the “equal” 1:1 drawdown during drought conditions. At the same time, Possum Kingdom would be more than 2 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 obtain public feedback. On April 18, 2011, staff presented the proposal and public
comment to the BRA’s Board of Directors, which voted unanimously 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
plan in place, the BRA has been 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