What does it take to fill a lake?

April 6, 2013

Showers this week brought the level up about 1/2 foot at Lake Granbury. But it’s a different kind of rain that’s needed to fill the lake, still over 5 feet low.

Five inches of heavy rain over the drainage area upstream from Granbury could do the trick.

“The drainage area upstream that produces runoff to Lake Granbury covers about 16,000 square miles and extends west to the area around Lubbock,” said Brad Brunet, water services manager for the Brazos River Authority.

“Five inches of intense rain over this entire area in a short period of time would produce enough runoff to completely refill the lake from empty many times over.

“However, if the five inches of rain is spread out over an extended period of time, say several weeks, there would be little runoff that would make it to the reservoir due to the exceptional drought conditions we are experiencing. Most of the rain would soak into the ground.”

The amount of rain needed to fill up Lake Granbury depends on a number of factors:

Where the rain falls.

How large of an area the rain covers.

How fast the rain falls.

How much gets captured in upstream lakes and ponds that are also low.

While there’s been rain over the last three months, it has not produced any significant inflow to either Possum Kingdom Lake or Lake Granbury, Brunet noted.

State Climatologist John Nielsen-Gammon recently said that with the exception of East Texas, the state is entering its third year of drought. Lake Granbury was reported to be 71 percent full at the end of March. With the summer months ahead, the climatologist estimates that reservoir levels could decline to a statewide average of 50 percent full by early September.


On Jan. 24, 2012, Lake Granbury was more than 4 feet low. Remarkably, a steady downpour, mostly on Jan. 25, 2012, produced enough runoff to completely refill the lake in less than two days.

A reported 3-5 inches of rain fell in less than 24 hours over a large portion of Lake Granbury’s drainage area upstream to Possum Kingdom.

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