Rain offers relief, but how big is the impact on reservoirs – and flooding?

Rain offers relief, but how big is the impact on reservoirs – and flooding?

Much of the Brazos River basin has received a good deal of rain, helping to relieve a region that had been parched by a long summer of hot weather and low precipitation, but bringing flooding to other areas. So how big of an impact did the rainfall have on the reservoirs throughout the basin, and what can we expect during the next few months of fall?

The first thing to keep in mind is that dry conditions had depleted moisture in the soil, meaning much of the rainfall received – instead of immediately flowing into the Brazos River, its tributaries and reservoirs throughout the basin – was absorbed by the ground. That’s helpful because the soil had suffered from dehydration due to the lingering drought. But it also means that a significant portion of the rainfall did not initially flow into streams, rivers and reservoirs. Since the soil was able to be replenished by precipitation, future rainfall should be even more beneficial for river and reservoir levels.

A report from the West Gulf River Forecast Center, a division of the National Weather Service, has announced that because of the soaking rains received in early September, the soil is now saturated. As a result, the ground will not be able to absorb much more moisture, so any continuing rainfall should flow into creeks, streams and rivers.

As is so often the case in Texas, when conditions change they swing from one extreme to the other, from drought to flooding. Parts of the Gulf Coast, including Brazoria and Fort Bend counties, were bracing for possible flood conditions due to tropical storms in the Gulf of Mexico.

The good news is that the rainfall has begun to make a positive impact on reservoirs throughout the basin. The bad news is that any significant amount of rainfall – or potentially even moderate precipitation – could cause flooding to occur more quickly, the West Gulf Forecast Center notes.

“The upper zones of the soil profiles are at or near saturation levels,” the forecast center reports. “In this condition, rivers and tributaries are more prone to flooding due to the increase in surface runoff. With rains continuing to impact south and southeast Texas, the threat of additional flooding continues … in areas of excessive rainfall.”

After the heaviest rains had fallen through most of the basin, Possum Kingdom Lake was 94 percent full (2.2 feet below maximum capacity), Lake Granbury was 95 percent full (0.9 feet below capacity) and Lake Limestone, which received less rainfall, was 76 percent full (4.2 feet below capacity).

The reservoirs in the Brazos basin that remained at the lowest levels compared to maximum capacity were Lake Proctor and Lake Georgetown, each of which were 55 percent full. Lake Proctor was 6.3 feet low, while Lake Georgetown was 15.5 feet low.

If normal precipitation and evaporation rates continue through the end of October, the BRA’s Water Services Department estimates that PK will remain 94 percent full, Lake Granbury is expected to rise to 100 percent full and Lake Limestone, which is in an area that has remained more dry, should be at 70 percent full. While the water levels would be expected to rise slightly at Lake Georgetown in such a scenario, water levels would decline slightly at Lake Proctor. But if the basin receives an above normal precipitation rate, which is possible, the reservoirs would benefit.

Texas State Climatologist John Nielsen-Gammon said the wet weather that has drenched much of the state since the beginning of September is expected to linger.

“Wet tropical patterns will contribute more moisture,” he said, adding that more consistent rainfall is expected after drought conditions from May through August parched much of the state. That’s welcome news for reservoirs throughout the basin, which were diminished by hot and dry conditions during the height of summer.

Heading into the second weekend of September, areas upstream from Possum Kingdom Lake had received between 2 and 6 inches of precipitation. There were even some areas of Throckmorton and Haskell counties that had measured more than 8 inches of rainfall. The U.S. Geological Survey gage, located about 30 miles upstream from Possum Kingdom, was recording a streamflow of about 7,500 cubic feet per second at one point. The reservoir rose more than a foot thanks to the inflow of water, but was still more than 3 feet below capacity.

Lake Granbury was also the beneficiary of rain, and was about 1 foot below capacity. The area between Possum Kingdom and Lake Granbury was blessed with 2 to 4 inches of rain, and the streamflow at the Dennis USGS gage upstream from the lake was 470 cfs.

Ironically, despite the bountiful rainfall, 95 percent of the Brazos basin was still classified as being abnormally dry. But the severity of the drought has diminished significantly. As of Sept. 13, only 14 percent of the basin was experiencing severe drought conditions, a drop from 46 percent of the basin a week earlier.

While the rainfall was welcome through most of the basin, its clear more rain is needed to completely erase lingering drought conditions