SJRA Report on Spring Lowering of Lake Conroe

After three months of histrionic, apocalyptic debate about SJRA’s policy of seasonally lowering Lake Conroe, the lowering turned out to be neither the end of the world, nor the salvation of the Lake Houston Area. Simply put, no megastorms tested the policy. So there was no proof that it succeeded or failed.

Instead, regular smaller rains kept the lake close to its monthly averages. Neither drought nor excessive heat caused lake levels, property values, business or tax revenues to plummet. Everything functioned much as it normally has since Lake Conroe was built in 1973.

Lake Stayed Near Monthly Averages

In the two months between April 1 and May 31, the lake only dropped below 200 feet by an inch or two for three or four days and then promptly refilled. In fact, at the end of May, 2.5 inches of rain caused the lake to rise above its seasonal norm, prompting a second release.

Two releases and smaller, spread-out rains kept the Lake Houston Area safe and Lake Conroe close to its monthly averages (200.32 for April and 200.44 for May; see below). Source: SJRA Board Presentation 7/23/20.
Lake Conroe seasonal levels by month for 46 years. In April and May, Lake Conroe averages 200.32 and 200.44 respectively. Source SJRA Board Presentation.

During that time, the Lake Houston Area never flooded. Yet Lake Conroe was above its monthly average for the first three weeks of April. And it was only down about three inches for about three weeks in May.

The assertion that Lake Conroe is “normally” at 201 is a myth. That is the level at which the SJRA normally starts releases. Due to evaporation, which can take an inch a day, the lake is almost always well below that. A more accurate term for 201 would be “full pool.”

River Levels Up Slightly, but No Flooding

West Fork river levels rarely rose more than 2 or 3 feet even when rainfall was added to the release rate from Lake Conroe.

Release rates averaged about 600 cubic feet per second (cfs) and rarely exceeded 1000 cfs.

If the exercise proved anything, it was that releasing at those rates didn’t flood anyone.

River levels vs. rainfall for the last two weeks of the spring lowering. Source: SJRA Board Presentation 7/23/20.

Fall Lowering Starts This Saturday

The Houston Area lucked out with Hanna. Had the storm veered towards us, we would have received the torrential rains that swamped the Valley.

This fall, the lowering will be split into two phases: to 200 feet in August and 199.5 in September. The City of Houston has the right to call for an additional half foot in the event of a named storm.

Restatement of SJRA lake lowering policy for this year. Source: SJRA Board Presentation 7/23/20.

Even though Hanna missed us and Gonzalo fizzled, the next storm may not.

Near-Term Tropical Outlook

The NHC gives this disturbance an 80% chance of formation in the next five days. It’s moving west northwest at 15 to 20 mph and should impact the leeward islands late Wednesday.

The fact that Lake Conroe rebounded so quickly will likely calm debate in the future.

The Lake Conroe level at this instant stands at 200.22 feet. That’s less than half an inch below the July average for the last 46 years. Note that that’s also 3.5 inches above the August average.

Finally, note that the target level for August (200 feet) is about an inch ABOVE the normal monthly average. So if evaporation does its job, the SJRA will not have to lower the lake.

Posted by Bob Rehak on 7/27/2020

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