What determines outflows during normal operations?

On a June 11 post we solicited input from readers on topics they would like to see covered on this blog. One of the responses we received asked for more information on how release rates are determined during normal operations. Here is the inquiry I hope to address in this post:

“Summer lake level management for Hartwell and Thurmond. I note from an earlier post that, ‘According to our water control manual, the reservoirs are in normal conditions during the summer when water levels are within the first four feet of conservation storage’. Release rates are defined after levels drop four feet. But what factors determine release rates in the first four feet?”

Related to this, several stakeholders have noticed the most recent projection predicts a gradual decline in levels at Hartwell and Thurmond over the next 10 weeks. They have noted the projection includes an assumption that rainfall will return to 100 percent of normal at both Hartwell and Thurmond. These questions naturally follow: ‘If we receive 100 percent of normal rainfall, why the decline?’ ‘Why aren’t release rates adjusted to compensate?’ The remainder of this post aims to answer these two questions.

The charts below show the projections for both Hartwell and Thurmond:

Hartwell Projection

Thurmond Projection

So, what is ‘normal?’

Many of our upper-basin stakeholders consider full summer pool to be normal. And given the basin’s recent history with drought, an atmosphere of unease will understandably emerge whenever reservoirs levels descend below full-summer pool. But if normal is defined by what is average, the best indicator of “normal” reservoir levels is the dark red line on the charts above. This line shows the average level of the reservoirs over the years data has been collected.

It is practically impossible to maintain a precise and steady level, and the engineers who designed these reservoirs understood that. So when the designers established a guide curve and defined what would be full summer pool, it was understood that this line would be the upper limits of normal operations. The lower limit of summer normal operations levels is four feet below full pool, or drought trigger level 1. (There are minor adjustments to this range during the descent and ascent of the winter draw down.)

Water managers are responsible for ensuring the basin is operated in a manner that all seven project purposes are achieved. Available data demonstrates that all seven of the project’s purposes are most efficiently met when pool levels are in the normal operating range. Therefore water managers have an interest in keeping the pools within four feet of full. Any time reservoir levels rise out, or fall out of this level, adjustments to release rates must be made to compensate for the risk. To put it simply, full pool sits on the threshold of high water. Four feet below full sits on the threshold of mild drought. Theoretically then, the optimal level to maintain is right in the middle of the normal operations spectrum. In reality, we do our best to maintain levels as close to full as safety allows for the purpose of conserving water.

What determines release rates while in normal?

In the simplest terms, three things determine release rates when pool levels are within four feet of full: a) inflow volume, b) hydropower demand and c) environmental & municipal demands. For example, let’s imagine a situation where pool levels are normal and inflows are well below normal. Even if there were no hydropower demand, Environmental needs downstream define a minimum release rate in order to ensure plants, wildlife, industry and municipalities have enough water. There will always be a demand for a minimum flow even if hydropower wasn’t factored into release rates.

Why does the projection show a drop?

The current projection predicts a gradual three foot descent in the next 10 weeks. Since levels are currently in the normal operating range, two known factors account for the drop: the expected demand for peaking hydropower in the coming weeks, and upper-basin environmental demands. July and August are the two months most known for declining reservoir levels because of the associated hot weather. Historical records indicate descent is normal for these months. There is a great demand for peaking power during July and August due to the heat because homes and business are drawing on the grid more heavily to cool their buildings. Combined with this are high evaporation rates and the heavy demands for water from trees and plants. See our post on evaporation and transpiration to get an idea for how much these phenomena account for substantial water loss.  In order to avoid a descent in reservoir levels in July and August, the basin would need to receive higher-than-normal rainfall for the same period.

Any sign of drought?

Based on the data, a drop in reservoir levels in July and August is normal. Additionally, we do not have any reliable sign that we are facing drought this season. The most recent 90-day National Drought Outlook (released in June) predicts all states east of the Mississippi will likely remain drought free through the end of September. The U.S. Drought Portal  also is a useful resource to reference on drought conditions.

Thanks for allowing us an opportunity to address your questions, and as always we welcome your comments.

~Russell Wicke, Corporate Communications Officer

About U.S. Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multimillion dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on Facebook.com/SavannahCorps
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