WRAL WeatherCenter Blog

On Lake Levels...

Posted May 1, 2006

The designation of severe hydrologic and agricultural drought across much of central NC, along with imposition of mandatory water conservation rules in some areas, has prompted a number of questions regarding lake levels and their relation to water supplies. In particular, some have wondered why water supply concerns were so serious when lake levels really didn't seem to be all that far below "normal" levels, or in some cases were a little above. While a truly detailed discussion would take more space and time than we have here, I'll just briefly take note of a few of the issues involved.

One of the first items of note is that part of the reason lake levels have not fallen lower than they have in spite of a large first quarter rainfall deficit for most of the area is that conservation measures have been implemented for some time, including a deviation that involves releasing somewhat less water than normal from the lakes. This is something the Corps of Engineers (Kerr, Falls and Jordan Lakes) and Dominion Power company (Gaston Lake) must manage carefully in light of upstream and downstream rainfall and water usage in order to maintain critical activities along downstream waterways like the Roanoke, Neuse and Cape Fear Rivers. This leads to one of the misconceptions some people have about lake levels, i.e. why do the levels not always respond rapidly and directly to rainfall or a lack thereof? Since inflow to the lakes depends on how much rain fell over how much of the watershed, and takes some time to make its way into the lake, and since outflow from the lakes is being controlled to meet fluctuating demands for municipal water, for electric power generation, for downstream water supplies, for recreational purposes, and for wildlife management (for example, the level of Kerr Lake is raised for April and May, and the release of water to the Roanoke is modulated through those months, to best accomodate striped bass spawning season), there isn't always a neat and simple correspondence between rain and lake levels.

Another issue that arises in times of drought is what kind of rainfall is expected over the coming months and seasons and how that relates to future water supplies. Given that long range forecasts are both very general and often of low confidence, if rainfall has already been limited and streamflow into the lakes is running well below normal, it is prudent to hedge toward the potential that water supplies will decline. As of a couple of weeks ago, for example, streams were running very low in central and western NC, with inflow to Falls Lake as low as 9% of normal for the month at that time (78 cubic feet per second versus an April normal of 879). In addition, long range outlooks leaned toward below normal rainfall well into the summer, when both water use and evaporation increase substantially.

Given this, even though lake levels were within a foot or so of normal at all the area lakes at that time, Corps of Engineers projections for levels by mid-summer and early fall were alarmingly low. Using Falls Lake as an example, starting from a current elevation of 250.5 feet above sea level at that time, the level was projected to fall to 241.5 feet by September. The "normal" level for the lake (also called the "Guide Curve") is 251.5 feet. An important point, pointed out in a lake status message from the Corps of Engineers, is that the usable range for water quality and supply for the lake goes as low 236.5 feet. However, owing to the sloping shape of the lake bottom, about HALF of the actual water supply is represented by the upper five feet of this range, so if we drop as low 246.5 feet (5 feet higher than the September projection) we're already down to 50% of the normal water supply.

As of this writing, we have benefited significantly over much of our viewing area from notable rains during the past ten days or so. In addition, extended outlooks from the Climate Prediction Center have been adjusted to show an equal chance of above, below, or near normal rainfall through the summer season as opposed to the earlier "below normal" forecast. Accordingly, drought outlook graphics now indicate possible long-term improvement for central North Carolina. We'll see in the next couple of weeks whether the current Severe Drought designation for our area, as well as Corp of Engineers lake projections, will become more favorable, and whether any changes in water restrictions will be implemented. In the meantime, think rain!
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