New way of gauging reservoir evaporation

From Orlando, Fla., at a meeting of the American Meteorological Society

The U.S. Army Corps of Engineers is responsible for managing the water level of large human-made reservoirs in the western United States, including several behind dams on the upper reaches of the Missouri River. Evaporation, especially during the height of the summer, can significantly affect the rate at which water should be released from these reservoirs.

Now, corps scientists have developed a new method to estimate the evaporation of water. The technique could replace a labor-intensive procedure based on decades-old technology.

In the old method, an observer estimates evaporation by measuring daily changes in the water level in a 25-centimeter-deep pan that has a 1-square-meter surface area. This measurement is then multiplied by a reservoir-specific factor–determined by many years observations–to get a ballpark estimate of evaporation for the entire body of water.

Although this technique has been used for more than 50 years, it has many disadvantages, says Edgar L. Andreas, an atmospheric physicist with the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, N.H. No measurements are made on the weekends or on winter days when the pan freezes. In the summer, algae grow in the pans and birds bathe in them.

Now, Andreas and his colleagues have calculated lake evaporation by adapting formulas used to estimate evaporation from the open ocean and sea ice. Both sets of equations depend on factors such as wind speed, relative humidity, and the difference in temperature between the air above the water and the water surface itself.

The researchers put their new method to the test by analyzing weather data from three locations on the Oahe Reservoir, a long, narrow body of water that stretches about 350 kilometers from Pierre, S.D., to Bismarck, N.D. They found that the evaporation rate estimated with the new technique generally falls within a factor of three of the rate estimated with the current technique. This level of error is tolerable most of the year because rain and other factors affect water level more than evaporation does.

If the new technique is adopted, evaporation estimates could be updated constantly because data from many weather stations near reservoirs are available through the Internet 24 hours a day. For the Oahe Reservoir, Andreas contends, the biggest benefit from the new method would be increased efficiency. The evaporation pan used to estimate the water loss is located on a bluff above the lake at Pierre, and it takes the corps office about an hour each day to make measurements at the site.

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