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Scientists Measure Evaporation At Lighthouse

TRAVERSE CITY, MI (AP) - For more than a century, the Stannard Rock lighthouse existed for one purpose: warning mariners away from a deadly reef lurking just below the surface of Lake Superior.

Now it has a new mission. Scientists have placed equipment atop the 110-foot-high structure, hoping to determine how much of the giant lake’s water is being sucked into the atmosphere.

Hydrologists say stepped-up evaporation, possibly linked to global warming, is a leading reason that Great Lakes levels have receded over the past decade - at times hitting record lows. Milder temperatures have shrunk the ice cover that historically covered much of the lakes during winter, when evaporation rates are highest.

Low water has caused heavy losses for shippers, marinas and other sectors of the regional economy while stoking conspiracy theories about secret pipelines to Arizona.

It’s widely believed that evaporation causes about half the water loss each year from Lake Superior, said Jay Austin, assistant professor with the University of Minnesota at Duluth’s Large Lake Observatory. Superior is the biggest of the Great Lakes and a feeder for the others.

But the numbers are based on computer models. The Stannard Rock experiment is intended to provide the first direct evaporation measurements.

“ It’s one of the areas where our information is rather weak,” said Ted Yuzyk, co-chairman of a U.S.-Canadian panel studying water levels on the upper Great Lakes.

It’s not simply an academic question. The study was requested by regulators caught up in a debate over what, if anything, can be done about low water.

The International Joint Commission, a U.S.-Canadian advisory agency, is considering whether to change the rules that determine how much water is released from Lake Superior to the other lakes through gates, hydroelectric plants and locks on the St. Marys River at Sault Ste. Marie.

Also, the commission is investigating a Canadian group’s contention that navigational dredging has opened a “drain hole” in the St. Clair River that is sending billions of extra gallons daily from Lake Huron to Lake Erie - and, eventually, through the St. Lawrence River to the Atlantic. The group wants obstacles placed in the St. Claire River to stem the flow.

To resolve such politically touchy issues, the commission needs the best data available about what factors influence water levels, spokesman John Nevin said.

“ Evaporation is an important piece of the puzzle,” he said. “We may be looking at an entirely different water supply scenario because of climate change.”

A team led by Christopher Spence, a research scientist with Environment Canada’s Water and Science Technology Branch, installed the equipment on the isolated Stannard Rock lighthouse, nearly 50 miles north of Marquette. The lighthouse sits on a concrete base anchored to the reef.

The scientists are using a technique called eddy covariance, which combines humidity and air velocity data to calculate how fast water vapor rises from the lake surface. It’s similar to observing the way smoke wafts upward and outward after candles on a birthday cake are blown out, Spence said.

The gear includes a sonic anemometer, which produces three-dimensional wind measurements, and an analyzer that gauges water vapor in the air at intervals lasting only tenths of a second. Sensors keep track of air and water temperature, solar radiation, relative humidity and wind speed and direction.

It’s a complex process, and location is crucial. It can’t be too close to land, where conditions can be much different from over open water.

The researchers plan to compile data from Stannard Lighthouse for two years, extrapolating the findings to cover the entire lake by adding information from other sources to the equation.

Some hydrologists question whether the project will produce an accurate reading of evaporation beyond the immediate area where the instruments are located.

Variables such as wind speed and humidity can differ sharply from one point to another on Lake Superior, which has about the same surface area as South Carolina, said Tom Croley of the Great Lakes Environmental Research Laboratory in Ann Arbor.

The Great Lakes lab, part of the National Oceanic and Atmospheric Administration, doesn’t take direct evaporation measurements. It has a computer model that produces estimates using data from across the lake, gathered by satellites, buoys, aircraft and other means.

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