After more than a century of guarding mariners from a deadly reef below the surface of Lake Superior, the Stannard Rock Lighthouse now has an additional mission; housing equipment to determine how much of the great lake’s water is being lost to evaporation. Scientists believe that increased evaporation, possibly caused by global climate change, might be responsible for lower water levels across the Great Lakes, including historic low levels observed in August and September of 2007. Low water levels can cause heavy economic losses to shippers, marinas, and other sectors of local economies. Warmer temperatures have reduced the ice cover during the winter, when evaporation rates are highest.
The evaporative processes are currently modeled by the National Oceanographic and Atmospheric Association (NOAA); the inability to collect data over the lakes, however, means that estimates of total evaporation may vary by as much as plus or minus 25%. The average daily evaporation rate for Lake Superior is estimated at approximately 41 billion gallons per day (and possibly twice that volume during the winter). To try to reduce this margin of error, Dr. Christopher Spence and Newell Hedstrom of Environment Canada installed instrumentation, including the LI-7500 Open Path CO2/H2O Analyzer, at the Stannard Rock Lighthouse in Lake Superior, approximately 45 miles (72 km) north of Marquette, Michigan. A two-year field program designed to make direct measurements of evaporation over Lake Superior is the first of its kind; a second site (without the LI-7500) is located at the Spectacle Reef Lighthouse in Lake Huron. The LI-7500 makes high frequency measurements of carbon dioxide and water vapor density. When combined with vertical wind speed measurements from a sonic anemometer, the fast measurements of water vapor density allow computing the latent heat fluxes using the eddy covariance technique. The latter are easily converted from energy units (W/m2) to water units, the equivalent amount of evaporated water in millimeters, by dividing the energy value by latent heat of vaporization. Other measurements being made at the lighthouse include rainfall, horizontal wind speed, wind direction, air temperature, relative humidity, incoming solar and long wave radiation, and water temperature. These data will be combined with weather buoy measurements, satellite imagery, and climate model data to extrapolate evaporation measurements over the entire lake surface (the largest freshwater lake by surface area in the world). By improving the accuracy of the current estimation techniques, Spence and Hedstrom hope to incorporate the data into current models to improve predictions of the lake’s future water budget, and therefore, how water levels are managed. Ultimately, officials may use this information to change recommendations for releasing water from Lake Superior to the other Great Lakes, through gates, hydroelectric plants, and locks on the St. Marys River at Sault Ste. Marie.
Hedstrom chose the LI-7500 because of its reliability, and past successes when used in remote lake experiments in the Northwest Territories, Yukon, Ontario, and Saskatchewan. In addition, Hedstrom used the LI-7500’s SDM data output option to connect directly to existing Campbell Scientific Inc. dataloggers.
The LI-7500 is now improved, and available as the LI-7500A. The LI-7500A uses a new Analyzer Interface Unit (the LI-7550), which provides additional inputs for external sensors (e.g. sonic anemometer), additional data output options, including Ethernet capability, removable USB storage devices, and completely synchronized data sets. Existing LI-7500’s can be upgraded in the field with a simple upgrade kit; the kit includes the LI-7550 and hardware to enable connection of the LI-7500 sensor head to the LI-7550. For more information, visit http://www.licor.com/LI-7500A
“The Loneliest Place in the World”
Finished in 1883, the Stannard Rock Lighthouse is located on an underwater reef that was the most serious hazard to navigation on Lake Superior. It was one of several “stag stations”, manned only by men, and was given the nickname “The Loneliest Place in the World”. The exposed crib on which the lighthouse sits was rated by the National Park Service as one of the top 10 engineering feats in the United States; the lighthouse took 5 years to complete, and used 126 tons of iron, 76 tons of bricks, and 7276 tons of concrete. Listed on the National Register of Historic Places in 1971, the lighthouse was automated in 1962, and closed to the public. The U.S. Coast Guard and Michigan State Historical Preservation Office were responsible for getting needed permissions to install the equipment on the lighthouse.
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