VARIATION OF GREAT-LAKES WATER LEVELS DERIVED FROM GEOSAT ALTIMETRY

A technique for using satellite radar altimetry data to estimate the t emporal variation of the water level in moderate to large lakes and en closed seas is described. Great Lakes data from the first 2 years of t he U.S. Navy's Geosat Exact Repeat Mission (November 1986 to November 1988), for which there is an improved orbit, are used to demonstrate t he technique. The Geosat results are compared to the lake level data c ollected by the Great Lakes Section, National Ocean Service, National Oceanic and Atmospheric Administration, and are found to reproduce the temporal variations of the five major lakes with root-mean-square err or (rms) ranging from 9.4 to 13.8 cm and a combined average of 11.1 cm . Geosat data are also analyzed for Lake St. Clair, representing a mod erate-sized lake, with a resulting rms of 17.0 cm. During this study p eriod, the water level in the Great Lakes varied in a typical annual c ycle of about 0.2 m (0.5 m for Lake Ontario) superimposed on a general decline of approximately 0.5 m. The altimeter data reproduced the gen eral decline reasonably well for the lakes, but the annual cycle was o bscured in some lakes due to systematic errors in the altimeter data. Current and future altimetry missions will have markedly improved accu racy which will permit many moderate (25 km diameter) or larger lakes or enclosed seas to be routinely monitored.