Variation of Great I.akes Water I.cvcls Derived From Gcosat Altimetry

A tc.chniquc for using satellite radar altimetry data to estimate the temporal variation of the water ICVC1 in moderate to large inland bodies of wa[cr (Iakcs, in particular) is dcscribcd. Grcal Lakes data from the first two years of the U. S. Navy’s Gcosat Exact Repeat Mission (November 1986November 1988), for which there is an improved orbit, arc used to dcmonstra[c the technique. The Gcosat results arc compared to the lake lCVC1 data collcctcd by the Great Lakes Section, NOAA/National {kcan Scrvicc and arc found 10 rcproducc the temporal variations of the five major Iakcs with root mean square error (RMS) ranging from 9.5 to 14.0 ccnlimctcrs and a combirrcd average of 11.3 ccntimctcrs. ,Gcosat data arc also analyzed for Lake St. Clair, rcprcscnting a moderate-sized Iakc, with a rcsuhing RMS of 17.5 ccntimctcrs. During this s[udy period, the waler ICVC1 in the Great Lakes varied in a typical annual cycle of abou[ 0.2 mclcrs (0.5 meters for Lake Ontario) superimposed on a general dcclinc of approximately half a meter. The altimeter data reproduced the general dcdinc reasonably WCII for all the lakes, but the annual cycle was obscured in some lakes duc to systcma[ic errors in the al timctcr data. Current and future altimetry missions will have markcdl y improved accuracy which will pcrmi[ many moderate (25 kilometers) to Iargc (> 100 kilometers) inland bodies of water LO bc routinely monitored,