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~~ j ~ S Traditional monitoring programs are usually limited to monitoring only a fraction of the lakes in the Twin Cities each year due to logistics and cost. Satellite technology can complement on-the- ground monitoring programs by providing a spatially complete snapshot of water quality for all lakes in the Twin Cities region at comparatively little cost. Satellite remote sensing uses visible and infrared sensors to form digital images of the earth's surface by detecting solar radiation reflected from the ground. Assessment of water clarity by satellite imagery requires the development of a mathematical relationship between satellite observations of bright- ness in the red and blue regions of the spectrum and simultane- ously collected ground measurements of water clarity for 25 to 50 lakes. This relationship can then used to extrapolate water clarity for nearly all the region's lakes. w aa~ ~s_~ a 3~1h ~ ~s~~ ..' ~~;~ ~. o ~.;; r ` y ' :I;Kc `~ S `~ 4~ Q. J `:~ In 2004, the Council, with the use of volunteer help, conducted monitoring on 145 lakes; however, nearly five times the lakes were assessed using satellite data. Thirty-five percent of the ground- monitored lakes received an overall water quality grade of "A" or "B" compared with 32% for the satellite assessment. The percent of lakes receiving a grade of "C" was 37% for the ground-based monitoring program and 27% for satellite assessment. Lakes receiving a poor water quality grade of "D" or "F" accounted for 28% of the ground-monitored lakes, but 41 % of the satellite assessed lakes. One likely explanation for the differences in the lake grades is that the lakes enrolled in the ground-monitoring program tend to be the larger and deeper lakes, which are gener- ally characterized by better water quality. A comparison between the 2003 and 2004 satellite analysis of area lake water quality seems to indicate that the region experienced better water quality in 2004 in contrast to that of 2003. This also holds true when comparing the 2003 and 2004 ground-based monitoring data. di: \~ <_ --- -- ~ t Water clarity is an important attribute of lakes because it is strong- ly related to human perception of lake quality, particularly its suit- ability for swimming and boating. Water clarity is traditionally measured using a white metal disk, called a Secchi disk, which is lowered into a lake to the point where it is no longer visible. The depth at which the disk disappears serves as a good index of lake water quality. It is one of three measures used to characterize the trophic status (or degree of eutrophication) of a lake (the others are chlorophyll-a and total phosphorus concentrations). _ ~,~t ®~, a® a® In general, the relationship between satellite data and water clarity, as measured by Secchi disk transparency (SDT), is strong. For this year's assessment, more than half (59%) of the satellite estimated summer mean SDT values were within ±0.5 meters of the ground-observed summer mean SDT and 79% of the satellite estimates were within ±1.0 meter. In terms of lake water clarity grade, the satellite-estimated grade was within ±1 letter grade of the ground-observed grade 89% of the time. 5 ~, U a ~~ , a m e ~ B 4 e a . e ~ t a Sstrlt~te ssti's'.--;.=.e S~T~m, Information about the Council's lake monitoring program can be found at http://vvwvx 'I.orq/envirartment Satellite water clarity assessment data for specific lakes can be found at nttp://tsacyis.urvm,.eda,/water/reyiona~ water da.ity/~,tentnakebrowsers.ntar For more information about the Council's satellite lake assessment program contact Steve ~oitxr at (651)2.1056 ar steve.kl~ker a~troetcstate.rrsn.us For more information about the Council's lake monitoring programs contact Randy dnharn at (651 X602.8743 ar randytanhoe~e a~r~etastate.rm.us 11~~tr~p~taxx ~t>'aixucil A F3 C D F S:~tetitte Water CtarPty Grade