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,_;~ ~:-~ 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 lake water quality by satellite imagery requires the development of mathematical relationships between satellite observations of brightness in the red and blue region of the spectrum and simultaneously collected ground measurements of water e!arity far about 25 t~ 50 lakes. This relationship can then be used to extrapo- late water clarity for nearly all of the region's lakes. °~ m to fA y 40°io Q 35% ~ 30% ~ 25% '-~ 20°/ ~ 15% .. ~ 10% ~ 5% a 0% In 2003, the Council, with volunteer help, conducted monitoring on 130 lakes; however, nearly 1000 lakes were assessed using satellite data. Thirty percent of the ground-monitored lakes received an overall water quality grade of "A" or "8" compared with just f 5% for the satellite assessment. The percent of lakes receiving a grade of "C" was 37% for the ground-based monitoring program and 33% for the satellite assessment. Lakes receiving a poor water quality grade of "D" or "F" accounted for 33% of the ground-monitored lakes, but 50% of the satellite assessed lakes. Ono likely explanation for the differences in lake grades is that the lakes enrolled in the ground monitoring program tend to be the larger and deeper lakes, which are generally characterized by better water quality. Water clarity is an important attribute of lakes because it strongly relates to human perception of lake quality, particularly its suitability 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 loke (the others are chlorophyll-a and total phosphorus concentrations). "~?~~ _ _ - 1<`~ In general, the relationship between satellite data and water quality clarity, measured as Secehi disk transparency (SDT) is strong. For this assessment, morn than half (5b%) of the satellite estimated summer mean SDT values were with- in ±0.5 meters of the ground-observed summer mean SDT and 81% of the satellite estimates were within ±1.0 meter. In terms of lake water quality grade, the satellite-estimated grade was within ±1 letter grade of the ground- observedgrade 90% of the time. 0.00 2.D0 4.00 6.00 8.00 Satellite Estimated SDT (m) 0 information about the Council's lake monitoring program can be found at :// .aaae~ratcawaacil.axg/~rovirama Satellite water clarity assessment data for specific lakes can be found at h9vp:!/resaa;.0iis.u .ealu/wander/regi~aaal watd~ cl ity/cwN~ast/I~kcbr ears, For more information about the Council's satellite lake assessment program contact Sdeve KI®i ad (651)502-1056 ®r s@eme.klaber@eo~Bcs~ceteanaa.us For more information about the Cauncil's lake monitoring programs contact Rmaedy Aeshn tad (651)602-8743 er randya:aah~aa c.sdarPe acs 8 E F. 6 0 v m Z ~ 4 O a 0 ~ 2 ~~z•~~®lit C® cil A E; C D F