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A collaboration of the University of Minnesota's Remote Sensing and Geospatial Analysis Laboratory and Water Resources Center

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Regional Level
Minnesota, Wisconsin, and Michigan

Statewide lake water clarity mapping in Minnesota was expanded to the States of Wisconsin and Michigan as part of the Upper Midwest Regional Earth Sciences Application Center (RESAC) project.

A 2000 regional census of the lake clarity conditions in Minnesota, Wisconsin and Michigan has been produced which includes information on thousands of lakes that were previously unmonitored. For relatively little cost, the efforts of resource monitoring agencies can be greatly extended through the application of satellite remote sensing. Click on 2000 Regional Lake Water Clarity in the right column to view the regional lake water clarity map.

Interactive web maps present the lake water clarity results in an easy to use interface. Users can zoom-in and -out and turn layers of information on and off. Visit these interactive web maps by clicking on their links in the right column.

Expanding to utilize MODIS data

Current regional research is also being conducted using a third satellite platform: MODIS. MODIS data is well suited for the assessment of large lakes and offers better spectral sensitivity and temporal (daily) coverage than Landsat which has a 16 day overpass interval. However due to the relatively low spatial resolution (250, 500 and 1000 m), the number of lakes that can be assessed using MODIS is considerable lower than for Landsat's 30 m resolution. Preliminary analyses using 500 m MODIS data indicate that only about 100 Minnesota lakes will be able to be assessed using MODIS 500 m data. This is only 1 percent of the lakes that can be assessed using Landsat imagery.

At 250 m resolution, the most useful one for lake monitoring, the spectral sensitivity of MODIS is low, with only two bands, one in the red portion of the spectrum, and one in the near infrared. Spectral sensitivity increases as spatial resolution decreases. Unfortunately, the number of lakes that can be studied with each of the resolutions also decreases sharply. Click on MODIS and Landsat Comparison in the right column to view the difference between Landsat ETM+, MODIS 250, 500 and 1000 m data.

To explore the potential of MODIS data for regional lake water quality monitoring, researchers at the University of Wisconsin selected an image in which Wisconsin was near the nadir of the sensor, cloud cover was low and a sufficient number of ground-based Secchi disk depth observations was available. The selected image was acquired on September 17, 2000, and contained 32 Secchi disk depth observations in cloud free areas. Of these, 17 were used in our analysis, because lakes smaller than 160 ha in surface area were excluded.

The 250 m resolution MODIS image was sampled manually to extract the pixel with the minimum radiance inside the lake of interest; then, observed Secchi disk depth observations were regressed against minimum radiance in the red and near-infrared bands. The best model used only radiance in the red band, and "raw" (not log-transformed) data. The relationship presented below is only slightly lower than the results using Landsat data. Click on MODIS vs. Secchi Disk Measurement to view this relationship.

MODIS holds promise for monitoring lake water clarity of relatively large lakes on a large regional scale.

At this regional scale, what can influence lake water clarity?

  • Ecoregion (soils, geology, climate)

  • Land use

  • Drainage/stormwater

  • Sewage treatment

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