Although several satellite remote sensing systems have been used for water quality assessment, the relatively low cost, temporal coverage, spatial resolution, and data availability of the Landsat system make it particularly useful for assessment of inland lakes. Several studies have demonstrated a strong relationship between Landsat Multispectral Scanner (MSS) or Thematic Mapper (TM) data and ground observations of water clarity and chlorophyll a. However, use of satellite remote sensing for water clarity assessments as a routine application has moved slowly. This may be due to a lack of familiarity among inland aquatic scientists with remote sensing technology and a perception that the data are expensive and difficult to process. In 1999 with the launch of Landsat 7, data distribution was returned to the public sector, and the cost of data acquisition dropped significantly. Along with today’s powerful desktop computers and sophisticated software, processing and analysis of satellite imagery has become relatively inexpensive and easy to perform. Click on the first picture below to view the spatial coverage of one Landsat TM image centered over the Twin Cities Metropolitan Area (TCMA). This one Landsat image covers an area of around 12,000 square miles and contains spectral radiometric data for over 2000 lakes. It would take over 300 IKONOS high resolution images to cover the same area as one Landsat image and would cost around $300,000 as compared to around $600 for one Landsat image. Click on the second picture to view the spatial coverage of a IKONOS image.
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Click on the images below to see the Landsat image zoomed into the Lake Minnetonka area. The first image is the Landsat image displayed in infrared (TM bands 4,2,1) the second picture is the Landsat image displayed as true color (TM bands 3,2,1). In these images you can see some differences in water characteristics. |
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| In the picture below the terrestrial features have been masked and the radiometric values measured by the Landsat TM sensor have been stretched to span the spectral range your monitor can display. In this picture you can see even more difference between waters with different characteristics. These differences in spectral radiometric response measured by the Landsat TM sensor allow us to assess water characteristics. |
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Our initial work using Landsat imagery for water quality assessments focused on the seven-county Twin Cities (Minneapolis and St. Paul) Metropolitan Area (TCMA), a 7,800 km2 area that contains more than 600 small and medium-sized lakes. This study focused on developing and applying an image-processing and assessment methodology. Method development issues that were addressed included defining timing and frequency requirements for an assessment program, selecting satellite images, data extraction from images, selecting ground observation data, and developing an appropriate standardized regression model. For more information check out our Slide Show or Publications pages.
We applied the image-processing and assessment methodology to create a comprehensive water clarity database of over 500 lakes in the TCMA. From the pool of available images we selected 14 Landsat TM and MSS images of the Twin Cities Metropolitan Area from 1973-1998. Images were selected from years with relatively normal climatic conditions except for one dry year (1988) and one wet year (1993). Four images from the same year (June - September 1991) were selected to analyze seasonal variability, and TM and MSS images from the same day (September 4, 1991) were acquired to evaluate the compatibility of the two sensors for water clarity assessments. Click on the first picture below to view an example Lake Water Clarity or Trophic State Map of the TCMA. The entire database can be accessed through our Lakebrowser. We have recently assessed the water clarity of all lakes 20 acres or larger in the State of Minnesota. Click on the second image to view the Minnesota Water Clarity Map.
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