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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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Metro Level
Twin Cities (Minneapolis-St. Paul), Minnesota

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 the initial image-processing and assessment methodology, which would support our efforts in mapping lake water clarity at a city and state scale.  To develop this method, we had to address several issues including:

  • Defining timing and frequency requirements for an assessment program

  • Selecting satellite images

  • Data extraction from images

  • Selecting ground observation data

  • Developing an appropriate estimation model

To create the TCMA water clarity map, we used Landsat satellite data acquired on September 7, 1998 and obtained Secchi disk measurements.  Next, the spectral signatures of selected lakes (i.e., the lakes for which we had Secchi disk readings) were extracted to model water clarity for all lakes in the TCMA.

As with the city-scale study with IKONOS imagery, the model was developed using the band 1:3 ratio and band 1 as the independent variables and the natural log of Secchi measurements as the dependent variable. The agreement between the Landsat measurements and an independent set of Secchi disk measurements had an R2 = .87 (again, an R2 value of 0 would indicate no relationship and a value of 1 would indicate a perfect agreement). Click on Landsat vs. Secchi Disk Measurements in the right column to view this relationship.

In the last step, a lake level map of the water clarity was created for all lakes in the TCMA.  Click on the 1998 Landsat Classification of Lake Water Clarity map in the right column to view a lake water clarity map based on 1998 Landsat data.

We used 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 TCMA 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.

Visit the TCMA LakeBrowser to access the entire database of 500 lakes in the TCMA and review the 10 dates of satellite data.

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

  • Land use

  • Lake morphology

  • Drainage/stormwater connectivity to lakes

  • Individual sewage treatment systems

  • Lawn management

  • Imperviousness

  • Recent development (sediment)

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