Using Satellites to Monitor Water Quality Donald Kentner

1
Using Satellites to Monitor Water QualityDonald
Kentner Walter Goedecke
2
The Great Lakes Region

• The Upper Great Lakes Region has over 30,000
  smaller lakes
• Minnesota, known as the land of 10,000 lakes, has
  most of these lakes
• Ground surveys were not sufficient to monitor
  every lake only enough resources to monitor only
  a small percentage of the total lakes, 50 to 100
  lakes, less than 1
• Most of the lakes were clean

3
Water Quality Problem

• Polluted or unclean lakes were largely in the
  southern Minnesota
• Researchers needed a faster method of identifying
  unclean lakes
• University of Minnesota decided to use Landsat
  satellites to map water clarity, at a fraction of
  the cost

4
Clean vs. Dirty

• Water clarity corresponds with water quality
• Lakes with good water clarity are of good
  quality, or oligotrophic
• Lakes with bad water clarity are of bad quality,
  or hypereutrophic, with murky water that is
  choked with algae.

5
Clean vs. Dirty

• Storm water runoff will often carry chemicals and
  sediments from farms and urban areas into nearby
  lakes.
• Sediments can cloud a lake, and nutrients in the
  sediments, particularly phosphorous, can cause
  unwanted algae to grow.
• Once a lake becomes eutorphic, reversal to
  oligotrophic is very difficult

6
Clean vs. Dirty Example

• Hallet Lake (left) and Schwanz Lake (right), both
  in Minnesota, are representative of very good and
  very bad water quality.
• Hallet lake is oligotrophic, with very clear
  water.
• Schwanz Lake is hypereutrophic, with murky algae
  choked water
• (Photographs courtesy Upper Great Lakes Regional
  Earth Science Applications Center)

7
Ground Visual Walter Clarity Test

• In 1865, Italian physicist Pietro Angelo Secchi
  made an 8-inch disk painted in an alternating
  black and white pattern.
• A person in a boat lowers the Secchi disk into
  the lake and records the depth at which it
  disappears from sight.
• This depth, known as the Secchi depth, is the
  measure of water clarity.

8
Orbital Visual Walter Clarity Test

• In Minnesota, oligotrophic lakes have a Secchi
  depth of 16 feet or greater, and hypereutrophic
  lakes have a Secchi depth of only a few feet or
  less.
• This test is difficult to do over the many lakes
  of this region.
• The Landsat Thematic Mapper now permits easy
  observation of the regions lakes from space
• The satellite images are calibrated to Secchi
  depth levels.

9
Early Satellites

• The early Landsat remote sensing satellites were
  launched in 1970s had a resolution of 80 meters
  in four spectral bands red, green, blue, and
  near infrared (NIR)
• Later versions of Landsat have improved sensors,
  increasing in power and accuracy
• Landsats 4 and 5 used an Enhanced Thematic Mapper
• The most recent of the series is Landsat 7,
  launched in 1999

10
Modern Landsat

• Current satellites receiver seven wavelengths of
  radiation, including thermal imagery, with a
  spatial resolution of 30 meters
• Capable of retrieving data that is more accurate
  than samples taken from people on the ground
• 100 of the lakes greater than 20 acres were
  analyzed by Land sat images
• Satellite imagery is the only practical way to
  measure water quality on such a large scale.

11

• Water quality in the counties surrounding
  Minneapolis and St. Paul, Minnesota.
• Dark blue, light blue, and green represent good
  water quality.
• Orange, red and purple indicate progressively
  worse water quality.

12
Lake Water Quality Example

• True color Landsat image of a clean Lake
• Dirty Lake

13
Lake Minnetonka and Surrounding Areain RGB and
in IR from Landsat
14
Lake Minnetonka and Surrounding AreaImages
Processed and Calibrated to Secchi Depths
Resolution Generalized to Lake
Pixel resolution
15
References

• Weirer, John, Testing the Waters Using
  Satellites to Monitor Water Quality, March 11,
  2002, http//earthobservatory.nasa.gov/Study/Water
  Quality/printall.php