Press Release: 13
January 2003
Contact: Thomas
Lillesand, (608) 263-3251, tmlilles@facstaff.wisc.edu
RESEARCHERS TRACK LAKE CLARITY
FROM SPACE
- TEXT -
Graphics are included in a separate file at
http://www.lakesat.org/lakes_from_space_graphics.php
MADISON – Assisted by hundreds of volunteers around the state, University
of Wisconsin-Madison researchers and their cooperators have developed
a method of assessing the water quality of Wisconsin’s lakes from space.
Using
images captured 438 miles above the earth, they have completed the
first satellite-based inventory of the clarity of the largest 8,000
lakes in the state. The inventory is available to the public in map
form on the Web, where it is possible to zoom in for a close look at
your favorite lake or group of lakes.
"We
are pleased to help usher in the Year of Water in Wisconsin with a
new tool to aid in monitoring lake water quality statewide," says Thomas
Lillesand, who led the effort as director of UW-Madison’s Environmental
Remote Sensing Center.
"Our
research aims to integrate satellite data into the state’s day-to-day
lake management programs. This won’t eliminate the need for conventional
water quality monitoring, but it will greatly increase the benefits
of ground-based sampling."
The
statewide effort is part of a multifaceted research project funded
by NASA called the Satellite Lake Observatory Initiative. The UW-Madison
Center for Limnology, Green Bay Metropolitan Sewerage District, Fox-Wolf
Watershed Alliance, Inc., and Wisconsin Department of Natural Resources
have all cooperated to provide "ground truth" water quality data necessary
to calibrate the satellite images.
As
part of the DNR’s Self-Help Citizen Lake Monitoring Program, volunteers
across Wisconsin routinely measure the clarity of their local lakes
with simple tools called secchi discs, which look like oversized CDs
with a bold black-and-white pattern on top. A metal secchi disc is
lowered on a tether into the water until it is just deep enough to
disappear from sight. At that point the user records the depth. The
clarity of water often is expressed in terms of secchi depths.
To
aid in the satellite inventory, Self-Help volunteers took secchi readings
on lakes for the past three years while the Landsat satellite passed
overhead, gathering its own electronic images of these and other lakes.
Back at UW-Madison, researchers correlated the conventional water-clarity
data with the corresponding Landsat data. Lillesand says in this way,
secchi readings from fewer than 400 lakes made it possible to estimate
the clarity of all other lakes in the satellite’s images without sampling
each of them by hand.
Landsat
uses picture elements, or pixels, that are 98 feet square at ground
level. It takes approximately 160 million of these pixels to cover
the entire state. Because of its relatively narrow field of view,
Landsat captures images of any given geographic area only once every
16 days.
Lillesand
says a new imaging system aboard the Terra and Aqua satellites, called
MODIS, has a much wider field of view and can provide coverage nearly
every day. Although MODIS data are "coarser," revealing far less detail
than Landsat’s, their broad coverage area and frequency permit scientists
to monitor the clarity of large water bodies like Lake Winnebago and
Green Bay daily except when clouds obscure them.
The
new statewide water clarity map and daily MODIS images of Wisconsin
are both viewable on the Web at www.ersc.wisc.edu.
The UW-Madison researchers also are working with their counterparts
at the University of Minnesota and Michigan State University to develop
a three-state regional picture of lake water clarity.
"Demonstrating
that lake clarity can be estimated over very large areas via satellite
data at this level of detail is just the beginning of our research,"
says Lillesand. "We want to be able to
answer such questions as how lake clarity has changed over time, where
lake management activities
might be most useful, and which lakes will be most subject to change
in the future due to such factors as changes in land use and climate." |