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?
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