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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