Mapping
Stream Quality
Use
of Airborne Remote
Sensing Imagery
for Water Quality
Assessment of
Minnesota's
Rivers
Minnesota
has around 92,000
miles of rivers
and streams. It
has been estimated
that 40 percent
may be impaired.
To date less than
10 percent of Minnesota
river and stream
miles have been
assessed. We are
exploring the use
of airborne remote
sensing as a cost-effective
way to gather the
information needed
for river assessments.
We previously have
had great success
assessing lake
water clarity using
reflectance information
from Landsat imagery
and have found
similar relationships
for large rivers.
However, compared
with lakes, rivers
and streams pose
a challenging set
of problems for
application of remote
sensing techniques
to water quality
assessment because:
- They
are temporally
more dynamic.
- The
resolution of
Landsat (30
m) is
too coarse
for small
rivers and
streams.
- And,
if
we want
more than
clarity, we
really need
a better
set of
spectral bands
than the
Landsat bands.
Our
solution has
been to
use airborne
high-resolution
hyperspectral
imagery obtained
from a
small aircraft
flying over
stretches of
rivers. For
calibration purposes,
water samples
were collected
concurrently
with
the fly-overs,
and to
provide a
range of
conditions for
calibrations,
we
focused our
initial measurements
around the
confluences of
river systems
in Minnesota
that have
different water
quality characteristics.
Methods
On
August 19, 2004
an aircraft fitted
with the AISA-Classic
sensor collected
high resolution
1-3 m hyperspectral
(35 well selected
bands) imagery
over six river
segments (identified
by red boxes on
the map). In a
collaborative
effort sampling
crews from the
Minnesota Pollution
Control Agency,
Metropolitan Council,
Minnesota Department
of Natural Resources,
Minnesota Department
of Agriculture
and the University
of Minnesota were
dispatched to collect
39 water samples.
These samples were
analyzed by the
Metropolitan
Council and Minnesota
Department of
Health laboratories
for a number of
water quality
characterization
variables.
On
August 15, 2005
an aircraft fitted
with the AISA-Eagle
(AE) Hyperspectral
Imager (VNIR)
collected high
resolution 2 m hyperspectral
(97 contiguous
bands ~2.5 nm from
435-724 and ~10
nm from 724-950
nm) imagery over
a fairly large
area along (36
mile stretch) the
Mississippi
River from Spring
Lake to Lake
Pepin (identified
by purple boxes
on the map). At
the same time sampling
crews from the Minnesota
Pollution Control
Agency and the
Metropolitan Council
collected 22 water
samples. The in-situ
water quality data
and remotely sensed
data are currently
being analyzed to
determine the best
model for each variable.
Preliminary single
band, band ratio
and multiple
band regression
analysis models
were used to create
the maps of each
water quality variable
for each
river segment.
Results
Preliminary
results are promising
with strong relationships
for a number of
important water
quality variables
(see water quality
table). With additional
statistical analysis
we anticipate
developing improved
models. Using
the best fit models
from our preliminary
assessment, we
were able to map
important
water quality variables
for river segments
throughout each
image. The maps
show the complex
interactions of
sediment and different
types of algae
in these important
river segments.
In
the future we anticipate
that
remote sensing
will be an important
tool in assessing
water and land
resources including
thousands of miles
of rivers. This
should enable us
to see more detailed
water quality patterns
than we could ever
sample with volunteers
or more advanced
field diagnostic
methods. Remote
sensing allows
us to see the big
picture of land
and water resources
as well as being
able to zoom in
and get a more detailed
view. This “complete
view” can
be used to detect problem areas
and help allocate limited field
monitoring resources to areas
that need additional attention.
Acknowledgements
This
research has been
conducted
by
the faculty and
staff of the University
of Minnesota, Department
of Civil Engineering
and College of
Natural Resources
-- Remote Sensing
and Geospatial
Analysis Laboratory
and Water Resources
Center, with support
from the
Legislative Commission
on Minnesota Resources,
Minnesota Pollution
Control Agency
and Metropolitan
Council.
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