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August
29, 2007
NASA SATELLITES EYE COASTAL WATER
QUALITY
Armed with data from two NASA satellites,
researchers have
invented a way to map the fleeting changes in coastal water quality
from space
- something that has long evaded researchers and coastal managers
relying only
on ground-based measurements.
Using data from instruments aboard NASA satellites, Zhiqiang Chen and
colleagues at the University
of South Florida
in St.
Petersburg, found that they can
monitor
water quality almost daily, rather than monthly. Such information has
direct
application for resource managers devising restoration plans for
coastal water
ecosystems and federal and state regulators in charge of defining water
quality
standards.
The team's findings will aid in the effort to tease out factors that
drive
changes in coastal water quality. For example, sediments entering the
water as
a result of coastal development or pollution can cause changes in water
turbidity
– a measure of the amount of particles suspended in the
water. Sediments
suspended from the bottom by strong winds or tides may also cause such
changes.
Knowing where the sediments come from is critical to managers because
turbidity
cuts off light to the bottom, thwarting the natural growth of plants.
"If we can track the source of turbidity, we can better understand why
turbidity is changing. And if the source is human related, we can try
to manage
that human activity," says Frank Muller-Karger, a study co-author from
the
University
of South Florida.
Satellites previously have observed turbidity in the open ocean by
monitoring
how much light is reflected and absorbed by the water. The technique
has not
been of much success in observing turbidity along the coast, however.
That’s
because shallow coastal waters and Earth’s atmosphere serve
up complicated
optical properties that make it difficult for researchers to determine
which
colors in a satellite image are related to turbidity, which to shallow
bottom
waters, and which to the atmosphere. Now with advances in satellite
sensors
combined with developments in how the data are analyzed, Chen and
colleagues
show it is possible to monitor turbidity of coastal waters via
satellite.
The traditional methods of monitoring coastal water quality require
scientists
to use boats to gather water samples, typically on a monthly basis
because of
the high costs of these surveys. The method is sufficient to capture
episodic
events affecting water quality, such as the seasonal freshwater runoff.
Chen
and colleagues suspected, however, that the monthly measurements were
not
capturing fast changes in factors that affect water quality, such as
winds,
tides and human influences including pollution and runoff.
The team set out to see if satellites could accurately measure two key
indicators of water quality - turbidity and water clarity –
in Tampa
Bay, Fla.
An analysis of turbidity takes into account water clarity, a measure of
how
much light can penetrate into deep water. Satellites, with their wide
coverage
and multiple passes per week, provided a solution to frequent looks and
measuring an entire estuary within seconds.
To determine water clarity in Tampa Bay, the
team looked at
more than eight years of imagery from GeoEYE’s Sea-viewing
Wide Field-of-view
Sensor (SeaWiFS) instrument, whose data is analyzed, processed, and
distributed
by NASA for research. The images give a measure of how much light is
reflected
by the water. The data were put through a two-step calculation to
arrive at a
measure of clarity. Similarly, data from NASA’s Moderate
Resolution Imaging
Spectroradiometer (MODIS) instrument onboard the Aqua satellite was
compared
with measurements of turbidity gathered on the ground and then applied
to each
whole image to make the maps.
When compared with results from independent field measurements,
collected with
the help from the U.S. Geological Survey, the researchers found that
the
satellites offered an accurate measure of water quality in the bay. The
method
can be applied to coastal waters worldwide with little change in
methods,
according to Muller-Karger.
Frequent measurements from space could resolve questions about the
specific
timing and nature of events that lead to decreases in water quality.
Seasonal
freshwater discharge from nearby rivers and runoff into the bay can
carry
nutrients. If these nutrients are not controlled, they can give rise to
large
and harmful phytoplankton blooms, which can kill sea grass. Wind
conditions,
however, are the driving force for a decline in water quality in the
dry season
between October and June, when bottom sediments are disturbed.
Chen thinks the new tools will help people understand how coastal
systems
change over time, which will provide important information for managing
the
health of coastal waters. "It’s important to look at baseline
conditions
and see how they change with the seasons and over the years, and
whether that
change is due to development, coastal erosion, the extraction and
dumping of
sediments, or digging a channel," Muller-Karger says.
SeaWiFS was launched aboard the OrbView-2 satellite in 1997. The sensor
collects ocean color data used to determine factors affecting global
change,
particularly ocean ecology and chemistry. MODIS was launched aboard the
Aqua
satellite in 2002. The instrument, together with its counterpart
instrument
aboard the Terra satellite, collects measurements from the entire Earth
surface
every one to two days.
The study was published July 30 in two papers in the journal Remote
Sensing of
Environment.
Writer: Kathryn Hansen, Goddard
Space Flight Center
##
Contact:
Lynn Chandler
Goddard Space Flight Center
301-286-2806
Lynn.Chandler-1@nasa.gov
This
text is derived from:
http://www.nasa.gov/centers/goddard/news/topstory/2007/coastal_waters.html
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