SUMMERLAND KEY, Fla.–
Three U.S. satellite remote sensing experts believe they have figured out how to monitor the cleanliness of coastal waters with enough detail to aid coastal water-quality managers in their search for sources of pollution.
The scientists warned, however, that performance questions surrounding
a forthcoming U.S. satellite instrument, the Visible/Infrared Imager Radiometer Suite (VIIRS), make it uncertain whether the new technique can be put into action.
In a paper dubbed
“Monitoring Turbidity in Tampa Bay Using MODIS/Aqua 250-m Imagery,” published in the July 30 issue of the journal Remote Sensing of
Environment, the scientists explain how they successfully converted readings from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite into measurements of suspended sediments, chlorophyll and dissolved organic matter. Those are the major contributors to the sometimes turbid, or dirty appearance, of water in the Tampa Bay estuary, said the authors. At the time of the study, all three authors worked at the University of South Florida Institute for Marine Remote Sensing in St. Petersburg.
High chlorophyll levels often indicate the presence of pollutants such as nitrogen, which can trigger algae blooms, whereas sediment levels are more a function of natural phenomena such as wind or tides, although coastal construction also can be a factor. If scientists cannot attribute a sudden increase in water turbidity to natural causes, such as a storm, they can start looking for sources of pollution
Most MODIS data is collected in 1-kilometer-wide picture elements, but scientists have figured out how to use the sensor’s 250-meter resolution imagery, which is sharp enough to help coastal managers spot sources of pollution.
Local coastal managers currently gather water-quality measurements by riding boats out onto the
tethered disks into the water to see how far they sink
for examination in laboratories back on shore. The work is funded under a water-quality monitoring program run by the U.S. Environmental Protection Agency and the state of Florida, said Frank Muller-Karger, one of the paper’s authors
and now Dean of the University of Massachusetts School of Marine Sciences and Technology in Dartmouth
Muller-Karger said he and his colleagues were able to mathematically process MODIS imagery into measurements corresponding
to those gathered by boat, but with a potentially great advantage. “The problem with a boat, of course, is that it’s people intensive. It takes about a month to sample all of Tampa Bay,” Muller-Karger said. “Now, within seconds we can see all of Tampa Bay. We can convert it to these products and get water-quality assessments in similar ways that they do from a boat.
Whether the method will make the transition from research project to
operational technique remains unclear, said Muller-Karger and Zhiqiang Chen, the paper’s lead author. The five-year-old MODIS instrument on the Aqua satellite is an experimental sensor. Its operational successor is VIIRS, which is part of the payload on a new generation of U.S. civil-military weather satellites slated to start launching around 2013. The initial version of VIIRS is slated to launch a few years earlier on a demonstration platform.
Still in development, the VIIRS sensor has been plagued by a problem with internally scattered light – a phenomenon called crosstalk – which has thrown off its readings in ground tests. “We’re very, very concerned,” Muller-Karger said.
The first VIIRS is scheduled for launch in 2009 or 2010 aboard the National Polar-orbiting Operational Environmental Satellite System Preparatory Project
, a NASA-funded platform that will demonstrate the key instruments for the operational weather satellites
Engineers at VIIRS contractor Raytheon Santa Barbara Remote Sensing
in Goleta, Calif., fixed a leaky light filter in the sensor and
now are addressing a remaining problem with internal scattering, said James Gleason,
project scientist for the demonstration mission at NASA’s Goddard Space Flight Center in Greenbelt, Md.
Muller-Karger and Chen are “right to be concerned,” Gleason said Sept. 5. “The concerns are still being analyzed and we will not know [the results] until we complete testing of the VIIRS, which is ongoing right now,” Gleason said.
In the MODIS study, the breakthrough was tapping the bands of reflected sunlight delivered by the instrument in 250-
, or picture elements. The MODIS 1-kilometer-resolution imagery was too coarse to help coastal managers identify sources of pollution and plug them, Chen and Muller-Karger said in a telephone interview.
The MODIS 250-meter resolution capability had been used mostly as a tool to sharpen the sensor’s 1-kilometer-resolution imagery, Chen and Muller-Karger said.
Chen and his colleagues wrote mathematical equations to correct for the effects of
atmospheric ozone, moisture and light reflected from the ocean bottom
in the 250-meter imagery. They compared their results to on-site water measurements to be certain they were not mistaking changes in the atmosphere for fluctuations in turbidity.
“Once you’ve been able to throw away all those other factors, you can tell whether the change in color or transparency of the water is due to chlorophyll or due to suspended sediments or due to dissolved organic matter,” Muller-Karger said.
Winds and tides can lift sediments from the bottom, and coastal managers need to distinguish those natural events from the effects of polluted runoff flowing into the ocean from rivers.
“You have to identify the source,” Muller-Karger said. “Once you are able to focus on the color of the water itself, you can start following up and looking at the variables, like wind or tidal cycle, and understand when you saw a change in the turbidity, whether that was driven by one of these other parameters,” he said.