David E. Steitz
Headquarters, Washington, DC
(Phone 202/358-1730)
Cynthia M. O’Carroll
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/614-5563)
RELEASE: 00-132
NASA has a new tool designed to keep a close watch over our
plants. What we see in the reflection of the vegetation may help
researchers do a better job of monitoring and, one day, predicting
periods of drought.
A new Multi-spectral Drought Index measures the impacts of
too little water or too much rainfall on vegetation. The index
will also be used to verify other existing drought-monitoring
products.
“What makes this data set unique is its unprecedented detail,
which provides a resolution four times that of current drought
prediction maps, and it is based on a 20-year data record,” said
Compton Tucker, the research scientist leading the project at
NASA’s Goddard Space Flight Center, Greenbelt, MD.
The new Multi-spectral Drought Index utilizes and improves
the data from the Normalized Difference Vegetation Index (NDVI)
and shows deviations from average vegetation levels. The NDVI is
an index created by Tucker 20 years ago to measure the absorption
and reflectance of sunlight by plants.
The NDVI data sets show the greening and browning of plants
as they relate to seasonal changes and conditions such as drought
or abundant rainfall. The data is gathered by the polar-orbiting
satellites built by NASA and operated by the National Oceanic and
Atmospheric Administration.
The satellites measure the reflectance and absorption
characteristics of plants at different wavelengths in the
electromagnetic spectrum. The data are registered in numerical
form, and translated by researchers into monthly maps of
vegetation color changes, which in turn indicate how much soil
moisture is available to plants. Since the global data set spans a
20-year time period, researchers can better determine what are
“normal” levels of plant growth, and what are unusually high or
low levels.
Sunlight can either be absorbed by leaves and needles or
scattered within and among them. By using red and infrared
wavelengths in the spectrum, multi-spectral imaging measures the
absorption rate of sunlight and identifies levels of chlorophyll
generated in vegetation. When more sunlight is absorbed, higher
levels of chlorophyll are generated in vegetation showing plant
growth. Conversely, when a plant is stressed from lack of
fertilizer or water, it will limit its chlorophyll production
compared to healthy plants.
“The new Multi-spectral Drought Index is used to generate
better vegetation anomaly maps than before,” according to Tucker.
Light brown on the drought map means there’s diminished plant
growth, green on the map indicates a higher than average plant
growth.
A map generated for July 2000 indicated a drought in the
western United States. “The data clearly shows why we’re having
wildfires,” Tucker said. “Soil conditions are dry, and the
diminished vitality of vegetation indicates that.”
Many drought products are based on water availability in
soils. The new index maps integrate climate variables such as soil
moisture, temperature and precipitation, and show how vegetation
responds to environmental conditions around the world.
“The bottom line is that the new Multi-spectral Drought Index
reflects the actual environmental conditions of the vegetation,
and at a much higher resolution than previously available, which
will be helpful in supplementing and validating the NOAA drought
forecast maps,” Tucker said.
The first data sets covering North America and Africa and are
currently available at:
http://earthobservatory.nasa.gov/Drought/
http://svs.gsfc.nasa.gov/imagewall/drought.html
Complete data sets including images from all continents are
expected to be released to the scientific community early next
year. This research is done in support of NASA’s Earth Science
Enterprise, Washington DC. The Enterprise is dedicated to the
long-term study of how human-induced and natural changes affect
our global environment. More information about the Enterprise can
be found at:
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