RELEASE NO: 01-126
December 20, 2001
MODIS instrument on NASAs Terra satellite improves global
vegetation mapping, makes new observations possible
Unique observations of Earths vegetation are coming down from
the Moderate-resolution Imaging Spectroradiometer (MODIS) on NASAs
Terra satellite. Scientists from the University of Arizona and Boston
University are using these observations to refine estimates of the
distribution and variability of Earths vegetation as well as to
produce some of the first-ever global maps of vital signs such as the
leaf area of plants and trees and the amount of sunlight they absorb.
Alfredo Huete and his colleagues at the University of Arizona are
using the new observations to produce the most commonly used
satellite-based indicator of vegetation densitythe Normalized
Difference Vegetation Index (NDVI)as well as an enhanced version that
makes use of new electromagnetic information collected daily by MODIS.
These maps display Earths vegetation on a scale from 0 to nearly
1, with higher values indicating more dense vegetation. Most of the
Sahara Desert earns a zero; rain forests hit the top of the scale. New
global maps are produced daily and then are combined into a single map
bi-weekly and monthly.
Traditional satellite-based mapping of vegetation vigor and amount is
based on the way vegetation interacts with red and infrared light.
Occasionally, however, those two signals are not enough. MODIS measures
light reflected from Earth at a variety of wavelengths, and the Arizona
researchers incorporate the additional information into their Enhanced
Vegetation Index (EVI).
Says Huete, We are currently conducting detailed monitoring of
both rainforests and semi-arid regions, two areas where we expect the
EVI to make its most significant contributions in assessment of climate-
and human-induced changes in vegetation. The EVI has increased
sensitivity within very dense vegetation, and it has built-in
corrections for several factors that can interfere with the
satellite-based vegetation mapping, like smoke and background noise
caused by light reflecting off soil.
The bi-weekly and monthly vegetation index maps have wide usability
by biologists, natural resources managers, and climate modelers. They
can track naturally occurring fluctuations in vegetation, such as
seasonal changes, as well as those that result from land use change,
such as deforestation. The EVI can also monitor changes in vegetation
resulting from climate change, such as expansion of deserts or extension
of growing seasons.
MODIS observations also allow scientists to track two
vital signs of Earths vegetation. At Boston
University, a team of researchers is using MODIS data to create global
estimates of the green leaf area of Earths vegetation and how much
sunlight the leaves are absorbing. Called LAI, for Leaf Area
Index, and FPAR, for Fraction of absorbed Photosynthetically
Active Radiation, both pieces of information are necessary for
understanding how sunlight interacts with the Earths vegetated
surfacesfrom the top layer, called the canopy, through the understory
vegetation, and down to the ground.
Animations
LAI animation (1.1 MB MPEG)
FPAR animation (1.1 MB MPEG)
High-Resolution Images
LAI September 2000 (3.9 MB JPEG)
LAI December 2000 (4.1 MB JPEG)
LAI mid-April to mid-May 2001 (4.2 MB JPEG)
FPAR September 2000 (4.0 MB JPEG)
FPAR December 2000 (3.9 MB JPEG)
FPAR mid-April to mid-May 2001 (4.0 MB JPEG)
In Africa, rainfall is the most important factor that affects where
people and animals live, and influences patterns of plant growth. Life
happens when rain falls! These LAI and FPAR animations show the cycle of
wet and dry seasons in Africa from September 2000 through May 2001 and
the corresponding variation in the green leaf area and how much sunlight
the leaves are absorbing over the course of a year. The seasons in the
Southern Hemisphere stand in direct opposition to those of the Northern
Hemisphere while meteorological patterns in the Northern Hemisphere
roughly mirror those in the Southern Hemisphere. For example, when
summer comes in the northern part of Africa in June, the winter (dry
season) takes over South Africa, drying out green leaves. Scientists
are concerned that Africa is becoming
drier and that the Sahara desert is getting bigger. The MODIS LAI maps
can help to better understand this desert creep. The images above compare LAI to
FPAR in December of 2000. (Credit NASA/GSFC/Boston University)
Scientist Yuri Knyazikhin is part of the Boston team using the
observations to create these first-ever global LAI and FPAR maps. Daily
observations are combined at weekly intervals into a single map that
shows leaf area and absorbed sunlight for every square kilometer of the
Earths land surface during the time period.
The interaction of sunlight with a forest canopy or other
vegetation has a large influence on climate. Knowing how much light is
absorbed and distributed among the canopy, the understory, and the
ground makes us better able to model and predict the energy exchange
between the earth and atmosphere, says Knyazikhin. That flow of
energy is what drives our climate. Year to year changes in LAI and FPAR
could be a signal of longer-term climate change.
To make the LAI/FPAR maps, Knyazikhin and his team have a computer
simulate what different types and amounts of vegetation would look like
from space. The computer then compares the images collected by MODIS to
the simulated images until it finds just the right match. Next it
calculates the corresponding leaf area and absorbed radiation. In
addition to their important scientific contributions, the maps have
real-world uses, like assessing the grazing potential of rangelands, or
identifying areas at risk for wildfires.
The Terra MODIS observations began in February 2000 and are expected
to continue through at least 2004. Another MODIS is planned for launch
aboard NASAs upcoming Aqua mission. In addition to its
observations of vegetation, MODIS collects information on atmospheric
and oceanic conditions, such as cloud cover and ocean temperature. The
EVI, LAI, and FPAR observations are available free of charge from the
Earth Resources Observation Systems Data Center in Sioux Falls, SD.
Launched December 18, 1999, NASAs Terra satellite is the
flagship of the Earth Observing System series of satellites, part of
NASAs Earth Science Enterprise, a long-term research program
dedicated to understanding how human-induced and natural changes affect
our global environment.
Contact:
Lynn Chandler
Phone: 301-286-2806
Lynn.Chandler.1@gsfc.nasa.gov
For more information about LAI & FPAR, visit http://cybele.bu.edu/modismisr/other.html
For more information about EVI, visit http://gaea.fcr.arizona.edu/projects/modis/
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Animation
Global EVI Animation (1.2 MB QT)
Hi-res Global EVI Animation (8.4 MB QT)
Hi-res Images
May 21-Jul 21, 2000 (410 KB JPEG)
Aug 21-Oct 21, 2000 (410 KB JPEG)
Nov 21, 2000-Jan 21, 2001 (410 KB JPEG)
Feb 21-Apr 21, 2001 (410 KB JPEG)
The MODIS Enhanced Vegetation Index (EVI) provides a look at vegetation around the globe year round.
The seasonal images in this animation were made from averaging two
months of MODIS EVI observations centered on the first day of each
season: a month before and after June 21, 2000; September 21, 2001;
December 21, 2000; and March 21, 2001. As the seasons change, the mirror
effect of seasonality is seen, with vegetation alternatively blooming
and fading, and one hemispheres vegetation is high while the other is
low. The images at top show EVI during two different seasons. Vegetation ranges from 0, indicating no vegetation, to nearly 1,
indicating densest vegetation. Gray areas indicate places where
observations were not collected. (Credit: NASA/GSFC/University of Arizona)
Animation
US EVI Animation (460 KB QT)
Hi-res US EVI Animation (12.4 MB MPEG)
Hi-res Images
Jan 1-16, 2001 (1.9 MB JPEG)
Mar 22-Apr 6, 2001 (2.1 MB JPEG)
May 25-Jun 9, 2001 (2.3 MB JPEG)
MODIS EVI observations from June of 2000 through May 2001 for the
United States shows the cycle of vegetation from one season to the
nextfrom vegetations summer peak, through the first winter snowfall,
to the new growth of spring, and the returning lushness of summer. EVI
observations can help determine how human- and climate-induced changes
are affecting vegetation in the U.S. and around the world. The three images above show spring
arriving in the United States. Vegetation
ranges from 0, indicating no vegetation, to nearly 1, indicating densest
vegetation. (Credit: NASA/GSFC/University of Arizona) |