From: Goddard Space Flight Center
Posted: Thursday, December 20, 2001
Unique observations of Earth's vegetation are coming down from the Moderate-resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Scientists from the University of Arizona and Boston University are using these observations to refine estimates of the distribution and variability of Earth's 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 density-the 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 Earth's 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 Earth's vegetation. At Boston University, a team of researchers is using MODIS data to create global estimates of the green leaf area of Earth's 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 Earth's vegetated surfaces-from the top layer, called the canopy, through the understory vegetation, and down to the ground.
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 Earth's 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 NASA's 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, S.D.
Launched December 18, 1999, NASA's Terra satellite is the flagship of the Earth Observing System series of satellites, part of NASA's Earth Science Enterprise, a long-term research program dedicated to understanding how human-induced and natural changes affect our global environment.
For pictures, animations and additional information, go to: http://earthobservatory.nasa.gov/Newsroom/EVI_LAI_FPAR
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