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.
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:
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/ |
Animation
Hi-res Images
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
Hi-res Images
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) |