On September 4, 2000, the Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) acquired these multi-spectral data over
the U.S. Department of Agriculture's Grazing Lands Research Laboratory
near El Reno, Oklahoma. This series of false-color composite images
demonstrates some of the many remote-sensing measurements that
scientists can make with ASTER's high-resolution (up to 15 square meters
per pixel), multi-spectral data.
In the top image, bright red colors indicate green
vegetation, which at this time of year only includes irrigated lands and
riparian zones. Gray-green colors represent harvested winter wheat
fields. Dendritic drainage patterns are clearly depicted in the lower
left and upper right portions of the scene. ASTER's three visible and
near-infrared bands were used to make this image.
The second two images show that there is a strong
correlation between the abundance of green vegetation (referred to as
Normalized Difference Vegetation Index, or NDVI) and land surface
temperature. Vegetated areas have NDVI values of greater than 0.3 (blue
and green pixels) and are relatively cool (315-320 Kelvin). Bare soil
surfaces have NDVI values close to zero (orange and yellow) and are
relatively hot (325-330 Kelvin). Water bodies have a very low NDVI of
-0.2 (red) and cool temperatures of about 300-305 Kelvin (blue).
The fourth and fifth images represent components of the surface energy
balance over the region at 11:30 a.m. local time. Surface energy
balance shows the relationship between incoming solar energy, energy
absorbed by the surface, and energy reflected or emitted from the
surface back up into the overlying atmosphere. These images provide
insights into the complex processes of direct radiation, conduction, and
convection that are important for scientists in studies of both
weather patterns and the water cycle. The fourth image shows sensible
heat and the fifth image shows latent heat, which represents energy
flowing from the Earth's surface into the atmospheric boundary layer.
Sensible heat is energy flow due to temperature gradients, while latent
heat is energy flow due to evapotranspiration. The ASTER Team derives
sensible and latent heat by combining measurements of surface
temperature and vegetation abundance (NDVI) with surface meteorological
measurements. Together, they show that heat flow from bare fields is
dominated by sensible heat, while heat flow from vegetated areas and
water bodies is dominated by latent heat.
The latent heat flux measurements derived from ASTER data can be
converted into rates of evaporation, shown in the sixth image, and is
therefore a direct measure of water lost to the atmosphere. Before
thermal infrared satelllite imagery became available, spatial changes in
evaporation could not be measured. But until the recent launch of
ASTER, the ability to accurately measure surface temperatures at high
resolutions from space did not exist. Reliable surface temperatures are
essential to monitoring evapotranspiration (the sum total of water
evaporated and transpired by plants into the atmosphere). At 90-meter
resolution, scientists can use ASTER's thermal infrared detectors to
accurately measure surface temperatures over a wide range of land
surfaces. This capability will greatly improve our knowledge of
patterns of evapotranspiration and of vegetation health.
ASTER's measurement capabilities may also prove useful for precision
farmers.
Images courtesy Andrew French, ASTER Science Team