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
