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IMPACT
OF WESTERN U.S. DROUGHT VISIBLE FROM SPACE
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| Colorado
and Oklahoma | | | The
drought plaguing the western U.S. is so intense, that its impacts on vegetation
can easily be seen from space. New satellite-derived images of vegetation confirm
extremely dry conditions exist that are ripe for fires in the western United States,
according to a data collaboration between NASA and the USDA Foreign Agricultural
Service. Images created from France's SPOT satellite show the health of vegetation
as severely weakened indicating very dry conditions over large areas of the western
U.S. The
images for May show the most severely dessicated regions to be over the western
Great Plains, especially southeastern Colorado near the Texas/Oklahoma panhandle
region. The data show severely dry conditions over the U.S. Southwest, and northward
over the Dakotas and Montana, reaching all the way up into Canada. There is a
strong correlation between the rainfall amount and vegetation "greenness"
particularly for semi-arid regions. Severe and persistent below normal vegetation
greenness over these regions implies drought. Dry
vegetation conditions associated with drought, shown in the images as brown and
red colors, also imply a higher than normal risk of fire danger for these regions
where the lushness of vegetation appears depressed. A
New Way to View Vegetation The
product that generated these images is the Normalized Difference Vegetation Index
(NDVI), which measures the health of plant life based on their levels of photosynthesis.
The NDVI was developed by Compton Tucker, a senior scientist at NASA/Goddard.
Several satellites now contribute remote sensing data to the NDVI by reading light
reflected back from the Earth in different bands of the spectrum. Normally,
plants absorb visible red wavelengths of light (from the Sun) when they are healthy.
When they are unhealthy, such as when they are stressed from drought conditions,
they absorb less red light. The NDVI index measures the red light that the plants
reflect. More red light than average is indicative of unhealthy plants, likely
due to drought or heat stress. The
color scale for the images indicates how much the lushness of vegetation has changed
from what is calculated as the average or normal state over the last 3 years.
For example, the range of normal conditions (+/-5%) on the scale is indicated
by a pale yellow-white color. Above normal greenness or more lush plant life is
indicated by green colors. Negative
changes are indicated by yellow, beige, brown and red colors on the scale, with
red being the most extreme Red areas suggest drought conditions and represent
vegetation that is 60% less healthy than normal. In addition to rainfall shortages,
summertime heat also causes the vegetation to become stressed and reduces its
photosynthetic activity. The
NDVI data created from a SPOT vegetation sensor aboard the French SPOT-4 satellite
provides a closer look at the details of the land than previous satellites. The
SPOT-4 provides a resolution of 1km (6/10ths of a mile). The
SPOT based NDVI images are currently prototype products. The NDVI provides a look
at how land surfaces and vegetation respond to climatic conditions and provides
land resource managers with another tool to measure drought and fire hazard conditions. Back
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