MONITORING VEGETATION CONDITION FROM NOAA OPERATIONAL
POLAR-ORBITING SATELLITES
Image Interpretation
The images are color-coded maps of vegetation condition (health) estimated by the Vegetation
and Temperature Condition Index (VT). The VT is a numerical index, which changes from 0 to
100 characterizing change in vegetation conditions from extremely poor (0) to excellent (100).
Fair conditions are coded by green color (50), which changes to brown and red when conditions
deteriorate and to blue when they improve. The VT reflects indirectly a combination of
chlorophyll and moisture content in the vegetation and also changes in thermal conditions at the
surface. This new approach combines the visible, near infrared, and thermal radiances in a
numerical index characterizing vegetation health. This approach is extremely useful in detecting
and monitoring such complex and difficult-to-identify phenomenon as drought. The VT values
below 35 are used for identifying vegetation stress which is an indirect drought indicator.
The VT is very useful for early drought
detection, assessing drought area coverage, duration, and intensity, and for monitoring drought
impacts on vegetation and agricultural crops.
Background of the Tool
Monitoring vegetation health (condition), including drought detection and watch, is based on
radiance measurements in the visible (VIS), near infrared (NIR), and 10.3-11.3 micrometers
thermal (T)
bands (channels) of the Advanced Very High Resolution Radiometer (AVHRR). These
measurements are processed to reduce long-term noise. The VIS and NIR values were
converted to the Normalized Difference Vegetation Index [NDVI=(NIR-VIS)/(NIR+VIS)] and
the T to brightness temperature (BT) using a look-up table. The NDVI and BT were filtered in
order to eliminate high frequency noise. They were also adjusted for a non-uniformity of the
land
surface due to climate and ecosystem differences using multi-year NDVI and BT data. The
NDVI
and BT were converted to the Vegetation Condition Index (VCI), Temperature Condition Index
(TCI), and their combination (VT).
Technical Characteristics
Satellites: NOAA-9 (1985-1988), NOAA-11 (1989-1994), NOAA-14 (1995- 2000), NOAA-16 (2001);
Data Set: Global Vegetation Index;
Sensor: Advanced Very High Resolution Radiometer (AVHRR);
Resolution: spatial - 16 by 16 km; temporal - one week.
Reference
Kogan, F.N., 1997: Global Drought Watch from Space. Bulletin
of the American Meteorological
Society, 78, 621-636.
ORA VCI Pages by Felix Kogan, Robert M. Carey and David Forsyth.
For information, contact
Felix.Kogan@noaa.gov