STAR - Global Vegetation Health Products : Technique Background
Monitoring Vegetation Condition From NOAA Operational Polar-Orbiting Satellites
Vegetation Health System
The images are color-coded maps of vegetation health (condition) estimated by the Vegetation Health (VH) Indices. The VH are indices, which range from 0 to 100 characterizing changes 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 VH reflects indirectly a combination of chlorophyll and moisture content in the vegetationhealth 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 VH values below 40 are used for identifying vegetation stress which is an indirect drought indicator. The VH is very useful for early drought detection, assessing drought area coverage, duration, and intensity, and for monitoring drought impacts on vegetation and agricultural crops.
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 Vegetation Health Index (VHI).
- NOAA-9 (1985-1988), NOAA-11 (1989-1994), NOAA-14 (1995-2000), NOAA-16 (2001);
- Data Set:
- Global Vegetation Index;
- Advanced Very High Resolution Radiometer (AVHRR);
- Spatial - 4 km; Temporal - one week.
Kogan, F.N., 1997: Global Drought Watch from Space. Bulletin of the American Meteorological Society, 78, 621-636.