# Weekly NDVI

To assess the condition of the vegetation for the purpose of rating fire danger in conterminous U.S., weekly images of Normalized Difference Vegetation Index (NDVI) are utilized. NDVI is derived using the following equation:

$$NDVI= \frac{Near Infrared-Red Visible Wavelength}{Near Infrared+Red Visible Wavelength}$$

NDVI ranges from -1.0 to 1.0, with features such as barren, sand, and snow resulting in low values (0.1 or less) and surfaces with sporadic vegetation such as grasslands and senescing vegetation registering moderate NDVI values (0.2 – 0.5). In contrast high and dense vegetation coverage on a surface gives high NDVI indexes (0.6 – 0.9). For the purposes of generating Fire Danger Forecast products, 1 km resolution, 7‑day Visible Infrared Imaging Radiometer Suite (VIIRS) NDVI composites are used (Figure 1).

Weekly Relative Greenness:

Relative greenness (RG) (Figure 2) is calculated from weekly composites of the NDVI previously derived from Moderate Resolution Imaging Spectrometer (MODIS) NDVI data, and more recently from VIIRS NDVI composites. RG indicates how green each pixel currently is in relation to the range of historical NDVI observations for it. RG values are scaled from 0 to 100, with low values indicating the vegetation is at or near its minimum greenness. The purpose of using relative greenness in the Wildland Fire Potential Index model is to define the proportion of live and dead vegetation. Specifically, RG is derived using the following equation:

RG=(NDo−NDmn)(NDmx−NDmn)∙100

Where, NDo = highest observed NDVI value for the 1-week composite period, NDmn = historical minimum NDVI value for a given pixel, NDmx = historical maximum NDVI value for a given pixel