Vegetation indices measure the amount of green vegetation over a given area and can be used to assess vegetation health. NASA’s Earth-observing satellites collect several different vegetation indices, including Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), Leaf Area Index (LAI), and Fraction of Photosynthetically Active Radiation (FPAR).

NDVI values range from -1 to 1, and represent the difference between near-infrared (NIR) and red reflectance divided by their sum. Low values of NDVI generally correspond to barren areas of rock, sand, exposed soils, or snow, while higher NDVI values indicate greener vegetation, including forests, croplands, and wetlands. EVI is similar to NDVI, but it minimizes canopy-soil variations and improves sensitivity over dense vegetation conditions.

LAI is the amount of leaf area in an ecosystem; more specifically, it is the one-sided green leaf area per unit ground area in broadleaf canopies, and is one-half of the total needle surface area per unit ground area in coniferous canopies. FPAR is the fraction of photosynthetically active radiation (400-700 nm) absorbed by green vegetation. Both of these measurements are used for calculating surface photosynthesis, evapotranspiration, and net primary production, which in turn are used to calculate terrestrial energy, carbon, water cycle processes, and biogeochemistry of vegetation.