What is NDVI?
Normalized Difference Vegetation Index
NDVI is a measure of “greenness” that can be computed for each map pixel. It is highly correlated with photosynthetically active vegetation (green biomass). Areas with higher NDVI values have greater amounts of green biomass.
Using NDVI to Measure Biomass
NDVI is calculated from the red and near-infrared light reflected by vegetation in the satellite images. Healthy vegetation (left) absorbs most of the red light that hits it and reflects a large portion of the near-infrared light. Unhealthy, senescing or sparse vegetation (right) reflects more red light and less near-infrared light.
Determining Vegetation Phenology
Tracking Growth Through Time
This illustrates how a vegetation phenology curve can be generated using the NDVI values for each 250 acre pixel within a region. Vegetation within a pixel that is stressed by drought has a curve that is different from the normal curve for that pixel. (click the image to the right to enlarge)
The Annual NDVI Cycle
On the left panel we have the Mauna Loa CO2 data from the corresponding period. The spinner is an annual plot. Notice the annual cycle of CO2 follows a distinct pattern that is a function of the green-up of the Northern Hemisphere. At the beginning of the year (where the red dot begins) we’re still accumulating CO2 in the atmosphere before spring comes on. Where it turns the corner is the beginning of May. Then, the CO2 starts to decline as the plants start to consume it, then late August/September, enough senescence has occurred that the CO2 reverses course and starts to build again.