The Normalized Difference Vegetation Index (NDVI) is a measure of the amount and vigor of vegetation on the land surface and NDVI spatial composite images are developed to more easily distinguish green vegetation from bare soils.

In general, NDVI values range from -1.0 to 1.0, with negative values indicating clouds and water, positive values near zero indicating bare soil, and higher positive values of NDVI ranging from sparse vegetation (0.1 - 0.5) to dense green vegetation (0.6 and above).

NDVI is also directly related to the:

• “leaf area index” (LAI), which is often used in crop growth models
• herbaceous or total green biomass (tons/ha) for given vegetation types,
• photosynthetic activity of the vegetation
• percent ground cover.

Indirectly, NDVI has been used to estimate the cumulative effective of rainfall on vegetation over a certain time period, rangeland carrying capacity, crop yields for different crop types, and the quality of the environment as habitat for various animals, pests and diseases.

NDVI is calculated from satellite imagery whereby the satellite’s spectrometer or radiometric sensor measures and stores reflectance values for both red and NIR bands on two separate channels or images. Kriegler, et al. (1969) were the first to propose NDVI and it is calculated by subtracting the red channel from the near-infrared (NIR) channel and dividing their difference by the sum of the two channels, or:

NDVI= (NIR - RED ) / (NIR + RED )

where, RED = the red portion of the electromagnetic spectrum (0.6-0.7 μm) and
NIR = the near infrared portion of the electromagnetic spectrum (0.75-1.5 μm).

In addition, for agricultural and vegetation condition monitoring, clouds are partially screened from NDVI images by producing Maximum Value Composites (MVC) over 10-day, 16-day, or 1-month periods, where the highest NDVI pixel value within the time period is retained under the assumption it represents the maximum vegetation "greenness" during the period.

The Global Agriculture Monitoring (GLAM) project collects and processes NDVI data for display in Crop Explorer and for use by USDA’s Office of Global Analysis (OGA) of the Foreign Agricultural Service (FAS) crop analysts. NDVI data in Crop Explorer is derived from the raw digital imagery for the red and NIR channels collected from several different satellites, namely the Advanced High Resolution Radiometer (AVHRR) sensor placed on-board the National Oceanic and Atmospheric Administration (NOAA) satellite series with Global Area Coverage (GAC) or 4.0-kilometer spatial resolution reprojected to 8-kilometer pixels; The VEGETATION instrument on the Systeme Probatoire pour l’Observation de la Terre (SPOT)-4 satellite with 1.1-kilometer spatial resolution; and the Moderate Resolution Imaging Spectroradiometer on-board NASA’s Terra and Aqua satellites with 250-meter spatial resolution.

The long-term and short-term NDVI averages for AVHRR-GAC data, SPOT-VEG, and MODIS are processed by the Global Environmental Monitoring and Modeling Studies (GIMMS) group from the National Aeronautics and Space Administration (NASA) by calibrating all NDVI composites for changes in satellite-series sensors, radiometric sensor decay, atmospheric changes caused by volcanic eruptions, etc. Regional droughts and wet seasons can then be easily determined by comparing the current NDVI composites with the short-term or long-term NDVI averages from the same period.

References
Kriegler, F.J., Malila, W.A., Nalepka, R.F. and Richardson, W., 1969, Preprocessing transformations and their effect on multispectral recognition, in: Proceedings of the sixth International Symposium on Remote Sensing of Environment, University of Michigan, Ann Arbor, MI, pp. 97-131

NDVI time-series graphs for specific crop regions can be viewed by clicking on the NDVI or NDVI departure images.

NDVI time-series graphs are useful for estimating crop stage or growth period (i.e., planting, vegetative development, flowering, grain-filling, etc.) and observing when periods of dryness or drought stress occurred during the growing season.

In general, when NDVI values begin to increase, it corresponds to the start of the growing season and the period of maximum NDVI approximately corresponds to the end of vegetative development or the beginning of the flowering stage for most climatic regions. It also is useful to monitor how long NDVI departures were below-average during the growing season and if any these dry periods occurred during critical crop stages that are especially sensitive to water stress, such as flowering or early ripening stages, for which several dry periods during the flowering and grain-filling stages can severely reduce potential grain yields.

Comparing current NDVI images with historical average NDVI images are useful to monitor positive and negative deviations during the growing season and assess the relative vegetation condition of the entire growing season.

An NDVI departure from average image is calculated by subtracting the current NDVI composite with the historical average NDVI composite, whereby the following average baseline periods are used for each sensor:

• GAC-NDVI (AVHRR-NOAA Long-term average): July 1981 - June 2006
• SPOT Vegetation NDVI (Short-term average): January 1999-December 2002 and June 2006-May 2008
• MODIS-NDVI (Short-term average): February 2002- Present

Since NDVI is as an indicator of the amount of vegetation greenness vigor within in a pixel, positive NDVI departures from average are shaded from light green to dark green to depict above-average vegetation conditions, and negative NDVI departures from average are shaded from light yellow to dark brown to depict below-average vegetation conditions. Correspondingly, pixels or regions shaded with dark brown colors tend to indicate vegetation that probably experienced water stress or drought for duration of several 10-day periods or more.

Comparing the current NDVI image with last year’s NDVI image for the same time period helps to determine if current crop conditions are better than last year.

Current NDVI departure from last year’s NDVI image is calculated by subtracting the current NDVI composite image with last year’s NDVI image for the same time period.

One should be careful when comparing NDVI images between current and last year because the start or rains for each year can be different by several weeks and hence the crop stage or greenness vigor also can be slightly different for the same time period.

Comparing the current NDVI image with the previous dDekadal (10-day) NDVI image helps to monitor the current season and determine if vegetation growth increased or declined during the past 10-days. Vegetation growth is typically caused by rainfall events that occurred during the previous 10-days because vegetation greenness response to rainfall (or lack of rainfall) tends to be delayed by 1-2 weeks.

Current NDVI departure from the pervious dekadal NDVI image is calculated by subtracting the current NDVI composite image with the previous dekadal NDVI composite image.

The VEGETATION sensor is onboard the Spot 4 satellite which was launched on March 24, 1998. It provides global coverage on an almost daily basis at a spatial resolution of 1 kilometer.

The SPOT-VEG program is co-funded by the European Union, Belgium, France, Italy and Sweden and led by French space agency CNES. The SPOT-VEG spectral bands were designed specifically to study vegetation cover and its temporal dynamics; red (0.61 to 0.68 µm), near-infrared (0.78 to 0.89 µm), short-wave infrared (1.58 to 1.75 µm); with a blue band (0.43 to 0.47 µm) for atmospheric corrections. The red and near-infrared bands are used for making maximum value Normalized Difference Vegetation Index (NDVI) composites every 10-days.

Additional SPOT-VEG information can be found at:

http://www.spotimage.fr/home/system/introsat
 /payload/vegetati/welcome.htm

http://spot4.cnes.fr/spot4_gb/