NDVI sensitivity to atmospheric water vapor as a function of spectral bandwidth

The Normalized Difference Vegetation Index (NDVI) has been an important data product of the AVHRR series of instruments that have flown on the NOAA polar orbiting satellites. This index has been extensively used to monitor global vegetation and to study deforestation. As planning begins for follow-on versions, a proposal has been made to improve the robustness of the index by narrowing the spectral bands that are used in the NDVI calculation to reduce sensitivity to atmospheric water vapor. Without dramatic improvements in instrument design, this narrowing will also degrade the instrument´s signal-to-noise ratio. In this paper, results are presented of an investigation into the sensitivity of the NDVI to water vapor as a function of the width of the spectral bands. Consideration was also given to the increase in instrument noise. This study concluded that spectral bandwidths of around 50 nanometers centered on two water vapor transmittance windows provide sufficient insensitivity to water vapor without significantly degrading the instrument´s noise performance. However, it should be noted that changing the bandwidths from the current AVHRR will complicate the use of the index for long term environmental studies by changing the values obtained for similar scenes