THE EFFECT OF WATER-VAPOR ON THE NORMALIZED DIFFERENCE VEGETATION INDEX DERIVED FOR THE SAHELIAN REGION FROM NOAA AVHRR DATA

The near-infrared channel of the NOAA advanced very high resolution radiometer (AVHRR) contains a water vapour absorption band that affects the determination of the normalized difference vegetation index (NDVI). Daily and seasonal variations in atmospheric water vapour within the Sahel are shown to affect the use of the NDVI for the estimation of primary production. This water vapour effect is quantified for the Sahel by radiative transfer modelling and empirically using observations made in Mali in 1986. In extreme cases, changes in water vapour are shown to result in a reduction of the NDVI by 0.1. Variations of the NDVI of 0.01 would result from typical low atmospheric water vapour days within the wet season. If these conditions were to persist throughout the season it would lead to an overestimate of production of 200 kg ha-1. The measurement of atmospheric water vapour using the AVHRR thermal channels, the high-resolution Infrared Sounder 2 (HIRS2), and the microwave sounding unit (MSU) sensors, which are all carried on the NOAA satellites, is discussed. A procedure for operational correction of the water vapour effect on the NDVI is suggested; however, additional studies over a wider range of Sahelian conditions are recommended.