Remote sensing of weather impacts on vegetation in non-homogeneous areas

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Abstract
Successful application of the normalized difference vegetation index (NDVI) for estimating weather impacts on vegetation is currently hindered in non-homogeneous areas. The problem is that the differences between the level of vegetation in these areas can be related, in addition to weather impacts, to the differences in geographic resources (climate, soil, vegetation type and topography). These differences should be eliminated when weather impacts on vegetation are estimated from NDVI data. This paper discusses a concept and a technique for eliminating that portion of the NDVI which is related to the contribution of geographic resources to the amount of vegetation. The Advanced Very High Resolution Radiometer (AVHRR) data of the Global Vegetation Index format were used for the 1984-1987 seasons in Sudan. The procedure suggests normalization of NDVI values relative to the absolute maximum and the absolute minimum of NDVI. These two criteria were shown to be an appropriate characteristic of geographic resources of an area. The modified NDVI was named the Vegetation Condition Index (VCI). Comparison between VCI, NDVI and precipitation dynamics showed that the VCI estimates better portray precipitation dynamics as compared to the NDVI. The VCI permits not only the desciption of vegetation but also estimation of spatial and temporal vegetation changes and weather impacts on vegetation.