Mapping vegetation-soil-climate complexes in southern Africa using temporal Fourier analysis of NOAA-AVHRR NDVI data
The distribution and phenology of vegetation is largely associated with climate, terrain characteristics and human activity. Satellite data provide the opportunity to monitor continuously the dynamics of vegetation, its changes and its impact on the environment. Monthly normalized difference vegetation index (NDVI) values which had been extracted from NOAA-AVHRR GAC data for Southern Africa (south of the Equator) from 1981 till 1991 were analysed using the Fast Fourier Transform (FFT). The FFT gives a new representation of the time series of images, i.e. a set of images of amplitude and phase (pixelwise) of periodic functions with different frequencies, which allows the analysis of the vegetation phenology using only the amplitude and phase of the most important periodic components. This approach is a powerful way to monitor various dynamic parameters of the vegetation in Southern Africa. The relationship between amplitude values and aridity and vegetation type was studied by correlating the amplitude and phase images with maps of the Budyko Aridity Index (ratio of net radiation to rainfall) and of the vegetation (White map). Images of amplitude and phase values were then used as attributes to map land units homogeneous as regards leaf display phenology. The map of these land units in Southern Africa, which has a 7.6 km resolution, was based on long term (9 years) information of 19 different vegetation-soil-climate complexes. Ten out of the 19 vegetation-soil-climate complexes are described in detail.
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