UTILITY OF AVHRR DATA FOR LAND DEGRADATION ASSESSMENT - A CASE-STUDY

AVHRR data are widely used to monitor vegetation greenness and to prov ide a gross measure of primary production throughout the world. This p aper examines whether AVHRR data can be used for determining the exten t of land degradation in arid rangelands under commercial grazing usin g models of vegetation dynamics and animal grazing behaviour developed for Landsat-MSS data. These models are applied after large rainfall e vents and either search for systematic change in average vegetation co ver across relatively uniform land-scapes with increasing distance fro m stock watering points or examine the magnitude of vegetation respons e to rainfall for each pixel. We applied the models where previous wor k with Landsat-MSS had demonstrated the extent of grazing impact. An i ndex of vegetation cover using adjusted AVHRR channel 1 values produce d trends in wet period average vegetation cover with increasing distan ce from water similar to, but less pronounced than, those obtained wit h MSS data. NDVI produced inconsistent and often ambiguous results whe n compared with the MSS data. AVHRR-derived vegetation indices were un usable in degradation assessment procedures which require pixel-scale vegetation response models. The large AVHRR pixel, even in LAC mode, c reates difficulties in detecting grazing impact. Landscape changes as a result of grazing occur at a finer scale and are therefore subsumed within the pixel. Mis-registration of multi-temporal images further re duces the ability to detect grazing impact on a pixel basis when such change is occurring within the pixel. We conclude that despite their c ost attractiveness, AVHRR data are inappropriate for the reliable dete ction of grazing impact using grazing gradient methods in the large pa ddocks of arid rangelands.