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Vegetation in arid community grazinglands shows monsoonal growth. Its matching phenology with crops
makes its detection difficult during July to September. While crops are harvested during September-October, using
satellite data thereafter for the natural vegetation seems most appropriate but by then it turns dry. An index capable of
sensing dry vegetation was needed since conventional NDVI is sensitive to greenness of vegetation. Performance of
NDVI vis-à-vis another index, PD54, based on cover was therefore compared in assessing degradation of grazinglands.
The PD54 was used to isolate anthropogenic impacts from environmental induced degradation by analyzing satellite
images from dry and wet seasons. Substantial absence of appreciable vegetation response indicated poor resilience and
severe degradation.
Five grazinglands in Shergarh tehsil of Jodhpur district in Rajasthan were studied following above approach.
Ground radiometric observations were recorded. Satellite data of IRS 1C/1D/P6 with LISS 3 sensor for both pre and
post monsoon season were acquired for three contrasting wet-dry season events.
These were geometrically registered and radiometrically calibrated to calculate an index of vegetation cover
PD54 as well as NDVI. PD54 is a perpendicular vegetation index based on the green and red spectral band width. The
PD54 and NDVI calculated from spectro-radiometer were related to vegetation cover measured on ground in permanent
plots. This confirmed that PD54 was superior index for estimating cover in arid dry grasslands.
These ground vegetation trends in a good rainfall year (2001) with drought year (2002) were related with
satellite data for a protected and four unprotected grazinglands. NDVI failed to detect any vegetation in protected areas
supporting excellent grass cover which was succinctly brought out by PD54. Successful validation of PD54 in detecting
degradation of 13 additional sites confirmed its efficacy. These findings have implication in forage availability
assessments, forage forecasting, drought preparedness, pastoralism and transhumance.
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