Monitoring Oasis Degradation Using Coarse Resolution Remotely Sensed Vegetation Index Data

Oases in the Sahara Desert are undergoing rapid physical and socio-economic changes. High spatial resolution remote sensing data from the Landsat Thematic Mapper have been used in an attempt to monitor these changes, but because of their sparse temporal frequency, coupled with the significant intra-annual vegetation dynamics displayed by oases, such data are a poor sample from which to estimate oasis productivity, and results of such interpretation can be very misleading. This study assesses the utility of relatively coarse resolution global vegetation index data to monitor oases. The Pathfinder AVHRR Land (PAL) dataset, which provides estimates of Normalised Difference Vegetation Index (NDVI) every 10 days on a 8km grid, is able to resolve details of the cropping cycle, and provides a record going back to July 1981. Several oases in the Libyan Fazzān are used as examples, contrasting traditional agricultural systems alongside modern projects based on centre pivot irrigation. In most cases, productivity is seen to vary significantly over the period 1982-2002, but one project appears to have maintained a more steady production throughout the same period. New remote sensing products hold promise for future monitoring, which is essential to warn of potential catastrophic collapse of these vulnerable agricultural systems, but the longterm record provided by PAL is extremely useful for determining the regional pattern of oases productivity over the last 20 years.