ASSESSING LAND COVER AND DEGRADATION IN CENTRAL ASIA DESERTS USING SATELLITE IMAGE PROCESSING AND GEOSTATISTICAL METHODS

摘要

Desertification around watering points has been well observed by satellite images in many drylands around the world. It can be recognized as radial brightness belts fading as a function of the distance from the wells. The primary goal of the study was to characterize the spatial and temporal land degradation/rehabilitation in the Central Asia drylands, in term of vegetation and soil patterns, in different time periods, with respect to the socio-economic changes before and after the collapse of the Soviet Union. More specific objectives of the study were: (1) to develop a geo-statistical model, based on the kriging technique and using high-resolution satellite image processing in order to assess spatial and temporal land cover patterns in three key different time periods (mid-late 1970's, late 1980's, and 2000); (2) to conduct a change detection analysis based on the geo-statistical products in order to assess the direction and intensity of changes between the study periods; and (3) to link the previous findings to the socio-economic situations before and after the collapse of the Soviet Union that influenced the grazing gradients and hence the landuse/ land-cover state of the study site. The Tassel-Cup's Brightness Index was found as the best spectral transformation for enhancing the contrast between the bright degraded areas close to the water wells and the darker surrounding areas far and in-between these wells. Empirical variograms were computed for each of the images and the exponential models were fitted. The Kriging geo-statistical technique utilized the variograms for creating brightness maps. The maps demonstrate the grazing gradient as levels of degrading belts around the wells. Change detection analysis, based on the Kriging maps, reveals some land rehabilitation between the 1975 and the 1987 images. However, mixed results, degradation and rehabilitation, were observed between the 1987 and the 2000 images. Degradation of the area occurs due to recent exploration and exploitation of the gas and oil reserves in the region. Consequently, large areas went through intensive 'technological desertification' that means utilizing large amount of heavy-duty equipments, large-scale plants, and vehicles that damage the soil surface. The rehabilitation of the rangelands can be explained by the historical events of the last decades. Following independence of the former Soviet states in 1991 and the imposition of difficult economic conditions with transition reforms, several major socio-economic changes occurred that caused drastic declines in livestock populations, with the major drop in the number of sheep and goats, and hence vegetation recovery and land rehabilitation.