| Oasis is a kind of intrazonal landscape developed under the steady water supply and background of desert and semi-desert region, which plays an important role in regional economic development. But the complete and simplex dependency of oasis on water sources is doomed to the frangibility of oasis eco-system. It is incidental that land degradation would occur with decline or irrational use of water resources, by which the stability, ecological and economic safeties of oasis would be threatened.The process of vegetation cover change and its driving mechanism in desertification of oasis are profoundly studied based on a case study carried out in Minqin Oasis of Gansu Province, China, by using integrated TM image processing, quantitative analysis of filed positioning observation data and model simulation techniques, and a new method for desertification assessment in oasis is put forward.One outcome of this study is that the NDVI may prove to be better than SAVI, MSAVI and DEMI for the detection of small quantities of green vegetation based on the relationship between spectral indices and fractional vegetation cover, and DEMI is more effective than other 3 indices to minimize the soil influences on the extraction of vegetation information from images. NDVI is identified as a first chose vegetation index for measure of vegetation quantities in desert-oasis regions after integrated analysis of vegetation features and spectral reaction characteristics between vegetation and soil.An approach to estimate the fractional vegetation cover over oasis area is put forward based on NDVI and a dense sub-pixel vegetation model for estimation of fractional vegetation cover from TM images. The quantitative and spatial characteristics of vegetation cover change over Minqin Oasis from 1987 to 2001 are studied on the basis of above approach, and the study results indicate that the area of medium and high cover vegetation has decreased greatly and the area of cultivated vegetation has steadily increased over Minqin Oasis, leading to deterioration of eco-environment quality in the Oasis. There is a obvious trend that vegetation landscape in Minqin Oasis is fragmentized by degrees, the diversity and heterogeneity of landscape is aroused, and the oasis is developing to unstable status induced by decline of medium and high cover vegetation and landscape fragmentation. The result also reveals that the relationship between NDVI orfractional vegetation cover and rainfall is stronger than the relationship between NDVI or fractional vegetation cover and groundwater table although both are statistically significant in linear or nonlinear correlations. This manifests the growth of vegetation highly relies on rainfall rather than groundwater at present; therefore, both natural brushwood and artificial sand fixing shrubs around the Oasis are unsteady vegetation landscapes under present hydro-environmental condition due to the annual change of rainfall, and the correlation between NDVI or fractional vegetation cover and groundwater table represents for the trend of vegetation degradation.Considering at the spatial scales for desertification assessment, a new method for desertification assessment in oasis in which the rain use efficiency of vegetation is applied as a key assessment indicator is put forward. This method overcomes the shortcoming that the assessment result is commonly influenced by fluctuation of rainfall when vegetation indicators are used. The new method has been applied in desertification assessment of Minqin Oasis successfully.Desertification in Minqin Oasis presents not only in expansion of desertified area but also in upgrading of desertification degree. Reclamation, decrease of river runoff to Minqin and disappearance of natural water system after building the reservoir are main causes for promoting desertification In Minqin Oasis. All of these three factors causes the lowering of groundwater table and drought of soil directly or indirectly, and finally leads to degradation of vegetation. This is mayor process o...
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