Remote Sensing-based Characteristics of Grassland Degradation in the Arid Western Three-River Sources Regions of Qinghai in the Past 30 Years

Using three-phase remote sensing images(i.e.,1970sMSS,1990sTM and 2004TM/ETM) and field investigation,this paper reconstructed spatial and temporal characteristics of grassland degradation across the arid western Three-River Sources regions of Qinghai for resent 30 years.The results indicated that the proportions of grassland degraded area to total grassland area were approximately 10% of the whole study stages.The area of slight degraded grassland coverage to total grassland degraded area was 80%,followed by slight desertification and salinization of grassland,accounting for 10%.At spatial patterns,the reduced tendency of grassland degraded area was along a southwest to northwest direction in the study site.In this orientation,the grassland degradation degree presented declined stages,and the grassland degradation types were transiting from complexity to single.At temporal possess,the increasing tends were observed,compared with the area of grassland degradation of two stages,especially grassland desertification.Significant differences are among grassland degraded areas under different altitudes,slope gradients and aspects.A number of grassland degraded areas are mainly distributed in the altitude of 4800-5100m,and the grassland degradated area at 4500-4800m was not obviously different to that above 5100m.Furthermore,the proportions of grassland degraded area to total grassland area occurred at the slope gradient of 2-8o,and were similar to the distribution of grassland area under different slope gradients.At the same time,there was more grassland degraded area in shady and on subsunny slope than on sunny slope.The reasons for these results were due to the limited factors, including vulnerable environment and interannual periodic fluctuations of extreme climate which controlled the trends and processes of grassland degradation,while human disturbances,as incentive forces,determined the intensity and speed of grassland degradation.In short,grassland desertification associated with natural conditions has been in a dominant position in the study site.Human activities were relatively weakening.The increasing trends of grassland degradation were not pronounced,and new grassland degradation phenomenon was not very obvious.