Dynamic Change of Vegetation Coverage in Water-wind Erosion Ecotone in the Loess Plateau

NDVI( normalized difference vegetation index) of GIMMS( global inventory modeling and mapping studies) was taken as the data source and the dimidiate pixel model was used to extract the data of vegetation coverage with different grades in water-wind erosion ecotone in the Loess Plateau in periods of 1982- 1989,1990- 1999and 2000- 2006. The transition matrix model was used to quantitatively analyze the change of vegetation coverage in the study area. The result showed that the vegetation coverage in the study area was holistically in an increase trend,and it was temporally increased at first and then decreased. Viewing from the spatial distribution,it was found that the proportion of moderate-low vegetation coverage was the highest,then that of low and moderate vegetation coverage,and that of high vegetation coverage was the lowest. In general,the development of vegetation coverage in the study area was virtuous from the period Ⅰ( 1982- 1989) to the period Ⅲ( 2000- 2006),and the environment was holistically improved except in some few regions. Analysis revealed that the center of low vegetation coverage was shifted southwestward,and that of moderate-high and high vegetation coverage was shifted eastward,which indicated that the vegetation in Ningxia and central Gansu in southwest region of the study area was degenerated due to the decrease of precipitation and the impact of urbanization in recent 25 years. However,the ecological construction including the conversion of cropland to forest and grassland has brought good ecological benefits in Wuqi,Zhidan,Ansai,Yan'an and Dongsheng in Mu Us Sandy Land in the eastern part of the study area