基于NDVI绿洲-荒漠过渡带宽度识别——以河西走廊中部荒漠绿洲为例

绿洲-荒漠过渡带是荒漠与绿洲之间的生态缓冲区,在维持绿洲能量流动、物质循环和景观稳定方面具有重要作用。过渡带宽度和属性直接影响到了其在整个绿洲系统中的功能发挥。以河西走廊中部张掖绿洲的Landsat(OLI)NDVI(归一化植被指数,Normalized Difference Vegetation Index)数据为基础,结合野外调查将过渡带类型划分为绿洲与石质裸山、砾质荒漠、沙质荒漠和人工固沙区4类。采用缓冲分析、分段线性趋势分析和尺度分别为30 m、90 m、210 m、330 m焦点分析等方法研究了不同类型过渡带宽度和尺度依赖特征。结果表明,在不同尺度上绿洲外缘NDVI变化存在二种线性回归趋势,趋势线交点至绿洲边界距离可确定为过渡带宽度。不同尺度分析表明,绿洲-石质裸山过渡带宽度为165—220 m,在其内NDVI线性变化趋势显著(P0.05)。绿洲-砾质荒漠过渡带宽度保持在330 m,在其内NDVI变化趋势极显著(P0.001)。绿洲-沙质荒漠过渡带宽度变化在230—290 m,NDVI变化趋势也为极显著(P0.001)。绿洲-人工固沙区过渡带宽度变化在570—580 m,与其它类型不同地是在过渡带内存在二种变化趋势,在210—240 m范围内变化趋势极显著(P0.001),超出此范围线性回归趋势不显著(P0.05)。

NDVI-based identification of oasis-desert transitional zone wideness: a case study in the central Hexi corridor

Oasis-desert transition zone, an ecological buffer area, plays an important role in maintaining the energy flow, material cycle and landscape stability in oasis ecosystem. The wideness and attributes of the transition zone directly affect its function within an entire oasis ecosystem. Based on the Landsat (OLI) NDVI data of Zhangye oasis in the middle of Hexi corridor, along with field investigation, this paper divides the oasis-desert transition zone into four types including oasis with stony bare mountain, gravel desert, sandy desert, and artificial sand-fixation zone. Then, buffer analysis, focal analysis and piecewise linear trend analysis were used to study the wideness and scale-dependence effects of different transition zones. The results showed that there were two kinds of linear regression trends in the variation of the NDVI at the outside edge of the oasis boundary, and the distance from the intersection of the trend line to the oasis boundary could be determined as the transition zone wideness. The focal analysis of different scales showed that the wideness between oasis and stony bare mountain was 165-220 m, where the linear variation trend of NDVI was significant (P < 0.05). The wideness between oasis and gravel desert transition zone was about 330 m in different focal scales, where the variation trend of NDVI was extremely significant (P < 0.001). The wideness between oasis and sandy desert varied from 230 m to 290 m, where the variation trend of NDVI was also extremely significant (P < 0.001). The wideness between oasis and artificial sand fixation varied from 570 m to 580 m, where two kinds of NDVI variation trends were observed, being extremely significant only within 210-240 m (P < 0.001), but not significant (P > 0.05) beyond this range.