南方典型红壤水土流失区地表裸土动态变化分析--以福建省长汀县为例

Dynamics of Bare Soil in A Typical Reddish Soil Loss Region of Southern China:Changting County,Fujian Province

基于遥感数据和改进的双重指数法,从裸土面积、裸露强度和破碎度三方面研究了福建省长汀县1988~2010年的地表裸土时空变化。研究表明,该县近22 a的水土流失治理已大大减轻了地表的裸露程度,地表裸土面积从175.5 km2减少到86.6 km2;地表裸露强度从0.487下降到0.383;裸土破碎度从0.103上升到0.207。在1988~1999年和1999~2010年的2个时间段里,裸土面积的变化呈现逐渐减少,减速加快的趋势,客观地反映了长汀县水土流失治理的重要扶持政策所产生的积极效应。

Changting County of Fujian Province is one of the most typical reddish soil loss regions in southern China. Serious soil erosion has resulted in environmental and ecosystem degradation in the county. Local people and governments have been struggling for the control of the soil loss for more than two decades. To investi- gate and assess the changes after the effort, this study used remote sensing technology to analyze spatiotemporal dynamics of the soil exposure degree in the county during the period from 1988 to 2010. The bare soil features were estimated from the Landsat TM/ETM＋ images of 1988, 1999, 2004 and 2010 based on two thematic indices-normalized difference soil index （NDSI） and normalized difference impervious surface index （NDISI）. The used four images were geometrically corrected and registered to allow spatial overlay analysis between im- ages. After the geometrical correction, the digital number （DN） values of the images were converted to at-satellite reflectance. The NDSI was used to enhance and extract soil features from the four used images with proper threshold values. However, the extracted soil features were found to have been mixed with built-up land fea- tures due largely to the spectral confusion between the two categories and the similar result in the NDSI perfor- mance. In order to improve the accuracy of the extracted soil feature, the NDISI was further used to filter out the unwanted built-up land features from the extracted soil features using a logical tree algorithm, as the NDISI is the index specially designed for the highlight of impervious surface features represented mainly by built-up lands. This significantly improved the estimate of bare soil and the extraction accuracy can reach up to 95.95%. Based on the extracted bare soil features, the exposure degree of bare soil and spatial and temporal changes of bare soil were analyzed. Results showed that the 22-year fight for treating soil loss has greatly re- duced exposed bare soils in Changting County. The area of bare soil has been greatly reduced from 175.5 km2 in 1988 to 85.6 km2 in 2010, bare-soil exposure intensity dropped down from 0.487 to 0.383, and fragmenta- tion degree of bares soil increased from 0.103 to 0.207. In the two observed periods （1988-1999 and 1999-2010）, the decrease in bare soil area was in an accelerate speed, which was -3.74 km2 per year during the 1988-1999 period and -4.34 km2 per year in the duration between 1999 and 2010. This corresponds closely to the two major policies issued by the Fujiang Provincial Govenment specially for the county＇ s soil loss treatment. Obviously, government policies accompanied with effective actions were the major driving forces to the reduction of the exposure degree of bare soil in the county during the study period.