漓江上游植被覆盖度时空变化对地表热场影响

INFLUENCE OF VEGETATION COVERAGE SPATIO-TEMPORAL VARIANCE ON LAND SURFACE THERMAL FIELD IN UPPER REACHES OF THE LIJIANG RIVER

自20世纪80年代以来,桂林漓江由于上游森林遭受破坏出现水生态环境快速恶化的趋势。地表热场的时空变化直接影响森林蒸散及水文效应。因此,对漓江上游1989～2006年5景TM/ETM+卫星图像,利用归一化植被指数反演植被覆盖度,利用单窗算法反演地表温度,提出归一化温度使不同时相地表热场具有可比性,分析植被覆盖度时空变化对地表热场的影响。结果表明,在空间上,地表温度随植被覆盖度的增加而降低,在植被覆盖度很高或很低时,这种影响相对变弱;在时间上,漓江上游1989～2000年植被中、高度覆盖面积从96.1%逐年下降到65.9%,导致归一化地表温度数值较高的像元比例相应增加;2000～2006年植被中、高度覆盖面积逐渐恢复上升到90.8%,但仍略低于1989年,使得归一化地表温度数值较高的像元比例相应减少。植被覆盖可以有效地降低漓江上游地表温度。

Ecological environment of the Lijiang River in Guilin has been degenerating quickly since the forest on its upper reaches was devastated for the eighties of last century.Spatio-temporal variance of land surface thermal field affects forest evapotranspiration and hydrological effect directly.Therefore,vegetation coverage was retrieved with normalized vegetation index,and land surface temperature was done with Mono-window algorithm respectively from five TM/ETM＋images of upper reaches of the Lijiang River during 1989 to 2006.Normalized temperature was proposed so that different temporal land surface thermal fields could be compared with each other.Effect that spatio-temporal variance of vegetation coverage affected land surface thermal field was analyzed.Results showed that land surface temperature decreased with vegetation coverage increasing.The effect became weak when vegetation coverage was very high or very low.From 1989 to 2000,the proportion of medium and high vegetation coverage area droped from 96.1% to 65.9%.It resulted in the increasing of the pixels with higher normalized surface temperatures.Although the proportion of medium and high vegetation coverage area rose to 90.8% from 2000 to 2006,it was still a little lower than that in 1989.It resulted in the decreasing of the pixels with higher normalized surface temperatures.Vegetation coverage could effectively reduce land surface temperature on the upper reaches of the Lijiang River.