黄河源区玛曲县不同沙化程度高寒草地光谱特征分析

Spectral characteristics of alpine grassland with different degrees of desertification in Maqu County,the source region of the Yellow River

高寒草地是黄河上游主要的生态屏障,受自然和人为的双重影响,草地沙化现象严重且监测和治理困难。因此,分析不同沙化程度的高寒草地光谱特征可为该区遥感监测及沙化程度精细识别提供充分的地面验证信息。研究基于玛曲县野外实测不同沙化程度高寒草地的高光谱信息,利用多种光谱变化方法对其光谱特征进行分析,结果显示:除极重度沙化的光谱曲线表现出典型荒漠化土壤的特征外,其他沙化程度草地的红谷、绿峰及三边参数均与绿色植被光谱特征保持一致,其中720-1000 nm是区分不同程度沙化类型草地的最佳波段,而在510-560和650-690 nm波段范围内的红谷和绿峰区分度不理想。利用一阶微分和包络线去除的光谱转化方法,对草地光谱反射率曲线进行处理,发现较好的可用于不同程度沙化草地区分的特征波段和参数有510-560、650-700 nm、红边幅值以及包络线去除后600-700 nm处的吸收深度,经过波段运算后的比值植被指数、绿红植被指数指数在不同程度沙化草地区分作用明显,同时对植被指数进行阈值化处理,可作为识别不同沙化程度草地的提供判别依据,研究成果为高寒区沙化监测及沙化程度精细识别提供数据支撑和参考。

Alpine grasslands are the ecological barrier in the upper reaches of the Yellow River.The increase in areas of bare sand following decreasing vegetation cover has become severe in grasslands,which is difficult to monitor and control because of natural and human influences.Analyzing the spectral characteristics of alpine grassland with different degrees of desertification can provide sufficient ground verification information for remote sensing monitoring and accurate identification of degrees of desertification in the alpine region,which is of considerable importance.Based on hyperspectral information from alpine grassland with different degrees of desertification in Maqu County field,the study analyzed the spectral characteristics by examining various mathematical variations.In terms of the results,the spectral curves were relatively complex and inefficient and showed representative characteristics of desertified soil.Meanwhile,the parameters of red valve ley,green peak,and the“three edges”of other sanded grasslands were consistent with the spectral characteristics of green vegetation.The red valley and green peaks in the 510-560 nm and 650-690 nm were not well differentiated.The first-order differential and envelope removal mathematical transformations were used to process the reflectance curves of the grassland spectra.It was found that the characteristic bands and parameters that could be used to differentiate between different levels of sandy grassland were 510-560 nm,650-700 nm,the red-edge amplitude,and the absorption depth at 600-700 nm after continuum removal.After the band calculation,the relative vegetation index(RVI)and green and red vegetation index(GRVI)indices were apparent in distinguishing different degrees of sandy grassland.Meanwhile,the vegetation indices were thresholded,which could be used as a basis for identifying grasslands with different degrees of desertification.The results provide data support and a key reference for monitoring loss of vegetation cover and effective analysis of the degree of desertification in alpine areas.