Wang Qiang, Yi Qiu-Xiang, Bao An-Ming, Zhao Jin
Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China.
Guang Pu Xue Yu Guang Pu Fen Xi. 2013 Feb;33(2):507-12.
Proper vegetation indices have decisive influences on the precision of hyperspectral estimation models for surface parameters. In the present paper, in order to find the proper hyperspectral indices for cotton canopy water content estimation, two water parameters for cotton canopy water content (EWT(canopy), equivalent water thickness; VWC, vegetation water content) and corresponding hyperspectra data were analyzed. A rigorous search procedure was used to determine the best index predictors of cotton canopy water. In the procedure, all possible ratio indices and normalized difference indices were derived from the canopy hyperspectra, involving all the two-band combinations between 350 nm and 2500 nm. Then the correlation between two water parameters and all combination indices were analyzed, and the best indices which produced maximum correlation coefficients were determined. Finally, the indices were compared with the published water indices for their performances in estimation of cotton canopy water content. The results showed that for the estimation of EWT(canopy), the new developed ratio index R1 475/R1 424 and normalized difference index (R1 475 -R1 424)/(R1 475 + R1 424) was the most proper one, and the correlation coefficient of the estimated and measured EWT(canopy) reached 0.849. For the estimation of VWC, the performance of published index was better than new developed index, the best suitable water indices for VWC estimation were (R835 - R1 650)/(R835 + R1 650), and the correlation coefficient of the estimated and measured VWC was 0.849.
合适的植被指数对地表参数的高光谱估计模型精度具有决定性影响。在本文中，为了找到用于棉花冠层含水量估计的合适高光谱指数，分析了两个用于棉花冠层含水量的水分参数（EWT(冠层)，等效水厚度；VWC，植被含水量）以及相应的高光谱数据。采用严格的搜索程序来确定棉花冠层水分的最佳指数预测因子。在该程序中，从冠层高光谱中导出所有可能的比值指数和归一化差异指数，涉及350纳米至2500纳米之间的所有双波段组合。然后分析两个水分参数与所有组合指数之间的相关性，并确定产生最大相关系数的最佳指数。最后，将这些指数与已发表的水分指数在估计棉花冠层含水量方面的性能进行比较。结果表明，对于EWT(冠层)的估计，新开发的比值指数R1 475/R1 424和归一化差异指数(R1 475 -R1 424)/(R1 475 + R1 424)是最合适的，估计的EWT(冠层)与实测值的相关系数达到0.849。对于VWC的估计，已发表指数的性能优于新开发的指数，用于VWC估计的最合适水分指数是(R835 - R1 650)/(R835 + R1 650)，估计的VWC与实测值的相关系数为0.849。
