叶倾角分布对离散冠层反射率的影响

Influence of Leaf Angle Distribution on Discrete Canopy Reflectance

叶倾角分布(leaf angle distribution,LAD)是植被冠层最重要的结构参数之一,直接决定冠层对太阳辐射的截获量,进而影响植被冠层在不同波段的反射率。相比基于实测数据开展的研究,计算机模型模拟能够较好地排除扰动因子的影响,实现特定冠层结构参数对冠层反射率的影响研究。文章基于DART(discrete anisotropic radiative transfer)模型,研究LAD对离散冠层多角度反射率的影响。结果表明,LAD对冠层反射率影响较大,且影响程度与模拟波段和观测天顶角等有密切关系。在“暗点”方向,近红外波段和红光波段水平分布的冠层反射率分别比喜直型分布冠层反射率高63%和25%。进一步研究发现,LAD对常见植被指数也有较大影响。当观测天顶角为0°时,水平分布的RVI(ratio vegetation index)、EVI(enhanced vegetation index)和NDVI(normalized difference vegetation index)值分别比喜直型分布对应的数值高52%、52%和13%;相比之下,PRI(photochemical reflectance index)对LAD变化的敏感度较低,但当在前向散射方向观测天顶角大于20°时,LAD对PRI的影响同样不容忽视。文章定量分析了不同LAD对冠层反射率及常见植被指数的影响,研究结果对离散冠层的参数反演有重要意义。

Leaf angle distribution(LAD)is one of the most important structural parameters of the canopy,which directly determines the interception of solar radiation by the canopy,further affects the reflectance of the canopy in different wavebands.Compared with research based on measured data,computer model simulation can better eliminate the influence of disturbing factors,and carry on the research on the influence of specific canopy structure parameters on canopy reflectance.Based on the DART(discrete anisotropic radiative transfer)model,this paper studies the influence of LAD on the multi-angle reflectance of the discrete canopy.The results show that LAD has a great influence on the canopy reflectance,and the degree of influence is closely related to the simulated waveband and the observation zenith angle.In the“dark spot”direction,the reflectance of the horizontal distribution canopy in the near-infrared band and the red-light band is 63%and 25%higher than that of the erectophile,respectively.Further research finds that LAD also has a great impact on common vegetation indices.When the observation zenith angle is 0°,the RVI(ratio vegetation index),EVI(enhanced vegetation index),and NDVI(normalized difference vegetation index)values of the horizontal distribution are 52%,52%,and 13%higher than the values corresponding to the erectophile,respectively;in contrast,PRI(photochemical reflectance index)is less sensitive to LAD changes,but when the observation zenith angle is bigger than 20°,the impact of LAD on PRI cannot be ignored.This paper quantitatively analyzes the effects of different LAD on canopy reflectance and common vegetation indices,and the results of the study are of great significance to the inversion of discrete canopy parameters.