Intercomparison and sensitivity analysis of Leaf Area Index retrievals from LAI-2000, AccuPAR, and digital hemispherical photography over croplands
Intercomparison and sensitivity analysis of Leaf Area Index retrievals from LAI-2000, AccuPAR, and digital hemispherical photography over croplands
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DOI：
10.1016/j.agrformet.2008.02.014
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发表时间：
2008-07-04
影响因子：
6.2
通讯作者：
Myneni, R. B.
中科院分区：
文献类型：
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作者：
Garriques, S.;Shabanov, N. V.;Myneni, R. B.
由中分辨率遥感观测得到的叶面积指数(LAI)的验证通常涉及到测量地面LAI的光学技术。由于目前的验证数据集是使用多种光学检索技术得出的，因此需要对这些技术之间的一致性进行评估。本研究利用LAI-2000、AccuPAR和数字半球像(DHPS)三种主要光学仪器对阿根廷科尔多瓦省曼弗雷迪的10种作物(大豆、玉米、紫花苜蓿、高粱、花生和牧草)的有效植物面积指数(PAI(Ef))进行了分析。研究的重点是量化PAI(Ef)对仪器类型、反演参数和林隙分数反演方法以及环境条件(冠层异质性、衰老植被、光照条件)的敏感性。结果表明，DHP方法对光照条件的敏感度较低(14%，而LAI-2000和AccuPAR分别为28%和86%)。PAI(Ef)反演结果的对比表明，短冠层光学技术之间存在较大差异，其中向下DHP技术优于LAI-2000和AccuPAR技术。没有衰老植被和低空间异质性的高大冠层具有更好的一致性。总体而言，仪器之间PAI(Ef)的差异主要是由于仪器占地面积、方位范围和天顶角空间分辨率(LAI-2000比DHP粗略)造成的仪器之间透过率的空间采样差异(在短的和不同的冠层上)。我们的结果表明，DHP是最稳健的技术，它对光照条件的敏感度低，透过率的空间采样准确，能够使用俯视照片获取短冠层上的林隙分数，独立于冠层光学辅助信息，并且有可能推导出聚集指数。因此，它可以应用于验证协议所要求的大范围的伞盖结构和环境条件。(C)2008爱思唯尔B.V.保留所有权利。
Validation of Leaf Area Index (LAI) derived from moderate resolution remote sensing observations generally involves optical technique to measure ground LAI. As the current validation datasets are derived using multiple optical retrieval techniques, assessment of the consistency between these techniques is required. in this study the effective Plant Area Index (PAI(eff)) retrievals by three major optical instruments, LAI-2000, AccuPAR, and Digital Hemispherical Photographs (DHPs), were analyzed over 10 crops (soybean, com, alfalfa, sorghum, peanut and pasture) at Manfredi site in Cordoba province, Argentina. The focus of research was on quantifying PAI(eff) sensitivity to the type of instrument, retrieval parameters and gap fraction inversion methods as well as environmental conditions (canopy heterogeneity, senescent vegetation, illumination conditions). Results indicate that sensitivity of DHP method to illumination conditions is low (14% compared to 28% and 86% for LAI-2000 and AccuPAR, respectively). The intercomparison of PAI(eff) retrievals indicates large discrepancies between optical techniques for short canopy over which downward-pointing DHP technique performs better than LAI-2000 and AccuPAR. Better agreement was found for tall canopy without senescent vegetation and low spatial heterogeneity. Overall, discrepancies in PAI(eff) between instruments are mainly explained by differences in spatial sampling of transmittance between instruments (over short and heterogeneous canopies) caused by variations in instrument footprint, azimuthal range, and zenith angle spatial resolution (coarser for LAI-2000 than DHP). Our results indicate that DHP is the most robust technique in terms of low sensitivity to illumination conditions, accurate spatial sampling of transmittance, ability to capture gap fraction over short canopy using downward-looking photographs, independence from canopy optical ancillary information, and potential to derive clumping index. It can thus be applied to a large range of canopy structures, and environmental conditions as required by validation protocols. (C) 2008 Elsevier B.V. All rights reserved.