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A multi-angular invariant spectral index for the estimation of leaf water content across a wide range of plant species in different growth stages
Remote Sensing of Environment ( IF 13.5 ) Pub Date : 2021-02-01 , DOI: 10.1016/j.rse.2020.112230
Xiao Li , Zhongqiu Sun , Shan Lu , Kenji Omasa

Abstract Plant leaf water content plays a key role in several biogeochemical processes, such as photosynthesis, evapotranspiration, and net primary production. Yet, the accurate estimation of leaf water content using multi-angular reflectance measurements across different plant species is still challenging. This study aims to propose a generic spectral index for accurately estimating equivalent water thickness (EWT) when multi-angular spectral reflection is considered. The index was selected to have the format of a difference ratio using three reflectance factors. The reflectance factor at 410 nm was used to reduce the specular reflection from the leaf surface in the 400–2500 nm range, and the ratio of the wavelengths in the 1268–1285 nm range (at non-water absorption wavelengths) to the wavelengths in the 1339–1346 nm range (at water absorption wavelengths) strengthened the relationship with EWT for all of the sampled plant species. The modified difference ratio (MDR) index, (R1271-R410)/(R1342-R410), was linearly proportional to EWT, with R2 > 0.90, when the leaves reflection data were collected from various viewing angles. However, the relationships between some existing indices (simple difference, simple ratio, normalized difference, double difference index and difference ratio indices) and EWT at the leaf level were weak and unstable for all of the 18 plant species (including 14 broadleaf, 3 shrub, and 1 liana species) at different angles under laboratory and field conditions. Moreover, validation results from six independent datasets (n = 1800) and one modeled dataset (n = 2375) further confirmed that the algorithm derived from the proposed index (based on multi-angular reflectance factors of leaves) was not only effective for EWT estimation across a diverse set of plant species with widely variable leaf structure and water content, but also insensitive to different measurement conditions (leaf clip, integrating spheres or multi-angle measurements). The algorithm developed from this new index is generic, does not require reparameterization for each species, and can be accurately used for nondestructive EWT estimations using a simple handheld laboratory or field instrument, and thus, is convenient for agricultural and ecological studies.

中文翻译:

用于估计不同生长阶段的各种植物物种的叶含水量的多角度不变光谱指数

摘要 植物叶片含水量在光合作用、蒸发蒸腾和净初级生产等多种生物地球化学过程中起着关键作用。然而,使用跨不同植物物种的多角度反射测量来准确估计叶片含水量仍然具有挑战性。本研究旨在提出一种通用光谱指数,用于在考虑多角度光谱反射时准确估计等效水厚度 (EWT)。指数被选择为具有使用三个反射系数的差异比的格式。410 nm 处的反射系数用于减少 400-2500 nm 范围内叶面的镜面反射,1268-1285 nm 范围内(非吸水波长)的波长与 1339-1346 nm 范围内(吸水波长)的波长之比加强了所有采样植物物种与 EWT 的关系。当从不同视角收集叶片反射数据时,修正差异比 (MDR) 指数 (R1271-R410)/(R1342-R410) 与 EWT 成线性比例,R2 > 0.90。然而,18种植物(包括14种阔叶植物、3种灌木植物)在叶水平上的一些现有指标(单差、单比、归一化、双差指数和差比指数)与EWT之间的关系较弱且不稳定。和 1 种藤本植物)在实验室和野外条件下的不同角度。而且,来自六个独立数据集 (n = 1800) 和一个建模数据集 (n = 2375) 的验证结果进一步证实,从提议的指数(基于叶片的多角度反射系数)导出的算法不仅对跨区域的 EWT 估计有效具有广泛可变的叶结构和含水量的多种植物物种,但对不同的测量条件(叶夹、积分球或多角度测量)不敏感。从这个新指标开发的算法是通用的,不需要对每个物种重新参数化,并且可以使用简单的手持实验室或现场仪器准确地用于无损 EWT 估计,因此便于农业和生态研究。
更新日期:2021-02-01
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