Xie, Qiaoyun, Dash, Jadu, Huang, Wenjiang, Peng, Dailiang, Qin, Qiming, Mortimer, Hugh, Casa, Raffaele, Pignatti, Stefano, Laneve, Giovanni, Pascucci, Simone, Dong, Yingying and Ye, Huichun (2018) Vegetation indices combining the red and red-edge spectral information for leaf area index retrieval. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. (doi:10.1109/JSTARS.2018.2813281).
Abstract
Leaf Area Index (LAI) is a crucial biophysical variable for agroecosystems monitoring. Conventional Vegetation Indices (VIs) based on red and near infrared regions of the electromagnetic spectrum, such as the Normalized Difference Vegetation Index (NDVI), are commonly used to estimate Leaf Area Index (LAI). However, these indices commonly saturate at moderate-to-dense canopies (e.g. NDVI saturates when LAI exceeds 3). Modified VIs have then been proposed to replace the typical red/green spectral region with the red-edge spectral region. One significant and often ignored aspect of this modification is that, the reflectance in the red-edge spectral region is comparatively sensitive to chlorophyll content which is highly variable between different crops and different phenological states. In this study, three improved indices are proposed combining reflectance both in red and red-edge spectral regions into the NDVI, the modified simple ratio index (MSR) and the green chlorophyll index (CIgreen) formula. These improved indices are termed NDVIred&RE (red and red-edge normalized difference vegetation index), MSRred&RE (red and red-edge modified simple ratio index) and CIred&RE (red and red-edge chlorophyll index). The indices were tested using RapidEye images and in-situ data from campaigns at Maccarese Farm (Central Rome, Italy), in which four crop types at four different growth stages were measured. We investigated the predictive power of nine vegetation indices for crop LAI estimation, including NDVI, MSR, CIgreen, the red-edge modified indices NDVIRed-edge, MSRRed-edge, CIRed-edge (generally represented by VIRed-edge) and the newly improved indices NDVIred&RE, MSRred&RE, and CIred&RE (generally represented by VIred&RE). The results show that VIred&RE improves the coefficient of determination (R2) for LAI estimation by 10% in comparison to VIRed-edge. The newly improved indices prove to be powerful alternatives for LAI estimation of crops with wide chlorophyll range, and may provide valuable information for satellites equipped with red-edge channels (such as Sentinel-2) when applied to precision agriculture.
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