Abstract
The leaf area index (LAI) is one of the most important Earth surface parameters used in the modeling of ecosystems and their interaction with climate. Numerous vegetation indices have been developed to estimate the LAI. However, because of the effects of the bi-directional reflectance distribution function (BRDF), most of these vegetation indices are also sensitive to the effect of BRDF. In this study, we aim to present a new BRDF-resistant vegetation index (BRVI), which is sensitive to the LAI but insensitive to the effect of BRDF. Firstly, the BRDF effects of different bands were investigated using both simulated data and in-situ measurements of winter wheat made at different growth stages. We found bi-directional shape similarity in the solar principal plane between the green and the near-infrared (NIR) bands and between the blue and red bands for farmland soil conditions and with medium chlorophyll content level. Secondly, the consistency of the shape of the BRDF across different bands was employed to develop a new BRDF-resistant vegetation index for estimating the LAI. The reflectance ratios of the NIR band to the green band and the blue band to the red band were reasonably assumed to be resistant to the BRDF effects. Nevertheless, the variation amplitude of the bi-directional reflectance in the solar principal plane was different for different bands. The divisors in the two reflectance ratios were improved by combining the reflectances at the red and green bands. The new BRVI was defined as a normalized combination of the two improved reflectance ratios. Finally, the potential of the proposed BRVI for estimation of the LAI was evaluated using both simulated data and in-situ measurements and also compared to other popular vegetation indices. The results showed that the influence of the BRDF on the BRVI was the weakest and that the BRVI retrieved LAI values well, with a coefficient of determination (R2) of 0.84 and an RMSE of 0.83 for the field data and with an R2 of 0.97 and an RMSE of 0.25 for the simulated data. It was concluded, therefore, that the new BRVI is resistant to BRDF effect and is also promising for use in estimating the LAI.
Highlights
The leaf area index (LAI) is one of the most important Earth surface parameters used in modeling ecosystems and their interaction with climate [1,2]
The purpose of our study was to develop a new bi-directional reflectance distribution function (BRDF)-resistant vegetation index which can be used to retrieve the LAI with a reduced influence by the BRDF effects; the other fixed parameters in the PROSAIL model were set to their default values with the exception of the LAI, the view zenith angle (VZA), the solar zenith angle (SZA) and the azimuthal difference between the solar and observation angles (RAA)
Multispectral indices employed for Definition retrieving LAI
Summary
The leaf area index (LAI) is one of the most important Earth surface parameters used in modeling ecosystems and their interaction with climate [1,2]. It provides key information about crop growth and is highly correlated with crop biomass and productivity [3,4,5]. Vegetation with different LAI values has different spectral reflectance characteristics [6,7]. Proposed a red-edge normalized difference index (NDI) that is correlated strongly with the LAI without saturating at larger values [8]. Viña et al tested the sensitivity of different indices to the LAI and found that the chlorophyll indices (CIGreen, CIRed-edge and the MERIS terrestrial chlorophyll index, MTCI)
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