*2.4. Tested Vegetation Indices*

A wide range of vegetation indices has been used to estimate vegetation canopy chlorophyll content, a product of LAI and *C*ab. In this study, twelve widely used vegetation indices that have been used to estimate chlorophyll content or LAI were evaluated (Table 2). Some of these use reflectance in VNIR: the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and its two-band version (EVI2), optimized soil-adjusted vegetation index (OSAVI), renormalized difference vegetation index (RDVI), pigmentspecific normalized difference index (PSND) and transformed chlorophyll absorption reflectance index/OSAVI (TCARI/OSAVI). These indices are used to extract one or more vegetation parameters, such as LAI, canopy cover fraction, biomass and pigment content. Other indices have been formulated with the red edge bands: the red-edge transformed chlorophyll absorption reflectance index/OSAVI (TCARI/OSAVIred edge), which has a red edge band instead of the NIR band, the MERIS terrestrial chlorophyll index (MTCI), two versions of normalized difference red-edge vegetation indices (NDRE1 and NDRE2, see Table 2 for details) and the red-edge chlorophyll index (CIred edge) (rows 1–12 in Table 2). These indices were used to extract chlorophyll content in previous studies. To identify the CCC-sensitive and MTA-insensitive band combinations, eleven general index types were selected from the literature next, including six two-band and five three-band formulations (Table 2): ratio index (RI), normalized difference index (NDI), difference index (DI), soil adjusted index (SAI), modified simple ratio (MSR) and modified soil adjusted index (MSAI), triangular index (TI), Gitelson three-band index (Git), Tian's three-band index (BSI-T), Verrelts's three-band index (BSI-V) and Wang's three-band index (BSI-W) (rows 13–23 in Table 2). When calculating TI, the central wavelength of the broadband was used to calculate the wavelength difference.

**Table 2.** The vegetation indices used in this study: indices 1–12 are existing indices with fixed wavelengths; 13–23 are general indices with wavelengths found by optimization.



**Table 2.** *Cont.*

The bands used for the test vegetation index calculations for Sentinel-2 are *RRed* (B4), *RGreen* (B3), *RBlue* (B2), *RRE*<sup>1</sup> (B5), *RRE*<sup>2</sup> (B6), *RRE*<sup>3</sup> (B7) and *RNIR* (B8); for GaoFen-6 *RRed* (B3), *RGreen* (B2), *RBlue* (B1), *RRE*<sup>1</sup> (B5), *RRE*<sup>2</sup> (B6) and *RNIR* (B4).

#### *2.5. Statistical Analysis*

The relationships between the CCC, MTA and vegetation indices were evaluated using the coefficients of determination (*R*2). The *R*<sup>2</sup> between vegetation indices and CCC is indicated as *R*<sup>2</sup> CCC and that relationship with MTA is indicated as *R*<sup>2</sup> MTA. The difference between *R*<sup>2</sup> CCC and *R*<sup>2</sup> MTA is used for the quantitative assessment of the CCC-sensitive and MTA-insensitive vegetation indices. The correlations between the CCC, MTA and individual band reflectance were also calculated.
