The GCOM-C/SGLI vegetation indices (NDVI, PRI, Chlorophyll Index) from the viewpoint of leaf optical property

Abstract

For long-term vegetation monitoring, satellite-derived vegetation indices (VI) are important in environmental science and, thus, have been generally used. Toward the contribution of long-term monitoring, the Global Change Observation Mission-Climate (GCOM-C) satellite, launched in 2017, provides the VI of a 250 m spatial resolution. This study aims to inform the characteristics of VI derived from the second-generation global imager (SGLI) on GCOM-C from the viewpoint of leaf optical properties and to propose a new VI sensitive to large chlorophyll content. To obtain a leaf-level VI, we measured the leaf reflectance of 25 species (including major plant functional types (PFT): evergreen needle and broad leaves; deciduous needle and broad leaves) using an integrating sphere and a spectral radiometer in summer and autumn. Each ‘leaf-level satellite VI’ was calculated using leaf reflectance and relative spectral response curves of satellite bands. As for the normalized difference vegetation index (NDVI), NDVI derived from SGLI and Moderate Resolution Imaging Spectroradiometer (MODIS) was similar to each other but has a discrepancy in soil moisture dependency and absolute value[1]. Therefore, we obtained the leaf-level correction factor between SGLI and MODIS, supposing their concatenated use in long-term research. The leaf-level correlation between SGLI NDVI and MODIS NDVI had no dependence on PFT and seasons. As for the photochemical reflectance index (PRI), we calculated the following ‘leaf-level satellite PRI’: SGLI PRI (using bands 5 and 6), MODIS PRI (bands 11 and 12), MODIS PRI (bands 11 and 1). Each was compared with the ‘leaf-level definition PRI’ calculated from the leaf reflectance at 531 nm and 570 nm wavelengths. As a result, the SGLI PRI (bands 5 and 6) showed the largest R2 with the definition PRI in summer and throughout seasons (R2=0.94 and R2=0.97, respectively). The MODIS PRI (bands 11 and 1) was better correlated with the definition PRI than MODIS PRI (bands 11 and 12) throughout seasons (R2=0.81 and R2=0.43, respectively) but depended on PFT with no correlation in summer (R2=0.0049). As for the chlorophyll index (CI), the CI using red edges is popular. However, the satellite-based time series of CI using SGLI bands 8 and 9 adjacent to the red edge (673.5 nm and 763 nm, respectively) saturated in small chlorophyll content in early summer. Thus, we investigated which SGLI bands are sensitive to large chlorophyll content using the SCOPE2.0 model[2][3], which combines radiative transfer in plant leaves, canopies, and soil with photosynthesis. In this study, we propose a new SGLI CI sensitive to large chlorophyll content.