A Comparison of Hyperspectral Chlorophyll Indices for Wheat Crop Chlorophyll Content Estimation Using Laboratory Reflectance Measurements

Abstract

The objective of this paper is to investigate the relationship between a wide range of hyperspectral chlorophyll indices and wheat crop chlorophyll content using laboratory measurements. These measurements included the GER-3700 spectroradiometric data, leaf chlorophyll content using the Soil-Plant Analyses Development (SPAD)-502 meter and leaf chlorophyll content estimated from chemical laboratory analysis. The SPAD-502 readings were correlated with leaf chlorophyll content extracted in the laboratory to establish calibration equations for the computation of chlorophyll-ab (Chl-ab) and chlorophyll-a (Chl-a) content. This resulted in a coefficient of determination (R2) of 0.72 for the Chl-ab content and 0.69 for the Chl-a content and a root mean square error (RMSE) of 3.53 and 1.94 mug/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. These estimates were used to establish relationships against hyperspectral chlorophyll indices calculated from the GER-3700 data. From the investigated indices, the NPCI showed the best results with R2 of 0.84 and RMSE of 11.0. The other indices, such as GNDVI, OSAVI, PSSR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> , PSND <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> , CAI, HNDVI, and MTCI did not perform satisfactorily. The better ones, but still showing a relatively week relationship with leaf chlorophyll content, are the indices NDPI, SIPI, PRI and SRPI with R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2'</sup> s of 0.56, 0.62, 0.54, and 0.57, respectively and RMSEs of 11.06, 10.27, 11.32, and 10.96 mug/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively.