Vegetation and soil lines in visible spectral space: a concept and technique for remote estimation of vegetation fraction

The goal of this study is to investigate the information content of reflectance spectra of crops in the visible and near infrared range of the spectrum and develop a technique for remote estimation of vegetation fraction (VF). For four wheat species with VF 100% in a wide range of pigment contents and compositions, a high degree of covariance was found for paired reflectances (R) at 550 nm versus 700 nm (R-550 versus R-700) and 500 nm versus 670 nm (R-500 versus R-670). Both relationships, defined as 'vegetation lines', were linear with determination coefficients r(2) >0.9 and the plotted points were tightly clustered. Using the same coordinates to plot reflectances for a variety of soils, a high degree of covariance (r(2)>0.94) and a distinct 'soil line' were found. The vegetation and soil lines define a two-dimensional spectral construct within which canopy reflectances, regardless of VF, may be located. Based on these optical properties of vegetation and soils, an attempt was made to estimate VF remotely for selected plant canopies. It is suggested that the coordinate location within the constructs, as defined by reflectances at 500 nm and 670 nm as well as at 550 nm and 700 nm, be used to measure VF. Algorithms for VF assessment in wheat for a wide range of soil brightness were devised and validated. The root mean square error (RMSE) of VF prediction was less than 10%. The technique was also validated by means of independent datasets taken above cornfields in Nebraska. The RMSE of VF prediction did not exceed 9.7%.