Spectral Determination of Plant Water Content of Wheat Canopies

The aim of this study is to analyse the relationship between spectral reflectance measurements and the water content of wheat canopies based on the Water Index WI (R900/R970) of Peñuelas et al. (i) and a modified version of this index. For this purpose, reflectance measurements over a full phenological cycle of four wheat canopies using an ASD FieldSpec II spectroradiometer have been performed. Canopy water content has been determined by the difference between fresh and dry weight of the harvested plant material. Reflectance spectra were used to calculate both the WI and the modified WI-version. Both indices were compared for their prediction power of canopy water content using an exponential regression model. Final estimation results with cross-validated r2 higher than 0.85 imply a possible operational application. Additionally, the reflectance spectra were resampled to the HyMAP channels and the WI was then calculated from these spectra. The resulting exponential regression model was applied to HyMAP image data for spatial prediction of wheat canopy water content.