Remote sensing of water content in Eucalyptus leaves

The spectral reflectance of leaves from several Eucalyptus species was measured over the 400-2500 nm wavelengths with a laboratory spectroradiometer. The relationship of reflectance with the gravimetric water content and equivalent water thickness (EWT) of the leaves was analysed. The results showed that EWT was strongly correlated with reflectance in several wavelength regions. No significant correlations could be obtained between reflectance and gravimetric water content. It was also possible to confirm theoretically that reflectance changes of leaves could be directly linked to changes in EWT but not to changes in gravimetric water content. Several existing reflectance indices were evaluated for estimation of leaf water content and some new indices were developed and tested. Two semi-empirical indices developed in this study, (R-850 - R-2218)/(R-850 - R-1928) and (R-850 - R-1788)/(R-850 - R-1928), were found to show significantly stronger correlations with EWT than all other indices tested. It was also shown that these new indices were least sensitive to the effects of radiation scatter. The indices (R-850 - R-2218)/(R-850 - R-1928) and (R-850 - R-1788)/(R-850 - R-1928) are therefore proposed as two new indices for the remote estimation of vegetation water content.