The radiative-equivalent water thickness of leaves

The spectral transmittance of pure liquid water was measured using reflectance spectroscopy to provide a physical standard of comparison for the spectral reflectance of leaves. The spectral reflectance of a white reference measured through a 356 μm ray pathlength of water compared favorably with values predicted for the same pathlength by the known absorption coefficients of water. Differences between predicted and measured reflectances were significant only near wavelengths of 1400 nm and in the 1965–2200 nm range. The maximum difference of 7% occurred at 1401 nm. Predicted reflectances indicated notable reflectance minima at wavelengths near 1450 nm, 1940 nm, and 2500 nm, even for a 5-μm ray pathlength. Predicted reflectances at wavelengths less than 900 nm remained approximately 100% for a 3160 μm pathlength. A method is provided by which leaf reflectance in the 1500–2500 nm range can be used to easily compute the radiative-equivalent water thickness (REWT) of leaves. The REWT is defined as the thickness (absorption pathlength) of pure liquid water that would be required to yield the same radiative properties as the leaf in the 1500–2500 nm range, and is a physical standard that may be used in the study of leaf infrared reflectance.