THE RADIATIVE-EQUIVALENT WATER THICKNESS OF LEAVES

The spectral transmittance of pure liquid water was measured using ref lectance spectroscopy to provide a physical standard of comparison for the spectral reflectance of leaves. The spectral reflectance of a whi te reference measured through a 356 mum ray pathlength of water compar ed favorably with values predicted for the same pathlength by the know n absorption coefficients of water. Differences between predicted and measured reflectances were significant only near wavelengths of 1400 n m and in the 1965-2200 nm range. The maximum difference of 7% occurred at 1401 nm. Predicted reflectances indicated notable reflectance mini ma at wavelengths near 1450 nm, 1940 nm, and 2500 nm, even for a 5-mum ray pathlength. Predicted reflectances at wavelengths less than 900 n m remained approximately 100% for a 3160 mum pathlength. A method is p rovided by which leaf reflectance in the 1500-2500 nm range can be use d 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 radiativ e 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.