Submitted to: American Society for Photogrammetry and Remote Sensing Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2004
Publication Date: May 24, 2004
Citation: Hunt, E.R. 2004. Equivalent water thickness of leaves estimated using the absorption coefficient of liquid water [abstract]. American Society of Photogrammetry and Remote Sensing Proceedings. p. 29. Technical Abstract: Equivalent water thickness (EWT), or vegetation water content, is usually estimated using remotely sensed indices such as the normalized difference infrared index [NDII = (R870 - R1650)/(R870 + R1650)] and the normalized difference water index [NDWI = (R870 -R1240)/(R870-R1240)]. With hyperspectral data, regressions of ln(reflectance) versus the absorption coefficient of liquid water is another method to estimate EWT. Spectral reflectances and transmittances were measured for leaves of various crop, weed, and deciduous-tree species, and EWT were measured from the leaf area, fresh weight and dry weight. NDII and NDWI were very highly correlated to estimated EWT from the regression of ln(reflectance), showing these indices have a strong biophysical basis. However, for every leaf, regressions using wavelengths around 1240 nm region had a different estimated EWT compared to the 1650 nm region, and regressions from both regions had estimated EWT different from the measured EWT. Therefore, NDWI and NDII have to be calibrated with measured EWT for each species in order to remotely sense vegetation water content.