A Simple Physical Model of Vegetation Reflectance for Standardising Optical Satellite Imagery

Monitoring vegetation using remote sensing is beset with difficulties. Primary among these are the variations of vegetation reflectance with sun zenith angle, view zenith angle, and terrain slope angle. To provide corrections of these effects, for visible and near-infrared light, we derive two simple physicial models of vegetation reflectance, both of which have three parameters. The first model, WAK I, is applicable when illumination is by direct light only; and the second model, WAK II, is applicable when illumination is by both diffuse and direct light. The models were tested by fitting them to reflectance data of Virginia pine, irrigated wheat, and alfalfa, at large sun zenith angles, and then comparing predictions of reflectance at small sun zenith angles with measurements. The process was then reversed to do the fitting on the small sun zenith measurements and the prediction of the large sun zenith measurements. Using this method of assessment, the WAK models were found to perform better than several other published models. The WAK models were used to derive a formula for the variation of reflectance with terrain slope angle, which was tested by measuring reflectances of microcanopies set up on different slopes in the laboratory. The formula permits topographic correction of vegetation reflectance without needing to know the bidirectional reflectance factor. The simplicity and robustness of the WAK models make them useful for general correction and calibration of remotely sensed imagery.