A Diurnal Reflectance Model Using Grass: Surface-Substrate Interaction and Inverse Solution.

ABSTRACT

We report an analysis of canopy reflectance (ρ) experiment, using hand-held radiometer to measure distribution of biomass in a grass field. The analysis: 1) separates the green-fraction from thatch and soil background, 2) accounts for the changing diurnal ρ with the sun elevation angle, and 3) predicts plant biomass in a model directly from a new ρ observation, which is the inverse solution of the original experiment. Previous studies addressed one or more of these needs separately. Because reflectance components are inter-dependent, difficulties remain in obtaining a combined inverse solution. We combined a conditional probability method with a novel experimental procedure to predict grass dry weight (dw). Using the ratio of the Near Infrared and Red wavebands, that is, Simple Ratio, (SR)=(NIR)/Red that varied with the normalized time T = (local time-sunset)/(day length), we predicted the difference in grass dry weight (Δdw= dw₁-dw₂) of two grass patches. SR₁ and dw₁ defined the first patch which included a background and the second, SR₂ and dw₂, a pre-defined background. The inverted solution for Δdw was an ellipse with axes formed by the diurnal reflectance SR₁ and SR₂ coordinates. It described previously studied “soil line” and the zone of canopy-canopy-ground interactions. The standard error of predicting Δdw was 17%. We separately tested for plant height SR=f(h) or fresh weight SR=f(fw) as functions of SR and for dry weight as a function of normalized difference vegetation index NDVI=f(dw). SR=f(dw) produced superior results. Potential applications include predicting other biophysical plant properties in a single or in hyper-spectral bidirectional reflectance in agronomic and ecological remote sensing.