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Remote Sensing

A Diurnal Reflectance Model Using Grass

Surface-Substrate Interaction and Inverse Solution

^a Western Ecology Division, NHEERL, U.S. Environmental Protection Agency, 200 SW 35th St., Corvallis, OR, 97333
^b Botany and Plant Pathology, Oregon State Univ., Corvallis, OR 97331-7306

^* Corresponding author (Shirazi.Mostafa{at}epa.gov)

The accuracy of using remote sensing data from earth orbiting radiometers can be improved by using a model that helps to separate the green-fraction in a canopy reflectance () from thatch and soil background, accounts for their diurnal changes, and inverts to a solution of a biophysical plant property of interest. Previous studies addressed one or more of these needs separately. Because reflectance components are interdependent, 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 simple ratio (SR) = near-infrared reflectance (NIR)/red that varied with the normalized time T = (local time – sunset)/daylength, we predicted the mean differential grass dry weight, dw = dw₁ – dw₂, of two grass patches. SR₁ and dw₁ defined the first patch, which included a background and the second, SR₂ and dw₂, a predefined background. The inverted solution for dw 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 x canopy x ground interactions. The standard error of predicting dw was 17%. We separately tested for plant height SR = f(h) or fresh weight SR = f(fw) using SR, and for dw as a function of normalized difference vegetation index NDVI = f(dw). SR = f(dw) produced superior results. Potential applications include noninvasive prediction of other biophysical plant properties in a single or in hyperspectral bidirectional reflectance in agronomic and ecological remote sensing.

Abbreviations: B, biophysical parameter for Canopy 1 (B₁) or Canopy 2 (B₂) • c1, clipped once • c2, clipped twice • cg, canopy cut to ground level • cstdv, conditional standard deviation • dw, biomass dry weight • E(|) conditional expectation • fw, biomass fresh weight • h, canopy height • ir, index of relationship • LAI, leaf area index • MSS4–MSS7, four Landsat wavelength bands • n1, Natural Canopy Number 1 • NDVI, normalized difference vegetation index • NIR, near-infrared reflectance • RI, reflectance index • SR, simple ratio • stdv, unconditional (commonly used) standard deviation • T, normalized local time • dw, differential biomass dry weight • _s, solar zenith (or elevation) angle • _v, viewing zenith angle • , canopy reflectance (_grass/_Labsphere) • _s, solar azimuth angle • _v, viewing azimuth angle • , canopy radiance (W m^–2 sr^–1) • _pxq, variance-covariance matrix p rows by q columns

Received for publication July 18, 2006.