Estimating the fraction of absorbed photosynthetically active radiation (fAPAR) at FIFE with airborne bidirectional spectral reflectance data

The relationship between the fraction of absorbed photosynthetically active radiation (f(APAR)) and spectral vegetation indices (SVIs) is affected by various factors, including background effects and the view zenith angle of the sensor. This study investigated the effects of estimating f(APAR) from multiple off-nadir airborne measurements acquired by the advanced solid-state array spectroradiometer (ASAS) during the First International Satellite Land Surface Climatology Project Field Experiment. Using atmospherically corrected data, f(APAR) was estimated with both directional SVIs (normalized difference vegetation index (NDVI) and simple ratio (SR)) and PAR hemispherical reflectance (R(PAR)) calculated from PAR bidirectional reflectance factors for the seven viewing angles of ASAS. Very weak linear relationships were found between the ground-measured total f(APAR) values and ASAS measurements, assumed to be due to the presence of varying amounts of senescent plant materials in the canopy. Exclusion of data collected in October when the canopy was fully senescent somewhat improved these relationships. However, correction of total f(APAR) values by the fraction of green vegetation present (green f(APAR)) provided substantial improvement. For both SVIs the highest coefficients of determination (r(2) = 0.839 for NDVI; r(2) = 0.890 for SR) were found at 15 degrees in the backscattering direction decreasing to minimum values at +/-45 degrees, a trend thought to be associated with the effect of canopy structure on the SVIs. Although R(PAR) was not strongly correlated with either the total or the green f(APAR), it was related to the independently measured PAR flux reflected from the canopy indicating a potential for deriving PAR hemispherical reflectance from directional measurements for natural cover types such as grasslands.