Estimation of canopy photosynthetically active radiation (PAR) interception using bidirectional aircraft reflectance measurements of the HAPEX-Sahel bush-fallow subsite

Visible and near-infrared bidirectional reflectance factors at three view zenith angles in the principal plane of the Sun were collected throughout the growing season of 1992 with airborne radiometers during the HAPEX-Sahel experiment near Niamey, Niger. A coupled radiative transfer model for a discontinuous bush canopy over a continuous herb layer was used to estimate the fractional interception of Photosynthetically Active Radiation (f_IPAR) by the vegetation canopies at several subsites. Results for the bush-fallow subsite located within the west central site are examined. The relationship between instantaneous f_IPAR and canopy reflectances was investigated using the Normalized Difference Vegetation Index (NDVI) and a novel Normalized Difference Relative Shadow Index (NDRSI) which made use of the additional information obtained by sampling the canopy bidirectional reflectance. The NDVI showed a strong positive relationship with increasing f_IPAR while the NDRSI showed an equally strong but negative relationship, indicating a potential for bidirectional indices such as NDRSI to estimate canopy variables such as f_IPAR