THE USE OF REMOTELY SENSED DATA IN ESTIMATION OF PAR USE EFFICIENCY AND BIOMASS PRODUCTION OF FLOODED RICE

A model of biomass production for flooded rice crops is proposed based on an energetic yield approach, in which PAR interception efficiency is estimated from vegetation index calculated from canopy reflectances at visible and near-infrared wavelengths. The relationship between interception efficiency (measured from hemispherical photographs) and the vegetation index results from coupling reflectance and PAR interception models based on the Beer-Lambert extinction law. This relationship does not depend explicitly on the canopy structure parameters but it assumes that the background reflectance is known. Although the background reflectance depends on the water state and depth variation in a way which cannot be explicitly quantified, it has been empirically described by its observed evolution fitted with time. After anthesis, this reflectance takes into account the reflectance of the senescent vegetation. Given the spectral profile of a crop (temporal evolution of its reflectance), its PAR absorption profile can be estimated. The PAR use efficiency (conversion of the absorbed energy into biomass) is then determined for every phenological period as the ratio of total absorbed PAR to total synthetized biomass during the period. For flooded rice, root extraction and therefore root biomass evaluation are rather easy, so that more realistic PAR use efficiencies can be estimated than for most crops. The obtained values for these efficiencies agree with those of the literature. Observed varietal differences agree with independent experimental evaluation of cultivar photosynthetic capabilities.