A DIRECT METHOD TO MEASURE BIDIRECTIONAL GAP FRACTION IN VEGETATION CANOPIES

Bidirectional gap fraction Po(z,OMEGA1,OMEGA2) is an important variabl e to be estimated for canopy reflectance calculations. In the recent y ears, models have been developed to describe bidirectional gap fractio n, taking into account the finite size of the phytoelements. However, we lack from experimental data the means to evaluate such models. In t his article, we present a method that enables direct measurement of bi directional gap fraction. The method is based on digital analysis of u pward-looking hemispherical photographs taken from inside the canopy. Measurements made on a maize canopy were compared to calculations with the model by Kuusk (1985). Agreement between calculated and experimen tal data was sufficiently good. However, experimental data show that s ome correlation between Po(z,OMEGA1) and Po(z,OMEGA2) subsists for a r ange of angle much wider than predicted by simulations.