MEASUREMENT AND MODELING OF RADIATION TRANSMISSION WITHIN A STAND OF MARITIME PINE (PINUS-PINASTER AIT)

A semi-empirical model of radiation penetration in a maritime pine canopy was developed so that mean solar (and net) radiation absorption by crowns and understorey could be estimated from above-canopy measurements only. Beam radiation R(b) was assumed to penetrate the canopy according to Beer's law with an extinction coefficient of 0.32; this figure was found using non-linear regression techniques. For diffuse sky radiation, Beer's law was integrated over the sky vault assuming a SOC (standard overcast sky) luminance model; the upward and downward scattered radiative fluxes were obtained using the Kubelka-Munk equations and measurements of needle transmittance and reflectance. The penetration of net radiation within the canopy was also modelled. The model predicts the measured albedo of the stand very well. The estimation of solar radiation transmitted by the canopy was also satisfactory with the maximum difference between this and the mean output of mobile sensors at ground level being only 18 Wm(-2). Due to the poor precision of net radiometers, the net radiation model could not be tested critically. However, as the modelled longwave radiation balance under the canopy is always between -10 and -20 Wm(-2), the below-canopy net radiation must be very close to the solar radiation balance.