A COMPUTERIZED NUMERICAL EXPERIMENTAL-STUDY OF AVERAGE SOLAR-RADIATION PENETRATION IN PLANT STANDS

An important problem in radiation measurements is the averaging of radiant fluxes in horizontal directions. The use of downward cumulative foliage-areal index averaged over a horizontal surface for the calculation of the average penetration of solar radiation in a plant stand underestimates penetration unless the stand is horizontally uniform. Using a computerised numerical experiment method we show that the underestimation of penetration varies with the downward cumulative foliage-areal index, the extinction coefficient, as well as the foliage distribution pattern. The results show that the underestimation is strongly pronounced for some combinations of extinction coefficient and foliage-areal index. It is also shown that the use of a penetration value averaged over a horizontal surface (P(AP)) to invert the stand foliage-areal index (LAI) by means of the logarithm formula LAI = -ln(P(AP))/k can cause large underestimation of LAI, especially in the case of large LAI. Numerical results are presented in this paper to provide a basis for calibration.