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

An important problem in radiation measurements is the averaging of rad iant fluxes in horizontal directions. The use of downward cumulative f oliage-areal index averaged over a horizontal surface for the calculat ion of the average penetration of solar radiation in a plant stand und erestimates penetration unless the stand is horizontally uniform. Usin g a computerised numerical experiment method we show that the underest imation of penetration varies with the downward cumulative foliage-are al index, the extinction coefficient, as well as the foliage distribut ion pattern. The results show that the underestimation is strongly pro nounced for some combinations of extinction coefficient and foliage-ar eal index. It is also shown that the use of a penetration value averag ed 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 LA I. Numerical results are presented in this paper to provide a basis fo r calibration.