Sg. Chen et al., A COMPUTERIZED NUMERICAL EXPERIMENTAL-STUDY OF AVERAGE SOLAR-RADIATION PENETRATION IN PLANT STANDS, Journal of quantitative spectroscopy & radiative transfer, 49(6), 1993, pp. 651-658
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.