The influence of the sky radiance distribution on the flux density in the shadow of a tree crown

Abstract

The simulation of the shortwave diffuse sky radiation in plant radiation regime models has traditionally used the uniform overcast (UOC) sky radiance distribution. It has been assumed that this distribution is a good first estimate to the actual sky radiance for average clear days. To test this assumption, a standard overcast (SOC), a uniform overcast, and a clear sky radiance distribution were simulated and the flux density due to sky radiation in the shadow of an ellipsoidal crown calculated. Results show that the choice of sky radiance distribution greatly influences the sky radiation reaching points in the shadow of the simulated tree. The results support the contention that the SOC and UOC sky radiance distributions are quite similar with respect to the radiant flux density in tree shadows. However, the clear-sky radiance distribution resulted in approximately four times less radiant flux density in the tree shadow than that found for the UOC or SOC sky radiance distributions. It was concluded that (1) the use of the UOC radiance distribution greatly overestimated the diffuse radiation in the shadow of a tree, and (2) a clear sky radiance distribution would enhance the prediction of the radiation regime in crown arrays.