Radiation balance of soybeans grown in Brazil

This paper describes an experimental study of the radiation balance over a growing soybean (Glycine max (L.) Merrill) crop at a tropical agricultural experimental station, Jaboticabal (22°15′S 48°18′W), São Paulo, Brazil. Incoming shortwave radiation, S, net radiation, R, and the shortwave reflection coefficient, α, were measured over this crop from November 1976 to April 1977, and net longwave radiation, L, was deduced from the radiation balance equation. Hourly variation of the radiation components, S, αS, R and L, are presented for 4 nearly clear sky-days during the different growing crop periods. The diurnal variation of α is also presented for these 4 days. The mean daily albedo values for this vegetative surface were between 0.12 and 0.26, and inversely related to the vegetation height. A regression analysis of the radiation balance equation has been made, and the heating coefficient, β, and the longwave exchange coefficient, λ, obtained. The results of the study indicate that both β and λ are found to be descriptively helpful parameters in agricultural meteorology. Predicted regression equations for net radiation from shortwave radiation are presented for the different periods.