Solar-radiation transmittance of flail-chopped corn residue layers.

The transmittance of solar (global) radiation through crop residues is required to model the influence of residue on the energy balance and temperature of the underlying soil in conservation tillage systems, but we could find no measurements of the transmittance of residue layers. The objective of this study was to devise ways to measure the transmittance of flail-chopped corn (Zea mays L.) residue layers, to measure the reflectance and transmittance of the residue elements, to measure the transmittance of residue layers of different areal densities, and to see if global radiation models developed for crop canopies would be suited to predicting transmittance of residue layers. The solar reflectance of dry flail-chopped corn stalk pieces was about 0.25 in the 400- to 700-nm wave band and about 0.6 in the 700- to 2600-nm band. The transmittance of residue pieces in these two bands was 0.005 and 0.02, respectively. The transmittance of layers of flail-chopped residue was almost independent of the solar zenith angle because the bulk of the residue was comprised of stalk wall fragments that were essentially flat strips lying parallel to the surface with randomly oriented axes. The extinction coefficient of the residue for global radiation was about 1.70. This is 1.7-fold greater than expect for randomly distributed flat, black elements. The extinction coefficient for solar radiation was not significantly different from that of thermal radiation (1.6), despite a large difference in element reflectance. We believe the large extinction coefficients resulted because residue elements were distributed uniformly rather than randomly. The similarity of extinction coefficients of solar and thermal radiation indicates forward scattering of solar radiation is negligible, probably because of the opacity and close packing of residue elements, which reduces interelement reflections. For crop radiation models to be applied to residue, they must be revised to account for both nonrandom element distribution and the close packing of residue elements on each other and on the soil.