Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 9, 1996
Publication Date: N/A
Interpretive Summary: The adoption of conservation tillage practices has increased the amounts of crop residues left on the soil surface. These residues, while protecting the soil from erosion, also reflect and absorb solar radiation that would otherwise reach the soil surface. The objective of this study was to measure the amount of sunlight reflected and transmitted by corn residue layers and whether these amounts change as the residue weathers. We found that fresh corn residue reflects more sunlight in the near-infrared waveband as compared to the visible waveband. The same held true in the spring except the reflectivity was lower in both wavebands. The amount of light transmitted through the residue layers was directly proportional to the percent of residue cover. Small differences were found between the spring and fall measurements because the residue layer was compacted by snow over the winter. Both the amount of reflected and transmitted light were also affected by the amount of direct versus diffuse sunlight and whether the residue was wet. Any effort to predict the amount of sunlight reflected or transmitted by these residue layers on an hourly basis would have to consider these effects.
Technical Abstract: Crop residues on the soil surface absorb solar radiation and have reflectivity properties that may differ significantly from the underlying soil. The objective of this study was to measure the temporal variation in solar radiation reflectivity and transmissivity of a corn (Zea mays L.) residue layer under field conditions. Incident and reflected solar and visible radiation were measured in a field with standing corn stubble and corn residue prostrate on the soil surface. Transmitted solar and visible radiation were measured with line sensors placed beneath the prostrate residue. Reflectivity measurements were made from October 1994 until April 1995. Transmissivity measurements were made during snow-free periods over the same time interval. Mean reflectivity decreased from 0.20 +/- 0.02, 0.12 +/- 0.02, and 0.27 +/- 0.03 for the solar, visible, and near-infrared wavebands during the fall, to 0.17 +/- 0.01, 0.11 +/- 0.01, and 0.22 +/- 0.02 during the spring measurement period, respectively. For the winter period with snowcover, reflectivity averaged 0.66 +/- 0.17, 0.68 +/- 0.20, and 0.64 +/- 0.15 for the same wavebands. Transmissivity of solar and visible radiation through the corn residue layers was directly proportional to percent residue cover. Extinction coefficients for solar and visible radiation ranged from 0.79 to 0.96 and were higher in the fall for both wavebands. Diurnal patterns of reflectivity and transmissivity showed a sensitivity to the proportion of beam vs. diffuse radiation and wetness of the residue.