Reflected Radiation from a Soybean Crop1

Abstract

AbstractThe albedo, defined as the percentage of incoming solar radiation that is reflected, was measured continuously over a soybean crop [Glycine max. (L.) Merr.] during the 1968, 1969, and 1970 growing seasons at St. Paul, Minn. The study was undertaken to establish average albedo values for soybeans and to evaluate the effects of increasing crop cover on the albedo. The average daily albedo ranged from 24% to 27% with complete soybean cover. The albedo of the moist soil was approximately 10.5% with no cover and increased at the rate of about 1.3% for each 10% increase in crop cover. Because this relationship held true over the entire growing season, it should be possible to accurately estimate soybean cover from albedo measurements once the growth characteristics of a variety have been established.It was also possible during the study to examine the effects of soil moisture and lodging on the soybean albedo. It was found that with full cover the albedo decreased at low soil moisture because the soybean leaves wilted and became more vertically oriented, thereby permitting increased penetration of incoming solar radiation. Before canopy closure, particularly early in the season, low soil surface moisture caused a marked increase in albedo. Severe lodging during mid‐August 1970 caused a marked decrease in albedo.Failure to properly shield the inverted Eppley pyranometer was shown to cause an increase in the daily average albedo of as much as 7.5% because of internal reflection from the pyranometer dome.An average albedo of 25.9% was obtained for two dear days. It was noted on these days that morning albedoes were higher than evening albedoes at the same solar altitudes. This was attributed to temporarily wilted, drooping leaves, and to increased leaf flutter from stronger afternoon winds. The average daily albedo for two cloudy days was 24%.