The radiation balance of several field crops

Abstract

Radiation balance components have been measured over field polts of oats, beans, sunflower and corn. Daytime variations of incoming short-wave radiation, net radiation, net long-wave radiation and the reflection coefficient are discussed. Linear regression equations relating measurements of hourly averages of net radiation on incoming short-wave radiation and net short-wave radiation, for each crop and for all crops grouped in cloudless and in overcast days, were highly significant:r2>0.98. Daily totals of net radiation, reflected short-wave radiation and net long-wave radiation, as a percentage of incoming short-wave radiation, were nearly the same for each crop as well for cloudy as for cloudless days. About 25% of the daily total of incoming short-wave radiation was reflected, 18% was lost as long-wave radiation and net radiation amounted to 58%. The diurnal variation of the reflection coefficient was very important on cloudless days. There was an almost linear increase of the reflection coefficient with decreasing solar elevation. Calculations of a heating coefficient on cloudless days gave values around 0.25.