SURFACE-ENERGY BALANCE OF A CORN RESIDUE-COVERED FIELD

Crop residues on the soil surface have the potential to significantly affect the magnitude of individual components of the surface energy ba lance. Previous research has concentrated on residue effects on soil t emperature and moisture early in the growing season. The objective of this study was to measure each of the surface energy balance component s of a field during snow-free periods between successive growing seaso ns. A Bowen ratio system was used to measure surface fluxes within a n o-tillage corn (Zea mays L.) field near Ames, IA, USA. During the fall , large solar zenith angles and short daylengths resulted in < 5.1 MJ m(-2) d(-1) of available energy (R-n-G), On overcast days with a dry s urface, average daytime Bowen ratios (beta) were < 1.5 and 42% to 75% of the available energy was consumed by evaporation. On contiguous sun ny days, daytime beta values were > 2.3 and < 21% of the available ene rgy was partitioned to evaporation. When the surface was wet, there wa s little difference in average daytime beta values (1.0 and 1.5 on sun ny vs. 0.87 and 1.84 on overcast days) while less available energy was used to evaporate water on sunny days(< 19% vs. > 38% on overcast day s). More energy was available (up to 12.9 MJ m(-1) d(-1)) during the s pring measurement interval with daytime Bowen ratios averaging 1.7 and 0.8 on sunny and overcast days, respectively. With overcast condition s and wet soil, evaporation approached potential rates predicted by bo th the Priestley-Taylor and Penman-Monteith equations. With clear skie s and wet soil, Penman-Monteith estimates using a residue resistance t erm agreed well with measured values. Published by Elsevier Science B. V.