Submitted to: Agriculture Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/1997
Publication Date: N/A
Interpretive Summary: Crop residues left on the soil surfaces affect the movement of moisture and temperature changes in shallow soil layers. Generally, no-tillage fields with a full layer of crop residue are thought to have cool, wet soils in the spring. However, little is known about the effects of crop residues on soil moisture and temperature patterns over the winter. Interest in the over-winter processes is related to concerns over how agricultural chemicals (pesticides and fertilizers) move in the soil. The objective of this research was to study energy transfer and evaporation in a no-tillage corn field to improve our understanding of crop residue effects on water flow and soil temperature. Field measurements were made in central Iowa during periods without snowcover over the winter of 1994- 95. When the soil was dry but the residue was wet, it would take a couple of days to dry the residue and little evaporation occurred from the soil. But, when the soil is wet and the residue dries out, the residue layer prevents rapid evaporation from the wet soil, even on sunny days. Relationships were developed to help predict evaporation from residue-covered fields under varying moisture conditions.
Technical Abstract: Crop residues on the soil surface have the potential to significantly affect the magnitude of individual components of the surface energy balance. Previous research has concentrated on residues effects on soil temperature and moisture early in the growing season. The objective of this study was to measure each of the surface energy balance components of a field during snow-free periods between successive growing seasons. A Bowen ratio system was used to measure surface fluxes within a no-tillage corn (Zea mays L.) Field near Ames, IA USA. During the fall, large solar zenith angles and short day lengths resulted in <5.1 MJ m**-2d**-1 of available energy (Rn-G). On overcast days with a dry surface, average daytime Bowen ratios (beta) were ,1.5 and 42 to 75% of the available energy was consumed by evaporation. On contiguous sunny days, daytime beta values were >2.3 and <21% of the available energy was partitioned to evaporation. When the surface was wet, there was little difference in average daytime beta values (1.0 and 1.5 on sunny days 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 days). More energy was available (up to 12.9 MJm**-2d**-1) during the spring measurement interval with daytime Bowen ratios averaging 1.7 and 0.8 on sunny and overcast days, respectively. With overcast conditions and wet soil, evaporation approached potential rates predicted by the Priestley-Taylor equation. With clear skies and wet soil, Penman-Monteith estimates using a residue resistance term calculated from an equation presented by Tanner and Shen agreed well with measured values.