Water Vapor Transport through a Flail‐Chopped Corn Residue

Crop residue at the soil surface is known to decrease evaporation, and measurements have shown that wind increases evaporative loss through residues; however, to our knowledge, measurements of water vapor flux through residues have not defined their vapor conductance. The objective of this study was to measure the water vapor transport through crop residues in a wind tunnel and under natural wind and to parameterize the vapor conductance in a way useful for modeling the energy balance of conservation‐tillage systems. The vapor conductance of flail‐chopped corn ( Zea mays L.) residue laid over large vapor sources (0.15 by 1.2 m) at different vapor‐pressure gradients was measured in still air and under winds. In still air, the vapor conductance of 10 samples of residue (areal density = 0.4 kg m −2 ; average thickness = 11 mm) averaged 2.2 mm s −1 at 20 °C and was not significantly different than the conductance of a similar thickness of air. Error in the measurement (CV = 13%) was sufficient that porosity and tortuosity effects on conductance could not be determined. Buoyancy‐driven convection in the flail‐chopped residue was negligible under much greater buoyancy instabilities than are likely to occur in the field. The vapor conductance ( h ) of nine samples of residue measured in a wind tunnel increased linearly with wind speed ( V ) measured at 1 m above the residue: h = (2.2 mm s −1 ) (1 + 0.89 V ), with a standard error of estimate ( S yx ) of 1.82 mm s −1 . The regression coefficients of vapor conductance vs. wind found in the wind tunnel and under natural winds did not differ at P = 0.25. Based on our experience, measurements of conductance of other residues would be done much more easily in a suitable wind tunnel than in the field, and with the same results.