Measurement of heat and vapor transfer coefficients at the soil surface beneath a maize canopy using source plates

In order to further the understanding of energy and moisture transfer in canopy air space and thus canopy microclimate, it is of vital importance to accurately describe heat and mass transfer rates at the soil surface. The objective of this study was to develop a technique for independent measurement of soil heat and water vapor transfer coefficients at the soil surface beneath a plant canopy. Heat and vapor source plates were installed level with the soil surface to provide areas of known and controllable temperature and/or vapor pressure. Sensible heat flux density was determined from an energy budget analysis while evaporation from wetted felt fabric on one plate's surface was used to determine the source plate's latent heat flux density. Temperature and vapor pressure measurements at and 10 mm above the source plate surfaces were used to calculate interfacial heat and mass transfer coefficients. Measured heat and vapor transfer coefficients (hh and hv, respectively) ranged from 2 to 30 mm s−1 over wind speeds from 0.05 to 2.8 m s−1 measured 0.03 m above the plate surface. Log-profile estimates of surface transfer coefficients when the soil was bare or the canopy < 0.3 m tall, were comparable with calculated bulk soil surface and source plate transfer coefficients at moderate wind speeds but were much higher than the source plate values at wind speeds greater than approximately 1.0 m s−1.