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 impor tance to accurately describe heat and mass transfer rates at the soil surface. The objective of this study was to develop a technique for in dependent measurement of soil heat and water vapor transfer coefficien ts at the soil surface beneath a plant canopy. Heat and vapor source p lates were installed level with the soil surface to provide areas of k nown and controllable temperature and/or vapor pressure. Sensible heat flux density was determined from an energy budget analysis while evap oration from wetted felt fabric on one plate's surface was used to det ermine the source plate's latent heat flux density. Temperature and va por pressure measurements at and 10 mm above the source plate su;faces were used to calculate interfacial heat and mass transfer coefficient s. Measured heat and vapor transfer coefficients (h(h) and h(v), respe ctively) 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 estimat es of surface transfer coefficients when the soil was bare or the cano py < 0.3 m tall, were comparable with calculated bulk soil surface and source plate transfer coefficients at moderate wind speeds but were m uch higher than the source plate values at wind speeds greater than ap proximately 1.0 m s(-1).