Determination of sensible heat flux over sparse canopy using thermal infrared data

Surface temperatures, T_s, were estimated for a natural vegetative surface in Owens Valley, California, with infrared thermometric observations collected from an aircraft. The region is quite arid and is composed primarily of bushes (∼30%) and bare soil (∼70%). Application of the bulk transfer equation for the estimation of sensible heat, H, gave unsatisfactory values when compared to Bowen ratio and eddy correlation methods over a particular site. This was attributed to the inability with existing data to properly evaluate the resistance to heat transfer, r_ah. To obtain appropriate r_ah-values the added resistance to heat transfer, kB⁻¹, was allowed to vary although there is both theoretical and experimental evidence that kB⁻¹ for vegetative surfaces can be treated as constant. The present data indicate that for partial canopy cover under arid conditions kB⁻¹ may be a function of T_s measured radiometrically. The equation determining kB⁻¹ was simplified and tested over another arid site with good results; however, this had a limited data set (i.e., 6 data points). The dimensionless kB⁻¹ equation is simplified for use over full canopy cover and is shown to give satisfactory estimates of H over a fully-grown wheat crop.