FOLIAGE TEMPERATURE AND LATENT-HEAT FLUX OF IRRIGATED WHEAT

A series of 42 hourly measurements from a weighing lysimeter of foliage temperature provided a basis from which to evaluate the residual energy balance method for determining latent heat flux which incorporated foliage temperature measurements by an infrared thermometer. The measurements were taken regularly during daylight in a 2-ha field of irrigated wheat and covered a 14-day period ending at anthesis. The residual method (lambda-E(T)) underestimated lysimeter latent heat loss (lambda-E(L)) by 11% and accounted for 79% of the variance in lambda-E(L). Uncertainty in lambda-E(T) ranged from overestimates of approximately 35 W m-2 when advective effects were minimized with approximately 95% profile adjustment to underestimates as high as 20% of lambda-E(L) (approaching 200 W m-2) resulting from flux divergence in latent heat flux of comparable magnitude in causing advective enhancement of evaporation loss. This range in performance of lambda-E(T) was explained in part owing to a variable fetch requirement, contributed to by atmospheric conditions of stability in influencing boundary layer development. The greatest uncertainty in lambda-E(T) was observed during stable atmospheric conditions when foliage temperature lower than ambient accompanied significant local advective enhancement of crop evaporation.