Measurements of non-potential evaporation from wheat

Abstract The reliability of the energy balance and lysimetric methods for measuring evaporation has been examined in experiments in a field of wheat. Net radiation, soil heat flux and evaporation, E, were measured, the last by means of a weighing lysimeter. Sensible heat flux, H, was obtained by difference in the energy balance equation, and H and E used to construct an experimental estimate, β , of the Bowen ratio, β (= H/LE, with L the latent heat of vaporisation). Simultaneous measurements were obtained of differences in temperature, ΔT, and specific humidity, Δq, over a height interval, Δz, above the crop. These were used to calculate the function β c (= c p (ΔT + ΛΔz) (ΛΔq) , with cp the specific heat of air at constant pressure and Λ the adiabatic lapse rate). βc was compared with β on a variety of occasions when β ranged from 0.1 to 2.2, and the Richardson number at 42.5 cm above the crop from −0.001 to −0.026. The data conformed well to the assumption that β c = β , implying equality between the eddy diffusivities for heat and water vapour (at least close to the surface and over the range of instabilities examined). A total of 210 hourly comparisons were made between lysimeter evaporation and calculated evaporation, Eb, derived from the energy balance equation with β = βc. Differences between Eb and E were generally within ± 0.1 mm/h. This difference was of the same magnitude as the estimated experimental error, and it is concluded both that Eb was a reliable estimate of E, and that the lysimeter itself was adequately representative of its surroundings. The paper also reports studies over 4 1 2 years of seasonal changes in evaporation and soil water storage in an agricultural system typical of land-use in the Canberra area.