Some estimates of the amount and diurnal variation of evaporation from a clayland pasture in fair spring weather

Attention is drawn to a theoretical formula for evaporation in adiabatic conditions, which has been applied generally to the case of a natural land surface by Thornthwaite and Holzman. The formula is E=pk2(q1–q2)(u2–u1)/(loge z2/z1)2 where E is the rate of evaporation, q1, q2 and u1, u2 the specific humidities and wind velocities respectively at heights z1, z2, p the air density and “k” an aerodynamic constant of magnitude 0.4. A recent observational study described elsewhere by the writer has established the validity of the formula in adiabatic conditions and provided quantitative data on the errors which would arise on applying it in non‐adiabatic conditions. A simplified form of the equation is here employed as a justifiable approximation, in terms of near‐surface observations of the vertical distribution of wind velocity and vapour pressure, in evaluating the rates of evaporation at hourly intervals over a clayland pasture during a week of fair spring weather. These show a pronounced diurnal variation and the daily water losses evaluated therefrom lie between 1.4 and 3.6 mm. Demonstration is provided of the substantial error which could arise if for convenience the formula were to be evaluated in terms of daily mean values of the controlling factors.