Evaporation from an oat field

Evaporation from natural surfaces may be evaluated by a method based on aerodynamic considerations which lead to the simple equationin which E, p and k are the rate of evaporation, the air density and von Kármán's constant respectively, u 1 , u 2 , q 1 and q 2 are the wind speeds and specific humidities at the heights z 1 and z 2 , and d the so‐called surface zero displacement. This formula has been applied to observations taken over an oat field in order to determine the hourly values of E during a number of periods ranging in length from one to twelve days. The magnitude of d is shown to increase with increasing crop height and wind speed, the crop height increasing eightfold over the total period of observation. Daily water losses are calculated and found to vary from 0.05 mm to 5.65 mm. By using two sets of apparatus, the calculated values of E are shown, in unstable conditions, to depend on the height of exposure of the apparatus, decreasing with increasing height. A modified form of the equation, based on Deacon's generalized wind‐profile law, has been used in the last period of observation and found to reduce the dependence of the calculated values of E on the level at which the apparatus was placed. The general application of the modified expression was found to be impossible owing to insufficient precision in the measurements of the wind speeds and to the uncertainty in the value to be assigned to d. In general, the computed daily water losses are considered to be underestimated to an extent not exceeding 15 per cent.