The relationship between daylight evaporation from short vegetation and the USWB Class A pan

Knowledge of the mechanism of evaporation from a wire mesh-protected USWB Class A pan is imperative for accurately assessing evaporation from reference crop, TEr, and hence atmospheric evaporation demand, AED. An existing mathematical model to calculate evaporation from the A pan was appropriately modified. Micrometeorological measurements were used to explain the mechanism of evaporation from the A pan. The modifications include the calculation of thermal energy storage and the establishment of a wind function for the daylight period. The modified model yielded an index of agreement and mean absolute difference between modelled and measured values of 0.92 and 0.76 mm (28% of mean pan measurements) for the daylight period, respectively. The mean difference was 0.06 mm or 2% of mean pan measurements. This model was used to create functions for transforming pan evaporation to AED. The ratios of estimated TEr to partially transformed pan measurements and untransformed pan measurements were 1.13 and 4.62, respectively, for daylight. The coefficients of variation of 44 observations for the ratio of TEr to partially transformed and untransformed pan measurements were ±36 and ±222%, respectively.