Design and performance of a fully automated evaporation pan

Abstract

The design and operating characteristics of a fully automated evaporation pan are described. The objectives were to develop and test the feasibility of a system to automatically measure either evaporation or rainfall surplus as part of a fully automated station. The reference water level in the automated pan is controlled/detected by a liquid level switch, and the evaporation or rainfall surplus is calculated from the net change in mass of a supply reservoir or a surplus tank, before and after the refilling or emptying process, respectively. The supply reservoir and the surplus tank were individually installed with a precision load cell to measure changes in mass and with a refilling or emptying mechanism to extend the unattended operation period. The measuring sequences are controlled by a sophisticated data logger. Performance tests revealed that the liquid level switch has a dead zone of 0.05–0.08 mm between ON and OFF of the control relay, and is affected slightly by ambient temperature at a rate of approximately 0.013 mm °C −1. Excluding the temperature effect, an error of ±0.06 mm of the Class A evaporation pan at 99.7% probability limit was found to be attributed to the level switch and the load cells. Two years of evaporation and rainfall surplus results measured with the automated pan compared favourably with that of the manually operated pan, with a correlation coefficient greater than 0.98 and a probable error of ±1.0 mm. Major sources of error are attributed to different pan conditions, observer bias and slow responses of the fixed point gauge used in the manually operated station. Although the system is relatively high cost, its accuracy and reliability has made it very attractive for network use in major agricultural and forestry weather stations.