DESIGN AND PERFORMANCE OF A FULLY AUTOMATED EVAPORATION-PAN

The design and operating characteristics of a fully automated evaporat ion pan are described. The objectives were to develop and test the fea sibility of a system to automatically measure either evaporation or ra infall surplus as part of a fully automated station. The reference wat er level in the automated pan is controlled/detected by a liquid level switch, and the evaporation or rainfall surplus is calculated from th e net change in mass of a supply reservoir or a surplus tank, before a nd after the refilling or emptying process, respectively. The supply r eservoir and the surplus tank were individually installed with a preci sion load cell to measure changes in mass and with a refilling or empt ying mechanism to extend the unattended operation period. The measurin g 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 slight ly by ambient temperature at a rate of approximately 0.013 mm degrees- C-1. Excluding the temperature effect, an error of +/-0.06 mm of the C lass A evaporation pan at 99.7% probability limit was found to be attr ibuted to the level switch and the load cells. Two years of evaporatio n and rainfall surplus results measured with the automated pan compare d favourably with that of the manually operated pan, with a correlatio n coefficient greater than 0.98 and a probable error of +/-1.0 mm. Maj or sources of error are attributed to different pan conditions, observ er bias and slow responses of the fixed point gauge used in the manual ly operated station. Although the system is relatively high cost, its accuracy and reliability has made it very attractive for network use i n major agricultural and forestry weather stations.