Investigating the conversion coefficients for free water surface evaporation of different evaporation pans

The conversion coefficients of free water surface evaporation were investigated. Comparisons were made between numerous evaporation tanks (pans) and a 20 m(2) evaporation tank. Based on 6 years of evaporation data collected at Nansihu Lake Evaporation Station in Shandong Province, China, the following results have been obtained. First, conversion coefficients varied according to the evaporator type. The average yearly conversion coefficient was only 0.60 for phi20 evaporator (an evaporation pan with a diameter of 20 cm), whereas it was about 1.07 for the E601's pan, whose rim was about 7.5 cm above ground. Second, the conversion coefficient changes within a year had the same pattern among the evaporators evaluated. The monthly conversion coefficients had the minimum values in March or April, then increased with time, and reached their maximum values in August or September. After September, the conversion coefficients varied in different directions with time. Third, the conversion coefficients changed from year to year, but their variations between maximum and minimum values were between 0.06 and 0.16. Fourth, the installation modes, structures, and composition of the evaporators affected the evaporation amount. Fifth, the conversion coefficients at Nansihu and Yucheng stations for the same evaporator did not show a distinct difference. The E601 evaporator has almost the same conversion coefficients at six of seven stations spanning almost 20degrees in latitude from northern to southern China. Sixth, the yearly evaporation from a 100 m(2) evaporation tank was almost the same as that from the 20 m(2) evaporation tanks; however, the differences still existed in any given month. Copyright (C) 2004 John Wiley Sons, Ltd.