Wind-induced splash in Class A evaporation pan

Abstract

This study investigates the wind-induced splash in the Class A evaporation pan through a series of wind tunnel experiments. The experimental results revealed that high wind speed can generate seiche wave inside the pan and splash water out of the pan in several minutes. The splash loss increases as the wind speed increases, and the loss rate is at least one order of magnitude greater than the evaporation rate. In other words, the water loss from the pan is not entirely due to evaporation, and the evaporation rates under high wind speeds are over-estimated. By checking the wind speeds and evaporation rates from a four-year (2004-2008) field observation collected in northern Taiwan, it is found that the hourly evaporation rate was unusually high when wind speed was larger than 7ms -1. The splash-out criterion is set as: hourly average wind speed U7ms -1 and evaporation rate E>1.64mmhr -1. The ratio of the splash-out to the average evaporation rate is 0.75% at this site. In addition, this study examines the influence of the initial water depth in the pan on the evaporation rate. The results demonstrate that, because of the shelter effect caused by the rim of the pan, the evaporation rates for water depth less than 8cm are lower than that of standard water depth (20cm) when wind speed U=4 and 6ms -1. However, the shelter effect becomes insignificant when wind speed was U=2ms -1. Copyright 2012 by the American Geophysical Union.