The aerodynamics of pan evaporation

In response to worldwide observations reporting a decline in pan evaporation over the last 30-50 years, we developed an instrumented US Class A pan that replicated an operational pan at Canberra Airport in Australia. The aim of the experimental facility was to investigate the physics of pan evaporation under non-steady state conditions. By monitoring the water level at 5-min intervals we were able to calculate the evaporation rate and thereby determine the short-term mass balance of the pan. Over the same time intervals, we also monitored (short- and long-wave) radiation, temperature (air, water surface, bulk water, inner and outer pan wall), atmospheric pressure as well as the air vapour pressure and the wind speed at a standard reference height (2. m above ground level). The experimental pan was operated for three years (2007-2010).In this paper, we develop a framework for quantifying vapour transfer by coupling Fick's First Law of Diffusion with boundary layer theory. This approach adequately represented pan evaporation measurements over short time intervals (half-hourly) under non-steady state conditions provided that surface temperature measurements, that account for the substantial cooling associated with evaporation, are available. It involved estimating the boundary layer thickness and other properties of air above the evaporating surface for a pan. Our results are consistent with the "envelope of theoretical curves" concept for the wind function introduced by Thom et al. (1981).