The energy balance of a US Class A evaporation pan

Extending our earlier work on the aerodynamics of pan evaporation, we conducted a theoretical and experimental study on the energy balance of the pan under non-steady state conditions. The theory considers heat exchanges at both the pan water surface and the pan wall. We formulate the radiative balance based on the geometry of the system (the pan diameter, the height of the water level, the height of the rim and solar zenith angle) and account for differences in albedo and emissivity between the pan water surface and the pan wall. The theory is used to compute all relevant energy fluxes and thereby close the energy balance. Integration of the half-hourly fluxes to a daily basis showed that we were able to close the energy budget with a RMSE of ∼18 W m−2 or 7.5% of the net short-wave radiation. We find that evaporation from our pan is dominated (∼80%) by the radiative exchanges at the pan water surface; those at the pan wall are smaller but also important. Contrary to expectation, there was little if any sensible heat exchange across the pan wall. Instead we found that the major sensible heat flux occurs across the pan water surface. Importantly, evaporative cooling at the pan water surface can result in a transfer of sensible heat from the (warmer) air to the (cooler) surface of the water body. The sensible heat flux from the overlying air to the pan water surface is minimal at low evaporation rates, but at higher evaporation rates (e.g., >300 W m−2) it contributes the equivalent of up to 15% of the total evaporative flux.