Ramifications of the skin effect for crater lake heat budget analysis

Mass, energy and chemical balance analysis for volcanic crater lakes offers a means to interpret magmatic–hydrothermal processes. In addition to water temperature, several meteorological parameters, including windspeed, rainfall, solar irradiation, air temperature and humidity, need to be evaluated in order to balance heat fluxes. Conventional `contactʹ thermometry yields bulk water temperatures. However, these are typically a few degrees or tenths of a degree higher than the water surface temperatures which should be used in several bulk aerodynamic and other formulae for heat flux calculations. The magnitude of overestimation of derived evaporative flux that can arise from ignoring this `skin effectʹ is shown here to depend on bulk water temperature, air temperature and other meteorological factors. Field observations obtained at Kawah Ijen (Indonesia), with a lake bulk temperature of 43.1°C, suggest a bulk−skin temperature difference of around 1.5°C, leading to an overestimation of evaporative and sensible heat fluxes of ~10% if the skin effect is overlooked. This error can be minimised by assuming a value for the bulk–skin temperature difference, or eliminated by ensuring that evaporation formulae are parameterised with the temperatures, skin or bulk, for which they were originally defined.