Methods for assessing dam evaporation - an introductory paper

Abstract

[Abstract]: An evaluation the effectiveness of chemical monolayers, floating covers and shade structures in reducing dam evaporation is being undertaken at the National Centre for Engineering in Agriculture at the University of Southern Queensland. Evaporation is being assessed using high precision pressure sensor transducers to measure small changes in dam height. The evaporation rate is calculated as the residual in the dam water balance, taking into account in-flows and out-flows, and seepage which is assumed to equal the nighttime loss. As night-time evaporation is minimal compared to relatively large daytime evaporation rates experienced in warm semi-arid environments, this method is proving a successful and robust standard method for assessing the evaporation of farm dams in Australia. Alternative assessment methods include the use of evaporation pans, automatic weather stations, or more specialised Bowen Ratio equipment. However, these methods have a large fetch requirement (hundreds of metres) which makes them invalid and therefore of unknown accuracy for small farm dams. However, a method known as eddy correlation avoids the fetch requirement by directly measuring the upward flux of vapour from the water surface. Eddy correlation equipment is now readily available and may prove useful in routine assessments of small dam evaporation, and also in applied research to more fully understand the complicated array of aerodynamic and advective processes involved.