Evaporation from water supply reservoirs: An assessment of uncertainty

Many water resource investigations require an estimate of evaporation from water supply reservoirs. Despite its recognised limitations, the pan coefficient method for estimating reservoir evaporation is widely used in industry due to its moderate data requirements. This paper presents a framework to quantify the uncertainties associated with estimates of reservoir evaporation generated using the pan coefficient method. The uncertainties associated with each of the inputs are quantified using a combination of statistical techniques, including Bayesian statistics and subjective judgement; before being combined using Monte Carlo simulation. The framework is applied to three reservoirs in the Werribee River catchment, Australia. The 95% probability intervals surrounding the estimates of reservoir evaporation are as large as ±40% of the best estimate. The largest contributing factor to uncertainty is shown to be the estimation of pan evaporation at reservoirs without a climate station, followed by the uncertainty surrounding the annual pan coefficient itself. The assessment shows that the largest reductions in uncertainty can be achieved by installing an evaporation pan at the reservoir.