Comparison of atmospheric water vapour δ18O and δ2H estimated using evaporation pan, rainfall equilibrium and continuous measurements

For a period of 16 months in Sydney, Australia, the variations of H-2/H-1 and O-18/O-16 in atmospheric vapour (delta H-2(A) and delta O-18(A)) were estimated using an evaporation pan method as well as using the isotopic precipitation-equilibrium approach. These calculations were then compared with delta H-2(A) values measured at 10 m above ground surface using a Fourier Transform Infrared Spectrometer (FTIR). As pan isotopic composition was available on a weekly time scale, the evaporation rates were measured daily, and the atmospheric variables were available hourly, the weekly time scale was used to calculate the arithmetic averages of the atmospheric variables that were used in the estimation of the pan-derived delta H-2(A). Good agreement (r = 0.7, P-value = 0.00) was found between the pan-derived and the FTIR measured delta H-2(A) for weekly intervals, although individual differences ranged from similar to 25.0 to 20.4 parts per thousand, with the absolute difference averaging 8.0 parts per thousand. A sensitivity analysis showed that the determination of delta H-2(A) is most sensitive to air temperature, relative humidity and the isotopic composition of the pan water. While the precipitation-equilibrium approach only appears to be representative of atmospheric conditions close to times of precipitation events, the pan-derived isotopic composition of atmospheric vapour was found to be closer to the FTIR averages over longer periods including intervals with no precipitation. Overall, this means that the pan method is far more effective for uninterrupted estimation of delta H-2(A) and delta O-18(A) of atmospheric water vapour, as required for water budget studies, than the precipitation-equilibrium method, and it is more cost effective and robust than continuous measurement.