A comparison between direct and pan-derived measurements of the isotopic composition of atmospheric waters.
Azcurra, C.S., Hughes, C.E., Parkes, S., Hollins, S.E., Gibson, J.J., McCabe, M.F. and Evans, J.P.
In Chan, F., Marinova, D. and Anderssen, R.S. (eds) MODSIM2011, 19th International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand, December 2011, pp. 1652-1658. ISBN: 978-0-9872143-1-7.
The stable isotopes of water can be used to examine and quantify the contribution to
atmospheric moisture from evaporation, transpiration and surface-waters. However, obtaining extensive and
ongoing time series data of the isotopic composition of atmospheric moisture has been difficult. Presented
here is an alternate method using an isotope mass balance approach to estimate the isotopic composition of
atmospheric moisture using water samples collected from class A evaporation pans. While this evaporation
pan method does not provide the high-resolution time series data that can be obtained from an isotope
analyser taking in-situ measurements of atmospheric moisture, the method is relatively simple and
inexpensive to set-up and maintain.
In this preliminary investigation, a comparison between the isotopic composition of atmospheric moisture
estimated from the evaporation pan method and in-situ measurements of the isotopic composition of water
vapour using a Fourier Transform Infrared (FTIR) spectrometer deployed at the Lucas Heights weather
station in New South Wales is undertaken. Through comparison of the two series of hydrogen isotope data,
an assessment of the evaporation pan method can be made. Although there was some agreement between the
isotopic composition of vapour measured by the FTIR spectrometer and the estimation for the atmospheric
moisture (R2 = 0.49), the comparison is sensitive to climatic parameters that vary significantly within a 24-
hour period such as the relative humidity of air and the air and pan temperatures. Inverting the model to use
the FTIR spectrometer measurements at an hourly resolution improved the performance of the model (R2 =
0.57). However, this also revealed that the model produced more depleted values of the evaporation pan
water isotopes than those observed. In contrast, there was a variable relationship between the modelled and
observed isotope values of atmospheric moisture. These conflicting results will need to be resolved before
the evaporation pan method is broadly applied in isotope hydrology.
Figure 3: Time series of estimated δL at hourly intervals (solid line), as a weekly moving average (thin
broken line) and observed δL as determined by approximately weekly grab samples (circles).
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Last updated 31st January 2013