Estimation of the isotopic composition of lake evaporate

The use of evaporation pans for determining the isotopic composition of water evaporating from a lake, δ E, has been studied. It was found that useful values of the relevant parameters, m, the relative rate of isotope enrichment, and δ s, the heavy-isotope concentration attained as complete evaporation is approached ( V → 0), could be obtained with the size of pan used only when evaporation had proceeded under stable humidity conditions at least to V V 0 = 0.5 . In practice meteorological conditions usually changed before this was reached. One period of meteorologically stable conditions permitted field evaluation of m and δ s. Comparison with values calculated from: (a) free air relative humidities; and (b) relative humidities normalized to the evaporating water temperature indicates that the former corresponds more closely to field-measured values. Due to limitations in the field measurements of m and δ s under variable meteorological conditions, these parameters were calculated from the free air relative humidities to deduce day-to-day δ E values from isotope concentrations in the pans. Similarity of results from pans on the shore to those from a pan at the centre of the lake indicates that pan siting relative to the lake is not critical when proper account is taken of contributions to the ambient δ air by lake evaporate. The use of pairs of pans, one having an artificially elevated isotope concentration, was found to be a promising technique for field measurement of m with improved precision.