ESTIMATION OF THE ISOTOPIC COMPOSITION OF LAKE EVAPORATE

The use of evaporation pans for determining the isotopic composition of water evaporating from a lake, δE, has been studied. It was found that useful values of the relevant parameters, m, the relative rate of isotope enrichment, and δs, 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 V0 = 0.5. In practice meteorological conditions usually changed before this was reached. One period of meteorologically stable conditions permitted field evaluation of m and δs. 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 δs under variable meteorological conditions, these parameters were calculated from the free air relative humidities to deduce day-to-day δE 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 δair 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. © 1979.