EARTH SURFACE EVAPORATIVE PROCESS - A CASE-STUDY FROM THE GREAT-LAKESREGION OF THE UNITED-STATES BASED ON DEUTERIUM EXCESS IN PRECIPITATION

The ''deuterium excess'' parameter in rain, defined as d = delta D - 8 delta(18)O, is a powerful geophysical tool which can be used to estim ate secondary processes that influence atmospheric vapor content in th e evaporation-condensation cycle in nature. The d-excess has been calc ulated, based on the systematic analysis of the stable oxygen and hydr ogen isotope ratios, in precipitation samples from Kalamazoo, southwes t Michigan. The results show that moisture derived from the evaporatio n of Lake Michigan contributes between 9-16% during the summer months to the atmospheric vapor load in the region. Admittedly, this estimate is an upper value since in our model we have assumed that transpirati on from leaf surfaces is minimal compared to the evaporation from Lake Michigan. It is therefore obvious that large freshwater systems need to be regarded as a potentially significant source of atmospheric vapo r load. Isotopic investigations such as this will enable to attain qua ntitative estimates of this important aspect, a knowledge of which, wi ll enhance the usefulness of or perhaps even be essential in calculati ons based on many atmospheric circulation models.