Atmospheric water balance-the isotopic perspective

The isotopic composition of atmospheric moisture is determined foremost by the isotope fractionation that accompanies the evaporation occurring over the oceans. This subsequently is modified by rainout on the one hand, and on the other hand by the recycling of precipitation into the atmosphere as a result of evapo-transpiration from the continents or from droplets beneath the cloud base. The marine part of the atmospheric water cycle is described by the model of Craig and Gordon, showing the d-excess value to reflect the humidity gradient from the saturated vapour over the sea (a function of both temperature and salinity) to the ambient humidity. Average conditions correspond to a value of d = 10%. However, on the leeside of continents and over the Mediterranean Sea, where the humidity deficit in the dry and cold air is large, higher values of up to d = 30‰ are engendered. Similar results are found when the evaporated vapour from lakes and other open water bodies mixes into the atmosphere. A model enabling the quantitative evaluation of the contribution of the evaporation flux to atmospheric humidity is exemplified by the case of the North American Great Lakes. A regional atmospheric water balance model, based on the isotopic composition of runoff and precipitation is presented for the Amazon basin. Copyright (C) 2000 John Wiley and Sons, Ltd.