676
ERIKSSON AND BOLIN
therefore know rather well the meridional transfer of water vapor
associated with a large-scale motion of the atmosphere and have an
approximate idea about the importance of the meridional circulation
cells found in tropical latitudes and the large-scale eddy exchange
that is predominant in middle and high latitudes for bringing about this
transfer. Investigations into the moisture exchange between oceans and
continents are less complete. The occurrence of various nuclides in
water, such as ‘80, deuterium, and tritium, has given us another tool
for studies of the circulation of water in nature. Even though a considerable increase of the number of observations of these nuclides has
occurred during the last few years, particularly through the worldwide
network organized by the International Atomic Energy Agency it should
be emphasized that it is the combination of ordinary meteorological
observations of water vapor in the atmosphere andthe distribution of
these nuclides that will give us a better understanding of the water
cycle in nature. In this paper we shall therefore attempt to relate the
distribution of these nuclides and their transfer to the distribution and
transfer of ordinary water. It will be clear from the analysis to be
given that data are still insufficient to improve greatly on our present
knowledge of the water budget of the atmosphere. However, some
interesting results are presented, and a clearer picture of which observations are of particular interest in this connection is obtained.
More details are given in Refs. 2 and 3.
BASIC FEATURES OF WATER CIRCULATION
The main source for atmospheric water is the sea, which also receives most of the precipitation. Of the yearly evaporation from the
sea, about 90% is precipitated over it, whereas only 10% is brought
back to the sea in rivers from the continents. Precipitation over land
is about 2'4 times greater than the runoff; thus nearly 60% of the precipitation evaporates. This evaporation, which is also called: evapo-
transportation, is largely due to water consumption by vegetation. It is
of importance for a discussion of the occurrence of various nuclides in
natural waters to realize that most of the evaporation from land areas
takes place by evaporation from capillary systems. Such evaporation
will initially cause fractionation of the isotopes, but this raises the
concentration of the heavy isotopes in the evaporating parts whereby
the
rate of evaporation of these parts increases. An equilibrium is
reached when the flow through capillaries to the evaporating surface
equals the rate at which evaporation of the isotope takes place. Frac-
tionation therefore essentially depends on possible fractionation in the
capillary systems. Provided no such fractionation occurs, the groundwater acquires the average isotopic composition of precipitation.