THE ROLE OF MOISTURE TRANSPORT BETWEEN GROUND AND ATMOSPHERE IN GLOBAL CHANGE

Projections of the effect of climate change on future water availabili ty are examined by reviewing the formulations used to calculate moistu re transport between the ground and the atmosphere. General circulatio n models and climate change impact models have substantially different formulations for evapotranspiration, so their projections of future w ater availability often disagree, even though they use the same temper ature and precipitation forecasts. General circulation models forecast little change in tropical and subtropical water availability, while i mpact models show severe water and agricultural shortages. A compariso n of observations and modeling techniques shows that the parameterizat ions in general circulation models likely lead to an underestimate of the impacts of global warming on soil moisture and vegetation. Such er rors would crucially affect the temperature and precipitation forecast s used in impact models. Some impact model evaporation formulations ar e probably more appropriate than those in general circulation models, but important questions remain. More observations are needed, especial ly in the vicinity of forests, to determine appropriate parameterizati ons.