Geoscience Reference
In-Depth Information
2.
Contribution by land surfaces to atmospheric water availability
The hypothesis that land surfaces influence climate and weather by contribut-
ing to the water vapor in the atmosphere is entirely plausible. It requires only
that the amount of water vapor contributed regionally by evaporation from
land surfaces is significant in comparison with that contributed by oceans.
There are now numerous observations and model analyses that indicate the
recycling of water over land surfaces is significant. In fact this significance has
been apparent for decades in global water balance studies (e.g., Baumgartner
and Reichel, 1975; Korzun, 1978) and has been confirmed by more recent
studies (e.g., Oki and Kanae, 2006). As a global average only about 35-45% of
precipitation falling over land leaves as runoff, which implies that the remain-
der is re-evaporated. There is also long-established evidence from global mod-
eling studies (e.g., Shukla and Mintz, 1982) that the atmospheric water vapor
resulting from land surface evaporation has a large effect on modeled
continental precipitation.
Early isotope studies (e.g., Salati
et al.
, 1979) further demonstrated that in
certain areas such as the Amazon River basin about 30% of area-average
precipitation originates from evapotranspiration, and later studies using
reanalysis data sets (e.g. Brubaker
et al.
, 1993; Eltahir and Bras, 1996; Costa and
Foley, 1999; Bosilovich
et al.
, 2005) confirm recycled evaporation accounts for
20-27% of precipitation in this region. The analysis of Makarieva and Gorshkov
(2007) suggests that the efficiency of recycling by forests is greater than for other
land covers so their extensive presence may help maintain precipitation amounts
for greater distances away from coasts. As reanalysis data sets have become
increasingly available there have been numerous studies of atmospheric cycling
including demonstrations of its importance in the context of ecoclimatological
stability (Dominguez and Kumar, 2008) and in monsoon systems (Dominguez
et al.
, 2008).
Thus, the phenomenon of precipitation recycling is now a well-studied and
well-established facet of terrestrial hydrometeorology and its quantification
and consequences are increasingly well-defined. For this reason in Table 25.1 the
credibility of this mechanism providing the basis of a land surface influence on
climate and weather is assessed as being 'extremely likely. But quantification and
modeling of the mechanism is assessed as being of 'medium' quality because
further research is justified, with focus on achieving greater realism and accuracy
when modeling surface evaporation and especially the atmospheric mechanisms
involved in releasing precipitation.
B.
The influence of transient changes in land surfaces
1.
Effect of transient changes in soil moisture
It is physically plausible that moisture stored in the soil which entered from
precipitation can later become re-accessable to the atmosphere, often via plants.
