Geoscience Reference
In-Depth Information
climate system. Moreover, water is vital for human existence and is irreplaceable. It is
more than a natural resource that we exploit and often take for granted. Water plays a
crucial role in Earth's climate, functioning of ecosystems and environment.
Many studies on the global water cycle deal with only specific aspects (see the review
by Trenberth et al.
2007a
and other chapters in this monograph). Reliable data on the
surface water budget are often available only over certain regions. Relatively few studies
(e.g., Trenberth et al.
2007a
,
2011
; Oki and Kanae
2006
) have attempted to provide a
synthesized, quantitative view of the global water cycle, and our quantitative knowledge of
the various components, and their variability of the global water cycle is still fairly limited
because of a lack of reliable data for surface evapotranspiration, oceanic precipitation,
terrestrial runoff and several other fields. Regional closure of the water cycle over many
large river basins has been attempted by Vinukollu et al. (
2011
) and Sahoo et al. (
2011
)
using satellite data but, unless adjusted, they do not adequately close the water budget, and
the imbalances highlight the outstanding observational and modeling limitations.
Satellite-based observations provide global coverage but may lack continuous coverage
in time and generally require some kind of algorithm to produce a geophysical product that
inevitably has limitations, so the result must be verified against other independent mea-
surements such as in situ observations. However, as the number of analyzed fields grows
with ever increasing satellite data products, it is vital for these to be properly evaluated and
documented for their strengths and weaknesses along with quantifying their uncertainties.
Some of the satellite-based observations limitations may be overcome with in situ
observations because they measure directly the quantity desired, but it is likely a spot
Hydrological Cycle
Atmosphere
12.7
Ocean to land
Water vapor transport
40
Land
Precipitation
113
Ocean
Precipitation
373
Evaporation, t
r
anspiration
73
Ocean
Evaporation
413
Vegetation
Land
Ice
Percolation
26,350
Rivers
Lakes
178
Ocean
Surface flow
Soil moisture
122
40
Ocean
1,335,040
Permafrost
22
Ground water flow
Groundwater
15,300
Units: Thousand cubic km for storage, and
thousand cubic km/yr
for exchanges
Fig. 1 The global annual mean Earth's water cycle for the 1990s. The arrows indicate the schematic flow of
water substance in various forms. From Trenberth et al. (
2007a
,
b
)
