Geoscience Reference
In-Depth Information
Atmosphere
12.7
Ocean to land
Water vapor transport
40
Land
precipition
113
Ocean
precipitation
373
Evaporation, transpiration 73
Ocean
evaporation 413
Vegetation
Percolation
Land
Ice
26,350
Rivers
Lakes
178
Ocean
Surface flow
Soil moisture
122
40
Ocean
1 335 040
Permafrost
22
Ground water flow
Groundwater
15 300
Figure 1.1
The global annual average hydrological cycle including estimates of the main water reservoirs (in plain font in
units of 10
3
km
3
) and of the flow of moisture between stores (in italics in units of 10
3
km
3
yr
−1
). (From Trenberth
et al.,
2007,
published with permission.) See Plate 1 for a colour version of this image.
varies significantly both regionally and with season. Area-average runoff in the
semi-arid south western USA is, for example, commonly just a few percent. When
averaged over large continents and over a full year, variations in the fraction of
precipitation leaving as runoff are less. Table 1.2 gives an example of the estimated
annual water balance for the continents (Korzun 1978). Runoff ratios in the range
of 35 to 45% are the norm, but the extensive arid and semi-arid regions of Africa
reduce average runoff for that continent. Fractional runoff in the form of icebergs
from Antarctica is hard to quantify but may be 80% because sublimation from the
snow and ice covered surface is low.
Components of the global hydroclimate system
Understanding the hydroclimate of the Earth does not merely require knowledge
of hydrometeorological process in the atmosphere. Several different components
of the Earth system interact to control the way near-surface weather variables vary
