Geoscience Reference
In-Depth Information
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PRECIPITATION
CLOUD FORMATION
EVAPORATION
SNOWMELT
RUNOFF
FROM
FALLING RAIN
WT
FROM WET
VEGETATION
AND
PUDDLES
SPRING
OVERLAND
FLOW
IL
IN
T
ERCEPTION
TRANSPIRATION
FROM STREAMS
AND
OPEN WATER
INFILTRATION
DEPRESSION
STORAG
E
FROM
SOIL
WT
SOIL MOISTURE
WATER TABLE (WT)
WT
GROUNDWATER
BASE FLOW
STREAM
FLOW
LAKE
OR
SEA
IMPERMEABLE LAYER
(I L)
Fig. 1.1
Sketch of some of the main processes in the land phase of the water cycle.
flow
; in the absence of
storm flow
or
storm runoff
caused by precipitation, base flow is
also referred to as
drought flow
or
fair weather flow
.
Finally, the hydrologic cycle is closed by
evaporation
, which returns the water, while
in transit in the different flow paths and stages of storage along the way, back into
the atmosphere. When evaporation takes place through the stomates of vegetation, it
can be referred to as
transpiration
. Direct evaporation from open water or soil surfaces
and transpiration of biological water from plants are not easy to separate; therefore
the combined process is sometimes called
evapotranspiration
. Evaporation of ice is
commonly referred to as
sublimation
. While these distinctions are useful at times, the
term evaporation is usually adequate to describe all processes of vaporization. Some of
the main processes are drawn schematically in Figure 1.1.
1.3
SOME ESTIMATES OF THE GLOBAL WATER BALANCE
Numerous studies have been carried out to estimate the magnitude of the most important
components of the water budget equations on a global scale. Because the available data
base required for this purpose is still far from adequate, several of the methods used in
these estimates may be open to criticism. Nevertheless, there is a fair agreement among
