Geoscience Reference
In-Depth Information
Month
J
F
M
A
M
J
J
A
S
O
N
D
Amount (mm)
46
53
80
105
123
146
168
178
127
97
68
53
Source: Texas Development Board, monthly lake surface evaporation.
Note: The site is close to Amarillo (Figures 5.8 and 5.18). In all months evaporation is much greater than precipitation.
to prevailing conditions. Without doubt, the major
evaporative losses occur from the sea - possibly 85 per
cent of the global return to the atmosphere is from the
oceans. The reason is not only the great extent of the
oceans - some 70 per cent of the world's surface - but also
the fact that evaporation can continue at the potential rate.
Unlike evapotranspiration from the land, the process is
unhindered by water shortage. Even so, seasonal and
regional differences in evaporation can be seen, owing to
the effect of changing meteorological conditions.
Finally, evaporation may occur from the other main
storage component of the hydrological cycle - the
cryosphere. In general the losses are small, for it requires
large amounts of energy to convert ice to water vapour -
a process known as sublimation; some 2ยท83
CONCLUSION
The precipitation input is probably one of the most
important regulators of the hydrological cycle, for it
determines the intensity and distribution of many of the
processes operating within the system. It is closely related
to the rate of evapotranspiration and also influences
the pathways of run-off and underground flow and the
magnitude of stream flow. Through these processes, and
through the direct effects of the impact of rainfall on the
ground, it also takes part in many geomorphological
processes; it causes rain splash and soil erosion and it plays
a vital role in weathering and rock breakdown. The
distribution of rainfall across the globe therefore to a
large degree controls the operation of the landscape
system. Precipitation is similarly a vital input to the
ecosystem, and the distribution of vegetation, fauna and
population owes much to the pattern of rainfall.
For these reasons, and because of its ultimate impor-
tance to human activities, a great deal of attention has
been paid to measuring, mapping and predicting
precipitation. As we have seen, scarcity of data, particularly
in the more remote parts of the world, limits our ability
to gain an accurate picture of precipitation inputs. On the
whole, however, rainfall is one of the easiest components
of the hydrological cycle to measure. Conversely, evapo-
transpiration is one of the more difficult. Empirical
formulas are the most frequently used ways of obtaining
the information.
10
6
J are
needed to evaporate 1 kg of ice at 0
C. Sublimation does
occur in the marginal areas of glaciers and ice sheets,
however, where seasonal inputs of solar radiation may be
high; perhaps 2 per cent of the moisture is returned to the
atmosphere each year in this way. Moreover, in the past
the process was much more important. During the latter
parts of the glacial periods, for example, as the ice sheets
that had spread into the mid-latitudes began to retreat,
sublimation must have been one of the main processes
of stagnation and decay. Warm, turbulent and often
relatively dry air masses moved across the ice margins,
drawing vast quantities of moisture from the ice sheets
and causing them to retreat over areas of thousands of
square kilometres.
KEY POINTS
1
Precipitation is found in a variety of forms. Which form reaches the ground surface will depend upon many
factors: surface temperature, atmospheric moisture, the method and rate of cooling and the intensity of
updraughts, for example. Each type of precipitation has its own characteristics and consequences. The
distribution of precipitation varies greatly in time and space, and in quantity.
2
Evaporation and transpiration are complex processes which return moisture to the atmosphere. The rate
of evapotranspiration will depend largely on two factors: (a) how moist the ground is and (b) the capacity
of the atmosphere to absorb the moisture. Hence the greatest rates are over the tropical oceans, where
moisture is always available and the long hours of sunshine and steady trade winds evaporate vast
quantities of water.
