Environmental Engineering Reference
In-Depth Information
evaporation (Table 5.5), and, in arid areas especially, this may represent an irretrievable
loss of an important resource.
Table 5.5 Kempton Park reservoir evaporation
Month
J
F
M
A
M
J
J
A
S
O
N
D
Amount (mm)
15
18
28
48
76
94
107
94
74
58
33
18
Source
: 'Evaporation from a reservoir near London', J. Inst. Water Eng. 19 (1965), 163-81.
In vegetated areas the major process of moisture return is by transpiration. Rates of
transpiration vary according to the character of the vegetation and therefore change over
both space and time. They are at a maximum when the vegetation is in full leaf and the
soil is moist; they decline as the plants lose their leaves or the soil dries out. During the
course of a single year, therefore, transpiration losses may show complex fluctuations in
response 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 2·83 × 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.
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
