Environmental Engineering Reference
In-Depth Information
Figure 5.18 Schematic representation of the hydrological
cycle, showing the main losses of water to the atmosphere.
returning water to the atmosphere. So far we have discussed some of the processes
involved, but it is important to appreciate that evapotranspiration occurs in many
different stages of the hydrological cycle. Thus losses of water to the atmosphere may
take place at any point within the system (Figure 5.18).
One of the major losses, for example, occurs during precipitation. Considerable
amounts of moisture may be evaporated during rainfall and the small droplets in
particular may be totally evaporated before they reach the ground. Similarly, moisture
which is intercepted by the vegetation is also susceptible to direct return to the
atmosphere by evaporation. The amount of moisture retained on the vegetation during a
storm varies according to the character of the storm, the species of plant, the leaf density
(and therefore the time of year) and, of course, the vegetation density. In the case of
woodlands as much as 50 per cent of the incoming water may be retained in the canopy
and returned as evaporation. In the case of more low-lying vegetation, such as grass, the
amount of interception is not known with such certainty, partly because of the difficulty
of measuring interception in crops. Nevertheless, again it seems likely that interception
may reach 20-30 per cent.
A proportion of the water which reaches the soil is also returned by evaporation, for in
heavy storms rainfall often collects in surface depressions and these are gradually dried
by the sun. Similarly, some of the rainfall flows across the surface as run-off, and further
evaporation losses may occur at this stage. Rates are generally low, however, for
turbulence mixes the water and disperses the heat from solar radiation through the water
body. Thus much greater inputs of energy are needed to heat the water and less is
available to carry out evaporation. Nevertheless, losses at this stage are often important to
humanity. Open canals and reservoirs may lose considerable quantities of water through
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