Geoscience Reference
In-Depth Information
3000
(a) Sri Lanka
6° - 9° N
(b) Sierra Nevada, California
38° - 39° N
(c) Austria
47° N
2500
2000
1500
1000
500
0
0
2000
4000
6000
0
500
1000
1500
2000
500
1000
1500
2000
2500
Mean annual precipitation (mm)
Figure 5.16
Generalized curves showing the relationship between elevation and mean annual precipitation in different climatic
regimes.
Transpiration
Transpiration is a related process involving water loss
from plants. It occurs mainly by day, when small pores,
called
stomata
, on the leaves of the plants open up under
the influence of sunlight. They expose the moisture in the
leaves to the atmosphere and, if the vapour pressure of the
air is less than that in the leaf cells, the water is transpired.
As a result of this transpiration, the leaf becomes dry and
a moisture gradient is set up between the leaf and the base
of the plant. Moisture is drawn up through the plant and
from the soil into the roots (
Figure 5.17
).
Evaporation and transpiration form the major flows of
moisture away from Earth's surface. Because we can rarely
see the processes taking place it is easy to neglect this
component of the hydrological cycle, but it is an extremely
important one. It returns moisture to the air, replenishing
that lost by precipitation, and it also plays a part in the
global transfer of energy.
Processes
Evaporation
Evaporation can be defined as the process by which a
liquid is converted into a gaseous state. It involves the net
movement of individual water molecules from the surface
of Earth into the atmosphere, a process occurring
whenever there is a vapour pressure gradient from the
surface to the air. The rate of evaporation depends on the
balance between the vaporization of water molecules into
the atmosphere and the condensation rate from the
atmosphere. The process requires energy: 2·48
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evaporate each kilogram of water at 10°C. This energy is
normally derived from the sun, although sensible heat
from the atmosphere or from the ground may also be
significant. However, when the air reaches saturation (100
per cent relative humidity) no net evaporation takes place.
Wind is required to remove the layer of air near the
surface, which would otherwise become saturated and
stop net evaporation.
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water and gases by transpiration in plants.
