Environmental Engineering Reference
In-Depth Information
the canyon floor at 760 m to 400 mm on the southern rim of the canyon at 2100 m. On
the forested northern rim, 2600 m above sea level, rainfall totals over 600 mm.
Nevertheless, the progressive increase in rainfall with altitude does not always extend
to the summits of the mountains. The Sierra Nevada in California are no wetter on the
summit than they are 1200 m lower (Figure 5.13). In the subtropical trade-wind belt over
Hawaii the peaks of Mauna Loa and Mauna Kea receive far less rain (380 mm) than the
windward slopes, where maxima between 1000 and 1300 m amount to about 7500 mm
yr
−1
. It is also apparent that the relationship between altitude and precipitation varies from
one part of the world to another. In the tropics much of the precipitation is produced by
warm clouds whose upper limit is only 3000 m above the ground; thus the effect of
altitude is subdued (Figure 5.13a) and the maximum may even be close to sea level. In
contrast, in temperate areas a large proportion of the rainfall comes from deep stratiform
clouds that extend through a considerable part of the troposphere. Here the effect of
altitude on rainfall is more marked, though the increase on windward slopes is usually
greater than on leeward slopes. Comparisons are difficult, however, because some of the
precipitation on the mountains in temperate areas falls as snow and, as we have seen,
snow is impossible to measure accurately.
EVAPOTRANSPIRATION
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 takes 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
beween the vaporization of water molecules into the amosphere and the condensation rate
from the atmosphere. The process requires energy: 2·48 × 10
6
J to 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.
