Environmental Engineering Reference
In-Depth Information
Evaporation is greatest when the sea is warm in comparison with the air, such as along
the Gulf Stream in the Atlantic Ocean. In general this is the case, as air temperatures are
slightly below those of the sea over much of the globe for much of the time. Where
upwelling of cold water from depth occurs, however, the surface temperatures are greatly
reduced and the difference between sea and air temperatures becomes small; in some
cases the sea may even be cooler than the atmosphere. An example of this phenomenon
occurs off the coast of Peru, where the cold Humboldt current brings bottom waters to the
surface. As a result, the air is warm relative to the sea, it retains moisture, and so the
humidity gradient above the surface is low. This greatly reduces the rate of evaporation,
and, as Figure 5.16 shows, the effect continues some way out into the Pacific.
EVAPOTRANSPIRATION FROM LAND SURFACES
Because of the importance of the energy and water balance to growing crops, there have
been a large number of studies of evapotranspiration from vegetated surfaces. The
presence of a vegetated surface complicates the energy exchanges taking place, however,
for the plants intercept radiation and rainfall inputs, they affect the temperature and wind
profiles near the ground, and they also modify humidity. The degree of these effects
varies with the character of the vegetation, so evapotranspiration from a vegetated surface
often differs markedly from PE.
Within a mature crop we can identify three layers: the upper layer, or canopy, the main
stem zone and the ground surface. During the day most of the incoming radiation is
absorbed by the canopy. The air space between the canopy and the ground acts as an
insulator, so that it is the top of the vegetation rather than the soil that acts as the active
surface. Consequently, transpiration rather than evaporation takes place.
So long as moisture is available in the soil, the plants are able to transpire at or very
close to the potential rate. Thus, in a moist soil, evapotranspiration proceeds unhindered,
water being drawn up the plant from the soil to replace that lost from the leaves. As the
soil dries out, however, the plant experiences increasing difficulty in extracting moisture
and the rate of transpiration cannot be maintained. Several changes take place. The plant
starts to suffer
