Environmental Engineering Reference
In-Depth Information
is available for heating and the leaves get warmer. Initially this may encourage growth,
but ultimately, if high temperatures are reached, it may damage the plant. The soil, too, is
heated more effectively, so the surface temperature rises. In one study in Wisconsin, with
an air temperature of 28° C, the surface of a dry sandy soil was 44° C, while the same soil
kept moist reached only 32° C. The difference was due to the fact that more energy was
used in evaporation from the wet soil.
The ultimate effect of continued drying of the soil is that plants can no longer obtain
any water and transpiration ceases. Since the water in the plant cells is important in
keeping the plants rigid (turgid), as the plants dry out they become limp and wilt. At that
stage, therefore, the soil is said to be at
wilting point
.
Of course, evapotranspiration does not always continue until wilting point is reached.
Instead renewed rainfall generally wets the soil and rejuvenates water uptake by the
plants and increases transpiration. Thus the soil acts as an important store of nutrients as
well as water.
Where the intervals between each period of precipitation are long, the ability of the
soil to supply water may be stretched to the limit and moisture stress may be a common
occurrence. In such circumstances the vegetation often adapts to the hydrological
conditions, by developing deeper roots, or by regulating water use in a variety of ways.
For example, the plants may have a dormant period during the dry season, completing
their growth cycle during the brief wet period. The sight of deserts blooming after a storm
is a most remarkable one (see Plate 27.4). Other plants adapt by controlling their stomata,
closing them during periods of dryness in order to reduce water loss. Others are able to
alter the orientation of their leaves so that the stomata are more sheltered from the hot
sun. Yet others, like the mesquite of the south-western United States, have thick, waxy
leaves which protect them from the radiation and slow down water losses.
MEASURING EVAPOTRANSPIRATION
One of the main needs of the farmer or irrigation engineer is to be able to predict when
the plants will suffer from moisture stress and how much water must be applied. This
involves being able to measure or calculate the rate of evapotranspiration. Knowledge of
evapotranspiration losses is also required by hydrologists who wish to plan water
management policies; they need to know what proportion of the precipitation will be
available to replenish ground water or run-off into streams. The measurement of
evapotranspiration is therefore important. Unfortunately it is also difficult. Several
approaches to measurement have been developed, including
direct measurement
(e.g.
with evaporation pans, lysimeters and eddy correlation systems),
meteorological
formulas
and
moisture budget
methods.
DIRECT MEASUREMENT
Possibly the most widely used method of direct measurement is the
evaporation pan
.
This consists of a shallow pan filled with water. The rate at which the water is lost
through evaporation is measured with a gauge (Plate 5.4). This procedure measures only
potential evaporation, for it does not allow for limitation of moisture supply, nor does it
directly determine transpiration losses. In addition, the results vary according to the size,
