Environmental Engineering Reference
In-Depth Information
Invisible drought can be dealt with relatively
easily by irrigation. In eastern Britain, for
example, supplementary moisture has been
supplied to sugar beet and potato crops since at
least the late 1950s to deal with that problem
(Balchin 1964).
evapotranspiration can take place. Yet even when
no water is available, the environment may retain
the ability to cause evapotranspiration through
such elements as temperature, radiation,
humidity and air movement. Thornthwaite
(1948) developed the concept of potential
evapotranspiration to recognize this situation.
Potential evapotranspiration may be considered
the amount of evaporation and transpiration that
would take place if sufficient moisture was
available to fill the environment's capacity for
evapotranspiration. In this way, the distinction
between measurable (actual) and theoretical
(potential) amounts of evapotranspiration can be
recognized. As long as precipitation exceeds
evapotranspiration, the actual and potential
values will be the same, but as soon as the
situation is reversed the two values begin to
deviate at a rate which depends upon the
availability of soil moisture. The difference
between actual and potential evapotranspiration
can be considered as a measure of the water
deficit, and agricultural areas experiencing
moisture deficiency depend upon this approach
to estimate the appropriate amount of moisture
required to combat drought or to allow crops to
grow at their full potential. The relationships
between elements such as precipitation, actual
and potential evapotranspiration, water surplus
and deficit, as identified by Thornthwaite, are
illustrated in Figure 3.4.
Modern methods of drought evaluation tend
to focus on the soil moisture deficit (SMD) rather
than the basic water deficit represented by the
excess of evapotranspiration over precipitation
(see e.g. Palutikof 1986). Such factors as the
nature and stage of development of the crop, the
storage capacity of a specific soil, and the ease
with which the crop can extract moisture from
the soil are all given greater attention than in
Thornthwaite's original approach to the problem.
The SMD in a region will not be a specific value,
but will vary according to crop and soil
conditions. A consistently high SMD over the
growing season, however, will ultimately lead to
drought unless the situation is recognized and
rectified.
Determination of moisture deficit
To establish the existence of a deficit in an area,
it is necessary to compare incoming and outgoing
moisture totals. The former is normally
represented by precipitation and the latter by
evaporation from the earth's surface plus
transpiration by plants, commonly combined into
one unit referred to as evapotranspiration. In the
simplest relationship, if precipitation exceeds
evapotranspiration, a water surplus will exist; a
water deficit results when the relationship is
reversed. Several factors complicate this simple
situation. For example, all soils have the ability
to store a certain amount of water, and this
disturbs the relationship between precipitation
and evapotranspiration. If the soil water storage
is full, the soil is said to be at field capacity, and
any additional precipitation not evaporated is
considered as run-off. The presence of soil water
will help to offset any water deficit, since, even
if the evapotranspiration exceeds precipitation,
some or all of the deficit may be made up from
the soil moisture storage. This has the effect of
delaying the onset of drought until the storage
capacity is exhausted, and illustrates one of the
dangers of measuring drought by precipitation
alone or even by some simple comparison of
precipitation and evapotranspiration. Once
precipitation is again in excess, the soil water
storage must be recharged before a moisture
surplus exists.
A second factor complicating the relationship
between precipitation and evapotranspiration,
and the existence of moisture deficiency, involves
the measurement of evapotranspiration.
Evapotranspiration will only occur if moisture
is available. Thus, if the moisture directly
available from precipitation and in storage in the
soil
is
completely
exhausted,
no
