Environmental Engineering Reference
In-Depth Information
very successfully, during the wet season, and even
during the dry season, if the moisture from the
preceding rainy season can be retained. If for
some reason the rainy season is curtailed,
however, the potential for drought is great, and
it is not surprising that areas such as the Sahel
and the Indian sub-continent have experienced
some of the world's most spectacular and
catastrophic droughts. The problem is intensified
in drier years by the irregularity with which the
rains fall, making planning difficult, if not
impossible.
Irregular and variable precipitation is also
characteristic of contingent drought. In
Thornthwaite's definition, this is experienced in
areas which normally have an adequate supply
of moisture to meet crop needs. Serious problems
arise because the agricultural system is not set
up to cope with unpredictable and lengthy
periods of inadequate precipitation. The interior
plains of North America have suffered from
contingent drought for hundreds of years, and
the droughts of 1975-76 and 1988-92 in Britain
would fit this category also.
The presence of these three types of drought
is indicated by physical changes in the soil and
vegetation in the areas affected, but there is also
a fourth type which is less obvious. This is the
so-called invisible drought, which often can be
identified only by sophisticated instrumentation
and statistical techniques. The crops appear to
be growing well, even to the experienced observer,
and there is no obvious lack of precipitation.
However, moisture requirements are not being
met, the crop is not growing at its optimum rate,
and the potential yield from the land is reduced.
Figure 3.4
Sample
climatic water budget for
a mid-latitude station in
North-America or
Europe, based on the
Thornthwaite model
