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).

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 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.

Thornthwaite's treatment of the measurement and classification of drought is only one of many. It may not meet all needs, but its broadly climatological approach lends itself to the geographical examination of drought-prone environments. The greatest human impact is felt in those areas which experience seasonal or contingent drought, although the nature of the impact is different in each case. The influence of the other two types of drought is limited. In areas of permanent drought, for example, populations are small, and may exist only under special circumstances, such as those at an oasis, where the effects of the drought are easily countered. Although invisible drought may have important consequences for individuals, it often passes unrecognized. It does not produce the life and death concerns that prevail in areas of seasonal drought, nor does it have the dire economic impacts that may be experienced by the inhabitants of areas of contingent drought.

Seasonal drought in the sub-tropics

Seasonal drought is most commonly experienced in the sub-tropics. There, the year includes a distinct dry season and a distinct wet season, associated with the north-south movement of the intertropical convergence zone (ITCZ) and its attendant wind and pressure belts (see Figure 2.13).

During the dry season, these areas are dominated by air masses originating in the subtropical high pressure systems—which characteristically contain limited moisture, and are dynamically unsuited to produce much precipitation. Anticyclonic subsidence prevents the vertical cloud development necessary to cause rain. In contrast, the rainy season is made possible by the migration of the ITCZ, behind which, the combination of strong convection and air mass convergence promotes the instability and strong vertical growth which leads to heavy rainfall. The passage of the ITCZ—in Africa, India, SE Asia and Australia—allows the incursion of moist, relatively unstable air from the ocean over the land to initiate the wet season. This is the basis of the monsoon circulation in these areas, and it is often the failure of this circulation that sets up the conditions necessary for drought. If, for example, the ITCZ fails to move as far polewards as it normally does, those regions, which depend upon it to provide the bulk of their yearly supply of water, will remain under the influence of the dry air masses, and receive little or no rainfall. Similarly, any increase in the stability of the airflow following the passage of the ITCZ will also cause a reduction in water supply. It is developments such as these that have set the stage for some of the worst droughts ever experienced.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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  • Beatrice
    What is the difference between permanent,seasonal ,contingent and invisible droughts?
    3 years ago

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