Arid Zone: Definitions and Characteristics

After reading this article you will learn about the definition and characteristics of arid zone.

Definition of Arid Zone:

The literature on the world’s arid zone demonstrates that there are many different definitions of the area referred to, and many different defini­tions of the aridity which is suggested to be the distinguishing characteristic. The differences stem partly from the fact that the criteria on which the definitions are based refer to the deficiency of a resource (basically moisture) in the lace of demands for the resource.

Because demands will vary and the thresholds of significance of the aridity characteristics will vary and the definition itself will vary as a result. Further differences in definition may result from the interpretation of different scholars concerned for different significance thresholds for aridity. Finally, definitions may be complicated by semantic confusion over the evolution of the meaning of the term ‘desert’.

These seem to be two separate types of definition of arid zones the one i series literary definitions based on the changing meanings of ‘desert’, the other a series of scientific attempts to establish a meaningful boundary to the arid zone.

Chanting definitions of ‘Desert’ (1225-1968):

1225 a desolate, barren region, waterless and treeless, with but scanty growth of herbage. ‘ J. A. H. Murray (ed.) A New English Dictionary on Historical principle, Oxford, 1897.

1398 ‘Place of wodes and mountayns that be not sowen be a calleyed desertes’ any wild uninhabited region, including forest land. Murray, ibid.

1880 ‘(desertus solitary), a term used to denote any portion of the earth’s surface which, from its barrenness, as in the case of the arid plains of Northern Africa and Arabia. The Desert proper may be said to signify the vast sandy plains of Africa and Arabia. Chamber’s Encyclopedia:

1909 A desert is a country with such an arid climate and such a scanty water supply that agriculture is impracticable and occupation is found possible only for a sparse population of pastoralists.’ J. W. Gregory: South Australia, London.

1924 ‘A desert has become by definition not naked sand and rock but a place of small rainfall with a spare and specialised plant and animal life.’ I. Bowman Desert Trails of Atacama, New York.

1968 1. (a) Archaic — ‘a wild uninhabited and uncultivated tract ; a desolate unoccupied plain or coast or pathless woodlands wilderness, waste

(b) ‘Any of the formerly unsettled regions of a the United states between the Mississippi River and the Rocky Mountains thought to be arid and uninhabitable.’

2. (a) ‘A region in which the vegetation is so scanty as to be incapable of supporting any considerable population as a region perpetually cold or covered with snow or ice or a region located in the interior of a continent by scanty rainfall especially of less than 10 inches annually.’

(b) ‘A more or less barren tract incapable of supporting any considerable population without an artificial water supply.’

(c) ‘An area of ocean believed to be devoid of marine life.’

3. ‘A secluded place of worship for Huguenots, 1715-1802.’

4. ‘A desolating or forbidding prospect (as from pathless emptiness, bleak unrelieved changelessness or monotony, futility of effort or destitution of mental or spiritual animation or stimulation). Webster’s Third Inter­national Dictionary.

Such variety illustrates both the virile nature of the language and its embellishment over time. As a basis for a scientific study of the arid zones however, the literary definitions of desert are inadequate.

The Arid Zone: The Scientific Definitions:

The scientific approaches to the definition of the arid zones have varied according to the aims of the enquiry. Arid Zones may be those .areas of the world where evaporation exceeded precipitation. This boundary, or ‘dry line’ comparable in some ways to the ‘snowline’ in mountainous areas.

E. de. Martonne and L. Aufrere in 1927 claimed that ‘the desert are “par excellence” the domain of interior-basin drainage, or endoreism’ because these areas did not drain to the sea, due to inadequate precipitation and hence geomorphological characteristics of such areas were likely to be dif­ferent from those areas which did drain surface waters to the oceans.

For the soil scientists, those soils where precipitation is insufficient and where carbonates remain and often accumulate by evaporation of capillary induced solutions at or close to the surface are ‘arid’ soils-indicate ap­proximately 43 per cent of the global land area.

W. Koppen on the basis of the presence or absence of a seasonal precipitation distinguished the deserts as 12 per cent and steppes or semi- deserts as 14.3 per cent of the land area, giving a total of 26.3 per cent of the land area with dry climates. However, the classification did not account.

For the variation in efficiency of precipitation between seasons. It remained for C W. Thornthwaite (1948) to devise a series of indices to show the relationship between precipitation and evapotranspiration (the combined ire loss from an area by evaporation and the transpiration of plants).

Using Thornthwaite’s method of calculating potential evapotranspiration Meigs calculated a moisture index to represent the relationship between precipitation and evapotranspiration.

Meigs divided the moisture index according to the possibility of crop h and added a further subdivision of the driest classification for those area where no precipitation had been recorded for at least 12 consecutive is (Table 1.1).

His calculations increased the global proportion of the arids over both Koppen’s and Thornthwaite’s figures (Table 1.2) and have remained a useful global estimate of the significance of aridity for growth of temperate crops. Subsequently his threefold classification has been widely adopted and average precipitation figures suggested for boundaries between the zones.

The columns 1, 2, 3 and 4 of Table 11 indicate:

1. Relationship of precipitation to evapotranspiration.

2. Climatic zones.

3. Suitability of climatic zones for temperate grain crops without irrigation.

4. Estimates.

Arid Nation:

While climate knows no political boundaries, national climates vary considerably. Using Meigs’ classification, it is possible to group the world’s arid nations according to the proportion of their area which is arid or semi-arid, for some nations aridity is the dominant climatic characteristic, for others it is merely a regional condition.

Characteristics of Hot Arid Zone:

The salient characteristics of hot arid zones are following:

(1) Aridity index is high (more than 70%).

(2) High range of temperature and solar radiation; erratic and low rainfall; low atmospheric humidity and high wind velocity in summer.

(3) Famine of food, fodder and water i.e., “Trikal” is a permanent guest in every 3 or 5 years.

(4) Soil is sandy with low water holding capacity; low organic content, deficit in nitrogen and phosphorus contents.

(5) Vegetation is sparse and limited with low biomass production.

(6) Vegetation is xerophytic.

(7) Heavy pressure of men and animals (cutting, felling, looping, grazing, browsing etc.) on vegetation.

(8) Human population is thin except in Indian Arid Zone; which is densely populated arid zone of the world.