Introduction

The word arid (from arere to be dry) implies a deficiency of rainfall as the basic characteristic of a dry climate. Hence, the earliest attempts to classify dry climates were generally based on the annual amounts of rainfall. However, the amount of rainfall in dry regions cannot serve as an adequate measure of aridity unless it is related to its effectiveness, which in turn depends on many factors.

Arid: (of land or a climate) having little or no rain; too dry or barren to support vegetation.

Koppen considered vegetation to be the best expression of the totality of a climate; his criteria for determining climatic boundaries were suggested with vegetation units in mind. This is the best known system, and has gained wide acceptance because it is based on relatively simple values that can be measured or estimated with sufficient accuracy in most regions of the world, namely rainfall (amount and distribution) and temperature. This makes it useful for a general comparison of climatic resources and constraints of the various land utilization types. The dry climates are subdivided into deserts (or arid climates), in which rainfall is insufficient for arable crop production, and steppes (semi-arid climates), in which rainfall is sufficient for certain types of crops when adequate management techniques are adopted and grasses constitute an important element of the native vegetation.

A final subdivision is based on temperature: hot dry climates and cool dry climates, according to whether the mean annual temperature is above or below 18 C, respectively.

De Martonne \((1962)\) attempted to find an empirical relationship between precipitation and temperature, that would enable a more accurate and general definition of dry climates. This relationship is expressed in his ’aridity index", which can refer to a selected period of a few days, a month, a season, or an entire year.

\[I = \frac{np}{t + 10}\] Where;

\(I =\) aridity index
\(n =\) number of rainy days
\(p =\) mean precipitation per day
\(t =\) mean temperature of the selected period in C

When calculated on an annual basis, I<20 is characteristic of an arid climate and 20<I<30 defines a semi-arid climate.

Gaussen \((1954)\), from studies of natural vegetation, developed a classification of climates based on the number of arid months in a year. A month is considered arid if:

\[p<2t\] Where,

\(p=\) mean monthly precipitation

\(t=\) mean monthly temperature (C)

Thornthwaite \((1948)\) improved on Martonne’s approach by introducing the water balance concept in his classification system. A dry climate occurs where evapotranspiration is markedly in excess of precipitation.

He based his classification of climate on two elements:

Water supply in the form of precipitation
Water needs resulting from evapotranspiration

Precipitation and evapotranspiration are due to different meteorological causes and may be markedly different from each other both in quantity and in seasonal distribution. In principle, Thornthwaite defines a climate as moist when precipitation exceeds evapotranspiration;

A dry climate: occurs where evapotranspiration is markedly in excess of precipitation.

Thornthwaite further differentiates between actual evapotranspiration, which in an arid climate may be very low simply because the moisture supply is limited, and potential evapotranspiration, or ‘water need’, which is the amount of water that would be transpired and evaporated under ideal conditions of soil moisture and vegetative cover.

Types of dry climates

The major types of dry climates in terms of their location and thermal regimes are:

arid (or desert) climates with insufficient precipitation for normal crop production
semi-arid climates in which a dry season alternates with a wet season

There are three basically different semi-arid zones:

The tropical and subtropical frost-free savannah climate with summer rainfall
The middle-latitude steppe continental climate with definite warm and cold seasons and mostly summer rainfall
The Mediterranean climate, with mild winters, occasional frosts, winter rainfall and hot summers

All three types of semi-arid climates have fringe areas of relatively low mean annual precipitation. Because of the great variability of rainfall that is characteristic of these areas, cycles of years with sufficient precipitation for crop production alternate with cycles when insufficient precipitation causes crops to fail. These are the most hazardous areas for rainfed agriculture.

Arid (or desert) climates

Arid climates are those in which no arable crop can be grown, unless precipitation is supplemented by water harvesting and spreading, or by irrigation. They are deserts in the full sense of the word, and as such their agricultural potential, at best, is confined to livestock production under extensive conditions. The upper limit of 250 mm is generally used to characterize arid climates.

Rains are rare and completely unpredictable. Only rarely will they suffice for arable cropping without additional water, excepting possibly in favoured niches, such as depressions, sabhas, water courses, etc. Several years may sometimes elapse without rains. The occasional rains may be of high intensity and cause floods.

Solar intensities are very high, excepting when a high dust-laden atmosphere reduces the intensity of solar radiation. The ability to survive under desert conditions is an essential characteristic of the xerophytic natural vegetation of the arid regions. Typically, many of these plants have adapted themselves to arid conditions by a reduced transpiring surface resulting in dwarfed plants with very limited leaf surfaces.

Hot arid climates

These comprise deserts and semi-deserts with less than two humid months. Nomadic pastoralism is the dominant land use. Game is a resource that can be a source of income, either as a tourist attraction, and/or for systematic harvesting

Cool arid climates

Middle-latitude deserts are located either in regions that are remote from oceans or are on the leeward side of mountains. These climates are characterized by long cold winters, cold soils, short growing seasons, high summer insolations, high daytime temperatures in summer, and by potential evaporation exceeding precipitation for much of the growing season.

Cold arid climates

The cold deserts, in which the mean temperatures in the warmest months are less than \(10\) C, and where even irrigation cannot ensure a reasonable level of productivity, are not included in the scope of this work.

Semi-arid climates

The term semi-arid can be misleading, in that it implies a climate that is intermediate – between dry and humid – in the amount of precipitation received. Actually, semi-arid climates are seasonally arid, and are characterized by a completely dry season that alternates with a more or less humid season, in which the amounts of precipitation may well exceed the amount typical of a humid climate.

During the dry season, crop production is only possible if water has been stored in the soil during the rainy season, or if irrigation is provided.

Hot semi-arid climates (savannah zones)

These semi-arid regions are characterized by a dry season of four-and-a-half to seven months and a wet season with an annual precipitation of 500 to 1500 mm; rainfall is very variable, and often very intense, causing severe problems of soil erosion. Atmospheric demand for water is very high; coupled with low and variable rainfall, periods of transient drought occur frequently.

On the basis of temperature, the semi-arid tropics are warm throughout the year and suitable for the production of any crop that does not require a cold period in its lifecycle. Unreliable moisture is the key limiting factor.

The reliability of rainfall at the beginning of the season is generally low, and consequently there is a high degree of risk in establishing crops, because there are no soil reserves of moisture at this time.

Mediterranean-type climates

At latitudes of around 30° to 40°, on the west sides of the continents, are located the areas with a Mediterranean-type climate. In terms of latitudinal locations and temperatures, this is a subtropical climate, being dry in the summer and cool and wet in the winter.

The Mediterranean climate is characterized by mild winters and hot, but not oppressive, summers. Frosts and snow are rare. Rainfall is light to moderate, and generally occurs in heavy downpours. Sunshine is abundant even during the winter months.

Cool semi-arid climates

Parts of the cool dry climates have a precipitation that is sufficient for growing crops, ranging from \(250-300 mm\) (which permits growing a cereal crop in alternate years, by using a cropfallow rotation), to \(500-550mm\), enabling annual cropping.

Rainfall is mainly in summer, but a significant part of the precipitation is in the form of snow, which, on melting, contributes water to the soil or to the aquifer. Precipitation may vary considerably over short distances.

Moisture regimes

In each of the semi-arid regions outlined above are a number of distinct moisture regimes. The two major moisture regimes under which arable crops are grown in areas with adequate average rainfall are:

Stored moisture environment.: In all semi-arid regions, whether subtropical, Mediterranean or middle-latitude, crops can be grown during the dry season, completing their entire life cycle on precipitation stored during the preceding rainy season. At the time of planting, the farmer knows the amount of available soil water, so that he can chose appropriate management practices. These will be mainly aimed at minimizing ET during vegetative growth, so as to ensure an adequate moisture supply for the reproductive stage.
Variable moisture environment.: In this environment, crops are grown during the rainy season (winter crops in Mediterranean climates, summer crops in subtropical and middle-latitude climates). Sowing is either in dry soil, shortly before the expected onset of the rains (modern farming), or shortly after the first rains have wet the soil (subsistence agriculture). Even in years of above-average rainfall, occasional periods of drought occur during the growing season. Plants must be able to take advantage of the periodic rainfall, and to recover quickly from periods of intermittent stress, at whatever stage they occur.

The dry regions of the world

Africa

Northern Africa is characterized by the unusual longitudinal breadth of the dry regions and their extension towards the Equator, in contrast to the situation south of the Equator. As a consequence, in Africa, transitional wet-and-dry climates are much narrower on the northern side of the Equator than on the southern side.

The dry areas of Africa comprise three categories: hyperarid, arid and semi-arid; the criterion used to separate these categories is the ratio of annual precipitation to evapotranspiration.

The Sahara and its margins.: The Sahara is in the trade-wind belt: dry north-east winds prevail over major sectors, becoming hotter and drier as they cross the landmass. In the Sahara are large areas of barren, shifting sand, as well as stony wastes, hills and mountains, valleys and depressions. There are also a number of oases, the largest of which is the narrow valley of the Nile. The greater part of the Sahara has less than 100 mm of annual rainfall, mostly occurring in heavy showers.
The Sudano Sahel.: South of the Sahara is a savannah belt which is subdivided into ecologically distinct zones, in which rainfall increases steeply southward. The rainy season is monomodal, from May/June to September/October. Rain intensities are high, and often exceed 100 mm/hour. Storms are often followed by hot days with high evaporative demand, resulting in the rapid dessication of the soil surface. The beginning of the cropping season is marked by the first rainfall that is sufficient for crops to germinate. The length of the growing season is determined by the time between this first useful rainfall and the end of the rainy season, though transient droughts during this period can occur.
Southern Africa.: The most striking climatic peculiarity in Southern Africa is the large proportion of the total area in which dry and semi-arid climates predominate. This region comprises the extremely arid Namib Desert, and the less dry Namaland, Kalahari, and Karoo, covering some \(550\ 000 \ km^2\) in all. Two-thirds of the Republic of South Africa are in the arid or semi-arid area consisting of four main plateaux.

Eurasia

Eastern Mediterranean region.: This region, formerly known as the Levant, comprises Syria, Lebanon, Israel, and Jordan. Along the sea coast, the climate is typically Mediterranean, with a a regular pattern of winter rains (late October to May), and an absolute summer drought. Along the coast the winters are mild; frosts and snow are rare. The climate is more extreme in the mountains, and becomes semi-tropical in the Rift valley.
South-west Asia.: South-west Asia has vast areas of hot desert extending from Arabia to western India. Most of Arabia is a hot and barren desert plateau covering approximately \(2.5\) million \(km^2\); it has very little rainfall and what there is occurs mainly in the winter months. The northern part is a stony or sandy desert producing some pasturage; in the centre are mountain chains with alluvial cultivated valleys, whilst the south is in the rain shadow of these mountains and is therefore extremely arid desert.
The Indian subcontinent.: Arid and semi-arid regions cover the whole of Pakistan and approximately one-third of the surface area of India. Such regions are found in all the \(11\) states situated in the western half of the country. The most important source of irrigation water in this region is the Indus River system, so that most of the cultivated land is restricted to the area through which the Indus River and its tributaries flow, or in which their waters are carried by canals. The Indus plains have one of the world’s largest and oldest irrigation systems, including some \(12\) million hectares of irrigated land, with thousands of kilometres of canals. The annual water-flow of the catchment area of the Indus and its tributaries is more than double that of the Nile. The Indian subcontinent is primarily characterized by a tropical monsoon climate with four seasons.

In the arid and semi-arid regions of the subcontinent, the same seasonal rythm prevails, but variability in space and time is much greater.

Rainfall: Mean rainfall usually ranges from \(0\) to \(200\) mm in arid regions, and \(400\) to \(700\) mm in the semi-arid regions. The main rainy season is from June to September, during which period \(90-95\%\) of the total annual precipitation occurs, under the influence of the south-west monsoon. This is the hottest period of the year, thereby reducing the efficiency of the rain received. On average, the maximum number of rainy days per year is 20 in the arid region, and varies from \(20-50\) in the semi-arid region.

Air humidity: Average relative air humidity values are more or less similar in the two regions. Mean annual evaporation is \(200-300\ mm\) in the arid regions, and \(200-350\ mm\) in the semi-arid regions. Daily evaporation rates during summer exceed \(12\ mm\) in the arid regions.

Temperatures: Winters are cool, with occasional frosts. The hot season begins in early April; June is the hottest month, with daytime temperatures reaching \(50\ ^{\circ}C\) or even higher during heat waves. The diurnal variation in temperatures in the arid regions may be as much as \(11-17 \ ^{\circ}C\).

Winds: Hot winds are frequent during the summer months. Very high wind velocities may be recorded throughout the year, except during the post-monsoon season. Occurrence of dust haze and dust storms is characteristic for the region. In the arid parts of Pakistan and some parts of India, dust clouds have been recorded at a height of \(6000-9000\ m\).

China.: Aridity increases steeply from the coast inland as the monsoon from the Pacific Ocean progressively loses its moisture while passing over the land mass. Drylands, ranging from subarid to superarid, account for over \(40\%\) of the total area of China, mainly located in the north-west of the country. Rainfall increases from \(200\) to \(300\ mm\) in the northern part of the steppe to \(300-450\ mm\).

North America

The dry farming areas of North America are clustered within three major geographical regions lying in the \(17\) western states of the USA and the three prairie provinces of Canada: the Pacific Valleys, The Intermountain Region and the Great Plains. USA part of the Great Plains comprises three subdivisions: Northern, Central and Southern.

The Pacific valleys

Pacific Northwest.: The Pacific Northwest has a maritime climate that is determined primarily by prevailing westerly winds from the Pacific Ocean. Winters are cool and moist, and \(75\%\) of the precipitation occurs from November to April. In the grain producing areas, precipitation ranges from \(200\) to \(750\ mm\).
Pacific Southwest.: The Pacific Southwest comprises California, Nevada, and Arizona. The two latter are mostly arid. Almost all the rainfall occurs in winter. The extreme heat and low relative humidity are the reasons for the very high evapotranspiration rates that characterize the area, which is mostly irrigated.

The Intermountain region

The Intermountain region lies in the rain shadow of the Sierra Nevada mountain range. Summer temperatures are high; winters, whilst cold, have temperatures that are somewhat milder than might be expected at this latitude. A dominant climatic feature is the biannual rainfall variation: one maximum occurring in winter, the second during the warm season.

The Great Plains

The Great Plains extend from Canada to the United States-Mexico border. The Rocky Mountains form their western border. Winter temperatures vary widely, averaging from \(10\) to \(20\ ^{\circ}C\) in the south to below \(0\ ^{\circ}C\) in the north, the differences in summer temperature are far less marked. Average annual rainfall increases from \(350\ mm\) in the west to \(500\ mm\) in the east. Rainfall is of the continental pattern, most of it occurs in summer. High winds and the long hours of sunshine contribute to high evapotranspiration rates and reduce the effectiveness of precipitation, so that light rains are of little value.

The Canadian prairies

On the northern fringe of the Great Plains is the Canadian dry belt, comprising southern Alberta and Saskatchewan, in which rainfall is still lower than normal for the general region, and falls to as low as \(300\ mm\).

South America

The desert zone.: The arid zone of South America forms a long narrow strip along the western coast between the sea and the Andes, reaching almost to the Equator. From northern Peru to northern Chile the highest average rainfall in this zone is under \(50\ mm\), while in parts no rainfall has been recorded over periods of one or more decades. A unique feature of this arid region is that occasionally, once in several years, frequent torrential rains may occur during one or more summer months, so that, practically overnight the desert takes on the characteristics of an area of tropical wet climate. These heavy rains are extremely destructive, as they flood the irrigated valleys, destroying crops, roads, bridges, and even entire villages.
The semi-arid areas.: In the rain shadow of the Andes is a semi-arid region extending inland to the east; this includes the pampas plains of the Gran Chaco, with mild winters and moderate winter rainfall. The area of drought varies considerably in extent and location from year to year; fairly extensive areas have under \(600\ mm\) of annual precipitation while there are restricted areas with less than \(400\ mm\). The rains occur in the form of heavy showers extending over a small area at a time.
Mediterranean-type climate.: In central Chile, the climate is typically Mediterranean, with a long summer drought followed by a short winter rainy season; in the north, annual rainfall amounts to \(200-250\ mm\) and increases towards the south up to \(1400\ mm\). Frosts are rare. During summer, temperatures are usually between \(32\) and \(35\ ^{\circ}C\).

Australia

Dry climates occupy the entire mainland of the continent of Australia except in its northern, eastern, and part of its southern, margins. About \(40\%\) of Australia, or some \(3.2\ million\ km^2\), consists of hot desert, with mild to hot winters. This desert occurs not only in the region of the subtropical belts of the trade winds, but also in the rain shadow of the mountain ranges of the eastern coast. Although the Australian desert is one of the most extensive in the world, precipitation in it is higher than in other low-latitude deserts. Thus the driest parts of the Australian desert average about 125 mm of rain annually (McDonald, 1959)..Whilst in most other low-latitude deserts the annual rainfall increases inland, the opposite is true in Australia.

Groundwater is the major, and often the only, source of water in arid Australia; large reserves of groundwater occur in sedimentary basins. The Great Artesian Basin, e.g., supplies water for livestock and domestic use from about \(20,000\) bores over an area of \(1.74\ million\ km^2\).

Causes of aridity

Rainfall generally occurs when the moisture-laden air rises, expands, and cools. As a result, vapour condenses and precipitation occurs. The distribution of world rainfall is influenced by a great number of factors, of which the principal ones are: the characteristics of the general circulation of the atmosphere, relief and the distribution of land and sea.

General circulation of the atmosphere

The hydrological cycle

The earth’s moisture is involved in a cycle of continuous circulation called the water cycle or hydrological cycle. Water is evaporated from the soil and from the water surfaces by solar energy, and transported as vapor by the general circulation of the atmosphere. Eventually the moist air is cooled to the point of condensation and may fall as precipitation.

Schematic presentation of the circulation pattern of the atmosphere between the Equator and 30°C latitude north and south.

Cause.: The basic cause for the general circulation of the atmosphere is the unequal heating of the earth by the sun. Expressed in its most simple terms, the air heated over the equatorial belt would rise, flow polewards in the upper levels of the atmosphere while progressively losing heat by radiation, and then subside over the poles. The cooled air would then return to the Equator near the earth’s surface.

Zonal systems

In the equatorial region there is a belt of relatively low and very uniform pressure, called the Doldrums.
In the subtropical latitudes there is a broad belt of high uniform pressure. In the northern hemisphere this high-pressure belt lies approximately between latitudes \(25^{\circ}\) and \(35^{\circ}N\), and is confined mainly to the oceans.
In the vicinity of the Arctic and Antarctic Circles, there are belts of low pressure.

Main circulation systems

The combined effect of the meridional temperature gradient, the earth’s rotation, and the pressure belts, results in three main circulation systems of the earth’s atmosphere.

Equatorial low-pressure belt.: The trough of low pressure in the equatorial belt causes surface air to move in from the subtropical high-pressure belts. As this air flows over warm oceans and tropical jungles which evaporate huge quantities of water, it becomes moisture-laden. The heated land mass of the equatorial region in turn heats the damp surface air, which expands and therefore becomes lighter and rises. As the heated air rises it continues to expand, thereby using up some of its energy and becoming progressively cooler. When the air has been cooled to below dew point, cloud formation occurs. Continued cooling then causes precipitation, resulting in heavy rainfall in the afternoon or evening. This occurs practically all the year around, and is typical for the equatorial belt.
Subtropical high-pressure belts.: The air that has ascended from the equatorial belt, after losing much of its moisture, flows poleward north and south in the upper regions of the atmosphere. By the time it is above latitude 30°N or S, the airstream is deflected by the earth’s rotation. This causes an accumulation of air above these latitudes (called the Horse Latitudes) and produces belts of high pressure on the earth’s surface.
The Polar Circle low-pressure belts.: Air moves polewards from the subtropical high-pressure belts to the subpolar lows mainly at the surface. As a result of deflection by the earth’s rotation, the winds develop into the so-called ‘mid-latitude westerlies’ or, simply, ‘westerlies’. These are neither constant nor steady in direction, as they are affected by various secondary circulation factors. The regions of prevailing westerlies lie between 35° and 60° in each hemisphere, and comprise the cool, humid climates of the earth.

Relief

Orographic rain

When rising ground, or mountains running at right angles to the prevailing winds, deflect moisture-laden air upwards, the air cools, resulting in condensation and subsequent precipitation. This is called orographic rain. When the air moves down the leeside of the mountain range, it has already lost most of its moisture. During subsidence, the air again becomes warmer and its relative humidity is decreased further, which explains why the leeside of a mountain has little rainfall (Figure 2).

Effect of relief on rainfall: orographic rain.

Distribution of land and sea

Winds may also arise as a result of the seasonal heating and cooling of the continents, which warm up rapidly in summer and cool rapidly in winter. Such winds are the so-called monsoons, which in summer are moist and blow landwards whereas in winter the direction is reversed. This latter outflow of dry air is characterized by lack of precipitation.

The greater part of the earth is covered by oceans; the distribution of land and sea will, therefore, have a marked effect on climatic patterns only in regions in which large land masses are involved – such as Asia and, to a lesser degree, Africa, the Americas, and Australia – and in which the zonal circulation pattern may be entirely obscured by the great thermal differences between land and sea. Air moving over warm oceans becomes moisture-laden. When it moves inland, any factor that causes this air to ascend, such as rising ground or a wedge-shaped mass of cold air, will result in cooling, condensation, and precipitation (Figure 3).

Effect of warm moisture-laden air encountering a cold air front.

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