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HYDROLOGY, FLOODS AND DROUGHTS | Deserts and Desertification
Deserts and Desertification VP Tchakerian, Texas A&M University, College Station, TX, USA Ó 2015 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by G Wang, G S Jenkins, volume 2, pp 633–640, Ó 2003, Elsevier Ltd.
Synopsis Deserts cover about 35% of the land surface area of the world and are typically located between and astride the Tropic of Cancer (35 N) and the Tropic of Capricorn (35 S). About 20% of the world’s population resides in this geographic region. The location of this global arid zone is primarily the result of the semi-permanent high-pressure cells that dominate this region along with such other factors as rain-shadow effects, continentality or remoteness from moisture sources, upwelling of cold currents that suppress the development of precipitation, and the nature and direction of the prevailing winds. Most deserts exhibit a combination of the above factors. Desertification refers to land degradation in the global arid zone owing to a series of complex climatic, biophysical, and anthropogenic factors and became a major global topic during the severe drought of the Sahel region in northern Africa in the 1970s. Desertification has been erroneously represented as the irreversible march of the desert and is now believed to result primarily from the degradation of arid ecosystems largely because of human induced factors along with natural climatic oscillations and drought cycles.
Introduction Deserts (arid lands/drylands) constitute about 35% of the land areas of the world and are typically characterized by rainfall scarcity, higher temperatures and evapotranspiration, lower humidity, and a general paucity of vegetation cover. A unique combination of atmospheric, geologic, and geomorphic conditions is responsible for the formation of deserts primarily between the Tropics of Cancer and Capricorn (Figure 1). Compared to humid lands, the relative importance of desert atmospheric and geomorphic processes and the magnitude and frequency of their operation is rather distinctive. Arid lands
Figure 1
comprise the most widespread terrestrial biome on Earth and are home to over 20% of the world’s people. In the twentieth century, a combination of natural and anthropogenic factors gave rise to the concept of desertification – first as a rather simplified ‘march of the desert’ into bordering semiarid regions, and then recently as a more complex phenomenon that includes both natural and anthropogenic causes, the latter primarily the consequence of increased population numbers in the semiarid regions of the world. The distinctive natural environment in arid lands, coupled with the growing human populations in drylands, is one of the primary reasons behind the recent upsurge in the global study of deserts.
Global distribution of deserts. Reproduced from Goudie, A.S., 2002. Great Warm Deserts of the World. Oxford University Press, Oxford.
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Deserts of the World Drylands of the world can be either classified as arid, based on climate or desert, based on surface characteristics (landforms, vegetation, etc.). For the purpose of this review, we will use the climatic classification adopted by UNESCO in 1979. Deserts thus can be classified as (1) semiarid (precipitation less than 500 mm); (2) arid (precipitation less than 250 mm); and (3) hyperarid (precipitation less than 25 mm). Approximately 16% of the global arid zone is semiarid, 15% arid, and 4% hyperarid. The division between the three is somewhat arbitrary and based on limited climatic data owing to the fact that there are large variations in annual precipitation regimes and that natural and anthropogenic activities have shifted those boundaries. Africa contains the greatest proportion of the global arid zone at about 37%, while Australia is the most arid continent, with about 75% of the land area being arid or semiarid. Contrary to public opinion, sand dunes and sand seas (ergs) are not the dominant landform type in deserts, although dunes cover about 40% of the surface area of Australia (most are stabilized, relict dunes), constituting 38% of the world’s dune fields. A recent landform map of North Africa (mostly the Sahara Desert) produced through moderate resolution imaging spectroradiometer, indicated that the two most dominant landform types were stone pavements (hamada, serir, reg, desert pavement) at about 25% cover, followed by sand seas (ergs) at about 20%. On the other hand, the North American arid zone includes extensive areas of alluvial fans, mountains, desert flats, playas, and arroyos, while sand dunes and sheets constitute less than 5% of the arid zone. There are a number of reasons for the global arid zone. The majority of the world’s deserts are subtropical in distribution, covering about 20% of the Earth’s land surface and located between the Tropic of Cancer (23.5 N) and the Tropic of Capricorn (23.5 S). The strong subsidence of air in these regions is largely the result of the descending branch of the Hadley cell, which causes air at the surface to be hot and dry. These are the locations for some of the most famous deserts in the world such as the Sahara Desert, the Rub’al Khali in the Arabian Peninsula, the Thar desert of India and Pakistan, the Kalahari Desert in southern Africa, the Sonoran and Chihuahuan Deserts of North America and the Australian deserts – Simpson, Gibson, Great Sandy, Tanami, and the Great Victoria (Figure 1). These deserts typically exhibit very high insolation values, very low humidity, very high evapotranspiration rates and extreme spatially and temporally variable precipitation regimes, with occasional heavy, short-lived thunderstorms, largely associated with the seasonal movements of the intertropical convergence zone (ITCZ). Another type of desert is formed when moisture is prevented from reaching continental interior locations because of either the distance from water bodies or the presence of mountain ranges. Examples of continental interior deserts include most of the midlatitude Asian deserts such as the Taklimakan and the Gobi Desert in China, the KaraKum deserts in Kazakhstan and Uzbekistan, and parts of the Great Basin and the Colorado Plateau in the United States. For example, moist air masses that originate around Scotland (wet) can move over Poland (moist) and will most likely be dry by the time they cross the Volga River on their way to central Asia. Other deserts owe their
existence because of their location on the lee side of major topographic barriers such as the Mojave Desert being on the rain shadow of the Sierra Nevada Mountains and the Transverse Ranges of southern California. Parts of the Great Basin and the Colorado Plateau in the United States are also considered rain-shadow deserts as well as the Patagonian Desert in Argentina. The north–south orientation of the Great Dividing Range in eastern Australia also contributes to the lee side aridity of the Simpson Desert, as easterly trade winds are prevented from bringing their moisture past the Great Divide. Deserts are also formed on the western coastline regions of continents, where the upwelling of cold, ocean currents, suppresses any precipitation potential. Other climatic conditions include low sea-surface evaporation, high atmospheric humidity, low annual temperature ranges, and extremely low rainfall amounts. Warm air as it moves over these cool waters is chilled/condensed (only a thin layer is affected) forming mostly fog and dew, which for many years is the only source of moisture for plants. Examples include the Namib Desert, located mostly in Namibia (Benguela Current), Baja Deserts in Mexico (California Current), the Atacama Desert of Chile and Peru (Humboldt or Peru Current), and smaller arid zones off the coasts of Mauritania, Somalia, and NW Australia. The Atacama Desert of northern Chile is considered ‘the driest place in the world,’ with less than 10 mm of annual precipitation, and runs roughly about 4000 km from north to south. In addition, winds in this region typically blow parallel to the coast and thus inhibit the eastern movement of moist air from the Pacific to the Atacama region. In summary, the general causes of aridity and hence the presence of deserts in the world can be summarized as (1) atmospheric stability (subtropical highs/Hadley cell circulation); (2) rain-shadow effects; (3) upwelling cold currents off west coasts; (4) prevailing winds parallel to coasts; and (5) remoteness from moisture sources (continentality). It should be noted that most deserts are arid because of a combination of the above five factors.
Desert Hydroclimatology Clear skies, lack of cloud cover, and low water vapor content are responsible for the high persistent temperatures that characterize most desert environments (variations discussed under microclimates) with maximum temperatures commonly between 45 and 50 C. Subtropical deserts tend to experience hot summers and cool winters, while midlatitude deserts tend to have hot summers but very cold winters. Annual temperature fluctuations tend to be highest in midlatitude deserts, while diurnal temperature changes can be extreme in all desert environments, except the cool, coastal deserts (Figure 2). During the daytime, the incoming solar radiation heats up surfaces very rapidly causing the temperature (sensible heat) to rise. During the night, when terrestrial long-wave radiation dominates the surface energy budget, the surfaces cool very rapidly owing to the fact that clear, dry, and cloudless skies cannot trap the outgoing terrestrial radiation, and thus the very large diurnal temperature fluctuations. A >30 C range is very typical in subtropical deserts and a 50 C diurnal range has been reported from dark, basaltic rocks in the central Sahara Desert. The breakdown of rocks as a result of volumetric
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Figure 2 (a) January and (b) July long-term observed temperature. Temperatures greater than 25 C are shaded. The darkest shading represents temperatures greater than 30 C.
changes from thermal expansion and contraction has been proposed as one of the physical weathering processes in hyperarid deserts (most likely it is the individual minerals that are found in rocks that will respond to these thermal changes, leading to granular disaggregation. Unequal thermal expansion of the dominant minerals is the controlling factor). Deserts typically receive less than 1 mm per day rainfall when averaged on an annual basis (Figure 3). Precipitation in deserts typically occurs in short durations but at high intensities, with low overall amounts, at irregular intervals, often with a strong seasonal bias and usually with a very large interannual variability. Enhanced precipitation owing to orographic (mountain) effects is especially prominent and has a strong impact on the spatial distribution of flora and fauna. Storms typically form as discrete convective cells and are unlikely to affect the entire drainage network within a desert – hence storms have low frequencies and high magnitudes and are
typically discontinuous in space and time. The rain falls on ground with a sparse or nonexistent vegetation cover, which is irregular in its distribution and especially adapted to collect rainfall. Interception rates are low and highly variable and rapid direct evaporation of excess surface water is characteristic. Evaporation rates from exposed surfaces are high in subtropical deserts, particularly during the summer months. Additionally, infiltration is largely controlled by the bare surface characteristics, which range from sands and alluvium to organic crusts and from stone (desert) pavements to duricrusts (a product of processes acting within the zone of weathering to cause the accumulation of iron and aluminum oxides, silica, calcium carbonate, or less commonly gypsum), such as calcrete (caliche/calcium carbonate). Most runoff in deserts occurs as overland flow (Hortonian overland flow) with vegetation type and densities as well as surface and subsurface (soil) characteristics controlling the
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Figure 3
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Annual precipitation rates for areas that receive less than 2 mm day1 of rain. The darkest shaded regions receive less than 0.5 mm day1 of rain.
intensity and duration of overland flow. Most overland flow is ephemeral, lasting only hours or days. Desert streams exhibit flashy hydrographs with short recessional limbs because of the predominance of overland flow, and seepage and transmission losses into the underlying alluvial stream beds are common, thus the importance of groundwater in providing moisture to plants and people. Although water paucity is the norm in deserts, there are water supplies that when used wisely, can sustain people and ecosystems in the global arid zone. Some of these sources are as follows: (1) perennial and ephemeral rivers, (2) wadis and arroyos, (3) groundwater from shallow (alluvial fans) and deep aquifers (mostly fossil aquifers), (4) lakes and playas, (5) fog, dew, and snow, (6) desalination plants, and (7) dams and reservoirs. The hydrological conditions in deserts leads to less weathering and leaching and hence soils tend to be shallow with coarse textures, high aeolian content, and retain many soluble substances (such as carbonates and salts). Soil pedogenesis (formation) takes a long time and paleosols (ancient soils) are rather common in many global deserts and can be used as proxy data for past climates. Deserts also exhibit certain geomorphic processes and landforms that are rather unique or operate more favorably than in other environments. Some of these attributes include a rather open and exposed landform assemblages in part because of the limited vegetation cover with high and frequent changes in relative relief, many superimposed landforms and sediments from previous geologic periods – hence a veritable laboratory for studying past climates (such as lake cores and sediments), and the greater efficacy of wind as a geomorphological agent of erosion and deposition (hence the vast expanses of sand dunes and sand seas), and dust entrainment and transport.
Desert Microclimates Microclimates are significant within deserts because they offer less arid conditions for plants, animals, and humans. Some
examples include (1) modification of relative humidity – mostly by nocturnal radiation and the shade effects or mixing with cooler air masses. The drops in temperatures increase the relative humidity of the air and the chances of moisture condensation as either fogs or dew (Namib Desert – up to 150 mm of moisture has been calculated). This process is vital for the survival of rich desert ecosystems such as in the Namib, Atacama, and Baja California deserts; (2) reduction of temperature extremes – any shade-giving object produces direct reduction of air temperatures in the arid lands because of the importance of the direct radiation component in the cloud-free atmosphere; and (3) reduction of wind speeds – shelter from wind movement reduces the amount of moisture loss from evapotranspiration – vegetation is sometimes used as a wind break; however, its competition for soil moisture may reduce crop yields close to the barrier (economic factors to be considered). The single most important microclimate is provided by mountains, which offer the maximum modifications with respect to overall climatic variables. These include the reduction of air temperatures with altitude, shade effects (plants can grow which themselves provide shade – pinyon–juniper trees), increased chances of precipitation (orographic effects), as well as air drainage among the basins and the ranges (peaks and valleys), which can ameliorate diurnal temperatures and humidity.
Desertification Desertification refers to land degradation in the global arid zone resulting primarily from various anthropogenic (human land use) and biophysical factors (climatic variations). In 1978 at the first UN Conference on Desertification (UNCOD) the following definition was proposed “Desertification is the diminution or destruction of the biological potential of the land which can lead ultimately to desert-like
Hydrology, Floods and Droughts j Deserts and Desertification conditions.” Desertification ultimately reduces the sustainability of arid lands whereby agriculturally productive lands become barren and thus prone to wind and water erosion and other forms of land degradation. For example, UNCOD estimates that moderate desertification can lead to a 10–25% drop in agricultural productivity. Contrary to public opinion, desertification does not refer to the expansion of deserts – although the margins of deserts are known to oscillate north and south owing to natural perturbations in climate and the resulting response of ecosystems. Desertification has occurred because desert ecosystems, which cover over one third of the world’s land area, are extremely vulnerable to overexploitation and inappropriate land use, compounded by poverty, political instability, deforestation, overgrazing, and wasteful irrigation practices. According to UNCOD, over 250 million people are directly affected by desertification. In addition, some 1000 million (or 1 billion) people in over 100 countries are at risk. These people include many of the world’s poorest, most marginalized, and politically weak citizens. Another way for looking at desertification is to analyze the ‘5Ds’ specifically and these include drylands, drought, desiccation, degradation, and desertification. ‘Drylands’ are the world’s arid lands (semiarid, arid, and hyperarid) that are inherently prone to natural perturbations throughout geologic time; ‘drought’ is a short-term (a few years) and natural decline in precipitation and desert ecosystems and economic systems (people) adapt to those changes and eventually there is full recovery during moister times; ‘desiccation’ is drought conditions that lasts over an extended period of time (such as decadal), and has an adverse impact on both natural and cultural ecosystems with some systems never recovering or needing many years to reestablish (such as certain plant species or transborder migration of peoples); ‘degradation’ is the end result of drought and desiccation with the land losing its agricultural productivity leading to water and wind erosion, salinization on one hand, and the loss of the natural vegetation as a result of overgrazing, firewood collection, and groundwater
removal on the other; and ‘desertification’ would then be the ultimate end of this cycle whereby desert conditions overwhelm the whole ecosystem. Although the system above seems like a positive feedback scenario, land degradation and desertification are more likely to operate on a negative feedback method, whereby eventually the system will revert back to its original position albeit having crossed a number of thresholds and experienced a few positive feedbacks during its cyclical journey.
Desertification and the Sahel It was the severe droughts beginning in the late 1960s in the Sahel (the areas immediately to the south of the Sahara Desert) and its subsequent socioeconomic consequences that enabled desertification to be firmly established within the global community as one of the most consequential environmental events of the late twentieth century (Figure 4). We will use the Sahel as a case study since it contains all the variables necessary for understanding global desertification. Three interrelated factors contributed to the environmental disaster of the Sahel: 1. Climatic – the Sahel is a transitional geographic region between the dry, desert climates of the Sahara and the moist and humid savanna environments to the south, with significant variability in the spatial and temporal distribution of precipitation. A short rainy season associated with the northward movement of the ITCZ is separated by long stretches of dry and dusty weather. Any prolonged interruptions in the arrival of the rainy season can lead to drought and desiccation. This transitional climatic zone is thus highly susceptible to both long-term and short-term climatic oscillations, and has undergone major climatic swings since the Last Glacial Maximum at about 18 ka. During the last Ice Age (Wisconsin Age in North America), the Sahel experienced an intense arid period characterized by major dune building episodes and dust deposition, followed by a much cooler and wetter period from 11 to 8 ka (the period of major rock art in the Sahara with petroglyphs
Sahel precipitation anomalies 1900−2012 5 4
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June through October averages over 20−10 N, 20 W−10 E. 1900−2012 climatology NOAA NCDC Global Historical Climatology Network data Figure 4
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Sahel precipitation anomalies 1900–2012. Reproduced from http://jisao.washington.edu/data/sahel/.
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of crocodiles and giraffes painted in rock shelters and caves). This period was then followed by a brief arid episode and then another wetter than present period from 7 to 5 ka. Since 5 ka, aridity has slowly returned to the Sahara–Sahel, as the subtropical high-pressure cells assumed their current position, with today’s hyperarid central Saharan core region well established by 2 ka. Desert margins, such as the Sahel, are thus very prone to extreme climatic variability, with many decades of good wet years followed by short to decadal length drought periods. The natural vegetation is rather resilient and adapted to these chronic environmental stresses and will typically recover after brief or even decadal perturbations in rainfall regime. However, twentieth century changes in land use and land cover from increased population pressures on the landscape have altered the natural ecosystem cycle (more below). Rainfall in the Sahel appears to be strongly influenced by the combined effects of North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO). The interannual variability in the position of the northern boundary of the Sahel (southern boundary of the Sahara), as represented by the 200 mm isohyet, can be explained in large measure by changes in NAO and ENSO. Various studies have indicated that up to 75% of the interannual variation in the extent of the Sahara Desert is accounted for by the combined effects of NAO and ENSO. The drier years in the Sahel tend to be associated
with warm sea surface temperatures in the southern oceans and Indian Ocean, and anomalously cold sea surface temperatures to the west of the continent. Two recent studies have advanced our understanding of the physical factors controlling long-term, persistent drought in the Sahel. One study has shown that the Sahel drought of the last 40 years was likely initiated by a change in worldwide ocean temperatures (multidecadal variations in sea-surface temperatures (SSTs)), which reduced the strength of the African monsoon, shifted the ITCZ, and was exacerbated by land–atmosphere feedbacks through natural vegetation and land cover change (Figure 5). Land use changes by humans may have also played an important role. Another study, using laminated sediment cores from a lake, found that the recent Sahelian droughts of the 1960s and 1970s, are characteristic of the monsoon and are linked to natural variations in Atlantic SSTs and, furthermore, these droughts have occurred many times during the past three millennia, and although the most recent multidecadal drought of the 1970s had widespread ecological, political, and socioeconomic impacts, the climatic oscillations of the past 3000 years are capable of much more severe and longer drought cycles. In summary, large-scale atmospheric circulation changes brought on by multidecadal oscillations in global sea-surface temperatures, seem to be the primary biophysical mechanism (in addition to the human
Figure 5 Complex feedbacks. The recent Sahel drought was likely initiated by a change in worldwide ocean temperatures, which reduced the strength of the African monsoon, and was exacerbated by land–atmosphere feedbacks through natural vegetation and land cover change. Land use changes by humans may have also played an important role. SST, sea-surface temperature; ITCZ, intertropical convergence zone. Reproduced from Zeng, N., Meyerson, J., 2003. Drought in the Sahel. Science 302 (5647), 999–1000.
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Figure 6 Global distribution of sand seas for (a) the present and (b) the Last Glacial Maximum (LGM), 18 ka (after Sarnthein, 1978). H denotes humid conditions. Reproduced from Tchakerian, V.P., 2009. Paleoclimatic interpretations from desert dunes and sediments. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments. Springer–Verlag, New York, pp. 757–772.
component discussed later) that contributes to desertification, and that their length, severity, and origin can be traced back at least 3000 years. 2. Geomorphic – underlying the natural surface of the Sahel is a mantle of relict dune sands (ergs) from the Pleistocene presently stabilized and anchored by the vegetation (Figure 6). The removal of the vegetation cover by either anthropogenic or natural causes will lead to a significant increase in aeolian activity and in sand/dust storms. Owing to the nature of the aeolian sand, the topsoil is thin, with shallow root systems, and the water table very low and highly susceptible to increases in water use as well as to pedogenic carbonate formation (which can affect water quality and translocation of nutrients). Aeolian activity is further enhanced in this region by the presence of many winds such as the trade winds, Harmattan, and Khamsin, and other convective mesoscale systems. The Harmattan, is a dry and dusty Sahelian tradewind that sometimes extends all the way to the ITCZ. It blows south from Sahara into the Gulf of Guinea from November to March. Over the Sahara, it picks up fine dust particles some of which might end up in the Caribbean and even North America. Since sediment transport varies with the cube of the wind speed, a slight increase in wind speed will result in a threefold increase in sand and dust transport. Reactivation of some of the
stabilized Sahelian dormant/relict dune systems has been the result of both increased population pressure in the region (stabilized dunes provide a richer plant cover for grazing and firewood gathering, and are also easier to cultivate), and to long-term decadal drought owing to the atmospheric perturbations from global sea-surface temperature changes. 3. Anthropogenic – factors such as overgrazing and conversion of woodland to agriculture are among features that have been proposed as human-induced causes for desertification. Persistent, decadal drought, and desiccation leads to increased pressure on land use and land cover, compounded by a concomitant increase in population (from high birth rates, immigration, and population movements because of conflict). The demand for water and energy also soar putting additional pressure on people and governments. The human-induced stresses lead to such manifestations of desertification as (1) soil erosion and salinization (and/or waterlogging) as a result of the population exceeding the environmental thresholds of agricultural sustainability owing to overgrazing, overcultivation, deforestation, etc.; (2) wind erosion leading to increased frequencies of dust and sand storms (including loss of topsoil); (3) drawdown in groundwater, wells, and diminished use of dams and other irrigation schemes; (4) introduction of exotic species;
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Figure 7
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Schematic mechanism for the enhancement of desertification through biosphere–atmosphere interactions.
(5) war and civil unrest with all its socioeconomic ramifications – the Sahelian population is doubling every 20 years. All the above ultimately changes the ecosystem with a marked loss in biodiversity and agricultural productivity leading to a prolonged hiatus whereby a return to the original state may take decades or longer to attain. The environmental effects accompanying desertification are many and widespread and include such factors as (1) increase in surface albedo (less sunlight is absorbed), (2) decrease of evaporation and transpiration, (3) reduction in the moisture supply to the atmosphere (less water vapor available for condensation), (4) decrease of soil moisture, (5) decrease in precipitation (compounded by increases in albedo and reduced moisture in the atmosphere), which ultimately leads to less favorable conditions for plant growth, and (6) surface temperature changes tend to be highly seasonal and closely related to the hydrologic cycle. On one hand, an increase in surface albedo will result in a decrease of surface net radiation, hence cooling the surface, while on the other hand, a decrease in latent heat as a result of lowered evapotranspiration can lead to the warming of the surface – thus an increase or decrease in surface temperatures will depend largely whether it is the dry or the wet season in the Sahel. Increased dust production from denuded dry soils and dry dune sands as well as from biomass burning (mineral aerosols) can have a number of effects on the environment including influencing the atmospheric radiative transfer directly by scattering and absorbing solar radiation, and indirectly by modifying the optical property and lifetime of clouds. Biomass burning from firewood, charcoal, and animal
dung releases significant amount of greenhouse gases as well as black carbon (soot), which could affect both atmospheric phenomena and human health. Atmosphere–biosphere interactions can also lead to enhancement or a positive feedback loop and become self-perpetuating until a certain threshold is reached. Drought-induced ecosystem degradation will then reinforce the initial anthropogenically driven changes and thus ultimately changing the regional climate, leading to a selfdegradation of the land surface as well as persistent drought (Figure 7). Only a return to a decadal or longer above average hydrologic conditions accompanied with changes in land use can jolt (threshold) the system to its former stage.
Further Reading Giannini, A., Saravanan, R., Chang, P., 2003. Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. Science 302, 1027–1031. Goudie, A.S., 2002. Great Warm Deserts of the World. Oxford University Press, Oxford. Laity, J.E., 2008. Deserts and Desert Environments. Wiley-Blackwell Publishers. Shanahan, T.M., Overpeck, J.T., Anchukaitis, K.J., Beck, J.W., Cole, J.E., Dettman, D.L., Peck, J.A., Scholz, C.A., King, J.W., 2009. Atlantic forcing of persistent drought in West Africa. Science 324, 377–380. Tchakerian, V.P., 1999. Dune palaeoenvironments. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. John Wiley & Sons, New York, pp. 261–292. Tchakerian, V.P., 2009. Palaeoclimatic interpretations from desert dunes and sediments. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments. Springer–Verlag, New York, pp. 757–772. Thomas, D.S.G., Middleton, N.J., 1994. Desertification: Exploding the Myth. John Wiley & Sons, Chichester.
Synopsis Deserts cover about 35% of the land surface area of the world and are typically located between and astride the Tropic of Cancer (35 N) and the Tropic of Capricorn (35 S). About 20% of the world’s population resides in this geographic region. The location of this global arid zone is primarily the result of the semi-permanent high-pressure cells that dominate this region along with such other factors as rain-shadow effects, continentality or remoteness from moisture sources, upwelling of cold currents that suppress the development of precipitation, and the nature and direction of the prevailing winds. Most deserts exhibit a combination of the above factors. Desertification refers to land degradation in the global arid zone owing to a series of complex climatic, biophysical, and anthropogenic factors and became a major global topic during the severe drought of the Sahel region in northern Africa in the 1970s. Desertification has been erroneously represented as the irreversible march of the desert and is now believed to result primarily from the degradation of arid ecosystems largely because of human induced factors along with natural climatic oscillations and drought cycles.
Introduction Deserts (arid lands/drylands) constitute about 35% of the land areas of the world and are typically characterized by rainfall scarcity, higher temperatures and evapotranspiration, lower humidity, and a general paucity of vegetation cover. A unique combination of atmospheric, geologic, and geomorphic conditions is responsible for the formation of deserts primarily between the Tropics of Cancer and Capricorn (Figure 1). Compared to humid lands, the relative importance of desert atmospheric and geomorphic processes and the magnitude and frequency of their operation is rather distinctive. Arid lands
Figure 1
comprise the most widespread terrestrial biome on Earth and are home to over 20% of the world’s people. In the twentieth century, a combination of natural and anthropogenic factors gave rise to the concept of desertification – first as a rather simplified ‘march of the desert’ into bordering semiarid regions, and then recently as a more complex phenomenon that includes both natural and anthropogenic causes, the latter primarily the consequence of increased population numbers in the semiarid regions of the world. The distinctive natural environment in arid lands, coupled with the growing human populations in drylands, is one of the primary reasons behind the recent upsurge in the global study of deserts.
Global distribution of deserts. Reproduced from Goudie, A.S., 2002. Great Warm Deserts of the World. Oxford University Press, Oxford.
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Deserts of the World Drylands of the world can be either classified as arid, based on climate or desert, based on surface characteristics (landforms, vegetation, etc.). For the purpose of this review, we will use the climatic classification adopted by UNESCO in 1979. Deserts thus can be classified as (1) semiarid (precipitation less than 500 mm); (2) arid (precipitation less than 250 mm); and (3) hyperarid (precipitation less than 25 mm). Approximately 16% of the global arid zone is semiarid, 15% arid, and 4% hyperarid. The division between the three is somewhat arbitrary and based on limited climatic data owing to the fact that there are large variations in annual precipitation regimes and that natural and anthropogenic activities have shifted those boundaries. Africa contains the greatest proportion of the global arid zone at about 37%, while Australia is the most arid continent, with about 75% of the land area being arid or semiarid. Contrary to public opinion, sand dunes and sand seas (ergs) are not the dominant landform type in deserts, although dunes cover about 40% of the surface area of Australia (most are stabilized, relict dunes), constituting 38% of the world’s dune fields. A recent landform map of North Africa (mostly the Sahara Desert) produced through moderate resolution imaging spectroradiometer, indicated that the two most dominant landform types were stone pavements (hamada, serir, reg, desert pavement) at about 25% cover, followed by sand seas (ergs) at about 20%. On the other hand, the North American arid zone includes extensive areas of alluvial fans, mountains, desert flats, playas, and arroyos, while sand dunes and sheets constitute less than 5% of the arid zone. There are a number of reasons for the global arid zone. The majority of the world’s deserts are subtropical in distribution, covering about 20% of the Earth’s land surface and located between the Tropic of Cancer (23.5 N) and the Tropic of Capricorn (23.5 S). The strong subsidence of air in these regions is largely the result of the descending branch of the Hadley cell, which causes air at the surface to be hot and dry. These are the locations for some of the most famous deserts in the world such as the Sahara Desert, the Rub’al Khali in the Arabian Peninsula, the Thar desert of India and Pakistan, the Kalahari Desert in southern Africa, the Sonoran and Chihuahuan Deserts of North America and the Australian deserts – Simpson, Gibson, Great Sandy, Tanami, and the Great Victoria (Figure 1). These deserts typically exhibit very high insolation values, very low humidity, very high evapotranspiration rates and extreme spatially and temporally variable precipitation regimes, with occasional heavy, short-lived thunderstorms, largely associated with the seasonal movements of the intertropical convergence zone (ITCZ). Another type of desert is formed when moisture is prevented from reaching continental interior locations because of either the distance from water bodies or the presence of mountain ranges. Examples of continental interior deserts include most of the midlatitude Asian deserts such as the Taklimakan and the Gobi Desert in China, the KaraKum deserts in Kazakhstan and Uzbekistan, and parts of the Great Basin and the Colorado Plateau in the United States. For example, moist air masses that originate around Scotland (wet) can move over Poland (moist) and will most likely be dry by the time they cross the Volga River on their way to central Asia. Other deserts owe their
existence because of their location on the lee side of major topographic barriers such as the Mojave Desert being on the rain shadow of the Sierra Nevada Mountains and the Transverse Ranges of southern California. Parts of the Great Basin and the Colorado Plateau in the United States are also considered rain-shadow deserts as well as the Patagonian Desert in Argentina. The north–south orientation of the Great Dividing Range in eastern Australia also contributes to the lee side aridity of the Simpson Desert, as easterly trade winds are prevented from bringing their moisture past the Great Divide. Deserts are also formed on the western coastline regions of continents, where the upwelling of cold, ocean currents, suppresses any precipitation potential. Other climatic conditions include low sea-surface evaporation, high atmospheric humidity, low annual temperature ranges, and extremely low rainfall amounts. Warm air as it moves over these cool waters is chilled/condensed (only a thin layer is affected) forming mostly fog and dew, which for many years is the only source of moisture for plants. Examples include the Namib Desert, located mostly in Namibia (Benguela Current), Baja Deserts in Mexico (California Current), the Atacama Desert of Chile and Peru (Humboldt or Peru Current), and smaller arid zones off the coasts of Mauritania, Somalia, and NW Australia. The Atacama Desert of northern Chile is considered ‘the driest place in the world,’ with less than 10 mm of annual precipitation, and runs roughly about 4000 km from north to south. In addition, winds in this region typically blow parallel to the coast and thus inhibit the eastern movement of moist air from the Pacific to the Atacama region. In summary, the general causes of aridity and hence the presence of deserts in the world can be summarized as (1) atmospheric stability (subtropical highs/Hadley cell circulation); (2) rain-shadow effects; (3) upwelling cold currents off west coasts; (4) prevailing winds parallel to coasts; and (5) remoteness from moisture sources (continentality). It should be noted that most deserts are arid because of a combination of the above five factors.
Desert Hydroclimatology Clear skies, lack of cloud cover, and low water vapor content are responsible for the high persistent temperatures that characterize most desert environments (variations discussed under microclimates) with maximum temperatures commonly between 45 and 50 C. Subtropical deserts tend to experience hot summers and cool winters, while midlatitude deserts tend to have hot summers but very cold winters. Annual temperature fluctuations tend to be highest in midlatitude deserts, while diurnal temperature changes can be extreme in all desert environments, except the cool, coastal deserts (Figure 2). During the daytime, the incoming solar radiation heats up surfaces very rapidly causing the temperature (sensible heat) to rise. During the night, when terrestrial long-wave radiation dominates the surface energy budget, the surfaces cool very rapidly owing to the fact that clear, dry, and cloudless skies cannot trap the outgoing terrestrial radiation, and thus the very large diurnal temperature fluctuations. A >30 C range is very typical in subtropical deserts and a 50 C diurnal range has been reported from dark, basaltic rocks in the central Sahara Desert. The breakdown of rocks as a result of volumetric
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Figure 2 (a) January and (b) July long-term observed temperature. Temperatures greater than 25 C are shaded. The darkest shading represents temperatures greater than 30 C.
changes from thermal expansion and contraction has been proposed as one of the physical weathering processes in hyperarid deserts (most likely it is the individual minerals that are found in rocks that will respond to these thermal changes, leading to granular disaggregation. Unequal thermal expansion of the dominant minerals is the controlling factor). Deserts typically receive less than 1 mm per day rainfall when averaged on an annual basis (Figure 3). Precipitation in deserts typically occurs in short durations but at high intensities, with low overall amounts, at irregular intervals, often with a strong seasonal bias and usually with a very large interannual variability. Enhanced precipitation owing to orographic (mountain) effects is especially prominent and has a strong impact on the spatial distribution of flora and fauna. Storms typically form as discrete convective cells and are unlikely to affect the entire drainage network within a desert – hence storms have low frequencies and high magnitudes and are
typically discontinuous in space and time. The rain falls on ground with a sparse or nonexistent vegetation cover, which is irregular in its distribution and especially adapted to collect rainfall. Interception rates are low and highly variable and rapid direct evaporation of excess surface water is characteristic. Evaporation rates from exposed surfaces are high in subtropical deserts, particularly during the summer months. Additionally, infiltration is largely controlled by the bare surface characteristics, which range from sands and alluvium to organic crusts and from stone (desert) pavements to duricrusts (a product of processes acting within the zone of weathering to cause the accumulation of iron and aluminum oxides, silica, calcium carbonate, or less commonly gypsum), such as calcrete (caliche/calcium carbonate). Most runoff in deserts occurs as overland flow (Hortonian overland flow) with vegetation type and densities as well as surface and subsurface (soil) characteristics controlling the
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Annual precipitation rates for areas that receive less than 2 mm day1 of rain. The darkest shaded regions receive less than 0.5 mm day1 of rain.
intensity and duration of overland flow. Most overland flow is ephemeral, lasting only hours or days. Desert streams exhibit flashy hydrographs with short recessional limbs because of the predominance of overland flow, and seepage and transmission losses into the underlying alluvial stream beds are common, thus the importance of groundwater in providing moisture to plants and people. Although water paucity is the norm in deserts, there are water supplies that when used wisely, can sustain people and ecosystems in the global arid zone. Some of these sources are as follows: (1) perennial and ephemeral rivers, (2) wadis and arroyos, (3) groundwater from shallow (alluvial fans) and deep aquifers (mostly fossil aquifers), (4) lakes and playas, (5) fog, dew, and snow, (6) desalination plants, and (7) dams and reservoirs. The hydrological conditions in deserts leads to less weathering and leaching and hence soils tend to be shallow with coarse textures, high aeolian content, and retain many soluble substances (such as carbonates and salts). Soil pedogenesis (formation) takes a long time and paleosols (ancient soils) are rather common in many global deserts and can be used as proxy data for past climates. Deserts also exhibit certain geomorphic processes and landforms that are rather unique or operate more favorably than in other environments. Some of these attributes include a rather open and exposed landform assemblages in part because of the limited vegetation cover with high and frequent changes in relative relief, many superimposed landforms and sediments from previous geologic periods – hence a veritable laboratory for studying past climates (such as lake cores and sediments), and the greater efficacy of wind as a geomorphological agent of erosion and deposition (hence the vast expanses of sand dunes and sand seas), and dust entrainment and transport.
Desert Microclimates Microclimates are significant within deserts because they offer less arid conditions for plants, animals, and humans. Some
examples include (1) modification of relative humidity – mostly by nocturnal radiation and the shade effects or mixing with cooler air masses. The drops in temperatures increase the relative humidity of the air and the chances of moisture condensation as either fogs or dew (Namib Desert – up to 150 mm of moisture has been calculated). This process is vital for the survival of rich desert ecosystems such as in the Namib, Atacama, and Baja California deserts; (2) reduction of temperature extremes – any shade-giving object produces direct reduction of air temperatures in the arid lands because of the importance of the direct radiation component in the cloud-free atmosphere; and (3) reduction of wind speeds – shelter from wind movement reduces the amount of moisture loss from evapotranspiration – vegetation is sometimes used as a wind break; however, its competition for soil moisture may reduce crop yields close to the barrier (economic factors to be considered). The single most important microclimate is provided by mountains, which offer the maximum modifications with respect to overall climatic variables. These include the reduction of air temperatures with altitude, shade effects (plants can grow which themselves provide shade – pinyon–juniper trees), increased chances of precipitation (orographic effects), as well as air drainage among the basins and the ranges (peaks and valleys), which can ameliorate diurnal temperatures and humidity.
Desertification Desertification refers to land degradation in the global arid zone resulting primarily from various anthropogenic (human land use) and biophysical factors (climatic variations). In 1978 at the first UN Conference on Desertification (UNCOD) the following definition was proposed “Desertification is the diminution or destruction of the biological potential of the land which can lead ultimately to desert-like
Hydrology, Floods and Droughts j Deserts and Desertification conditions.” Desertification ultimately reduces the sustainability of arid lands whereby agriculturally productive lands become barren and thus prone to wind and water erosion and other forms of land degradation. For example, UNCOD estimates that moderate desertification can lead to a 10–25% drop in agricultural productivity. Contrary to public opinion, desertification does not refer to the expansion of deserts – although the margins of deserts are known to oscillate north and south owing to natural perturbations in climate and the resulting response of ecosystems. Desertification has occurred because desert ecosystems, which cover over one third of the world’s land area, are extremely vulnerable to overexploitation and inappropriate land use, compounded by poverty, political instability, deforestation, overgrazing, and wasteful irrigation practices. According to UNCOD, over 250 million people are directly affected by desertification. In addition, some 1000 million (or 1 billion) people in over 100 countries are at risk. These people include many of the world’s poorest, most marginalized, and politically weak citizens. Another way for looking at desertification is to analyze the ‘5Ds’ specifically and these include drylands, drought, desiccation, degradation, and desertification. ‘Drylands’ are the world’s arid lands (semiarid, arid, and hyperarid) that are inherently prone to natural perturbations throughout geologic time; ‘drought’ is a short-term (a few years) and natural decline in precipitation and desert ecosystems and economic systems (people) adapt to those changes and eventually there is full recovery during moister times; ‘desiccation’ is drought conditions that lasts over an extended period of time (such as decadal), and has an adverse impact on both natural and cultural ecosystems with some systems never recovering or needing many years to reestablish (such as certain plant species or transborder migration of peoples); ‘degradation’ is the end result of drought and desiccation with the land losing its agricultural productivity leading to water and wind erosion, salinization on one hand, and the loss of the natural vegetation as a result of overgrazing, firewood collection, and groundwater
removal on the other; and ‘desertification’ would then be the ultimate end of this cycle whereby desert conditions overwhelm the whole ecosystem. Although the system above seems like a positive feedback scenario, land degradation and desertification are more likely to operate on a negative feedback method, whereby eventually the system will revert back to its original position albeit having crossed a number of thresholds and experienced a few positive feedbacks during its cyclical journey.
Desertification and the Sahel It was the severe droughts beginning in the late 1960s in the Sahel (the areas immediately to the south of the Sahara Desert) and its subsequent socioeconomic consequences that enabled desertification to be firmly established within the global community as one of the most consequential environmental events of the late twentieth century (Figure 4). We will use the Sahel as a case study since it contains all the variables necessary for understanding global desertification. Three interrelated factors contributed to the environmental disaster of the Sahel: 1. Climatic – the Sahel is a transitional geographic region between the dry, desert climates of the Sahara and the moist and humid savanna environments to the south, with significant variability in the spatial and temporal distribution of precipitation. A short rainy season associated with the northward movement of the ITCZ is separated by long stretches of dry and dusty weather. Any prolonged interruptions in the arrival of the rainy season can lead to drought and desiccation. This transitional climatic zone is thus highly susceptible to both long-term and short-term climatic oscillations, and has undergone major climatic swings since the Last Glacial Maximum at about 18 ka. During the last Ice Age (Wisconsin Age in North America), the Sahel experienced an intense arid period characterized by major dune building episodes and dust deposition, followed by a much cooler and wetter period from 11 to 8 ka (the period of major rock art in the Sahara with petroglyphs
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Sahel precipitation anomalies 1900–2012. Reproduced from http://jisao.washington.edu/data/sahel/.
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of crocodiles and giraffes painted in rock shelters and caves). This period was then followed by a brief arid episode and then another wetter than present period from 7 to 5 ka. Since 5 ka, aridity has slowly returned to the Sahara–Sahel, as the subtropical high-pressure cells assumed their current position, with today’s hyperarid central Saharan core region well established by 2 ka. Desert margins, such as the Sahel, are thus very prone to extreme climatic variability, with many decades of good wet years followed by short to decadal length drought periods. The natural vegetation is rather resilient and adapted to these chronic environmental stresses and will typically recover after brief or even decadal perturbations in rainfall regime. However, twentieth century changes in land use and land cover from increased population pressures on the landscape have altered the natural ecosystem cycle (more below). Rainfall in the Sahel appears to be strongly influenced by the combined effects of North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO). The interannual variability in the position of the northern boundary of the Sahel (southern boundary of the Sahara), as represented by the 200 mm isohyet, can be explained in large measure by changes in NAO and ENSO. Various studies have indicated that up to 75% of the interannual variation in the extent of the Sahara Desert is accounted for by the combined effects of NAO and ENSO. The drier years in the Sahel tend to be associated
with warm sea surface temperatures in the southern oceans and Indian Ocean, and anomalously cold sea surface temperatures to the west of the continent. Two recent studies have advanced our understanding of the physical factors controlling long-term, persistent drought in the Sahel. One study has shown that the Sahel drought of the last 40 years was likely initiated by a change in worldwide ocean temperatures (multidecadal variations in sea-surface temperatures (SSTs)), which reduced the strength of the African monsoon, shifted the ITCZ, and was exacerbated by land–atmosphere feedbacks through natural vegetation and land cover change (Figure 5). Land use changes by humans may have also played an important role. Another study, using laminated sediment cores from a lake, found that the recent Sahelian droughts of the 1960s and 1970s, are characteristic of the monsoon and are linked to natural variations in Atlantic SSTs and, furthermore, these droughts have occurred many times during the past three millennia, and although the most recent multidecadal drought of the 1970s had widespread ecological, political, and socioeconomic impacts, the climatic oscillations of the past 3000 years are capable of much more severe and longer drought cycles. In summary, large-scale atmospheric circulation changes brought on by multidecadal oscillations in global sea-surface temperatures, seem to be the primary biophysical mechanism (in addition to the human
Figure 5 Complex feedbacks. The recent Sahel drought was likely initiated by a change in worldwide ocean temperatures, which reduced the strength of the African monsoon, and was exacerbated by land–atmosphere feedbacks through natural vegetation and land cover change. Land use changes by humans may have also played an important role. SST, sea-surface temperature; ITCZ, intertropical convergence zone. Reproduced from Zeng, N., Meyerson, J., 2003. Drought in the Sahel. Science 302 (5647), 999–1000.
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Figure 6 Global distribution of sand seas for (a) the present and (b) the Last Glacial Maximum (LGM), 18 ka (after Sarnthein, 1978). H denotes humid conditions. Reproduced from Tchakerian, V.P., 2009. Paleoclimatic interpretations from desert dunes and sediments. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments. Springer–Verlag, New York, pp. 757–772.
component discussed later) that contributes to desertification, and that their length, severity, and origin can be traced back at least 3000 years. 2. Geomorphic – underlying the natural surface of the Sahel is a mantle of relict dune sands (ergs) from the Pleistocene presently stabilized and anchored by the vegetation (Figure 6). The removal of the vegetation cover by either anthropogenic or natural causes will lead to a significant increase in aeolian activity and in sand/dust storms. Owing to the nature of the aeolian sand, the topsoil is thin, with shallow root systems, and the water table very low and highly susceptible to increases in water use as well as to pedogenic carbonate formation (which can affect water quality and translocation of nutrients). Aeolian activity is further enhanced in this region by the presence of many winds such as the trade winds, Harmattan, and Khamsin, and other convective mesoscale systems. The Harmattan, is a dry and dusty Sahelian tradewind that sometimes extends all the way to the ITCZ. It blows south from Sahara into the Gulf of Guinea from November to March. Over the Sahara, it picks up fine dust particles some of which might end up in the Caribbean and even North America. Since sediment transport varies with the cube of the wind speed, a slight increase in wind speed will result in a threefold increase in sand and dust transport. Reactivation of some of the
stabilized Sahelian dormant/relict dune systems has been the result of both increased population pressure in the region (stabilized dunes provide a richer plant cover for grazing and firewood gathering, and are also easier to cultivate), and to long-term decadal drought owing to the atmospheric perturbations from global sea-surface temperature changes. 3. Anthropogenic – factors such as overgrazing and conversion of woodland to agriculture are among features that have been proposed as human-induced causes for desertification. Persistent, decadal drought, and desiccation leads to increased pressure on land use and land cover, compounded by a concomitant increase in population (from high birth rates, immigration, and population movements because of conflict). The demand for water and energy also soar putting additional pressure on people and governments. The human-induced stresses lead to such manifestations of desertification as (1) soil erosion and salinization (and/or waterlogging) as a result of the population exceeding the environmental thresholds of agricultural sustainability owing to overgrazing, overcultivation, deforestation, etc.; (2) wind erosion leading to increased frequencies of dust and sand storms (including loss of topsoil); (3) drawdown in groundwater, wells, and diminished use of dams and other irrigation schemes; (4) introduction of exotic species;
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Schematic mechanism for the enhancement of desertification through biosphere–atmosphere interactions.
(5) war and civil unrest with all its socioeconomic ramifications – the Sahelian population is doubling every 20 years. All the above ultimately changes the ecosystem with a marked loss in biodiversity and agricultural productivity leading to a prolonged hiatus whereby a return to the original state may take decades or longer to attain. The environmental effects accompanying desertification are many and widespread and include such factors as (1) increase in surface albedo (less sunlight is absorbed), (2) decrease of evaporation and transpiration, (3) reduction in the moisture supply to the atmosphere (less water vapor available for condensation), (4) decrease of soil moisture, (5) decrease in precipitation (compounded by increases in albedo and reduced moisture in the atmosphere), which ultimately leads to less favorable conditions for plant growth, and (6) surface temperature changes tend to be highly seasonal and closely related to the hydrologic cycle. On one hand, an increase in surface albedo will result in a decrease of surface net radiation, hence cooling the surface, while on the other hand, a decrease in latent heat as a result of lowered evapotranspiration can lead to the warming of the surface – thus an increase or decrease in surface temperatures will depend largely whether it is the dry or the wet season in the Sahel. Increased dust production from denuded dry soils and dry dune sands as well as from biomass burning (mineral aerosols) can have a number of effects on the environment including influencing the atmospheric radiative transfer directly by scattering and absorbing solar radiation, and indirectly by modifying the optical property and lifetime of clouds. Biomass burning from firewood, charcoal, and animal
dung releases significant amount of greenhouse gases as well as black carbon (soot), which could affect both atmospheric phenomena and human health. Atmosphere–biosphere interactions can also lead to enhancement or a positive feedback loop and become self-perpetuating until a certain threshold is reached. Drought-induced ecosystem degradation will then reinforce the initial anthropogenically driven changes and thus ultimately changing the regional climate, leading to a selfdegradation of the land surface as well as persistent drought (Figure 7). Only a return to a decadal or longer above average hydrologic conditions accompanied with changes in land use can jolt (threshold) the system to its former stage.
Further Reading Giannini, A., Saravanan, R., Chang, P., 2003. Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. Science 302, 1027–1031. Goudie, A.S., 2002. Great Warm Deserts of the World. Oxford University Press, Oxford. Laity, J.E., 2008. Deserts and Desert Environments. Wiley-Blackwell Publishers. Shanahan, T.M., Overpeck, J.T., Anchukaitis, K.J., Beck, J.W., Cole, J.E., Dettman, D.L., Peck, J.A., Scholz, C.A., King, J.W., 2009. Atlantic forcing of persistent drought in West Africa. Science 324, 377–380. Tchakerian, V.P., 1999. Dune palaeoenvironments. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. John Wiley & Sons, New York, pp. 261–292. Tchakerian, V.P., 2009. Palaeoclimatic interpretations from desert dunes and sediments. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments. Springer–Verlag, New York, pp. 757–772. Thomas, D.S.G., Middleton, N.J., 1994. Desertification: Exploding the Myth. John Wiley & Sons, Chichester.