Arid environments

Where arid environments are, and what their climates are like

Hot deserts lie in two belts, one in each hemisphere, at roughly 15 to 30 degrees north and south of the Equator. This is not where air is rising and raining but where it is sinking and drying. The northern belt runs through the Sahara, the Arabian and Iranian deserts and the Thar; the southern belt includes the Kalahari and Namib in southern Africa, the Atacama in South America and the Great Victoria and Simpson deserts of Australia. With the semi-arid margins around them, drylands cover close to a third of the world's land surface.

Aridity is defined by the water balance, not by a rainfall total. A place is arid when precipitation is less than potential evapotranspiration, so that there is a permanent water deficit: the water that would evaporate from a free surface, and that a plant could transpire, exceeds the water arriving. In practice a hot desert receives less than about 250 mm a year and a hot semi-arid environment roughly 250 to 500 mm, but 250 mm falling in a cool place is not a desert at all, because far less evaporates.

Rainfall is not only small in total but unreliable and intense. Annual totals swing widely from year to year, and when rain comes it usually arrives in a few violent convective storms separated by months of drought, falling in an afternoon on a surface that is baked hard and sheds it. Variability matters more than the average, because plants, farmers and herders have to survive the bad years rather than the typical one.

Temperatures are high in summer and the diurnal range is very large. Under clear skies and dry air, a great deal of solar energy reaches the ground by day, and at night the same dry, cloudless air lets that heat radiate away quickly, so the temperature can fall by 20 to 30 degrees C between afternoon and dawn. In continental interiors and in semi-arid margins, winter nights can drop below freezing and winter days are cool.

Seasonally, temperatures peak in early summer or during the summer rains, and the annual range increases with distance from the sea. The seasonal pattern of rain is controlled by the migration of the Inter-Tropical Convergence Zone (ITCZ) in the semi-arid tropics, which brings one wet season as it passes overhead, and by mid-latitude depressions on the poleward margins of the deserts, which bring a little winter rain.

These are also high-wind-energy environments. With little vegetation to slow the wind, intense surface heating producing strong convection, and steep local pressure gradients, surface winds are faster and gustier than in most humid landscapes. Because the surface is dry, loose and unprotected, the wind can lift material it could not move in a humid environment, which is why dust storms and sand storms are a defining feature rather than an occasional event.

Four causes of aridity

The first cause is the global pressure and wind system. Air that rose at the Equator in the Hadley cell has lost its moisture and, having spread polewards at altitude, is descending over the subtropics at about 20 to 30 degrees. Sinking air is compressed and warms adiabatically, and because warm air holds more water vapour than cool air its relative humidity falls, so cloud is suppressed and rain cannot form. The subtropical high pressure belt is therefore a belt of clear skies and settled sinking air, and the trade winds that blow out of it towards the Equator carry air that has already crossed a continent and lost most of its moisture.

The second is continentality, or distance from the sea. Oceans are the source of water vapour, so the further inland a place lies, the less moisture the air reaching it can carry and the more it has shed on the way. The same fact explains the very large annual temperature range of continental deserts, because land heats and cools faster than water. Central Australia illustrates it: Alice Springs is roughly 1 000 km from the nearest ocean in every direction and receives only about 280 mm of rain a year.

The third is cold ocean currents. Where a cold current flows along a coast, the air above it is chilled from below, which makes it stable and creates a temperature inversion a few hundred metres up. Moisture then condenses as fog rather than rising to form rain cloud, and the cold sea surface evaporates little water in the first place. This is why the coastal deserts beside the Benguela Current (the Namib), the Humboldt Current (the Atacama), the Canary Current (the western Sahara) and the California Current (Baja California) are among the driest places on Earth, even where the air above them is often foggy.

The fourth is the rain shadow. Air approaching a mountain range is forced to rise, cools at the dry adiabatic lapse rate until it reaches its dew point, and then condenses and rains on the windward side. Having lost its moisture the same air descends on the leeward side and warms as it sinks, so it arrives dry and must rise a long way before it can rain again. The Atacama is dry partly because it lies in the rain shadow of the Andes, and Death Valley lies in the rain shadow of the Sierra Nevada.

Almost no desert is explained by one cause alone, and the strongest answers combine them. The Namib is arid because it sits under the subtropical high, is chilled by the cold Benguela Current offshore, and lies in the rain shadow of the Great Escarpment, so much of its moisture arrives as fog rather than rain.

Weathering: how rock breaks down in a dry climate

Weathering is the breakdown of rock in situ, without the material being carried away, and in hot deserts it is dominated by physical (mechanical) processes. Water, the usual agent of chemical weathering, is scarce, but the temperature range is enormous and salts are abundant. Weathering matters here for two reasons: it produces the regolith that wind and water then transport, and it shapes landforms directly by splitting, rounding and hollowing rock before erosion reaches it.

Heating and cooling: thermal fracture and exfoliation. Rock conducts heat poorly, so the outer few centimetres expand in the afternoon sun while the rock beneath stays cool. Different minerals expand by different amounts and the grains are cemented together, so the surface layer is put under stress. Repeated daily cycles, and the rapid cooling at night, open cracks roughly parallel to the surface, and the outer shell eventually peels away in sheets - exfoliation, also called onion-skin weathering. In coarser rocks individual grains work loose instead, giving granular disintegration and a surface that crumbles to sand.

Freeze-thaw (frost shattering). Water in a joint or pore freezes, expands by about 9 per cent, and prises the rock apart; when it thaws the water penetrates further and the next freeze repeats the process. This needs the temperature to cross 0 degrees C often, so it is not the dominant process in the hottest deserts, but it is important in continental and high-altitude deserts - the Iranian plateau, the Gobi, the high Atacama - and in desert winters. It leaves angular blocks, slabs and scree below cliffs.

Salt crystal growth. Saline groundwater is drawn to the surface by capillary action and evaporates there, so salts crystallise in the pores. As the crystals grow they push the surrounding grains apart, and the cycle repeats each time the rock wets and dries. The result is honeycomb weathering (tafoni) on cliff faces, salt flakes on stone and a fine dust that is easily deflated. It is most active beside playas, salt lakes and coasts, where the groundwater is saline.

Chemical weathering is limited but not absent. Dew, fog and the occasional storm keep water on the rock long enough for reactions to run, and that water is not pure. Hydrolysis attacks feldspars in granite and other silicate rocks, converting them to clay minerals and releasing soluble ions, so the rock loses strength and crumbles to a coarse sandy residue called grus. Carbonation happens when carbon dioxide from the air and soil dissolves in water to form weak carbonic acid, which dissolves calcium carbonate; limestone and chalk are taken slowly into solution, and where the water evaporates the carbonate is reprecipitated near the surface as a hard layer of calcrete (caliche).

The significance for landforms is direct. Exfoliation rounds granite into domed core stones; salt and thermal weathering hollow out softer beds in a cliff so that a resistant cap stands proud; calcrete forms a hard crust that protects what lies beneath and later caps mesas and buttes; and the clay produced by hydrolysis is exactly the fine material that deflation lifts, so chemical weathering supplies much of the dust that wind erosion removes.

Wind (aeolian) processes

Wind is a weak agent compared with water, because air is roughly 800 times less dense, so it moves only relatively fine material: sand, silt and clay. Its importance in deserts comes from the absence of vegetation and the dryness of the surface, which leave loose material exposed, combined with frequent strong winds. Three sets of processes matter: erosion, transportation and deposition.

Deflation is the removal of loose, dry, fine material so that the surface is lowered. It is selective: the wind takes the sand and silt and leaves the gravel behind, which is why so much stony desert is covered by a closely packed lag of pebbles, a desert pavement (reg). Deflation stops where the surface is damp - the water table acts as a base level - or where a gravel armour protects the fines beneath, and where it is concentrated it hollows out a deflation hollow.

Abrasion (corrasion) is the sand-blasting of rock by grains carried in the wind. It is most effective within about a metre of the ground, because that is where most saltating sand travels, and this single fact explains the family of undercut desert landforms: rock is worn at its base and left intact above, so pedestals, mushroom rocks and undercut cliffs all form. Abrasion also polishes and facets exposed stone, and a pebble that stays put is worn into a ventifact with flat, smoothed faces and sharp edges.

Transportation sorts material by size. Traction (surface creep) rolls and slides the coarser grains along the ground. Saltation is the main mechanism for sand: grains of roughly 0.1 to 0.5 mm are lifted a few centimetres, fall back, dislodge others and bounce downwind in a series of hops. Suspension carries the finest silt and clay high into the air for great distances - Saharan dust crosses the Atlantic to the Amazon basin and reaches Europe - which is why desert dust turns up in soils thousands of kilometres away.

Deposition occurs wherever the wind slows or loses its load: behind an obstacle, in the lee of a ridge, on reaching damp ground or vegetation, where the surface becomes rougher, or where two wind directions meet. Wind-deposited sand is well sorted, its grains are rounded and frosted by repeated collisions, and it builds up in sloping layers that produce cross-bedding in the sandstone eventually formed from it.

Water (fluvial) processes and the desert hydrological regime

Water is scarce in a desert but it is the more powerful geomorphic agent when it arrives, because the landscape is not built to absorb it. Vegetation is sparse and soils are thin, baked and often sealed by a surface crust, so infiltration capacity is low and a high proportion of an intense storm becomes overland flow. Almost all of the water's work is done in a few hours each year, which is why desert landforms often look sharp and fresh rather than rounded.

Erosion begins with raindrop impact, which detaches particles and seals the surface, and continues as sheet wash, a thin unconfined film moving across a slope and removing material fairly evenly. Where the sheet concentrates it cuts rills and then gullies, and in a channel flood water erodes by hydraulic action, by abrasion with its load of sand and gravel, and by solution; channels also widen when saturated banks collapse.

Transport is by traction of the coarse bedload, saltation, suspension of the silt and clay that colour desert floodwater brown, and solution. Because the floods are short and steep, the load travels only a short distance before it is deposited: where the gradient falls as a channel leaves the hills, where the channel widens and the water spreads and slows, where flow meets an obstacle or a standing lake, or where the water soaks into the ground.

The hydrological regime is ephemeral. The channel is dry for most of the year and flows only for hours or days after rain. The hydrograph is flashy: a lag time of minutes or a couple of hours, a steep rising limb, a high peak discharge for the size of the catchment, and a rapid recession. Transmission losses are large because water infiltrates into the permeable bed, so discharge can fall to nothing within a few kilometres of where the flood began. Drainage is often internal (endorheic), ending in a basin rather than the sea, where the water evaporates.

Sheet floods spread across gently sloping surfaces such as pediments after a heavy storm, moving as a broad shallow sheet still capable of carrying gravel. Flash floods are confined to channels and far more dangerous: a storm may fall on bare upland kilometres away and the water arrives as a wall of mud and boulders with no warning, so a wadi that was dry in the morning is impassable by afternoon. Urbanisation, road building and the loss of vegetation all shorten the lag time and raise the peak.

• Ephemeral flow: channels dry for most of the year, flowing only after rain.
• Flashy hydrograph: short lag time, steep rising limb, high peak, rapid recession.
• Transmission losses: water infiltrates into the channel bed, so discharge falls downstream.
• Endorheic drainage: streams end in an inland basin and lose their water by evaporation.
• Sheet flood: unconfined flow across a pediment or slope after an intense storm.
• Flash flood: a sudden, short-lived, high-energy flow in a wadi, often from a storm falling far upstream.
• Groundwater: the water table feeds springs and oases, and acts as the base level for deflation.
• Rainfall intensity, not annual total, controls how much of the water runs off rather than soaking in.

Wind erosional and depositional landforms

A deflation hollow is a shallow depression blown out of unconsolidated material where the surface is unprotected. Its floor usually lies close to the water table, because damp sand cannot be lifted, and its long axis tends to follow the prevailing wind. Small examples are blowouts on sandy plains and in coastal dunes; the Qattara Depression in Egypt is the classic large example, a vast hollow below sea level whose floor lies near the water table, although deflation there worked alongside salt weathering and other processes.

A ventifact is a pebble or boulder whose faces have been flattened and polished by abrasion, the faces meeting at sharp edges, so its shape records the wind directions that blasted it. A rock pedestal (mushroom rock) is the larger version: a resistant cap of hard rock sits above softer rock, abrasion at about a metre above the ground cuts into the softer material, and the result is a narrow stem carrying a wide overhanging top. Alternating hard and soft beds produce a stack of waists, one for each soft layer.

Yardangs are streamlined ridges carved out of soft, fairly uniform rock, often old lake clays or weakly cemented sandstone, separated by parallel troughs. They are aligned parallel to the prevailing wind, long relative to their height, blunt at the windward end and tapering to a tail; the troughs between them are widened by abrasion and by wind funnelled along them. The Lut Desert in Iran has some of the finest, standing tens of metres high.

Zeugen are tabular masses of rock standing above the desert floor, each protected by a resistant cap rock resting on softer material. Wind and water remove the softer rock along joints, so the gaps widen and deepen and the blocks are left with their long axis running parallel to the wind. The structural difference matters: a zeugen is a remnant of layered rock protected by a hard cap, whereas a yardang is carved out of a thick mass of uniform soft rock.

Sand dunes form where there is a supply of sand, a wind strong enough to move it, a dry surface and somewhere for it to be trapped. Their shape records the wind regime, which is why three types are worth learning separately.

Barchan dunes are crescent-shaped with two horns pointing downwind and a steep slip face on the leeward side at the angle of repose of dry sand, about 32 to 34 degrees. They form where sand supply is limited, the ground beneath is firm and the wind blows consistently from one direction; the dune migrates as sand is carried up the gentle windward slope and avalanches down the slip face. Seif dunes are long narrow ridges running parallel to the wind, sometimes for tens of kilometres, separated by bare corridors; they form where sand is more abundant and the wind is bimodal, so sand is shuffled along the ridge rather than across it. Star dunes are pyramidal, with three or more arms radiating from a high central peak, and form where wind blows from several directions through the year, so sand is piled up rather than carried away; they migrate least and grow tallest.

Dune sand is well sorted and rounded, and its internal structure is a set of steeply dipping cross-beds recording each advance of a slip face. Dune fields carry Arabic names in much of the literature: an erg is a sand sea, a reg a gravel plain and a hamada a bare rocky plateau.

Water erosional and depositional landforms

A wadi (North Africa and Arabia) or arroyo (south-western United States) is a steep-sided valley with a flat floor of alluvium, cut by episodic floods. The channel is dry for most of the year; when a flood comes it deepens and widens the channel, carries the bedload a short way and deposits it on the floor, and undercuts the banks so that they collapse. Because the floods are violent, wadis extend headwards quickly by gullying at their heads, and they end abruptly where they spread onto a plain.

A pediment is a gently sloping surface of bare bedrock at the foot of a mountain front, planed off by sheet flow and by weathering of the rock in place, usually covered by a thin veneer of angular gravel. It is concave in profile, steepest at the mountain front and flattening into the basin, and it forms as the mountain front retreats under weathering while the debris is carried across the bedrock surface. Merging pediments form a broad pediplain, and the resistant hills left standing above it are inselbergs.

Mesas, buttes and inselbergs are residual hills left by erosion of the surrounding land. A mesa is flat-topped with steep sides, capped by a resistant layer that protects the softer rock beneath, so the cap and the cliff retreat together and the hill shrinks rather than lowering. A butte is the same feature reduced in size, narrower than it is wide, so it is a tower rather than a table. An inselberg ('island mountain') is an isolated residual hill of resistant rock rising abruptly from a plain, such as Uluru in central Australia or the granite domes of the Namib; many are thought to be exhumed cores of rock that resisted deep weathering under an earlier, wetter climate.

An alluvial fan builds where a stream leaves a steep upland channel and runs onto a plain. The gradient falls abruptly, the channel widens and splits into distributaries, the water spreads and slows, and the load is dropped: coarse gravel and boulders at the apex near the mountain front, sand and silt at the toe, which is why the deposit is a sorted fan. Fans are steep, braided and reworked by debris flows in flash floods, and where several grow into one another along a mountain front they form a continuous apron called a bajada.

A playa is the flat floor of a dry lake in a closed basin, made of silt and clay laid down by floods and cracked into polygonal mud cracks as it dries, with a crust of salt where the water has evaporated. A salt lake is the same basin with water still standing in it - the Dead Sea and Lake Eyre are examples, and the Salar de Uyuni in Bolivia is the largest salt flat on Earth. These basins are internally drained, so everything dissolved in the water stays behind when it evaporates: that is why playas are mined for salt, borax, potash and lithium, and why their dry surfaces are a major source of dust.

Vegetation and soils

Desert vegetation is sparse, low and slow-growing, and productivity is low: there is little water for photosynthesis, so biomass per hectare is small and biodiversity is limited, although many species are highly specialised and found nowhere else. Nutrient cycling is limited too, because there is little litter, decomposition is slow in the dry season, and much of the nutrient capital sits in dead plant material and in the soil beneath shrubs rather than circulating. After rain the system switches on: ephemeral plants germinate, flower and set seed within weeks, then the desert shuts down again.

Against extreme temperature, plants use pale reflective stems and hairs, small or absent leaves and, in some cases, vertical leaf orientation so that sunlight glances off rather than strikes. Many open their stomata at night and fix carbon dioxide then (CAM photosynthesis), so transpiration is not driven by daytime heat, and seeds and bulbs tolerate soil temperatures that would kill most plants.

Against physical drought - a genuine shortage of water in the environment - the adaptations are the classic xerophyte set. Some send a deep tap root down to the water table (acacia, mesquite); others spread a shallow, wide root system to catch every shower. Succulents store water in thick stems or leaves, and a thick waxy cuticle, sunken stomata and reduced leaves, often as spines, cut loss to a minimum. Many shed their leaves or die back to a tuber in the dry season, and ephemerals avoid drought altogether by existing as seed.

Physiological drought is different: water may be present but the plant cannot take it up, because the soil solution is too salty, because the ground is frozen, or because roots cannot keep pace with transpiration. Plants adapted to this are halophytes, which accumulate solutes to keep the osmotic gradient in their favour and excrete salt through salt glands in the leaves. In cold high deserts the same problem appears as frozen ground in winter.

Climate and people both shape the cover. Drought years thin the vegetation and kill seedlings, but overgrazing, the cutting of shrubs for fuelwood, cultivation of marginal land and trampling by livestock remove it faster than it can regrow. Loss of cover is a threshold: once the plants and their root mats have gone, the soil is exposed, infiltration falls, runoff rises and the surface erodes far more easily.

Arid soils are thin, stony, pale and low in organic matter, because there is little vegetation to supply humus and little water to move material within the profile. Leaching is weak, so salts and carbonates stay where they are, and evaporation pulls water upwards for much of the year. Calcium carbonate therefore accumulates in the subsoil as calcrete, and where groundwater is saline, salts are drawn to the surface by capillary action and left as a crust. Soils are often coarse, alkaline and nutrient-poor, with a gravel pavement where the wind has removed the fines.

Two named types matter. A solonchak is a saline soil: it contains a high concentration of soluble salts, often with a white crust at the surface, and forms in depressions and former lake beds where salty water collects and evaporates. A solonetz is a sodic soil: sodium ions are held on the clay particles, making the subsoil a dense, impermeable, columnar mass that is hard when dry and sticky when wet, with very poor drainage. Both are alkaline, both are poor for crops, and both are made worse by irrigation without drainage.

Climate, vegetation and human activity all leave their mark. A wetter climate would leach the salts away, which is why these soils are a product of the dry water balance; a denser cover would add humus and improve structure; and people change them fastest, by irrigating (which salinises), by grazing (which compacts) and by stripping vegetation (which exposes the surface to wind and to rain that runs off instead of soaking in).

Changes and challenges: population, desertification and climate change

Arid and semi-arid environments are being used more intensively. Populations have grown quickly in dryland countries, and growth is concentrated in cities that need water pumped from ever further away. Around them land use has changed: rangeland has become irrigated cropland or rain-fed cultivation on land too dry for it, cash crops have replaced subsistence farming, groundwater is pumped for irrigation and towns, and mining, tourism and roads have opened up remote areas. Every one of these increases demand on a water supply already smaller than evaporation.

Desertification is the degradation of land in drylands - a lasting loss of biological productivity - and it is not sand physically advancing across farmland, which is the misconception that costs most marks. Its physical factors are the climate itself: low and highly variable rainfall, long droughts, high temperatures and potential evapotranspiration, thin soils, and wind that removes topsoil once it is bare. Its human factors are overgrazing, overcultivation, removal of trees and shrubs for fuel, poor irrigation and the pressure of a growing population on a fixed resource. The two interact: a drought that a well-managed landscape would survive destroys a degraded one.

Salinisation is the accumulation of salts in the soil and the most damaging form of dryland soil degradation. Where land is irrigated without adequate drainage, water percolates down, the water table rises, and capillary action draws the salty water back to the surface, where the sun evaporates it and leaves the salt behind. Yields fall, the surface crusts and the land is abandoned. On coasts the same problem arrives from the sea, as heavy abstraction lets salt water intrude into the aquifer. The Indus plain in Pakistan and the irrigation schemes around the Aral Sea are the standard examples.

Climate change adds to all of this. Temperatures in drylands are rising, which raises potential evapotranspiration and therefore the water deficit even if rainfall does not change; droughts are likely to become more frequent and severe in many dryland regions, and rain, where it falls, is expected to be more intense and more variable, so a larger share runs off in floods rather than soaking in. The changes are not uniform - some drylands may get slightly more rain - but the reliable feature is greater uncertainty, which is hardest on farmers deciding what to plant and herders deciding where to move.

For agriculture the threats are specific. Crops face heat stress at flowering, a shorter growing season where temperatures rise faster than rainfall, and a greater demand for irrigation at exactly the moment rivers and aquifers are lowest. Livestock lose grazing as pasture dries and water points fail. Salt water threatens coastal farmland. And variability is itself a threat, because a farmer who cannot predict the season cannot invest in seed, fertiliser or machinery, so yields stay low and the land stays poor.

Management works on both the physical and the human side. Water harvesting captures runoff where it falls - zai pits and half-moons in Burkina Faso, stone lines across slopes, check dams and sand dams in Kenya that store water in the sediment of a seasonal river - and concentrates it on the crop. Farmer-managed natural regeneration protects the seedlings that regrow from cut stumps and roots in fields, and in Niger it has restored tree cover over several million hectares. Agroforestry, mulching, rotational grazing and destocking protect the surface, drip irrigation with proper drainage reduces salinisation, and dune stabilisation with brushwood and planted grasses holds sand in place. At the largest scale the Great Green Wall aims to restore land across the Sahel, while early warning systems, land-use planning and secure land tenure address the human causes.

The honest evaluation is that cheap, local, farmer-led measures have worked best and large schemes least well. Zai pits, half-moons, natural regeneration and sand dams cost little, use local materials, can be copied by farmers themselves and show results within a few seasons, which is why they spread without large budgets. The failures are instructive too: the Great Green Wall has fallen far behind its target because planting trees across a continent is expensive and often fails where rainfall cannot support them, and its approach has shifted towards restoring land in a mosaic with the people who farm it. Irrigation without drainage has created salinity as fast as it has raised yields. Success depends less on the technology than on whether farmers see a return, whether they hold secure rights to land and trees, and whether the scheme is maintained after the donors leave.