Energy and sediment budgets

Carry the systems language over from the water and carbon cycles. A desert is an open system with inputs, stores, flows and outputs of both energy and sediment.

• Energy budget. Inputs: intense insolation under clear skies, plus the kinetic energy of wind and (rarely) running water. Because there is little cloud, little moisture and little vegetation, almost no energy is used in evaporation from a soil or in latent heat — so it goes into heating the ground by day and is radiated straight back to space by night. Hence the huge diurnal temperature range, which is itself a geomorphological driver.
• Sediment budget. Inputs: weathering of bedrock in situ, plus sediment blown or washed in from outside. Stores: regolith, dune fields (ergs), alluvial fans, playa floors. Transfers: aeolian entrainment and fluvial flash flows. Outputs: dust exported downwind, sediment carried out of the basin by an exotic river.
• Many desert basins are endoreic — internally draining, with no outlet to the sea. Sediment and salt that enter simply cannot leave. That single fact explains playas, salt crusts and the salinisation risk under irrigation.

Examiner's eye: "deserts are hot and dry" is worth nothing. "Low atmospheric moisture and sparse vegetation mean a very small latent-heat flux, so the surface energy balance is dominated by sensible heat and long-wave loss, producing the diurnal range that drives thermal fracturing" is A-level.

Reading the energy budget

Why deserts are where they are

Hot deserts cluster in two belts roughly 15°–30° north and south of the equator, on western continental margins and in continental interiors. Four causes explain almost all of them — and most real deserts are produced by more than one acting together.

Add the Coriolis twist: the descending limb of the Hadley cell feeds the trade winds, which blow from the continental interior towards the equator on the western side of continents. Air arriving over the desert is therefore already dry, and it is warming as it descends — so its relative humidity falls further and condensation becomes even less likely.

Explaining the Atacama

The aridity index

"Under 250 mm of rain a year" is a crude definition. Aridity is really about the balance between the water arriving and the water the atmosphere could evaporate away. The widely used aridity index (AI) expresses exactly that:

• Hyper-arid: AI below about 0.05 · Arid: roughly 0.05–0.20 · Semi-arid: roughly 0.20–0.50 · Dry sub-humid: roughly 0.50–0.65.
• The index is a ratio, so it is dimensionless — and it correctly identifies places with modest rainfall but colossal evaporative demand as arid.
• Drylands as a whole (arid plus semi-arid plus dry sub-humid) cover a very large share of the land surface and support a substantial share of the world's population — the exact percentages vary between sources and definitions, so quote them cautiously.

Why this matters for desertification. The zone at risk is not the hyper-arid core (there is nothing left to degrade) but the semi-arid margin, where rainfall is just high enough to tempt cultivation and grazing but too variable to sustain it. AI tells you where that knife-edge lies.

Your turn — the aridity index

An AI of 0.12 sits squarely in the arid band — dry, but not hyper-arid. Note the point: the rainfall alone (180 mm) does not tell you this. It is the ratio against evaporative demand that does.

Weathering, water or wind?

Tap a process, then tap the column it belongs to. Examiners routinely find candidates who credit the wind with work that water actually did — this is where that mark is lost.

Weathering — and the surprising power of water

Weathering in deserts is dominated by physical (mechanical) processes, because chemical reactions need water:

• Thermal fracturing (insolation weathering) — the huge diurnal range makes surface minerals expand and contract at different rates; repeated stress opens cracks. It works far better when a little moisture (dew, fog) is present, which is why lab experiments on bone-dry rock struggled to reproduce it.
• Exfoliation (onion-skin weathering) — the outer shell of a rock heats and cools faster than the interior, so curved sheets peel away, especially from granites and other coarse crystalline rocks.
• Salt weathering (crystallisation) — saline groundwater is drawn to the surface by capillary action and evaporates; salt crystals grow in pore spaces and prise the rock apart. Hydration of salts (e.g. anhydrite to gypsum) adds further expansion. This is the most effective chemical-mechanical process in most hot deserts.
• Block disintegration and granular disintegration — the outcomes, along the joints and around individual grains, that supply sediment to the wind.

Now the counter-intuitive part, and it is the single most examinable idea in this topic: most desert landforms are the work of water, not wind. Rain is rare, but when it falls it is intense and convectional, on a surface with no vegetation, thin soil and often a surface crust that has been baked hard. Infiltration capacity is minimal.

• Sheet flooding — an unconfined film of water spreading across a low-angle surface, capable of moving large volumes of fine sediment and of planing off a pediment.
• Flash floods and ephemeral channels — run-off concentrates in dry valleys (wadis) and produces a violent, high-magnitude, short-duration discharge with an enormous sediment load. The channel is dry again within hours or days.
• Exotic rivers (the Nile, the Colorado) cross the desert but rise outside it — proof that channel form here is inherited from a wetter regime.
• Relict landforms: many desert valleys are far too large for present-day flows. They were cut in pluvial periods during the Quaternary, when the region was much wetter. Modern processes are only retouching an inherited landscape.

The water paradox

What the wind actually does

Wind erodes in two ways and transports in three. Get the vocabulary exact.

• Deflation — the wind lifts and removes loose, dry, fine material. Where deflation strips out all the fines it leaves behind a lag of coarse pebbles: a desert pavement (reg). Where it excavates below the local water table it creates a deflation hollow.
• Abrasion — the sandblasting of rock surfaces by grains carried in the airflow. Because sand is rarely lifted more than a metre or two, abrasion is concentrated near the ground — which is why undercut, mushroom-shaped rocks are so characteristic.

Wind needs help. Aeolian transport requires dry, loose, fine, unvegetated sediment. The wind cannot pick a grain out of solid rock; weathering and the flash flood must deliver it first. That dependency is why "wind is the dominant agent in deserts" is a claim you should evaluate, not repeat.

Why the mushroom rock?

Reading a desert basin

Walk from a mountain front to the centre of a closed basin and you cross the classic sequence in order. It is the same sequence every time, because it is controlled by one variable: declining energy as the slope flattens and the flow spreads.

• Wadi — a steep-sided, flat-floored ephemeral valley, dry for most of the year, choked with poorly sorted sediment dumped when the last flash flood lost energy.
• Alluvial fan — where a confined wadi bursts out onto the open basin floor, the flow loses depth, width increases, velocity and competence collapse, and sediment is deposited as a cone, coarsest at the apex.
• Bahada (bajada) — adjacent fans grow and coalesce into a continuous apron of sediment along the mountain front.
• Pediment — a gently sloping bedrock surface at the mountain foot, with only a thin sediment veneer. Probably formed by sheet flooding and by the parallel retreat of the mountain front (backwearing), not by downwearing.
• Playa (salt pan) — the flat floor of the basin centre. Water collects after storms, evaporates, and precipitates a salt crust. Sediment here is the finest in the system.
• Inselberg — an isolated, steep-sided residual hill rising abruptly from a pediment: the last remnant of a retreating mountain front, often of more resistant or less jointed rock.

Name that landform

Tap a description on the left, then the landform it defines. Naming is AO1; the mechanism in the description is what actually earns AO2 — read each one properly.

What the wind leaves behind

Erosional forms (all products of near-ground abrasion plus deflation):

• Ventifact — an individual pebble or cobble with flat, polished facets, cut where the wind blasts one face; a shift in wind direction (or the stone being rolled) cuts a second facet, and the sharp ridge between them is diagnostic.
• Yardang — a streamlined ridge carved from softer rock, elongated parallel to the prevailing wind, with a blunt, steep upwind end and a tapering downwind tail. Yardangs come in fields, and they need a strong, unidirectional wind regime.
• Zeugen — a tabular mass of resistant caprock over a softer bed; abrasion attacks the soft rock near the surface, undercutting it into a mushroom or pedestal shape.

Depositional forms — dune type is controlled by sand supply, wind regime (unidirectional or variable) and vegetation:

• Barchan — a crescent dune formed under a unidirectional wind where sand supply is limited. A gentle windward slope (grains driven up by saltation), a sharp crest, and a steep slip face at the angle of repose (around 30–34°) on the lee side where grains avalanche. The horns point downwind, because the thinner edges migrate faster than the thick centre. The whole dune migrates downwind by erosion of the windward face and deposition on the lee.
• Seif (linear) dune — a long, sharp-crested ridge running roughly parallel to the resultant wind, formed under a bimodal wind regime where two prevailing directions steer sand along a line.
• Star dune — a pyramidal dune with arms radiating from a central peak, formed under a multidirectional wind regime; star dunes grow upward rather than migrating.

Evaluation hook: "Ergs" — true sand seas — occupy only a minority of most hot deserts. Stony (reg) and rocky (hamada) surfaces are far more extensive. A question asking whether wind is the dominant agent should be answered: wind dominates the transport of fine sediment and builds the most spectacular depositional forms, but water cut the valleys, built the fans and shaped the pediments — and much of the large-scale relief is inherited from wetter pluvial periods.

Reading a dune

Desertification: what it is, and what it is not

Desertification is land degradation in arid, semi-arid and dry sub-humid areas, resulting from climatic variation and human activities. Two things follow immediately from that definition, and both are examinable:

• It is degradation of productive capacity — loss of vegetation cover, loss of topsoil, loss of soil structure, salinisation, falling water tables. It is not the Sahara "advancing" as an invading wall of sand. That image is a myth; degradation appears as expanding patches around wells, settlements and fields.
• It is explicitly multi-causal: climate and people. Any answer blaming only one is capped.

Physical causes: high rainfall variability from year to year, so a run of dry years follows a run of wet ones; recurrent drought; high evaporative demand; fragile soils with little organic matter and weak structure; sparse, slow-growing vegetation with a long recovery time.

Human causes — nearly all of them are ways of removing the vegetation cover that holds the soil in place:

• Overgrazing — herd sizes expanded during the wet years (often supported by new boreholes and by veterinary care) exceed the carrying capacity when the dry years return. Trampling compacts the surface and reduces infiltration.
• Over-cultivation — shortened or abandoned fallow periods as population grows or as cash cropping pushes onto marginal land; nutrients are mined and soil structure collapses.
• Deforestation — clearance for fuelwood and charcoal removes the last root binding and windbreak.
• Salinisation from irrigation — irrigating in a high-evaporation environment without adequate drainage draws saline groundwater up by capillary action; evaporation leaves salt at the surface, poisoning the root zone. This is degradation caused by adding water.
• Population pressure and its drivers — rapid growth, poverty that forces short-term land use, conflict and displacement, land tenure that removes any incentive to invest in the soil, and market pressure to grow export crops.

The positive feedback that does the damage. Vegetation removed → higher surface albedo and less transpiration → less local moisture recycling and reduced convective uplift → less rain → less vegetation. Meanwhile bare soil is deflated and gullied by the next storm, so the seed bank and the fine, nutrient-rich fraction are physically exported. Degradation, once begun, tends to reinforce itself.

The Sahel — desert margin under pressure

The Sahel is the semi-arid belt stretching across Africa south of the Sahara, from Senegal and Mauritania in the west through Mali, Burkina Faso, Niger and Chad to Sudan. It is the textbook desert margin, and a genuine test of your ability to hold climate and people in the same explanation.

• The climatic setting. Rainfall is delivered in a single short summer season by the northward migration of the ITCZ. How far north the rain belt reaches varies from year to year, so rainfall is not merely low but highly variable — and the further north you go, the more variable it gets. The Sahel suffered severe, prolonged drought in the late 20th century, widely linked in the research literature to changes in ocean surface temperatures; there has been partial recovery of rainfall in recent decades, though its extent and permanence are debated.
• The human setting. Livelihoods are dominated by pastoralism and rain-fed agriculture — both directly exposed to that rainfall variability. Population has grown rapidly. Traditional coping strategies (transhumance, long fallows, mobility across the herding range) have been constrained by settlement, borders, cropland expansion and conflict.
• The interaction. Wet years permit larger herds and cultivation further north; the next dry phase then finds far more people and animals on land that cannot support them. Vegetation is stripped, soils are exposed to wind and to intense storm run-off, and productivity falls even after the rains return. This is the mechanism, and it is worth far more than any statistic.
• Consequences: declining crop and livestock yields, food insecurity and famine risk, falling water tables, loss of soil and of biodiversity, distress migration to cities and across borders, and competition between farmers and herders that has fed into violent conflict in parts of the central Sahel.

Management and mitigation — and how to evaluate it:

• Water harvesting and soil conservation — stone lines (diguettes) laid along the contour to slow run-off and trap sediment; planting pits (zaï) that concentrate water and manure at the seed; half-moon bunds; terracing. Cheap, local, labour-intensive, adopted willingly — but they need labour at exactly the season it is scarce, and they cannot manufacture rain.
• Farmer-managed natural regeneration and agroforestry — protecting and pruning the shoots of trees already in the soil rather than planting seedlings. Restores shade, litter, nitrogen and windbreak at very low cost. Widely reported as the most successful intervention in parts of Niger and Burkina Faso; the scale of its impact is estimated rather than precisely measured.
• Grazing management — rotational grazing, fodder banks, restoring mobility corridors so herds can track the rain rather than concentrate around a fixed borehole. Politically difficult where land tenure is contested.
• Large-scale schemes — the African Union's Great Green Wall initiative, originally conceived as a belt of planted trees and now reframed as a mosaic of restored land uses. Its early tree-planting phase suffered high seedling mortality; progress against its targets is widely described as behind schedule, and reported figures vary considerably between sources.
• Judgement: the interventions that work are the ones that increase the water and vegetation held on each field and that fit existing livelihoods — small, cheap, locally owned. Grand top-down engineering has a poor record here. But none of it addresses rainfall variability itself, and climate projections for the region remain uncertain — so resilience, not restoration to a former state, is the realistic aim.

Honesty in case studies: the Sahel is drowning in half-remembered statistics, many of them wrong or discredited (the "advancing Sahara" figures of the 1970s are the classic example). An examiner rewards a precise mechanism far more than a number you cannot source — and penalises confident invention. Say "widely reported as" and "estimates vary" when they do.

Diagnosing degradation

Writing the evaluative essay

Paper 1 essays are marked on AO1 (knowledge), AO2 (application and analysis) and — decisively — a conclusion that answers the question. Two question types dominate this option:

• "Wind is the dominant agent shaping hot desert landscapes." Discuss. The trap is to agree. Argue: wind dominates fine sediment transport and produces the classic depositional forms (barchan, seif, erg) and the near-ground erosional forms (yardang, zeugen, ventifact). But water cut the wadis, built the fans and bahadas, planed the pediments and floors the playas; and much of the large-scale relief is relict, inherited from Quaternary pluvial periods. Add the dependency argument: wind cannot act until weathering and water have supplied loose sediment. Judgement: dominance depends on which landform, which scale and which timescale you are asked about.
• "Assess the relative importance of physical and human causes of desertification." Do not "balance" mechanically. Argue that climatic variability supplies the trigger and the stress, while human land use supplies the vulnerability — and that the two are locked in a positive feedback. Then take a position: without drought, the same land use might be sustainable; without the land-use pressure, the same drought would be survivable. Degradation is a coincidence of the two, and policy can only realistically change one of them.

Habits that lift a script: (1) name the process, then explain the mechanism, then state the landform — in that order; (2) use the sorting gradient to link deposition to energy; (3) quantify only what you actually know; (4) end with a conditional judgement — "to this extent, on this evidence, at this scale".

Sharpening the judgement

The big ideas to know

That is the whole of AQA 3.1.2. Press Finish to see your score.