Aridity & the aridity index

Drylands are defined by aridity — a shortage of water because potential evapotranspiration (PET) exceeds precipitation. Geographers grade them:

• Hyper-arid — extremely dry, rainfall very low and unreliable.
• Arid — true deserts.
• Semi-arid — the margins, where rainfall is a little higher.

Widely used UNEP thresholds: hyper-arid < 0.05, arid 0.05–0.2, semi-arid 0.2–0.5. A ratio below 1 means evaporative demand is greater than rainfall, so there is a water deficit.

Your turn — aridity index

Your turn — water balance deficit

Reading the index

The dryland system

Although water is scarce, drylands are shaped by two interacting subsystems:

• Aeolian subsystem — wind is an effective agent because vegetation is sparse and the surface is dry and loose, so sand and dust are easily moved.
• Fluvial subsystem — water is rare but powerful; occasional intense storms produce flash floods that do a great deal of geomorphic work in a short time.

Key idea: desert landscapes are polygenetic — many were shaped partly under wetter past climates, and both wind and water processes leave their mark on the same landscape.

Why deserts occur

• Subtropical high pressure — around 30° N and S, air that rose at the equator descends (subsides). Sinking air warms and dries, suppressing cloud and rain (e.g. the great subtropical deserts).
• Rain shadow — air forced over a mountain range loses its moisture on the windward side, so the leeward side is dry.
• Continentality — deep in a continental interior, air masses have lost their moisture long before arriving.
• Cold ocean currents — cool coastal water chills the air above it, reducing evaporation and rainfall along some western coasts.

Which cause?

Desert weathering

Large day-to-night temperature swings and salty, occasionally damp conditions drive distinctive weathering:

• Thermal fracturing (insolation weathering) — repeated heating and cooling stresses the rock; outer layers peel off as exfoliation (onion-skin weathering).
• Salt weathering (haloclasty) — salts left as thin films of water evaporate crystallise in pores and cracks, prising grains apart.
• Block and granular disintegration — rock breaks into blocks along joints, or crumbles grain by grain, leaving loose material for the wind and water to remove.

Debate: insolation weathering may need at least a little moisture to work effectively — pure temperature change alone is a matter of scientific argument. Salt and moisture-assisted processes are widely accepted.

Crystals in the cracks

Wind (aeolian) processes

• Deflation — the wind lifts and removes loose, dry, fine material, lowering the surface and leaving hollows.
• Abrasion — sand carried near the ground sand-blasts rocks and boulders, sculpting and undercutting them.

The wind transports sediment in three ways, depending on grain size:

• Suspension — fine dust carried high in the air over long distances.
• Saltation — sand grains bounce along in a series of low hops.
• Surface creep — the largest grains roll and are nudged along the ground by saltating grains.

Wind (aeolian) landforms

• Barchan — a crescent-shaped dune with horns pointing downwind, forming where sand is limited and wind blows from one direction.
• Seif (linear) dunes — long ridges aligned roughly parallel to the wind; star dunes form under variable multi-directional winds.
• Ventifacts — pebbles faceted and polished by wind abrasion.
• Yardangs — streamlined ridges carved by abrasion parallel to the prevailing wind; zeugen are ridges of resistant caprock on softer rock.
• Desert pavement (reg) — a surface of stones left behind after deflation removes the fines; deflation hollows are the depressions this excavation leaves.

Read the dune

Water (fluvial) in the desert

Desert rivers are usually ephemeral — dry for most of the year, then flowing violently after rare, intense storms as flash floods and unchannelled sheetwash.

• Wadi — a normally dry, steep-sided desert channel that carries flash floods.
• Alluvial fan — a fan of sediment dropped where a wadi leaves a mountain front and loses energy; merged fans form a bajada.
• Playa — a flat, salt-crusted floor of a temporary desert lake that has dried out.
• Pediment — a gently sloping rock platform at a mountain foot; inselbergs are steep residual hills rising above it.
• Mesas and buttes — flat-topped uplands (mesa) and their smaller, more eroded remnants (butte), protected by a resistant caprock.

Wind, water or weathering?

Tap a term, then tap the group that made it.

Match each landform to its description

Tap a description on the left, then its matching landform on the right.

Desertification

Desertification is the degradation of drylands into desert-like conditions, driven by both physical and human factors:

• Physical — drought and rainfall variability, often linked to climate change, reducing vegetation cover.
• Human — overgrazing, deforestation for fuelwood, and over-cultivation that strips nutrients; population pressure intensifies all three.

Once vegetation is lost, soil is exposed to wind and rain, erosion accelerates and fertility collapses in a downward spiral.

Management: afforestation and shelter belts, contour bunds and water harvesting, rotational grazing and drought-resistant crops, plus international schemes such as the Great Green Wall along the Sahel margin.

A human cause

The big ideas to know

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