3.1.2.1 - Desert systems and global aridity

A landscape as an open system

A landform is an individual feature, such as a dune or alluvial fan. A landscape is the spatial arrangement of related landforms: a mountain front, channels, fans and a basin floor form a connected desert landscape. Its boundary is chosen for the enquiry, rather than assumed to be the edge of a country or national park.

For a dune field, sunlight and moving air are energy inputs; sand blown from an upstream floodplain is a material input. Sand stored in dunes is transferred by wind between ridges. Sand leaving the field is an output. Rainfall can enter, infiltrate into a soil-water store and leave through evaporation. Gravity transfers rock debris downhill. Energy enables work on sediment; it is not itself a sediment store. A closed drainage basin has no surface river outlet, but still exchanges water vapour, dust and energy with its surroundings.

Feedback and dynamic equilibrium

Feedback occurs when a change affects processes that act back on the original change. Positive feedback amplifies it: vegetation loss exposes soil, erosion removes water-retaining fine material, and reduced soil moisture makes vegetation recovery harder. Negative feedback opposes change: deflation removes fine sediment, leaving a coarse surface armour that resists further deflation. These are conditional mechanisms, not inevitable outcomes at every site.

Dynamic equilibrium means fluctuating inputs and outputs maintain an approximate balance over an appropriate period. A dune can migrate while its total sand store remains roughly stable. A storm may briefly increase a fan's sediment store without showing a lasting regime change. A sustained reduction in sand supply can instead shift the equilibrium and leave dunes smaller or less active.

Global patterns and atmospheric circulation

Large hot deserts occur mainly in the subtropics, commonly around 20–30° north and south: the Sahara in North Africa, Arabian deserts in southwest Asia, western/central Australia and the Namib and Kalahari in southern Africa. The Atacama lies on the Pacific side of northern Chile and southern Peru. Mid-latitude deserts also occur farther poleward, including the Mojave in the southwestern USA and continental interiors of Asia. Latitude alone is insufficient: maritime areas at similar latitudes may receive much more rain. Hot deserts are the focus here; polar deserts are dry for different reasons and are not hot-desert examples.