Deserts as natural systems - Geography (9635) - Oxford AQA A-Level

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Deserts as Natural Systems (3.1.2.1)

Hello Geographers! Welcome to the fascinating world of hot deserts. When we think of deserts, we often picture vast, empty, unchanging lands. However, this chapter reveals that deserts are incredibly dynamic and complex environments that we study using the concept of a natural system. Understanding deserts as systems helps us analyse how different elements interact to create these unique, arid landscapes. Ready to dive into the sand and heat? Let's go!

1. Applying Systems Concepts to Deserts

In geography, a system is simply a set of interconnected components working together. Desert landscapes can be studied as open systems, meaning they exchange both energy and matter (sediment, water) with the surrounding environment.

Key Components of the Desert System

Think of a desert system like a complex machine or, more simply, like a bank account:

Inputs: What is put into the system.
- Energy: Primarily Insolation (solar radiation) – this drives heating, evaporation, and wind processes.
- Matter: Water (precipitation, often sparse and episodic) and Sediment (delivered by wind or sometimes distant rivers).
Stores / Components: Where energy or matter is held temporarily.
- Regolith (weathered rock fragments).
- Groundwater (deep stores, often limited).
- Dune systems (stored sediment).
- Biomass (stored organic matter in plants/animals).
Flows / Transfers: Processes that move energy or matter between stores.
- Wind erosion and transportation (e.g., saltation).
- Weathering (breaking down rock).
- Runoff (water flowing briefly across the surface).
Outputs: What leaves the system.
- Evaporation (water lost back to the atmosphere).
- Sediment transfer (dust carried out of the desert margins).
- Heat radiation (energy lost to space).

Feedback Loops and Dynamic Equilibrium

Systems don't just sit still; they constantly react to changes. These reactions are called feedback loops.

1. Positive Feedback (Amplifying Change):

This makes the change bigger. In deserts, a common example relates to vegetation:

Less rainfall -> Fewer plants survive -> Less shade on the ground -> Higher surface temperatures -> Increased evaporation -> Drier soil -> Even fewer plants survive.

This loop accelerates the process of aridification.

2. Negative Feedback (Self-Regulating/Stabilising):

This works to maintain balance.

Increased wind erosion removes fine sand -> A protective layer of coarse gravel forms (a desert pavement) -> Gravel shields the soil beneath -> Erosion rate slows down.

When the system is balanced, continually adjusting through negative feedback, it is in Dynamic Equilibrium. This means the overall landscape looks stable, but all the internal processes (flows, transfers) are constantly active.

Quick Review: Landform vs. Landscape

Landform: A single, individual feature resulting from geomorphological processes (e.g., a specific barchan dune or a wadi).
Landscape: The overall appearance of an area, formed by the combination of many related landforms (e.g., a sandy erg landscape or a rocky hamada landscape).

Key Takeaway: Deserts are open systems driven by energy (insolation) and constrained by limited water inputs. Feedback loops ensure they are always adjusting, often maintaining a state of dynamic equilibrium.

2. Global Distribution and Characteristics

Distribution of Deserts

Deserts are generally classified based on their latitude:

1. Low Latitude Deserts (Tropical Deserts):

Found roughly between 5° and 30° North and South of the Equator.
Example: The Sahara Desert (Africa) and the Great Australian Desert.
These are typically the hottest deserts and their existence is primarily controlled by global atmospheric circulation (the Hadley Cell – see Section 4).

2. Mid Latitude Deserts (Continental Deserts):

Found further inland, usually between 30° and 50° North and South.
Example: The Gobi Desert (Asia) and the Patagonian Desert (South America).
These often experience a greater temperature range (hot summers, very cold winters) and are caused mainly by continentality or rain shadow effects.

Desert margins are known as semi-arid or steppe environments. These areas receive slightly more rainfall but are still characterised by low soil moisture and high evaporation. They are often highly vulnerable to desertification.

Characteristics of Hot Desert Environments

The interaction between climate, soils, and vegetation creates the distinct desert environment.

Climate

Precipitation: Extremely low (<250mm/year) and highly unreliable (sporadic and intense when it does occur).
Temperature: Very high daytime temperatures (>40°C) but often very cold nights (high diurnal range) due to clear skies and low atmospheric moisture, which allows heat to escape quickly.
Evaporation: Very high rates, exceeding precipitation rates significantly.

Soils (Pedology)

Desert soils are called Aridisols.
They are thin, coarse, and lack significant organic material (humus) because decomposition is slow.
Due to intense evaporation, water moves upwards through the soil, leaving behind salts and minerals (like calcium carbonate). This process is called salinisation, often leading to a hard, infertile crust called caliche.

Vegetation (Ecology)

Plants that survive here are highly adapted: Xerophytes (drought-resistant, like cacti) and Phreatophytes (deeply rooted plants accessing groundwater, like mesquite trees).
Adaptations include small, waxy, or hairy leaves (to reduce transpiration) and extensive root systems.
Vegetation cover is sparse, meaning soil is often unprotected from wind and water erosion.

Key Takeaway: Deserts (arid) and their margins (semi-arid) are classified by latitude, but their defining feature is the intense interaction between dry, high-range climates, thin, saline soils, and specialised, sparse vegetation.

3. Measuring Dryness: Water Balance and Aridity Index

How do geographers scientifically define a desert? We use water measurements.

Water Balance

The water balance is a simple equation showing the relationship between water supply and water demand:

\(\text{Water Balance} = \text{P} - \text{E}\)

Where:

P = Precipitation (water supply)
E = Evapotranspiration (water demand)

In hot deserts, the water balance is always negative: P is much less than E (or specifically, PET - Potential Evapotranspiration). This deficit means the environment loses more moisture than it gains.

Aridity Index (AI)

The most reliable quantitative measure of aridity is the Aridity Index (AI). This index compares the average annual water supply (P) to the water demand (PET).

\(\text{Aridity Index (AI)} = \frac{\text{P}}{\text{PET}}\)

Arid regions (True Deserts): Have an AI usually less than 0.2. This means precipitation is less than 20% of the potential water demand.
Semi-Arid regions: Have an AI between 0.2 and 0.5.

Did you know? Even if a place gets 500mm of rain, if it's hot enough (meaning PET is 3000mm), it would still be severely arid because the AI would be 500/3000 = 0.16. Temperature is as important as rainfall!

Key Takeaway: Aridity is quantified by the negative water balance (P < E) and scientifically defined by the Aridity Index (AI < 0.2 for true deserts).

4. The Causes of Aridity

Deserts are not randomly placed. Their existence is explained by five main geographical factors. You can remember these with the acronym CARC (plus Relief):

1. Atmospheric Processes (The Hadley Cell)

Most hot deserts (low latitude) are caused by global atmospheric circulation.

Warm, moist air rises at the Equator (0°).
As it rises, it cools, condenses, and causes heavy rainfall (creating rainforests).
This dry air is then pushed outwards to the North and South, sinking around 30° latitude (N and S).
As the air sinks, it compresses and warms up (adiabatic heating). This warm, sinking air creates a persistent area of high pressure.
High pressure suppresses the formation of clouds and rain, leading to clear skies and immense heating—creating the world's largest deserts, like the Sahara.

2. Continentality

This explains many mid-latitude deserts (like the Gobi).

Air masses gather moisture over the oceans.
As prevailing winds blow these air masses across large continents, the air loses its moisture through precipitation.
By the time the air reaches the centre of the landmass, it is completely dry.
The distance from any moisture source means rainfall is minimal.

3. Relief (Rain Shadow Effect)

When mountains block the path of rain-bearing winds, they create a 'shadow' of dryness on the leeward side.

Moist air hits the mountain range (windward side) and is forced to rise.
Rising air cools and releases moisture as rain or snow (orographic rainfall).
The air then passes over the top and descends the other side (leeward side).
The descending air warms rapidly (adiabatic warming) and becomes very dry, evaporating any remaining moisture. This dry zone is the rain shadow.
Example: The Andes mountains cause the extreme aridity of the Patagonian Desert in South America.

4. Cold Ocean Currents

This is an unusual cause, as it creates deserts right next to the ocean.

Cold ocean currents (like the Humboldt Current off South America or the Benguela Current off Namibia) cool the air above the water.
This cool air prevents evaporation from the ocean and creates a dense, stable layer of air near the surface.
Since the air is cold and stable, it cannot rise high enough to cool and form rain-bearing clouds (convection is prevented).
The only moisture that often forms is fog (when warm air meets the cold surface), which provides minimal water.
Example: The Atacama Desert in Chile, one of the driest places on Earth, is caused almost entirely by the cold Humboldt Current.

Key Takeaway: Aridity is caused by a combination of global factors (Hadley Cell sinking air), continental factors (distance from the ocean, mountain barriers), and oceanic factors (cold ocean currents inhibiting rainfall).