Welcome to Dryland Landscapes!
Hello there! Welcome to your study notes for Dryland Landscapes. This chapter is part of your Physical Systems unit. When many people hear "dryland," they immediately think of the Sahara Desert and scorching heat. While that’s part of it, you’re about to discover that drylands also include freezing polar regions and scrubby grasslands.
We are going to look at these landscapes as systems—think of them like a machine with parts moving in and out. Don't worry if some of the terms seem a bit "dry" at first; we’ll use plenty of analogies to keep things moving!
1. How can dryland landscapes be viewed as systems?
In Geography, a system is just a way of looking at how things (like energy and sand) move into an area, change it, and then leave.
Think of a dryland system like a bank account:
• Inputs: Money coming in (in deserts, this is energy from the sun, wind, or rare rainfall).
• Processes: Spending or moving money (this is erosion and transportation of sand).
• Outputs: Money leaving (this is deposition of sediment or water evaporating).
The Aridity Index
How do we decide if a place is actually a "dryland"? We use the UNEP Aridity Index. It’s a simple ratio that compares how much rain falls to how much water could evaporate if it were available.
The formula looks like this:
\( AI = \frac{P}{PET} \)
Where:
• \( P \) is average annual precipitation (rain/snow).
• \( PET \) is potential evapotranspiration (how much water the sun and plants want to suck up).
Common Mistake: Students often think deserts have zero rain. They don't! They just have very little rain compared to the high amount of evaporation.
Types of Drylands
The syllabus requires you to know three specific types:
1. Polar Drylands: Very cold, very dry. The "water" is locked away as ice (e.g., Antarctica).
2. Mid- and Low-latitude Deserts: The classic hot deserts (e.g., the Sahara or the Gobi).
3. Semi-arid Environments: "Desert-ish" areas that get a bit more rain and have some tough shrubs (e.g., the Sahel in Africa).
2. Factors Influencing the System
Why does one desert look like a sea of sand dunes while another looks like a pile of rocks? It comes down to a few physical factors. You can remember these with the mnemonic "C-G-L-A-R-S":
C - Climate: How much rain falls and how often? Is it windy? Wind is the "engine" of the desert.
G - Geology: Is the rock hard or soft? Lithology (rock type) and structure (cracks and layers) determine how fast the landscape erodes.
L - Latitude and Altitude: High altitude (mountains) or high latitude (poles) makes things colder.
A - Aspect: Which way does the slope face? A slope facing the sun will be much drier than one in the shade.
R - Relief: The shape of the land. Mountains can create a rain shadow, blocking moist air and creating a desert on the other side.
S - Sediment availability: You can't have sand dunes without sand!
3. How are landforms developed?
This is the part of the exam where you describe how the scenery was made. Landforms are created by two main types of work: Erosion (wearing away) and Deposition (dropping off).
Erosional Landforms (The "Take-Away" Crew)
• Wadis: Dry riverbeds that only fill up during "flash floods." They have steep sides and flat bottoms.
• Pedestal Rocks: Rocks that look like giant mushrooms. Wind carries sand close to the ground, so it erodes the bottom of the rock faster than the top.
• Ventifacts: Rocks that have been polished or "sandblasted" flat by wind-blown sand.
• Desert Pavements: A surface of closely packed stones. The wind blows away all the fine sand (a process called deflation), leaving the heavy rocks behind like a cobbled street.
Depositional Landforms (The "Delivery" Crew)
When the wind or water slows down, it drops its load. This creates:
• Barchan Dunes: Crescent-shaped dunes. The "horns" point in the direction the wind is blowing.
• Alluvial Fans: When a flash flood rushes out of a narrow canyon onto a flat plain, the water spreads out and drops its sediment in a fan shape.
• Bajadas: This is just what happens when a bunch of alluvial fans grow so big they merge together into one big ramp of sediment at the base of a mountain.
4. Evolution Over Time: Climate Change
Don't worry if this seems tricky at first: You just need to remember that deserts haven't always been dry! Over millions of years, the climate has swung back and forth.
Pluvial Periods (Rainy Times): In the past, some deserts had massive rivers and lakes. This created landforms like:
• Inselbergs: Isolated "island mountains" (like Uluru) left behind after the surrounding land was eroded by ancient water.
• Pediments: Gently sloping rock platforms at the base of mountains, carved by retreating water.
Colder Periods: Sometimes, deserts were near freezing. This caused frost shattering (water gets into cracks, freezes, expands, and breaks the rock). This is why some deserts are covered in jagged scree (broken rock) rather than sand.
5. Human Activity and Drylands
Humans are constantly trying to change drylands to suit our needs, but this often causes big problems.
1. Water Supply and Dams:
We build dams to store water for farming.
• The Impact: Dams trap sediment. This means downstream landforms like wadis or alluvial fans stop growing because their "building material" is stuck behind a wall.
2. Tourism and Economic Activity:
Think of dune buggies or quad bikes.
• The Problem: These vehicles crush the cryptobiotic crust (a thin layer of tiny organisms that holds the sand together). Once this crust is broken, the wind can blow the sand away much easier, leading to massive erosion.
3. Loess Accumulation:
When we farm on the edges of deserts, we often disturb the soil. This dust gets blown away and settles elsewhere as loess (fine, fertile silt). While loess is good for farming elsewhere, losing it turns the original land into a wasteland.
Final Quick Review - Top Tips for the Exam
• Scale matters: Always mention if a process takes seconds (flash floods) or millennia (pediment development).
• Systems thinking: Use words like input, output, store, and transfer.
• Case Studies: Make sure you have one mid-latitude desert (like the Sonoran) and one low-latitude desert (like the Sahara) ready to compare.
• Water vs. Wind: Even in deserts, water is often a more powerful shaper of the land than wind, even though it only appears rarely!