Wind/Aeolian landforms in arid environments

Case study: Namibia

Context:

• hyper-arid (<30mm ppt)
• very dry
• one of the oldest deserts 55Ma
• aridity maintained by the dry descending air of the Hadley Cell
• main source of ppt= Advection fog (contact of warm land air with cold sea air)

Landforms:

1) Ventifacts & Desert Pavements 

2) Yardangs

3) Sand dunes

1. Ventifacts & Desert Pavements (wind erosion)

Location: Walvis Bay

• Occurs in every desert
• Takes a while to form 
• Size and energy of the wind relationship

Desert Pavements= The wind removes the fine materials and deflation continues until an accumulation of stones virtually blankets the surface. 

Ventifacts= Are small stones that have been shaped by attrition and abrasion due to wind blown sediment (Saltation). Flat faces develop in the prevailing wind direction. They are separated from the lee side by a sharp edge. (If there are different flat surfaces (facets) developed, there is more than one wind direction prevailing. Ventifacts with 3 facets are called dreikanters.

2. Yardangs

Location: Mouth of the orange river

The trough is formed either from deflation of the softer rock (geology) or from the erosion of weaknesses and joints (Lithology). The abrasion of the sides goes up to the height of 0.5m as that is the highest that rocks are transported via saltation. Are formed where there is a uni-directional prevailing wind.

3. Sand dunes 

Location: Namib Sand Sea/Cunene Sand Sea

NEED:

1) Large amount of loose sand

2) A wind or breeze

3) An obstacle

Formation:

1) Friction stops sediment leading to deposition

2) Positive feedback loop

3) Migration

This is how a sand dune may actually move over time 100m/yr, rolls along maintaining its shape as it goes.

5 types:

1) Transverse

2) Barchans

3) Linear (longitudinal dune)

4) Star

5) Parabolic

Transverse and Barchans are the most common.

Physical and Human factors which make arid ecosystems vulnerable

Physical:

• Harsh Climate
• Wind and water (flash flood)- They can remove unprotected soils and uproot vegetation.
• Lightning strikes can trigger fires (convection storms due to HP systems)
• Dust storms bury vegetation (high winds reduces photosynthesis as dust covers the leaves.

Human:

• Urbanisation and associated activities e.g. road building= separate out areas decreases migration for breeding.
• Water abstraction from groundwater= plants have long tap roots, lower the groundwater, the tap roots can no longer reach and the plants die.
• Grazing livestock= trampling.
• Introduction of non-native species both plant and animals= this changes the ecosystems they out compete local species.

Animal adaptations in arid environments

Kangaroo rat:

Found: South-West USA and Mexico
Height: 100-200mm 

Characteristics:

Behavioral:

1. Licks fur to keep cool
2. Lives in burrow during day

Physical:

1. Does not perspire
2. Highly efficient kidneys that concentrate urine into dry droppings

Fennec Fox:

Found: Sahara

Height: 200mm

Characteristics:

Behavioral:

1. Lives in burrows during the day to avoid the heat

Physical:

1. Large ears with blood vessels close to the surface to lose body heat (moisture gained from prey).
2. Light-coloured coat reflects heat.
3. Excretes highly concentrated urine.
4. Soles of feet protected by thick fur to run across hot sand.

Cryptobiotic soil crusts

Definition= A community of cyanobacteria (green algae, lichen, fungi and mosses) that form a fragile ground cover a few centimetres thick.

Found: Colorado Plateau & southwest USA (Account for >70% of the living ground cover)

Functions:

1) Important nurseries for seedlings
2) Slows run-off (rough surface)
3) Inputs organic matter (i.e. essential minerals) into the soil for plants.
4) Increase soil infiltration and absorb and stores rainwater (filaments of blue-green algae store up to 10 times their volume).
5) Bind together loose soil material from wind and water erosion (through dense network of filaments).

Human Impacts:

• Easily crushed and damaged (e.g. by vehicles, mountain bikes, hikers, livestock).
• Damage takes 50-250years for small areas to recover.
• Damaged crusts contribute less essential minerals.