Drylands

Question 1

What is aridity?

Answer 1

• The main characteristic of deserts and drylands

- Means lack of available water

Question 2

Are drylands always hot?

Answer 2

• No
• Winters are cold – winter snowfall can be an important precipitation source
• Clear cloudless skies cause high diumal range - range between temp

Question 3

What is the moisture balance like in drylands?

Answer 3

• Net negative moisture balance
• Evapotranspiration > precipitation
• P/PET ratio is below 0.5
• This ratio is a measurement of potential stress from lack of water

Question 4

What are hyper arid areas?

Answer 4

• True deserts
• P/PET less than 0.05
• Central Sahara, Arabian
• Periods more than 12 months with no rainfall

Question 5

What are arid areas?

Answer 5

• P/PET between 0.05 and 0.2

- Central Australia and fringes of Sahara

Question 6

What are semi arid areas?

Answer 6

• P/PET between 0.2 and 0.5

- Western interior of North America

Question 7

What are dry-subhumid areas?

Answer 7

• P/PET between 0.5 and 0.65
• Added to classification by UNEP because subject to drought
• Southern Russia, Canadian Prairies

Question 8

Outline precipitation variation in drylands

Answer 8

• All dry lands subject to large year to year precipitation variations
• High inter-annual rainfall variation is common
• All regions are drought susceptible

Question 9

What are the 4 main locations of dry lands

Answer 9

1) Sub tropical regions - stable depending air with high pressure belts
2) Interior continental regions - far from oceans
3) Lee of mountain zones - rain shadow effects
4) Certain western effect of land masses - cold ocean currents suppress sea-surface evaporation

Question 10

What factors control drylands rock weathering?

Answer 10

• Bare rock surfaces = exposure
• High diurnal temp ranges – thermal weathering
• Excess of evapotranspiration – salt weathering
• Different salts have different susceptibilities

Question 11

What weathering landforms are formed in drylands?

Answer 11

• Honeycomb weathering features

- Cracked ephemeral drylands lakes - playas

Question 12

Why can water be effective in drylands?

Answer 12

• Rainfall events are frequently intensive
• Short bursts of rainfall transport high yields of sediment
• Little vegetation cover causes high run off
• However, infiltration capacities can be high in sandy sediments

Question 13

What is channel flow like in drylands?

Answer 13

• Usually in short duration and irregular
• Localised nature of drylands rainfall events
• Localised high infiltration in unsaturated conditions

Question 14

What are the flow features of a channel in drylands?

Answer 14

• Irregular and unpredictable
• High event to event variability
• High water losses through evaporation and stream bed
• Peaked hydrograph
• High velocity

Question 15

What are the geomorphic and ecological implications of the flow features in drylands?

Answer 15

• Wide channels, short distance changes in channel
• Systems can change rapidly
• High sediment loads

Question 16

Outline wind processes and forms in drylands

Answer 16

• Wind can be effective
• Limited vegetation
• Wind erosion features due to abrasion
• Ventifacts and Yardangs

Question 17

How is sand transported in dry lands?

Answer 17

• Transported primarily by saltation (hopping) and creep (rolling
• Also, reputation – grains set into a low hopping motion due to high velocity impact of descending saltating grains

Question 18

Why is the relationship between surface cover and entrainment complex?

Answer 18

• Surface causes a frictional drag – velocity profile develops;
• Need to overcome this friction before particle entrainment
• Turbulent airflow increases entrainment
• Caused by variations in surface topography

Question 19

How do sand dunes form?

Answer 19

• Dunes form where the rate of arrival of sand exceeds rate of loss
• E.g. where an obstacle disrupts wind flow

Question 20

What does the rate of sand dune development depend on?

Answer 20

Sand transport capacity of the wind and sediment availability

Question 21

What are the different types of sand dune?

Answer 21

• Barchan (parabolic)
• Seif (longitudinal)
• Star dunes (largest)
• Transverse (most mobile)

Question 22

What is desertification?

Answer 22

Land degradation in arid, semi-arid and dry-sub humid areas resulting from various factors, including climatic variations and human actions

Question 23

What does desertification result in?

Answer 23

1) ecosystem degradation
- Difficult to distinguish from natural variability where no monitoring occurs and since some recovery may occur when pressures are removed

2) soil degradation
- Increased wind and water erosion where no vegetation cover
- Internal degradation in physical and chemical properties – nutrient loss

Question 24

What causes desertification?

Answer 24

• Primary factor is poor land management and over intensive land use
• Exacerbated by natural and anthropogenic climate change

Question 25

What does desertification lead to?

Answer 25

• Soil less usable and lower crop yields
• Leads to famine as food supply is reduced
• Leads to political, social and economic melt down