Drylands Flashcards
What is aridity?
- The main characteristic of deserts and drylands
- Means lack of available water
Are drylands always hot?
- No
- Winters are cold – winter snowfall can be an important precipitation source
- Clear cloudless skies cause high diumal range - range between temp
What is the moisture balance like in drylands?
- 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
What are hyper arid areas?
- True deserts
- P/PET less than 0.05
- Central Sahara, Arabian
- Periods more than 12 months with no rainfall
What are arid areas?
- P/PET between 0.05 and 0.2
- Central Australia and fringes of Sahara
What are semi arid areas?
- P/PET between 0.2 and 0.5
- Western interior of North America
What are dry-subhumid areas?
- P/PET between 0.5 and 0.65
- Added to classification by UNEP because subject to drought
- Southern Russia, Canadian Prairies
Outline precipitation variation in drylands
- All dry lands subject to large year to year precipitation variations
- High inter-annual rainfall variation is common
- All regions are drought susceptible
What are the 4 main locations of dry lands
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
What factors control drylands rock weathering?
- Bare rock surfaces = exposure
- High diurnal temp ranges – thermal weathering
- Excess of evapotranspiration – salt weathering
- Different salts have different susceptibilities
What weathering landforms are formed in drylands?
- Honeycomb weathering features
- Cracked ephemeral drylands lakes - playas
Why can water be effective in drylands?
- 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
What is channel flow like in drylands?
- Usually in short duration and irregular
- Localised nature of drylands rainfall events
- Localised high infiltration in unsaturated conditions
What are the flow features of a channel in drylands?
- Irregular and unpredictable
- High event to event variability
- High water losses through evaporation and stream bed
- Peaked hydrograph
- High velocity
What are the geomorphic and ecological implications of the flow features in drylands?
- Wide channels, short distance changes in channel
- Systems can change rapidly
- High sediment loads
Outline wind processes and forms in drylands
- Wind can be effective
- Limited vegetation
- Wind erosion features due to abrasion
- Ventifacts and Yardangs
How is sand transported in dry lands?
- 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
Why is the relationship between surface cover and entrainment complex?
- 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
How do sand dunes form?
- Dunes form where the rate of arrival of sand exceeds rate of loss
- E.g. where an obstacle disrupts wind flow
What does the rate of sand dune development depend on?
Sand transport capacity of the wind and sediment availability
What are the different types of sand dune?
- Barchan (parabolic)
- Seif (longitudinal)
- Star dunes (largest)
- Transverse (most mobile)
What is desertification?
Land degradation in arid, semi-arid and dry-sub humid areas resulting from various factors, including climatic variations and human actions
What does desertification result in?
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
What causes desertification?
- Primary factor is poor land management and over intensive land use
- Exacerbated by natural and anthropogenic climate change
What does desertification lead to?
- Soil less usable and lower crop yields
- Leads to famine as food supply is reduced
- Leads to political, social and economic melt down
