arid environments Flashcards
What are the 4 processes in how arid environments are formed?
Pressure systems, continentality, offshore currents and rain shadow effect
What are the key features of arid environments?
High-wind energy environments, diurnal and seasonal variations in precipitation and in temp
What are the main weathering processes which operate in an arid environment?
Thermal fracture, exfoliation, salt weathering and chemical weathering
Weathering in deserts is _______ and _______ ________
Superficial and highly selective
What are the two types of salt crystal growth?
Temperature fluctuation leading to expansion of crystals by 300% and evaporation of water leaving salt crystals behind
What is the rate of disintegration of rocks closely related to?
Porosity and permeability
What is thermal fracturing?
Refers to the break-up of rock as a result of repeated changes in temperature over a prolonged period of time
Where does the stress occur in rock?
Outer layers of rock
What did Griggs (1936) show?
Moisture is essential for thermal fracturing to occur
How does granular disintegration occur?
Certain grains are more prone to expansion and contraction than others – exerts great pressure on the grains surrounding them and forces them to break off
What is hydration?
When certain minerals absorb water, expand and change
Give an example of hydration?
Anhydrite to Gypsum
When does freeze thaw occur?
When water in joints and cracks freeze and expand by 10%
What are the processes of erosion, transport and deposition by wind?
Corrasion/abrasion, deflation, traction, saltation and suspension
What is sand mainly made of?
Quartz which is 7 out of 10 on the Mohs hardness scale
What is the zone of maximum abrasion called?
Saltation zone
The form taken by deposited material is influenced by?
The nature of any surface irregularity, the amount and type of material carried by the wind, the flow pattern of the dominant wind and the presence or absence of vegetation and groundwater
What are zeugens?
Develop where differing rock strata lie horizontally – eroded by the wind
What are Yardangs?
Occur when hard and soft rocks lie side by side; the softer rocks are worn down to form troughs, while the harder rocks stand up as wind-worn ridges or yardangs
What is a hydrological regime?
The annual and seasonal pattern of river flow and can be both irregular and unpredictable
What contributes to a flash flood environment?
Intense rainfall + impermeable soil
What are the three types of rivers in arid environments?
Exogenous, Endoric and Ephemeral
How is a wadi formed?
Formed from intense storms during pluvial times and are further eroded by infrequent flash floods
What are the characteristics of a wadi?
A broad, flat bottom, heavily braided channels formed on thick beds of sediment, steep-sided banks and rills and gullies
What is an alluvial fan?
A cone of sediment occurring between a mountain and a lowland plain
What is a bajada?
When a number of alluvial fans merge together
What are pediments?
Gently sloping areas of bare rock where there is a distinct break with the mountain region
What is a playa?
A thick crust of salt which has formed after water has flowed into a depression and then evaporated, leaving behind a crust
What is a Mesa?
plateau-like features with steep sides at their edges
what are Buttes?
- smaller versions of Mesas
- they are thinner landforms that have very steep sides and a flat top
what are inselbergs?
freestanding mounds of hard rock
what is Equifinality?
different processes produce the same result
what does pluvial mean?
a geological change, process or feature that is the result of the action or effects of rain
Evidence for Pluvials in desert regions
- river systems now blocked by sand dunes
- animal and plant remains found in areas too arid to support such species
- Lake Chad said to have been 120m deeper than it is now and 100s of km further north
- numerous Wadis now in hot arid environments
- sunken civilisations - the Garamantes in North Sahara lived from 500BC to 500AD
what does interpluvial mean
dry periods between periods of greater precipitation
interpluvial evidence in desert regions
- sand dune systems in areas now too wet for sand movement to occur
- if rainfall is over 150mm then it supports enough vegetation to restrict the formation of sand dunes
- satellite imagery suggests areas of savanna and rainforest with over 750mm of rainfall have shown evidence for degraded sand dunes
what is soil?
the top layer of the earth that is composed of disintegrated rock particles, humus, water and air
what is litter?
organic matter in and on the soil, it includes humus and leaf litter
what is Biomass?
- the total mass of living organisms, mainly plant tissue, per unit area
- it is a store of energy and is also known as standing crop
what is leaching?
removal of nutrients from the top soil
what is biomass productivity?
the amount of new organic matter produced each year by the existing biomass
what is physical drought?
describes water shortages over a long period of time
what is physiological drought?
- describes water shortages experienced by plants despite there being sufficient water moisture available
- mainly caused by high evapotranspiration rates
do deserts have a high or low biomass productivity and do they have great or limited biodiversity?
- Low
- net primary productivity of 90g/m2/year
- Limited biodiversity
characteristics of desert vegetation
- ephemeral (appears or flowers after rain)
- some have very short life-cycle (some less than 8 weeks)
- generally shallow-rooted
- small size
- small leaves
characteristics of Semi-arid vegetation
- succulent
- more vegetation occurring near to sources of water
temperature adaptations of plants
- changing the orientation of the whole body
- light colours maximise the reflection of solar radiation
- surface growth can absorb/reflect heat which keeps the under-surface cooler
- smaller body size - large surface-volume ratio
how do plants avoid intense solar radiation and overheating
- more vertical orientation of leaves
- rolling their leaves along the axis
- reflective leaf hairs
- waxy leaf surfaces
- smaller, deeply dissected leaves to minimise the boundary layer thickness, maximising heat loss by convection and conduction
examples of temperature adapted plants
Agaves:
- thick leaves to hold water and protect from sunburn
cacti:
- leaves are reduced to spines to reduce water loss through transpiration
water loss adaptations in plants
- small surface area to volume ratio
- water regulation by plants can be controlled by diurnal closure of the stomata
- xerophytic plants have a mix of thick, waxy cuticles, sunken stomata and lead hairs
- stores water in leaves
- rolled structure - stomata not exposed
- hairs/pits - traps any water lost and holds onto it increasing water concentration outside leaves
examples of water loss adapted plants
cacti:
- thick, waxy cuticle stops water evaporating too easily
- deep roots called ‘tap’ roots
- extensive shallow root system with large surface area
- thick, fleshy stems, which means they can store water inside
Creosole bush:
- shedding of its leaves
- small leaves
- low surface area
- high water potential tissues
- greater root pressure (faster uptake of water)
examples of water loss adapted plants
cacti:
- thick, waxy cuticle stops water evaporating too easily
- deep roots called ‘tap’ roots
- extensive shallow root system with large surface area
- thick, fleshy stems, which means they can store water inside
Creosote bush:
- shedding of its leaves
- small leaves
- low surface area
- high water potential tissues
- greater root pressure (faster uptake of water)
what is an aridisol?
desert soil with a low organic content
What are Solonchaks?
soils with a saline horizon of Sodium chloride (also known as white alkali soils)
what are Solonetz?
soils with a horizon of sodium carbonate (also known as black alkali soils)
what are duricrusts?
when the accumulation of salts become so great they form a distinctive crust on the surface
what are factors leading to salinisation?
- lack of rainfall and high temperatures resulting in potential evaporation exceeding rainfall amounts
- this brings soluble salts up into the soil from the water table
- the poor structure of the soils, lacking in humus
- clay rich soils
- over irrigation which brings the water table near the surface
- irrigating with water rich in salts; this includes from inland lakes and from coastal areas
what is salinisation?
the concentration of saline salts, usually in the upper horizon of a soil which can be toxic to plants
what is soil degradation?
decline in quantity and quality of soil
what are the causes of soil degradation?
- water erosion
- wind erosion
- acidification
- Eutrophication (nutrient enrichment)
- salt affected soils
- atmospheric deposition
- climate change
Human activities and their impact on the soil
removal of woodland or ploughing existing pasture:
- the vegetation cover is removed, roots binding the soil die and the soil is exposed to wind and water
- particularly susceptible to erosion if on slopes
Cultivation:
- exposure of bare soil surface before planting and harvesting
- cultivation on slopes can generate large amounts of runoff and create rills and gullies
Grazing:
- overgrazing can severely reduce the vegetation cover and leave the surface vulnerable to erosion
- grouping of animals can lead to over trampling and creation o bare patches
- dry patches are particularly susceptible to wind erosion
Roads:
- they collect water due to reduced infiltration that can cause rills and gullies to form
Mining:
- exposure of the bare soil
Causes of salinisation
- the presence of soluble salts, such as sulphates of sodium, calcium, and magnesium in the soil
- A high water table - The water table lies underground and is the level at which the soil and gravel are completely saturated with water. This means the salt cannot effectively drain away
- A high rate of evaporation - Causing salt crystals to be left behind on the soil surface.
- Low annual rainfall meaning salts are not leached away
Problems with salinisation
- Has impacts on native vegetation especially on floodplains and wetlands which decreases biodiversity
- High levels of salt in the soil have a similar effect as drought by making water less available for uptake by plant roots
- Agricultural land suffers since crops have difficulty surviving with low water has meant that many salt-affected lands have become non-arable
- In the hot and dry regions of the world the soils are frequently saline with low agricultural potential. In these areas most crops are grown under irrigation, and to exacerbate the problem, inadequate irrigation management leads to secondary salinization that affects 20% of irrigated land worldwide
- Irrigation systems contain salt which may remain on the soil surface
- These factors lead to food insecurity and food poverty
- Soil erosion - Salinity is often associated with prolonged wetness and lack of surface cover and therefore increases the vulnerability of soils to erosion
- Without plant roots to hold the soil in place, the soil washes away causing erosion of land, leading to desertification
Prevention/management of salinisation
- Maintaining adequate vegetation cover
- using crop rotations and conservation farming methods in cropping areas
- maintaining soil health to maximise plant growth
Egypt case study - salinsation
- Every year the Nile River overflows with summer rains
- Due to the river dams upstream the local water table have increased
- So when the river floods, excess salt cannot leach away as it once did
- Up to 40% of all land around the river delta is suffering from soil salinization due to inadequate drainage
- Improved drainage systems- with help from both public and private sectors, 50% of cultivated area in Egypt now has subsurface drainage
- Saline agriculture- A recent study on tomato irrigation found that the crops irrigated with higher salinity levels produced more fruits per plant and had a longer shelf-life. This also includes informing local farmers of these benefits
- These solutions have not yet come to full fruition, so only time will tell if they are successful or not
what is desertification?
land degradation in drylands resulting from human actions
Causes of desertification
Climatic factors:
- High rainfall variability
- High temperatures
Soil and Water factors:
- Low clay content
- Humid zones
Biological factors:
- High disease and pest occurrence
Social factors:
- uncertain tenure
- Overuse of land
Environmental consequences of desertification
- loss of nutrients through wind and water erosion
- Changes in composition of vegetation and loss of biodiversity
- reduction in land available for cropping and pasture
- increased sedimentation of streams due to soil erosion
- expansion of areas under sand dunes
Economic consequences of desertification
- reduced income form traditional economy
- decreased availability of fuelwood (have to buy alternatives such as oil)
- increased dependence on food aid
- increased rural poverty
Social/cultural consequences of desertification
- loss of traditional knowledge and skills
- forced migration due to food scarcity
- social tensions in reception areas for migrants
Human causes of desertification
- overgrazing
- overcultivation
- deforestation
strategies for prevention of human causes of desertification
Overgrazing:
- improved stock quality - vaccination programs and use of better breeds provides a greater meat, wool and milk yield without increasing the herd size
- better management - reducing herd sizes and grazing over wider areas would reduce soil damage
- Problems: Vaccination programs increase survival rate so increase herd sizes and population pressure often prevents these measures
Overcultivation:
- Use of fertilisers - increases crop yields without using more land
- New and improved crops - high yielding and drought resistant
- improved farming methods - crop rotation, irrigation and grain storage can all increase and hence reduce pressure on the land
Problems:
- cost to farmers
- artificial fertilisers may damage the soil
- some crops require expensive fertiliser
- some methods require special technology and expertise
Deforestation:
- no deforestation
Successes and failures of attempts to prevent desertification in China
Successes:
- Natural forest Conservation program - launched in 2000 to conserve natural forests and reduce deforestation - reduced deforestation rates and forest cover has increased by 1.3% between 2000 and 2013
- Grain for Green program - launched in 1999 to convert marginal agricultural land into forest and grassland - led to the conversion of over 24 million hectares of cropland into forests and grasslands
- Ecological compensation mechanisms - encourages forest conservation and management - in Guangxi Province, the gov has provided financial incentives to farmers who have established forest plantations
Failures:
- illegal Logging - still remains a significant problem and it undermines the conservation work
- forest degradation - still remains a problem partly due to the conversion of natural forests into monoculture plantations, which can lead to soil erosion and biodiversity loss
- enforcement challenges - challenging due to inadequate monitoring and law enforcement capacity - in some cases has been undermined by corruption and a lack of accountability
