Deserts

Definition: A desert is classified as an area receiving less than 250mm rainfall a year, or one where evaporation exceeds precipitation.

• Deserts cover 25% of the earth's land surface
• Deserts possess unique landforms as a result of their location and the agents of erosion or weathering acting within them
• The physical environment within them varies - variety of climates, ecosystems, soil and water availability.

Hot deserts are located along the latitude lines 30° North and South and at the Tropic of Cancer (23.5 degrees North) and the Tropic of Capricorn (23.5 degrees South).

Major deserts include:

• Mojave Desert, Colorado Desert, Sonoran Desert, Mexican Desert - North America
• Atacama Desert - South America
• Namib Desert, Kalahari Desert, Sahara Desert - Africa
• Arabian Desert - Asia
• Australian Desert - Australia

Hyper Arid = less than 100mm rainfall per year

Arid = less than 250mm rainfall per year

Semi-arid = 250mm - 500mm rainfall per year

Big ranges in annual temperature and diurnal temperature.

Daytime = very high temperatures due to high levels of insolation - incoming solar radiation not impeded by cloud cover

Night = Clear skies allow long-wave radiation to escape so ground temperatues fall rapidly e.g. below 0 degrees.

Diurnal ranges of over 30 degrees are not uncommon - deserts close to the sea have a lower diurnal range due to cold offshore ocean currents.

Annual ranges - Hot in summer, Cool in winter

Latitude

• Deserts are at lower latitudes between 10 and 30 degrees North and South
• The sun is overhead at the equator (0 degrees)
• Due to the curvature of the Earth, the sun's rays are concentrated on a smaller surface area = increased amount of solar insolation (heat) received.
• The sun's rays pass through less atmosphere so there's less reflection from clouds and absorption by CO2, ozone, water vapour and dust in the atmosphere.

Seasons

• Sun is directly overhead at the Tropic of Cancer on the 21st June because the Earth rotates on a tilt of 23.5 degrees - the Northern Hemisphere is tilted towards the sun so this is the Northern Hemisphere's deserts' summer
• On the 21st December, the sun is overhead at the Tropic of Capricorn so the Southern Hemisphere is tilted towards the sun - their summer.

Rainfall is low and unreliable. The lower the rainfall, the less reliable it is. In some places, it may not rain for long periods.

When it does rain, there is rapid surface runoff, which together with low infiltration and high evaporation minimises its effectiveness for vegetation.

Deserts are subject to extreme rainfall events where more than the yearly average rainfall can fall in  a few hours.

Deserts have the lowest organic productivity level of any biome. Many areas of deserts have some form of vegetation. Plants have adapted to arid conditions in a variety of ways:

• physical characteristics that prevent moisture loss
• ability to store moisture in stem or leaves
• deep/wide-ranging roots to maximise water gathering
• short life cycle which follows sporadic rainstorms

Features of desert plants:

• fleshy stems and swollen leaves
• thick, waxy cuticles
• thick, protective bark
• small, spiky or waxy leaves
• bulbous roots
• salt tolerance

Ephemerals

• Short life cycle
• Remain dormant for long periods
• Come to life when it rains
• Complete the cycle by germinating, flowering and dispersing seeds in up to 3 weeks
• Example: Boerhavia - southern Sahara, life cycle of 1 week 

Xerophytes

• Adapted to withstand drought
• Succulents - ability to close their stomata during the hottest parts of the day in order to reduce transpiration levels.
• Also developed capacity to store water in fleshy stems and leaves e.g. cactus

Phreatophytes

• Plants which evade drought by having long roots
• Penetrate to depth to tap groundwater sources
• E.g. - Tamarisk tree

Halophytes

• Plants that are adapted to salty conditions found in and around salt flats
• E.g. - Saltbush

• Soils in arid environments are inhibited by lack of moisture, high rates of evaporation and sparse vegetation cover.
• When soils develop they are often poor, thin, lacking in organic content and highly saline.
• In the few places where the water table is close to the surface, moisture can be drawn upwards by capillary action - salts and mineral bases are also drawn upwards and deposited in the upper layers to give a slightly alkaline soil.

Aridisol

• Forms in areas where water table is close to surface (see above)
• Forms very slowly
• Yellow-red or grey-brown colour
• Very thin
• Low organic content and high pH value
• Unproductive but not infertile - when irrigated, can yield high-quality produce

Solonetz

• Form in areas where there is sufficient rain to cause some leaching, but not enough to wash mineral bases out of the soil
• Surface layers not saline
• B horizon is rich in sodium and v. clayey so difficult to cultivate

Solonchaks

• Found in areas with high evaporation rates
• High concentrations of salts brought to the surface where they form a thick crust rendering the soil infertile