Aridity and Water Balance (AQA A Level Geography)

• Aridity is affected by:
  • The global circulation of air
  • Topography/rainshadow effect
  • Cold ocean currents
  • Prevailing winds
  • Continentality
• The global circulation of air:
  • Hot arid and semi-arid deserts are found in the tropics and subtropics due to two large convective cells called Hadley Cells that control the air circulating between the equator and the tropics in the northern and southern hemispheres
  • The area where the dry air descends is a zone of high pressure known as the sub-tropical high

Diagram showing the impact of the descending limbs of the Hadley Cells on the atmospheric pressure system of hot deserts

• Note that as the cool, dense, dry air descends, it is being warmed by high levels of insolation 

• Hot air rises and cooler air sinks through the process of convection
• The irregular heating of Earth’s surface creates various pressure cells, each generating different weather patterns
• The movement of air within each cell is roughly circular and moves surplus heat from equatorial regions to other parts the Earth

• Winds blow outwards from the sub-tropical high-pressure area, leaving the air dry and preventing moisture from being brought in by winds
• This results in low precipitation rates

Topography/rainshadow effect

• Also called the orographic effect and prevents moisture-laden air from arriving
• Air is forced to rise over mountains, which then cools, condenses and forms clouds and rain
• This leaves only dry air to carry on over the crest of the mountain, creating a dry area on the other side called a rain shadow
• For example, the Atacama desert is due to the Andes acting as a barrier, and stopping the rain-bearing, south-east trade winds from Argentina and the Atlantic

Image showing the orographic effect helping to form the Atacama Desert, Chile

Cold ocean currents

• • Cool air above ocean currents holds less moisture, these currents travel close to land
  • These bring cool dense air that displaces warmer air over the land
  • Once this cool air hits the land it heats up and evaporates any moisture in it, leaving a coastal fog, which brings some moisture
  • Air that travels further inland is dry and warm
  • Ocean currents are a major factor in increasing the aridity in the Namib, Atacama and Sonoran deserts

Image showing how cold ocean currents do not bring much moisture to the interior of continents

Prevailing winds 

• In many hot, arid areas, prevailing winds blow from land to sea and carry little moisture
• E.g.NE trade winds from N. Africa to the Atlantic influence the Sahara desert
• Winds are strong and gusty, influencing landforms
• At the centre of the high-pressure cell, winds are calm but at the edges of the system, there are the trade winds and westerlies meaning it can be very calm or very windy in a desert

Continentality

• Many deserts are in the middle of continents far away from rain-bearing winds
• Large distance means that offshore winds can't carry moisture for that length of time, so most is lost before it reaches inland areas
• The only wind that reaches, is dry and warm

Reasons for location

• Hot semi-arid deserts are found between the equator and hot deserts, they fringe hot deserts and experience seasonal rainfall in summer
• The reason is that the rain-bearing Hadley cells migrate with the movement of the sun between the Tropic of Cancer and Capricorn called the Intertropical Convergence Zone (ITCZ) 
• The ITCZ is a low-pressure zone that normally lies near the geographic equator and divides the global air circulation patterns into two mirror images to the north and south of the equator
• The ITCZ forms where the Northeast Trade Winds meet the Southeast Trade Winds just north of the earth's equator
• The arid and semi-arid regions of Africa (Sahara and Sahel) lie north approx. 10°N, near the northern limit of the ITCZ, and receive one rainy season with very little precipitation (due to evaporation)

Image showing the ITCZ migration and seasonal rain in the Kalahari Desert

• CW Thornthwaite (1948) came up with an aridity index to classify climate
• The approach is based on the relationship between the input of water as precipitation (P) and the output of moisture from potential evapotranspiration (PET) over the year
• In deserts, P is lower than E and there is a water deficit, the size of the deficit gives the aridity index ratio
• Arid regions have a P/PET ratio of less than 0.20, meaning that precipitation supplies less than 20% of the amount of water that is needed to support optimum plant growth
• A low value means aridity is high and classed as hyper-arid with an index of <0.05 (less than 5%)
• Desert margins are semi-arid environments and have an index ratio between 0.2 - 0.5 (between 20-50% of necessary water needed)

Aridity index showing levels of precipitation and percentage of global coverage

Aridity and rainfall effectiveness

• All arid climates have a low mean annual precipitation
• Rainfall effectiveness is the amount of water that reaches the root zone and is available to plants
• Factors include:
  • Rates of evaporation: Temp and wind speed
  • Seasonality: Evaporation is less in winter
  • Rainfall intensity: Heavy = rapid runoff, little infiltration
  • Soil type: Clay poor absorption; sandy too porous

Water balance of hot deserts

• Water balance shows the relationship between precipitation, soil moisture storage, evapotranspiration and runoff
• The hot desert water balance is usually in severe water deficit with brief periods of soil water recharge and utilisation 
  • The water balance can be shown using the formula:
    • precipitation (P) = total runoff (O) + evapotranspiration (E) +/- changes in storage (S)

• When PET exceeds P - as in most deserts - there will be no runoff/surplus for domestic, agricultural or industrial use
• This water deficit results in water being transferred from one area to another or abstracted from fossil aquifers both of which are unsustainable in the long term

Image showing significant soil moisture deficit in Iran