Aridity Index:
- Water balance = relationship between mean annual precipitaion (P) and potential evapotranspiration (PET). If P>PET = water surplus, vice versa = water deficit.
- In deserts, P<PET (water deficit). The size of the deficit (difference between P and PET) gives the aridity index
- A low value = aridity is high – a value below 0.2 = desert. Desert margins (semi-arid) = 0.2-0.5
Where are most deserts found?
- Around 30 degrees N&S of the equator: Global atmospheric circulation explanation)
- Air moves in circular patterns between the equator called Hadley cells.
- In a Hadley cell, air rises at the equator. The air cools as it rises, and moisture condenses and falls as rain, leaving the air dry.
- The dry air descends around 30 degrees north and south of the equator.
- In areas where the air descends a zone of high pressure is created.
- Winds blow outwards from high pressure areas – so no moisture is brought in by the wind.
- This means that the area has very low precipitation E.g Sahara
- In the middle of continents: Continentality
- Moist wind from the sea moves inland and the moisture held is dropped as precipitation.
- So when the wind reaches the centre of the large continent it’s carrying very little moisture,
- g Turkestan desert in it’s central part of Asia.
- Next to mountain ranges: Rainshadow effect explanation
- Tall mountain ranges force winds upwards (up the windward side of the mountain)
- As air rises it cools and is less able to hold water.
- Any moisture held is dropped as precipitation over the mountains, so the wind that moves inland (down leeward side) has very little moisture
- For example, the Atacama Desert in South America because of the Andes mountains.
- Near cold ocean currents: In some places, cold ocean currents run along the coast.
- Wind is cooled as it travels over the cold water and its ability to hold moisture is reduced.
- Moisture stored in the atmosphere is released as rain over the ocean before reaching land.
- So, when the wind reaches the land there’s very little moisture left, so very little rain falls.
- For example, Namib desert in Africa exists because of the Benguela Current
Why is there little vegetation in Deserts?
- Lack of water > difficult for things to grow > low biomass
- Small shrubs, grasses and cacti species
- More vegetation, increases the further you go from the desert, because there’s more water