B. Causes of Aridity and Sonoran Case Study

Question 1

How is a rain shadow created, and how do these create deserts?

Answer 1

• Rain shadows are created when mountain ranges lie parallel to moist coastal areas
• Prevailing winds moving inland cool as air is forced to rise over the mountains
• Moisture falls on the slope facing winds
• When the winds move over the peak of the mountain they have lost most of their moisture and are therefore very dry.
• Without sufficient moisture, a desert forms on the other side of the mountain range, in the rain shadow

Question 2

Why does the air rising over mountains become very dry?

Answer 2

• Rain forms from the expansion and cooling of the air which is then lost over the mountains
• Ascending air makes it hard for additional clouds and precipitation to form

Question 3

How are inland deserts formed?

Answer 3

• Winds blowing across the sea carry masses of moisture for forming precipitation
• Inland deserts form in areas so removed inland from the sea that their continentality causes an extreme diurnal range in temperatures, leading to mass evaporation.
• Furthermore, a lack of precipitation due to distance from the sea leads to aridity.

Question 4

How are coastal deserts formed?

Answer 4

• Cold air currents from the coast cool the air currents moving inland, causing fog which cools the air above it and prevents the ascension of air and movement of precipitation into the desert.

Question 5

How are high-pressure deserts formed?

Answer 5

• At the equator, warm air rises and forms Hadley cells moving warm air to the North and South.
• There, the air cools down and sinks
• The sinking air pushes down on the Earth, creating areas of high pressure
• High pressure reduces the rising of precipitation due to large volumes of cold air, bringing sunny days and clear cloudless skies.
• The sinking limb of the Hadley cell creates aridity.

Question 6

How does overgrazing occur?

Answer 6

Overgrazing occurs when herd sizes exceed the carrying capacity of the pasture area i.e. the number of livestock that can graze an area without long-term environmental degradation occurring.

Question 7

What are the effects of overgrazing?

Answer 7

• Overgrazing alters the quality and quantity of vegetation, with palatable grasses being replaced by more (drought)-resistant species less able to bind soil together.
• Cause a decline in the quality of the soil - hooves of animals compact the soil and break down the soil structure, increasing the likelihood of soil erosion by acid rain / erosion etc.

This leads to desertification and the expansion of arid areas

Question 8

How does overcultivation occur?

Answer 8

Overcultivation occurs due to pressure on agriculture to produce more food / cash crops to support an increasing population or for sale abroad.

Alternatively, it also occurs due to cultivation of soils unsuitable for the growth of crops

Question 9

What are the effects of overcultivation?

Answer 9

Reduces soil fertility, leading to soil exhaustion, loss of nutrients and increased erosion.

Question 10

What is vegetation clearance and what are its effects?

Answer 10

• Clearance, either for land preparation or to harvest wood and other products
• Lowers the water table in an area making it more susceptible to aridity

Question 11

How do salinisation and waterlogging occur and what are their problems?

Answer 11

Both of these factors are associated with irrigation schemes in arid areas which were designed to reduce the problems of unreliable rainfall

If irrigated land is not drained adequately, the groundwater levels rise in the soil and evaporation increases the salt concentration in the soil. This impairs plant growth and may even result in complete waterlogging with and upper salt crust.

Question 12

Which physical factors make the Sonoran a likely location for a desert to form?

Answer 12

• Descending limb of Hadley cell (subtropical high) - high pressure area
• Winds blown from Pacific cross Californian range and Santa Anna mountains creating a rain shadow
• Cold currents in Pacific inhibit rising air currents needed for precipitation
• Winds blown from Gulf of Mexico travel 1700km over the Sierra Madres - no water left when the rains reach the West coast.

Question 13

How much rainfall does the Sonoran desert receive each year

Answer 13

<200m

Question 14

How much vegetation does the Sonoran desert have?

Answer 14

Large numbers of xerophytic plants - over 2000 species of plant

• Saguaro cactus only grows in the Sonoran desert
• Creosote bush - deep + wide root system and bitter, waxy leaves

Question 15

What are conditions like in the centre of the Sonoran desert?

Answer 15

Temperatures up to 50’C
Large diurnal range
<100mm rainfall per year