The Savanna Ecosystemand

Pressures on the Sahel

Desert

Savanna

Equatorial Rainforest

Savanna

Desert

Sahel

• In the savanna climate there is a distinct dry season, which is in In the savanna climate there is a distinct dry season, which is in the winter. Savannas get all their rain in the summer months. the winter. Savannas get all their rain in the summer months. During the distinct dry season of a savanna, most of the plants During the distinct dry season of a savanna, most of the plants shrivel up and die. Some rivers and streams dry up. Most of the shrivel up and die. Some rivers and streams dry up. Most of the animals migrate to find food.animals migrate to find food.

• In the wet season all of the plants are lush and the rivers flow In the wet season all of the plants are lush and the rivers flow freely. The animals migrate back to graze. In West Africa the rainy freely. The animals migrate back to graze. In West Africa the rainy season begins in May.season begins in May.

• It is usually cooler during the dry season by a few degrees. It is usually cooler during the dry season by a few degrees. Because it is in the tropical latitudes that is still hot enough. The Because it is in the tropical latitudes that is still hot enough. The savanna climate has a temperature range of 68° to 86° F (20° - savanna climate has a temperature range of 68° to 86° F (20° - 30° C). In the winter, it is usually about 68° to 78° F (20° - 25° C). 30° C). In the winter, it is usually about 68° to 78° F (20° - 25° C). In the summer the temperature ranges from 78° to 86° F (25° - In the summer the temperature ranges from 78° to 86° F (25° - 30° C). In a Savanna the temperature does not change a lot. When 30° C). In a Savanna the temperature does not change a lot. When it does, its very gradual and not drastic.it does, its very gradual and not drastic.

• There is an annual precipitation of 10 to 30 inches (30 to 100 cm) There is an annual precipitation of 10 to 30 inches (30 to 100 cm) of rain. From December to February hardly any rain falls at all.of rain. From December to February hardly any rain falls at all.

savanna climate savanna climate

DRY SEASON

(JANUARY)

WET SEASON

(JULY)

TEMPERATURE 20 to 25’C 25 to 30’C

RAINFALL NONE 100 to 150CM

Parts of the Sahel will not see rain for over 9 months and even if/when the rains do come they can bring insufficient rain.

Sahelian Africa• Sahel: A semiarid region of north-central Africa south of

the Sahara Desert.• Countries are amongst the poorest LEDC’s in the world

The ITCZ is an area where two air masses meet- it brings heavy rain to the areas it passes over.

It happens all round the world between the Equator and about 20 degrees N/S.

Its full name is the Inter Tropical Convergence Zone. It is part of the Hadley convection cells

The trade winds come into the zone from cooler areas in the southern mid- latitudes and have travelled over oceans; they are there- fore carrying a lot of moisture. This is their position in January.

Once in the hotter latitudes, they are energised into huge towering cumulo-nimbus thunderclouds. These can be anything up to 10kms across, and groups of clouds can form covering 1000kms. In between the clusters are often sunny cloud-free areas.

Wet warm tropical maritime air

HEAVY RAINS

S N

Gulf of Guinea Coastal areas- equatorial climate

Inland areas- savanna climate type

Sahara- Desert climate type

Moves this way

Hot dry tropical continental air

IN JANUARY

In January, the sun is overhead near the Tropic of Capricorn, in the southern hemisphere.

The ITCZ zone of meeting air lies well to the south, as seen here.

The rains brought by the zone are confined to the very coastal areas of Nigeria, Togo, Ghana and their neighbours.

ITCZ JANUARY

Case study area

Here, the moist warm mT air from the Atlantic Ocean meets the hot dry cT air coming out of the Sahara Desert. Note that the desert is an area of HIGH pressure in winter and particularly strong winds blow AWAY from such areas.

The cT wind is called the HARMATTAN and is extremely hot, dry and dusty. Of course, the Harmattan cannot bring any rain to the area it travels over, and all the northern part of Africa is influenced totally by it. No crops can grow.

In the south, however, the wet moist winds from the sea are forced upwards over the land where they drench the land in life-giving torrential rains. Here crops can be easily grown, providing the soil is fertile enough and not washed away.

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During the spring, the ITCZ moves slowly northwards, the Harmattan losing its dominance over the land bit by bit.

Places further and further inland get the rains that they so desperately need for people, crops and animals alike. The slight drawback is that the further the ITCZ travels north, the less water it can bring to the rainy area, so crops to the north get less than those to the south.

By mid spring, places like northern Ghana, northern Benin and central Nigeria are getting the rains. The wells are re-filled, the grass for feeding animals starts to grow and farmers can start off their crops.

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S N

Gulf of Guinea Coastal areas- equatorial climate

Inland areas- savanna climate type

Sahara- Desert climate type

Wet warm mT air

Hot dry cT air

‘Harmattan’ wind

IN JULY

Moves this way

HEAVY RAINS LIGHT RAINS

Compare the January and July diagrams.

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By July the ITCZ has reached as far north as it will go, reaching central Mali, northern Niger and northern Chad. There the Harmattan is confined to the very edges of the Sahara desert, the weakest it gets all year.

This is the Sahel zone, the area most prone to DESERTIFICATION.

Without these rains most years, the desert will take over yet more land, forcing poverty-stricken, hungry people to migrate southwards into other peoples’ territory, with their thirsty cattle and goats.

ITCZ JULY

The savanna grasslands ecosystem

In tropical parts of the world where the climate alternates between a long wet season and a long dry season, grasses tend to dominate the landscape.

The savanna grasslands are generally found between 5o and 15o latitude, both north and south of the equator. This belt of grasslands is squeezed between the equatorial rainforests and hot desert. Savanna grasslands are found in Venezuela, northern Australia, across Africa between the equator and the Sahara desert

In the wet season the vegetation grows and during this season the vegetation can include lush green grasses and wooded areas. As you move further away from the equator and its heavy rainfall, the grassland becomes drier and drier - particularly in the dry season. Savannah vegetation includes scrub, grasses, and occasional trees which grow near water holes, aquifers or the seasonal riversPlants and animals have to adapt to the long dry periods. Many plants are xerophytic - for example, the acacia tree with its small, waxy leaves and thorns. Plants may also store water (like the baobab tree) or have long roots that reach down to the water table. Animals may migrate great distances in search of food and water.

Baobab

Acacia

Xerophytic = drought tolerant

Why does the Sahel suffer from desertification?Why does the Sahel suffer from desertification?

Increase in population Increase in cattle

Deforestation for fire wood Grassland grazed more intensively

Roots no longer hold soil together Roots may be eaten as well as grass

Leaves no longer protect soil from weatherLess vegetation means less protection from weather

Loose top soil blown away by wind

(Soil Erosion)

=

Loose top soil blown away by wind

(Soil Erosion)

=DESERTIFICATION

DESERTIFICATION