the-savanna-ecosystem-1229011393678230-1

8/4/2019 the-savanna-ecosystem-1229011393678230-1

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Ecosystemand

Pressures on theSahel


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Desert

Savanna

Equatorial Rainforest

Savanna

Desert

Sahel


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In the savanna climate there is a distinct dry season, which is in theIn the savanna climate there is a distinct dry season, which is in the

winter. Savannas get all their rain in the summer months. During thewinter. Savannas get all their rain in the summer months. During thedistinct dry season of a savanna, most of the plants shrivel up and die.distinct dry season of a savanna, most of the plants shrivel up and die.Some rivers and streams dry up. Most of the animals migrate to findSome rivers and streams dry up. Most of the animals migrate to findfood.food.

In the wet season all of the plants are lush and the rivers flow freely.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 seasonThe animals migrate back to graze. In West Africa the rainy season

begins in May.begins in May. It is usually cooler during the dry season by a few degrees. Because itIt is usually cooler during the dry season by a few degrees. Because it

is in the tropical latitudes that is still hot enough. The savanna climateis in the tropical latitudes that is still hot enough. The savanna climatehas a temperature range of 68 to 86 F (20 - 30 C). In the winter, ithas a temperature range of 68 to 86 F (20 - 30 C). In the winter, itis usually about 68 to 78 F (20 - 25 C). In the summer theis usually about 68 to 78 F (20 - 25 C). In the summer the

temperature ranges from 78 to 86 F (25 - 30 C). In a Savanna thetemperature ranges from 78 to 86 F (25 - 30 C). In a Savanna thetemperature does not change a lot. When it does, its very gradual andtemperature does not change a lot. When it does, its very gradual andnot drastic.not drastic.

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

savanna climatesavanna climate


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DRY SEASON

(JANUARY)

WET SEASON

(JULY)

TEMPERATURE 20 to 25C 25 to 30C

RAINFALL NONE 100 to 150CM

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


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Sahelian Africa

Sahel: A semiarid region of north-centralAfrica south of the Sahara Desert.

Countries are amongst the poorest LEDCs inthe world


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The ITCZ is an area where two air masses meet- it bringsheavy 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 theHadley convection cells


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The trade winds come into the zone

from cooler areas in the southern mid-latitudes and have travelled overoceans; they are there- forecarrying a lot of moisture. This is theirposition in January.

Once in the hotter latitudes, they areenergised into huge towering

cumulo-nimbus thunderclouds. Thesecan be anything up to 10kms across,and groups of clouds can formcovering 1000kms. In between theclusters are often sunny cloud-freeareas.


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Wetwarmtropicalmaritime

air

HEAVY RAINS

S N

Gulf ofGuinea

Coastalareas-equatorialclimate

Inland areas-savanna climatetype

Sahara-Desertclimatetype

Moves this way

Hot drytropicalcontinentalair

IN JANUARY


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In January, the sun is overheadnear the Tropic of Capricorn, inthe southern hemisphere.

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

The rains brought by the zone are

confined to the very coastal areasof Nigeria, Togo, Ghana and theirneighbours.

ITCZJANUARY

Case study area


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

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 cangrow.

In the south, however, the wet moist winds from the sea are forced upwardsover the land where they drench the land in life-giving torrential rains. Herecrops 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 desperatelyneed for people, crops and animals alike. The slight drawback is thatthe 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 centralNigeria are getting the rains. The wells are re-filled, the grass forfeeding animals starts to grow and farmers can start off their crops.


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S N

Gulf ofGuinea

Coastalareas-equatorialclimate

Inland areas-savanna climatetype

Sahara-Desertclimatetype

WetwarmmT air

Hotdry cTair

Harmattanwind

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 northas it will go, reaching central Mali,northern Niger and northern Chad.There

the Harmattan is confined to the veryedges of the Sahara desert, the weakest it

gets all year.

This is the Sahel zone, the area mostprone to DESERTIFICATION.

Without these rains most years, thedesert will take over yet more land,forcing poverty-stricken, hungry people tomigrate southwards into other peoplesterritory, with their thirsty cattle andgoats.

ITCZ JULY


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The savanna grasslandsecosystem


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In tropical parts of the worldwhere the climate alternatesbetween a long wet season and along dry season, grasses tend to

dominate the landscape.

The savanna grasslands aregenerally found between 5o and

15o latitude, both north and southof the equator. This belt ofgrasslands is squeezed betweenthe equatorial rainforests and hotdesert. Savanna grasslands arefound in Venezuela, northern


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In the wet season the vegetation grows and during thisseason the vegetation can include lush green grasses andwooded areas. As you move further away from the

equator and its heavy rainfall, the grassland becomesdrier and drier - particularly in the dry season. Savannahvegetation includes scrub, grasses, and occasional treeswhich grow near water holes, aquifers or the seasonalrivers

Plants and animals have to adapt to the long dry periods.Many plants are xerophytic - for example, the acacia treewith its small, waxy leaves and thorns. Plants may alsostore water (like the baobab tree) or have long roots thatreach down to the water table. Animals may migrate greatdistances in search of food and water.


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Baobab

Acacia

Xerophytic = drought


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Why does the Sahel suffer fromWhy does the Sahel suffer from

desertification?desertification?

Increase in population Increase in cattle

Deforestation for firewood Grassland grazed moreintensively

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

Leaves no longer protect soil from weather Less vegetation means lessprotection from weather

Loose top soil blown away by wind

(Soil Erosion)

=

Loose top soil blown away bywind

(Soil Erosion)DESERTIFICATION

DESERTIFICATION


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