Changing Rainfall and Water Availability in East Africa:Supplementary Material to Lesson 3 of the

“East Africa Climate Change Curriculum Unit”Available at http://www.eaclipse.msu.edu/teaching_materials.html

Lesson 3 by Dwight Sieggreen & Barbara Naess.PowerPoint based on materials by Jennifer M. Olson.

The Eaclipse Project is supported by

National Science Foundation Award No. BCS/CNH 0709671.

http://eaclipse.msu.edu

© 2010 Michigan State University Board of Trustees

Lesson 3: Changing Rainfall and Water Availability in East Africa

Supplementary Material

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In this lesson you will

• Learn the main factors that affect rainfall patterns in Africa

• Compare the water cycle in two different ecosystems

• Use the water cycle to understand where water is lost from each ecosystem

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Major influences on Africa’s climate zones

1. Intertropical Convergence Zone (ITCZ): As the ITCZ moves north and south, it determines the rainy seasons

2. Elevation: The mountains and highlands are cooler and wetter, and the windward side of a mountain gets a lot of rain (orographic effect)

3. Nearness to the coast: there is more rain on the coast 4

• The ITCZ is a belt of rainfall that governs the rainy seasons around the equator (between Tropics of Cancer and Capricorn)

• It is an area of low pressure: the sun heats the earth’s surface, the surface air warms and expands, and it rises up like a fountain

• As the hot air rises, it precipitates out water rainfall

The Intertropical Convergence Zone (ITCZ)

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ITCZ Rainbelt in July

• The ITCZ moves, within the tropics, with the peak of the sun• In July it is at its farthest northern point

WINDITCZ

MSU LUCID: Bilal Butt © MSU Board of Trustees 2010

Adapted from Stock1995. Africa South of the Sahara, p31.

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Rainfall

ITCZ Rainbelt in January

• In January, the ITCZ is at its most southern point• Maritime southwesterly (wet) winds cause heavy rain • Northern Africa is dry

WINDITCZ

Adapted from Stock1995. Africa South of the Sahara, p31. MSU LUCID: Bilal Butt

© MSU Board of Trustees 2010

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Rainfall

MSU LUCID: Bilal Butt © MSU Board of Trustees 20108

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Orographic Rainfall

• Elevation affects rainfall. As winds hit a mountainside on the windward side, air is forced upwards and cools. The cool air cannot hold moisture and rain precipitates out.

• The wind, now dry, flows over the mountain peak and descends on the leeward side. It doesn’t rain and leaves that side of the mountain dry.

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Source: http://rst.gsfc.nasa.gov/Sect14/Sect14_1d.html

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Moist Air Rising and Forming Clouds Along a Mountain Side

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Mt. Kenya

Windward side

Leeward side - dry

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Proximity to the Coast

• Coastal East Africa receives torrential rains from moist air masses moving inland from the Indian Ocean

• This means that coastal areas have significantly more rainfall than inland areas

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Map source: U.S. Central Intelligence Agency, 2003

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Source: Tanzania Meteorological Agency 27

Source: Tanzania Meteorological Agency28

Summary of factors affecting rainfall

• Tropical areas receive highly seasonal rainfall due to ITCZ– Some equatorial regions receive almost

constant rainfall– North & south of equator, there are

mirrored belts of decreasing rainfall

• Rainfall varies according to elevation –orographic effect

• Rainfall is higher on the coast

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How will rainfall affect vegetation?

• Vegetation growth is closely associated with rainfall

• Reliability: In general, the less rain a region receives in an average year, the less reliable the rainfall

• Drought: savanna livelihood systems must be capable of withstanding extreme variability of rainfall, including droughts

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Review Questions

• What is the Intertropical Convergence Zone (ITCZ) and how does it affect rainfall in East Africa?

• What is orographic rainfall and how does it affect rainfall on mountainsides?

• How does the ocean affect rainfall in locations on the coast?

• What differences did you see in the change in average rainfall in Arusha (highland savanna) and Zanzibar (coast)?

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Source: U.S. Geological Surveyhttp://ga.water.usgs.gov/edu/watercyclehi.html

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