Hydrogeology of Madagascar: Difference between revisions
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Revision as of 16:46, 14 November 2017
Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Madagascar
Context
Groundwater is of great economic and social importance in Madagascar. Groundwater is a major source of public supply, with several towns entirely reliant on groundwater, and is the only available water source in the semi-arid south and other dry areas. It is difficult to find good, reliable data on the hydrogeology of Madagascar. Even if they exist, groundwater data are often hard to access from the organisations that hold the data, such as government, international organisations, NGOs, and drilling and engineering companies.
Compilers
Dr Kirsty Upton, Brighid Ó Dochartaigh, British Geological Survey, UK Mathieu Monteleone, Bushproof, Madagascar
Please cite this page as: Upton, Ó Dochartaigh and Monteleone, 2016.
Bibliographic reference: Upton, K., Ó Dochartaigh, B.É. and Moneoleone, M. 2017. Africa Groundwater Atlas: Hydrogeology of Madagascar. British Geological Survey. Accessed [date you accessed the information]. https://earthwise.bgs.ac.uk/index.php/Hydrogeology_of_Madagascar
Terms and conditions
The Africa Groundwater Atlas is hosted by the British Geological Survey (BGS) and includes information from third party sources. Your use of information provided by this website is at your own risk. If reproducing diagrams that include third party information, please cite both the Africa Groundwater Atlas and the third party sources. Please see the Terms of use for more information.
Setting
Estimated Population in 2013* | 22,924,851 |
Rural Population (% of total) (2013)* | 66.2% |
Total Surface Area* | 581,795 sq km |
Agricultural Land (% of total area) (2012)* | 71.2% |
Capital City | Antananarivo |
Region | Eastern Africa |
Border Countries | none - Indian Ocean. |
Annual Freshwater Withdrawal (2013)* | 16,500 Million cubic metres |
Annual Freshwater Withdrawal for Agriculture (2013)* | 97.8% |
Annual Freshwater Withdrawal for Domestic Use (2013)* | 1.4% |
Annual Freshwater Withdrawal for Industry (2013)* | 0.8% |
Rural Population with Access to Improved Water Source (2012)* | 35.4% |
Urban Population with Access to Improved Water Source (2012)* | 78.2% |
* Source: World Bank
Geography
Madagascar has an area of 592,000 km2; is 1600 km from north to south, and has a maximum width of 600 km from west to east. It lies 300 km east of the east African coast, in the intertropical zone. There are two main geomorphological regions in Madagascar: - A highland area, which covers two-thirds of Madagascar, formed by Precambrian basement rocks. This landscape has rounded hills separated by plains, with river valleys. The highest part is the Tsaratanana massif, which reaches 2,876 m, and the highlands have an average altitude of 2000 m. - Lowland sedimentary basins around the coasts, which cover one third of the country. The west coast of Madagascar is dominated by mangroves and swamps; the southwest and extreme south by sand dunes; and the east by sandy beaches.
Climate
There are different climate zones across Madagascar. Rainfall and temperature vary across the different zones depending on altitude and location relative to the coast and prevailing winds/monsoon rains. The highest rainfall is along the northeast coast, where annual rainfall can exceed 3000 mm. The lowest rainfall is in the southwest, which is semi arid, with less than 400 mm rainfall annually. There are two distinct seasons: a cool, dry season from May to October, dominated by trade winds from the southeast, during which only the east coast receives any significant rain; and a hot, rainy season from November to April, dominated by the northwest monsoon (Rakotondrainibe 2006).
More information on average rainfall and temperature for each of the climate zones in Madagascar can be seen at the Madagascar climate page.
These maps and graphs were developed from the CRU TS 3.21 dataset produced by the Climatic Research Unit at the University of East Anglia, UK. For more information see the climate resource page.
Surface water
The rivers of Madagascar rise in the Highlands and flow dominantly towards the coast to the west, south and east. There are few rivers in the north (Rakotondrainibe 2006).
Rivers flowing west carry large amounts of sediment, draining across muddy plains and mangrove swamps into the Mozambique Channel. The rivers flowing east are generally short, but can have high flows. The rivers flowing south are typically ephemeral, or ‘wadis’ – dry in the dry season, flowing only after heavy rains (Rakotondrainibe 2006). There are many lakes in the country, including: Alaotra, Itasy and Andraikiba lakes in the Highlands; Kinkony, Hima, Ihotry and Tsimanampatsotsa lakes in the west, and the salt lake of Ihode in the south (Rakotondrainibe 2006).
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Soil
Land cover
Land cover is dominated by pasture and woodland, each covering around 40% of the land area. The dominant arable crops as coffee, vanilla, sugar cane, cocoa, rice and cassava (BGS 2002).
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Geology
This section provides an overview of the geology of Madagascar.
The geology map on this page shows a simplified version of the geology at a national scale (see the Geology resource page for more details).
A higher resolution geological map of Madagascar is the Carte géologique de Madagascar, echelle 1:1,000,000. Scanned versions of this map in 3 tiles are available: Feuille du Nord, Feuille du Centre et Feuille du Sud.
More detailed geological information is available in the following reports: UN (1988); Rakotondrainibe (2006) and BRGM (1968).
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Summary
About two-thirds of Madagascar – all the Highlands area – is underlain by ancient Precambrian metamorphic rocks, also called crystalline basement. The remaining third of the country – the coastal lowlands and valleys – is underlain by younger sedimentary and some volcanic rocks, from Carboniferous to Quaternary in age. There are four main coastal sedimentary basins, each with a mixed sequence of sands, clays, sandstones, and abundant carbonate rocks (limestones and marls), as well as intercalations of volcanic rocks, usually of basalt (BGS 2002). The basins are: the Western Basin, which extends almost the whole length of the island; the Diego-Suarez Basin in the extreme north; the East Coast (Cote Est) Basin and the extreme South Basin. The geological sequence of these four basins is summarised in the table below, largely based on information from Rakotondrainibe (2006) and UN (1988).
Geological period | Key formations and lithology | Geological environment | |
Western Basin | |||
---|---|---|---|
Quaternary | Alluvial deposits in valleys; sandy shells and mangrove swamp deposits along the coast | Unconsolidated sedimentary | |
Tertiary | Eocene limestones with areas where there are some calcareous dolomite, marl, calcareous marl and marly limestones.
Local outcrops of Oligocene marls. Neogene continental soft sandstone with sandy clay crops out across the coastal region, with a cover of sandy shells. |
Unconsolidated to semi-consolidated sedimentary | |
Cretaceous | Upper Cretaceous –sandstones – sometimes clayey, marl and limestone – sometimes chalky
Middle Cretaceous – sandstones – sometimes glauconitic, marl, limestone, clays. Lower Cretaceous – marls and clays; sandstone – sometimes glauconitic, limestone, |
Consolidated to semi-consolidated sedimentary | |
Jurassic | Upper Jurassic – limestones – sometimes marly; sandstones – sometimes glauconitic; marls
Middle Jurassic – marine karstic limestones forming large plateaus, e.g. at Ankara and Kelifely of Bemahara. |
Consolidated sedimentary | |
Karoo system (Upper Carboniferous – Middle Jurassic | Isalo Group (Triassic to Middle Jurassic) – continental sandstones, often poorly cemented and conglomeratic, with marine intercalations of marl and limestone. In total, several thousand metres thick
Sakamena (Permian) – largely continental but with marine intercalations - micaceous sandstones and shales and clays. Sakoa (Upper Carboniferous) – found only in the southwest. Tillites, black shales, coaliferous sandstone, clay and minor limestones. |
Consolidated sedimentary | |
Diego-Suarez Basin | |||
Quaternary | Coral reef deposits and sand dunes | Unconsolidated sedimentary | |
Tertiary | Eocene dolomitic limestones overlain by karstic limestones
Neogene marine sequence of alternating limestones, sandstones and basaltic tuff, covered by basaltic flows. |
Unconsolidated to Consolidated sedimentary | |
Cretaceous | Upper Cretaceous – sandstones, sometimes with calcareous beds, marly chalk
Middle Cretaceous – mainly marls, sometimes with gypsum Lower Cretaceous – marls and clays and continental sandstones |
Consolidated to semi-consolidated sedimentary | |
Jurassic | Upper Jurassic – marls
Middle Jurassic – limestones and dolomitic limestones of the plateau of Analamena and Ankara Lower Jurassic – limestones and marly limestones |
Consolidated sedimentary | |
Permian | Continental clayey sandstones and shales | Consolidated sedimentary | |
Cote-Est Basin | |||
Quaternary | Sands, dunes and alluvium | Unconsolidated sedimentary | |
Tertiary | Pliocene shale beds
Neogene – continental sandstones interbedded with clays and basaltic flows |
Unconsolidated to semi-consolidated sedimentary; some volcanic | |
Cretaceous | Upper Cretaceous: volcanic deposits
Maastrichtian – marls and limestones |
Consolidated to semi-consolidated sedimentary; volcanic | |
Southern Basin | |||
Quaternary | Dominantly sand dunes, of three periods – ancient, middle and recent. The sands are probably of aeolian origin, variably cemented by calcareous cement. Some calcareous crusts. Also alluvium in valleys. | Unconsolidated to semi-consolidated sedimentary | |
Tertiary | Neogene – continental clays, argillites, sands, clayey sands and clayey sandstone | Unconsolidated to semi-consolidated sedimentary | |
Highlands | |||
Precambrian | Metamorphic and igneous intrusive (magmatic) rocks
Metamorphic rocks are mainly migmatites, gneisses, leptynites, amphibolites, micaschists, cipolins, quartzites. Igneous intrusive (magmatic) rocks are granites and basalts |
Crystalline basement; igneous intrusive |
Hydrogeology
The hydrogeology map below shows a simplified overview of the type and productivity of the main aquifers at a national scale (see the Hydrogeology map resource page for more details).
More information on the hydrogeology of Madagascar is available in the report United Nations (1988) (see References section, below).
Madagascar is also covered by the SADC hydrogeological map and atlas (2010), available through the SADC Groundwater Information Portal.
Groundwater management
There was information on more than 3,000 water boreholes across the country in 2009. Geological logs for these boreholes are not available. There was no information about large diameter wells.
References
Other references with more information on the geology and hydrogeology of Madagascar may be accessed through the Africa Groundwater Literature Archive.
Online resources
SADC Groundwater Information Portal
General information on surface water and groundwater resources in SADC
Documents
United Nations. 1989. Groundwater in Eastern, Central and Southern Africa: Madagascar. United Nations Department of Technical Cooperation for Development.
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Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Madagascar