Hydrology in Madagascar: Tracing River Basins on the “Red Island”

BWI’s blog post on hydrology in Madagascar introduces you to some of the most prominent river basins on the world’s fourth-largest island. Although Madagascar has long been considered water-abundant, the variation in wet and dry season precipitation patterns significantly impacts both water availability and quality. BWI focuses on five key river basins, exploring their significance, noteworthy characteristics, and the challenges they face. Lastly, the blog post concludes with a brief section addressing the impact of climate change on these delicate river networks.

Madagascar’s Water Landscape 

Madagascar, located on the eastern coast of Africa, is home to 12 major river basins, each larger than 10,000 km². These are further divided into 533 subbasins across 22 regions. Most runoff flows through the western and eastern slopes. Furthermore, rivers in the highlands, east coast, and north supply water during the rainy season, while lakes and springs serve central highland communities.

The island’s rivers originate in the high plateaus and flow towards the west, south, and east. Despite not being classified as water-stressed nationally, water availability and quality vary geographically and seasonally. Madagascar experiences two main seasons:

• Hot and rainy: November to April
• Cool and dry: May to October

During the dry season, many southern rivers dry up, leading to significant water stress, especially as alternative groundwater sources are limited. In contrast, the wet season can bring high flow rates and rapid changes in river stages and flood levels. Additionally, deforestation in the central highlands caused by slash-and-burn agriculture or firewood collection has led to some of the highest levels of erosion and flooding (Migan, 2016). Moreover, the northern half of the island faces challenges too.  The increased precipitation and steep gullies “lavakas” deteriorate water quality in prospective regions. 

Thus, understanding the water situation at the river basin scale is crucial for effective water resource management. For this reason, BWI selected five representative river basins to foreground the complexities of Madagascar’s river network.

Key statistics pertaining to Madagascar’s freshwater resources include:

• Major economic sectors withdraw only 11% of Madagascar’s total freshwater resources.
• Agriculture accounts for 96% of all freshwater withdrawals, with 30% of cultivable land irrigated.
• Madagascar hosts 5% of the world’s biodiversity, requiring 217,500 million cubic meters of environmental flow for wetlands and rivers.
• Hydropower is a key non-consumptive use, generating 70% of the island’s electricity from just 11% of hydropower plants.
• 80% of domestic water use relies on surface water.
• The island’s average annual precipitation is 1,513 mm.    

(Zy et al., 2022; Winrock International, 2021)

Mangoky River Basin

Hydrology in Madagascar

Location
To begin with, the Mangoky River is the longest in Madagascar and is the primary watercourse in the dry and hot western lowlands. It originates in the central highlands and meanders through some of the country’s most remote and inaccessible regions before reaching the ocean just north of Morombe. The Mangoky Basin covers approximately 55,750 km², with the river itself stretching 564 km from the central highlands to the Mozambique Channel. The region experiences a mean annual rainfall of about 600 mm (Rabezanahary et al., 2021; DBpedia, 2024).

Significance
The Mangoky River Basin is one of Madagascar’s largest, covering around 58,000 km². It is a critical resource for irrigation in southwestern Madagascar, supporting extensive rice paddies and other agricultural activities in its fertile floodplains. These floodplains are among the most productive agricultural areas on the island, making a significant contribution to the local economy. Additionally, the river flow never reaches zero, allowing water to be taken from the current for irrigation all year around. The river also serves as a crucial water source for domestic use. Lastly, it has frequently garnered attention due to its potential for hydropower development (UNICEF, 2023).

Challenges
Deforestation over the past 40 years, largely due to slash-and-burn practices by indigenous people, is one of the main hurdles in the Mangoky River Basin. This has led to severe soil erosion in the Mangoky Basin, evidenced by numerous sandbars in the river. On top of that, climate change further complicates the situation by altering rainfall patterns. Exacerbated flooding and drought conditions impact agriculture and water availability (MangokyReserve, 2024). 

Noteworthy
The Mangoky River flows through some of Madagascar’s densest and most pristine deciduous dry forests. Besides, they represent one of the last natural forest areas on the island. A remarkable isolation characterises this area, with only a few villages along the river. The Mangoky River is vital for communication and trade for these communities (ExplorersWeb, 2023).

Tsiribihina River Basin

Hydrology in Madagascar

Location
The Tsiribihina River Basin is the third largest in Madagascar, covering an area of 49,800 km² (Chaperon et al., 1993). It occupies the central portion of the island’s western side, bordered by the Betsiboka and Manambolo river basins to the north, and the Morondava and Matsiatra river basins to the south. To the east,  the Ankaratra Mountain, which reaches an altitude of 2,643 meters, borders the river. The basin’s terrain gradually descends from the central highlands of Madagascar toward the west coast, with an average elevation of 919 meters. Lastly, the river continues its journey through the calcareous plateau of Bemaraha and eventually reaches the ocean, passing through a low-gradient area with numerous lakes and floodplains (Andriambeloson et al., 2020).

Importance
The Tsiribihina River Basin is economically significant due to its substantial surface water and groundwater resources, which make it highly suitable for agriculture. Furthermore, the cultivable lands in the basin are used for growing food, cash crops, and industrial crops, supported by extensive hydro-agricultural developments. Although the river is no longer regularly monitored, its irrigation potential remains a vital asset for the region (Andriambeloson et al., 2020).

Challenges
The Tsiribihina River Basin is burdened by environmental issues like deforestation, soil erosion, and climate change. These problems contribute to greater sedimentation and an increased risk of flooding, endangering both the region’s ecological health and the livelihoods of those who rely on the river (Midi Madagasikara,  2023).

Noteworthy
A key highlight of the Tsiribihina River is its delta, a constantly evolving area shaped by sediment deposits. This delta supports mangrove forests and rich wetland ecosystems and is recognized as a Ramsar Wetland of International Importance, emphasizing its global ecological value.

Betsiboka River Basin

Hydrology in Madagascar

Location
The Betsiboka River is Madagascar’s largest river. It  stretches 600 km from the central highlands to the northwest coast, where it flows into the Mozambique Channel at Bombetoka Bay. In addition, the Betsiboka River passes through Madagascar’s capital, Antananarivo, and reaches Majunga (Mahajanga), the country’s second-largest port (Tsilavo et al.,2010). 

Significance
The Betsiboka River Basin is crucial for agriculture, covering an area of approximately 49,000 km². It serves as the primary drainage basin for farming activities in Madagascar. The estuary, formed by the confluence of various tributaries like the Ikopa, Menavava, and Kamoro rivers, is vital for the region’s rice fields, shrimp pens, and remaining mangrove forests. Also, Betsiboka is a significant pathway for transporting lateritic soils and sediments eroded from the central highlands. The freshwater from the Betsiboka mixes with the saltwater of the Mozambique Channel, with the river’s strong dynamics ensuring that freshwater dominates during the rainy season (Tsilavo et al.,2010).

Challenges
Over the past three decades, sediment transport and deposition in Bombetoka Bay have increased dramatically, leading to significant environmental and economic challenges. Firstly, the discharge and depth of the river near the estuary have been steadily decreasing over the past 30 years, impacting the basin’s overall productivity. Secondly, the rise in sedimentation, primarily due to large-scale deforestation, bushfires, and overgrazing, have intensified erosion in the river basin. The excessive sediment has adversely affected agriculture, fisheries, and transportation, particularly in the area around one of Madagascar’s largest ports. Additionally, the frequent passage of tropical storms causes prolonged floods, further disrupting the region (Chaperon et al, 1993).

Noteworthy
A striking feature of the Betsiboka River is its blood-red hue, caused by the lateritic sediments it carries from the highlands to the sea. This dramatic colouring is symbolic of the significant environmental impact the river has on the island. In consequence, the river transports vast amounts of soil, altering both the landscape and the ecosystem. The Betsiboka River and Bombetoka Bay are not only economically and biologically important but also geologically significant due to the ongoing processes of erosion and sediment deposition (Raharijaona, and Randrianarison, 1997).

Sofia River Basin 

Hydrology in Madagascar

Location
The Sofia River Basin, one of the major rivers in the northwestern part of Madagascar, begins in the Tsaratanana Massif, specifically in the Andranomadio area within the Bealanana district. It extends approximately 328 km and spans a drainage basin of about 27,315 km². The basin is bordered by the Ramena River in the north and by the Mahajamba River in the southwest.

Significance
Beyond agriculture, the Sofia River Basin plays a critical role in regional connectivity and trade, especially in areas with limited road infrastructure. The river acts as a vital waterway, enabling the transport of goods and people. In doing so, it supports the livelihoods of remote communities. This accessibility enhances local markets and essential services. The basin also holds cultural importance owing to many traditional practices linked to this basin . The many festivals centered around the river reflect its integral role in the daily lives of the local population (Pravarini, 2024).

Challenges
Similar to the preceding ones, even this river basin faces several environmental challenges, such as soil erosion and seasonal flooding. Deforestation, in particular, contributes to increased sedimentation, which can obstruct waterways and reduce the efficiency of irrigation systems critical for agriculture. These issues collectively threaten the river’s health and the sustainability of agricultural activities in the region (Franc et al., 2008).

Noteworthy 
The Sofia River Basin is recognised for its untapped hydropower potential. The river’s consistent flow and the elevation variations in the region present opportunities for small-scale hydropower projects. These initiatives could provide renewable energy to local communities, reducing dependence on non-renewable energy sources and enhancing energy security in the area (WorldBank, 2017).

Onilahy River Basin

Hydrology in Madagascar

Location
The Onilahy River Basin is situated in southwestern Madagascar. The river begins its course in the Andringitra Massif and flows southwest for approximately 525 kilometers, eventually emptying into the Mozambique Channel near the town of Toliara (IRD, 2009; Floreonilahy, 2024).

Significance
The Onilahy River is a crucial water source in one of Madagascar’s driest regions, supporting agriculture, particularly in the lower basin where irrigation is vital for crops like maize and rice. It also sustains local fishing, which is essential for the livelihoods of communities along its banks. Additionally, the basin is home to unique biodiversity, including endemic plant and animal species, highlighting its importance for conservation efforts (IRD, 2009).

Challenges
The Onilahy River Basin faces significant environmental challenges, including deforestation. As a consequence, this leads to soil erosion and increased sedimentation in the river. Irregular rainfall patterns, exacerbated by climate change, further threaten water availability and agricultural productivity. Overfishing and pollution from agricultural runoff also place additional pressure on the river’s ecosystem (IRD, 2009).

Noteworthy Fact
An interesting feature of the Onilahy River is its role in sustaining the spiny forest, a unique ecosystem in southwestern Madagascar. This forest, characterised by drought-resistant plants like baobabs and various spiny succulents, depends on the river’s water to survive in an otherwise arid environment, underscoring the river’s ecological importance beyond human activities (Floreonilahy, 2024).

The impact of Climate Change On Hydrology in Madagascar 

Unfortunately, Madagascar is one of the world’s most vulnerable countries to climate change. Its significant impacts are already being felt across the island. Precipitation patterns are shifting, with overall rainfall expected to decrease even as the intensity of storms and flooding increases. The far north and east coasts receive over 3,500 mm of rain annually, while the drought-prone south receives just 380 mm, exacerbating water shortages. Daily temperatures have risen since the 1950s, particularly in the dry season, and by 2100, temperatures could increase by 2.5 to 3°C. The south and southwest, which have already endured seven droughts since 1981, are experiencing more severe and frequent droughts, devastating agriculture and pushing communities into famine (UNICEF, 2023). Cyclones are also expected to become more intense, having already affected 13 million people between 1990 and 2013 (Zy et al.,2022; Winrock International ,2021)

Overall, climate change is leading to wetter, storm-prone rainy seasons in humid regions and prolonged dry periods in the south.

Conclusion of Hydrology in Madagascar

These river basins are crucial to hydrology in Madagascar, agriculture, and biodiversity. Although varying in size and flow, each river, originating from the island’s diverse topography, plays a key role in sustaining ecosystems, supporting agriculture, and shaping the landscape. The challenges they face, from deforestation to climate change, underscore the delicate balance between Madagascar’s natural environment and human activity. The blog post emphasises the need for sustainable management to preserve this finite resource.

List of References 

A-I
Andriambeloson, J.A., Rakoto, S.V., Ravelonjatovo, A., and Raharinosy, V. (2020) ‘Re-initiating depth-discharge monitoring in small-sized ungauged watersheds by combining remote sensing and hydrological modelling: a case study in Madagascar.’, Hydrological Sciences Journal, 65(16), pp. 2709–2728. Available at:https://www.tandfonline.com/doi/full/10.1080/02626667.2020.1833013 
(Accessed: 13 August 2024).  

Chaperon, P., Danloux, J., and Ferry, L. (1993) ‘Monographie Hydrologiques de l’ORSTOM’, 10: Fleuves et rivières de Madagascar. Paris: ORSTOM Direction de la Météorologie et de l’Hydrologie.

DBpedia (2024) ‘Mangoky River.’ DBpedia. Available at: https://dbpedia.org/page/Mangoky_River (Accessed: 13 August 2024). 

ExplorersWeb (2023) ‘A hard start on Madagascar’s Mangoky River.’, ExplorersWeb. Available at: https://explorersweb.com/a-hard-start-on-madagascars-mangoky-river/ (Accessed: 13 August 2024).

Floreonilahy (2024) ‘Les paysages du bassin de l’Onilahy.’ Floreonilahy. Available at: https://floreonilahy.e-monsite.com/pages/les-paysages-du-bassin-de-l-onilahy.html (Accessed: 13 August 2024).

Franc, A., Soti, V., Tran, A., Duvallet, G., and Duranton, J.F. (2008) ‘Deforestation, new migration pathways and outbreaks of the red locust Nomadacris septemfasciata (Orthoptera: Acrididae) in the Sofia river basin (Madagascar)’ In: SAGEO 2008: Conférence SAGEO, 24-27 June 2008, Montpellier, France. s.l.: s.n., pp. 1–17. Available at: https://publications.cirad.fr/une_notice.php?dk=547186  (Accessed: 13 August 2024).

HolidaysPlease (2024). ‘Rafting the Mangoky River.’ Available at: https://www.holidaysplease.co.uk/attractions/madagascar/rafting-the-mangoky-river/ (Accessed: 13 August 2024).

IRD (2009) Etude de la dynamique de l’érosion dans le bassin versant de l’Onilahy, Madagascar. Available at: https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers09-11/010020465.pdf (Accessed: 13 August 2024).

M-Z
Madagascar Circuits Tours (2024) ‘Nous contacter.’ Madagascar Circuits Tours. Available at: https://www.madagascar-circuits-tours.com/nous-contacter/ (Accessed: 13 August 2024).

MangokyReserve (2024) ‘Mangoky Reserve.’ Mangoky Reserve. Available at: https://mangokyreserve.com/mangoky/(Accessed: 13 August 2024).

Midi Madagasikara (2023) ‘Menabe Antimena: Les défis de la sécurisation du noyau dur.’, Midi Madagasikara. Available at: https://midi-madagasikara.mg/menabe-antimena-les-defis-de-la-securisation-du-noyau-dur/ (Accessed: 13 August 2024).

Migan, S.A. (2016) ‘The impact of forest conversion on hydrology’, Technical Documents in Hydrology, no. 36. Available at: https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers16-07/010045015.pdf (Accessed: 13 August 2024).

Pravarini (2024) ‘Sofia.’ Pravarini. Available at: http://pravarini.free.fr/Sofia.htm (Accessed: 13 August 2024).

Rabezanahary Tanteliniaina, M.F., Rahaman, M.H., and Zhai, J. (2021) ‘Assessment of the future impact of climate change on the hydrology of the Mangoky River, Madagascar using ANN and SWAT.’, Water, 13(9), p. 1239. Available at: https://doi.org/10.3390/w13091239 (Accessed: 13 August 2024).

Raharijaona, R.L. and Randrianarison, J. (1997) ‘Facteurs géologiques et climatiques influençant l’érosion en lavaka et ensablement des rizières: le cas du massif d’Ambohitrandriampotsy du sud du lac Alaotra.’ In: Hurni, H. and Ramamojisoa, J. (eds.) IVè Conférence internationale de l’Association des Montagnes Africaines, 26 May-1 June 1997, Antananarivo. pp. 159–172.

Tsilavo Raharimahefa, T. and Kusky, T.M. (2010) ‘Environmental monitoring of Bombetoka Bay and the Betsiboka Estuary, Madagascar, using multi-temporal satellite data.’, Journal of Earth Science, 21(2), pp. 210–226. Available at: http://en.earth-science.net/en/article/doi/10.1007/s12583-010-0019-y (Accessed: 13 August 2024).

UNICEF (2023) ‘Improving safe water access in Madagascar.’, UNICEF Innovation. Available at: https://www.unicef.org/innovation/stories/improving-safe-water-access-madagascar (Accessed: 13 August 2024).

Winrock International (2021) Madagascar country profile. Available at: https://winrock.org/wp-content/uploads/2021/08/Madagascar_Country_Profile-Final.pdf (Accessed: 13 August 2024).

World Bank (2017) Hydropower atlas: Madagascar. Available at: https://documents1.worldbank.org/curated/en/270231504691891059/pdf/119399-V3-ESMAP-FRENCH-P145350-HYDROPOWER-ATLAS-PUBLIC-madagascar.pdf (Accessed: 13 August 2024). 

Zy Harifidy, R., Zy Misa Harivelo, R., Hiroshi, I., Jun, M., and Kazuyoshi, S. (2022) ‘A systematic review of water resources assessment at a large river basin scale: Case of the major river basins in Madagascar.’, Sustainability, 14, p. 12237. Available at: https://www.researchgate.net/publication/364050608_A_Systematic_Review_of_Water_Resources_Assessment_at_a_Large_River_Basin_Scale_Case_of_the_Major_River_Basins_in_Madagascar (Accessed: 13 August 2024).