Land clearance and hydrological change in the Sahel: SW Niger

River basin rainfall series and extensive river flow records are used to characterize and improve understanding of spatial and temporal variability in sub-Saharan African water resources during the last century. Nine major international river basins were chosen for examination primarily for their extensive, good quality flow records. A range of statistical descriptors highlight the substantial variability in rainfall and river flows [e.g., differences in rainfall (flows) of up to −14% (−51%) between 1931–60 and 1961–90 in West Africa], the marked regional differences, and the modest intraregional differences. On decadal time scales, sub-Saharan Africa exhibits drying across the Sahel after the early 1970s, relative stability punctuated by extreme wet years in East Africa, and periodic behavior underlying high interannual variability in southern Africa. Central Africa shows very modest decadal variability, with some similarities to the Sahel in the adjoining basins. No consistent signals in rainfa...

https://journals.ametsoc.org/doi/pdf/10.1175/2008JHM1004.1

Rainfall and Water Resources Variability in Sub-Saharan Africa during the Twentieth Century

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers16-06/010042157.pdf

The impact of land use change on soil water holding capacity and river flow modelling in the Nakambe River, Burkina-Faso

In this paper, we study the impact of climate change on river regimes in several parts of Africa, and we look at the most probable causes of these changes either climatically or anthropogenically driven. We study time series of updated monthly and annual runoff of rivers of North Africa, West Africa (Sahelian and humid tropical regions) and Central Africa, including the largest river basins: Niger and Volta rivers in West Africa, and Congo and Ogooue rivers in Central Africa. The recent years are studied in the perspective of multi-decadal variability. In West Africa and in a part of Central Africa, the climate has changed since 1970, and rainfall has not returned to previous annual amounts, except in Equatorial Africa. The consequences of the long-lasting drought are, depending on the area concerned, the modification of seasonal regimes (Equatorial area), the groundwater table decrease (Tropical humid area) and the land cover degradation (Sahelian area). The increasing number of dams and of agricultural areas also plays a major role on the modification of river regimes. The population increase will continue to impact on the environment: land cover change, deforestation, agriculture and increasing number of dams will be associated with a reduction of water and sediment discharges to the sea, and major impacts on downstream ecosystems and coastal areas. It seems necessary to share with stakeholders a comprehensive approach of the water cycle from the basin to the sea, to prevent long-lasting damages to ecosystems and infrastructures. Copyright © 2013 John Wiley & Sons, Ltd.

The rivers of Africa : witness of climate change and human impact on the environment

Despite the drought observed since 1968 in most of the West African Sahel, runoff and rivers discharges have been increasing in the same region. This trend is related with land use change rather than climate change. This paper aims to describe the regional extension of such a phenomenon and to demonstrate that the increase in runoff is observed from the point scale up to the regional scale. It highlights the opposition of functioning between a Sahelian zone, where the Sahel's paradox applies, and the Sudanian and Guinean areas, where runoff has been logically decreasing with the rainfall. The current OPEN ACCESS Water 2010, 2 171 trend is evidenced using experimental runoff plots and discharge data from the local to the regional scales.

https://www.mdpi.com/2073-4441/2/2/170/pdf

Increasing River Flows in the Sahel

Dans le contexte pluviometrique deficitaire de l'Afrique intertropicale, l'hydraulicite des fleuves soudano-saheliens a beaucoup diminue depuis 25 ans. Les fleuves des regions humides, relativement epargnes jusqu'en 1980, montrent aussi, avec un retard de plusieurs annees une hydraulicite qui baisse dans des proportions importantes, alors qu'un retour a la normale du regime des pluies parait amorce. Le deficit des apports est pour l'Afrique humide de 16% pour la decennie 80 (365 km3 an-1), contre 7% pour la decennie 70, et pour l'Afrique seche de 27% (65 km3 an-1) pour la decennie 80 contre 13% pour la decennie 70. Les maximums annuels de crue sont systematiquement plus faibles dans la periode recente. En regime equatorial, la crue de printemps est la plus affectee par le deficit hydrologique et la difference entre crue de printemps et crue d'automne a triple entre les annees 1950-1960 et les annees 1980. L'ensemble de la region connait des etiages exceptionnels repetes. En zone soudano-sahelienne, le tarissement s'est considerablement accelere et montre une vidange des ecoulements de base deux fois plus rapide dans la periode recente. Le phenomene est egalement sensible en Afrique humide et traduit un amenuisement des reserves souterraines du aux effets cumules des deficits pluviometriques et decale dans le temps, que l'on retrouve dans la baisse du niveau piezometrique des nappes phreatiques. Un effet memoire est donc observe sur les ecoulements de base, avec pour consequence un un appauvrissement durable des ressources hydriques. Celui-ci restera independant, pour un temps, d'une eventuelle amelioration des conditions climatiques, comme le montrent les observations des annees les plus recentes.

/pdf/evolution-recente-des-ressources-en-eau-de-l-afrique-56tmr63l7b.pdf

Évolution récente des ressources en eau de l'Afrique atlantique

The annual average of the groundwater levels in 27 wells is connected to the annual climate impact on the groundwater resources in the Bani river basin (Douna, 101 600 square kilometers). The groundwater level variations are well correlated to rainfall variations, and for the lowest levels of groundwater, the runoff stops in the dry season at Douna. The humid year of 1994 caused a rise in the groundwater level to the same level as 1981, together with a reduction of the depletion coefficient. But the runoff is still weak, showing that the rainfall–runoff relationships are modified over the long term.

Relations eaux de surface–eaux souterraines d'une rivière tropicale au Mali

Surface water and groundwater relationships in a tropical river of Mali.

The inner delta of the River Niger receives runoff from both the River Niger and the River Bani (249 000 km2) and is divided into a northern part (15 000 km2) and a southern part (58 000 km2). The average input and output discharges are approximately 1490 and 900 m3 s−1 (1955/1996), respectively. The annual average water loss is approximately 40% (24-48%). The losses are greater in the northern (10·5 km3) than in the southern delta (8·2 km3), but this situation was reversed in 1984. The correlation between the output and input discharges is high for the northern part of the delta, but very low for the southern part, where the change in flooded area is likely to be limited. The average flooded area, calculated using a method based on the hydrological balance, is estimated to be about 10 000 km2 (north) and 14 000 km2 (south), which is consistent with the estimates provided by the previous authors. Copyright © 2009 John Wiley & Sons, Ltd.

Water losses in the inner delta of the River Niger: water balance and flooded area

Resume Depuis une vingtaine d'annees, le bassin du fleuve Niger et ceux des principaux cours d'eau africains sont soumis a un deficit pluvio­ metrique important. La chronique des debits enregistres a Koulikoro (120000 krrr') montre une decroissance persistante des ecoulements. Cependant la baisse des pluies n'est que de 20% en valeur moyenne decennale entre 195111960et 198111989, alors que celle des ecoulements est de 55 %. Si les conditions du ruissellement superficiel ont ete peu modifiees durant la secheresse, l'ecoulement souterrain a lui fortement diminue comme l'indique l'augmentation du coefficient de tarissement. La reduction des modules annuels et des maximums de crue peut etre logiquement reliee a la diminution des apports souterrains. Cette hypothese est confirmee en 1994: des pluies de 15% superieures a la moyenne 1951-1989 (frequence vicennale) ne produisent qu'une crue moyenne tant en pointe qu'en module. Par rapport a la moyenne de cinq annees de pluies equivalentes sur le bassin versant, la lame ecoulee est plus faible de 28% en 1994/1995, tandis que le coefficient de tarissement reste eleve, et superieur a la normale. Recent climatic changes and modification of the hydrological regime of the Niger river at Koulikoro (Mali)

/pdf/changements-climatiques-recents-et-modification-du-regime-4xmo7nrldy.pdf

Changements climatiques récents et modification du régime hydrologique du fleuve Niger à Koulikoro (Mali)

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