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

Abstract

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.

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…This hydrological paradox is strictly Sahelian [2,6,7,9,26,45,46,51]. Paturel et al [52] and Amani and Paturel [53] pointed out the necessary changes to be processed in hydrological regional norms in order to supply engineers with actual figures of rainfall and runoff characteristics, and new methods, able to account for the non-stationarity in hydrological variables.…”

“…However, Roudier et al [42], conversely, considered that land use dynamics do not offset the effects of climate change on runoff in West Africa. At a broader scale, Mahé et al [26] recently proposed a synthesis of the hydrological behaviors observed throughout the whole Atlantic side of West Africa, ranging from the Maghreb to the Congo River mouths.…”

“…It includes basins for which data are available in the literature. Eleven are described by Nka et al [3], four by Opoku-Ankomah et al [25], two by Amoussou et al [27], one by Soro et al [28], two by Amogu et al [67], three by Badou et al [59], one by Genthon (personal communication, 2017), and three by Mahé et al [26] and Mahé and Paturel [9]. Figure 6 gathers the few hydrological series with available data for the period preceding the humid 1950-1967 period.…”

“…The 2nd Sahelian paradox (increase in runoff and increase in vegetation cover) is explained by both rainfall intensification and a reduction in soil WHC, probably linked to extension in cropping areas, as well as vegetation degradation and soil erosion over shallow soils [20,26,38,39,53,76,77,80].…”

ORCHIDEE-ROUTING: A new river routing scheme using a high resolution hydrological database

“…10c), in contrast, the RRS captures well the low flows under 500 m 3 .s occur only 5% of time and more than 50% of times, the flow is close to zero. It is difficult to capture the daily discharge at this station, and 25 the large agricultural water demand over the Chelif basin (Mahe et al, 2013) probably contributes to the errors. But the main error source seems to be the forcing datasets, since the observations fall in their wide uncertainty range.…”