The modifications in the hydrological regime have been the result of a combination of climate variability and climate change, increasing water demand, land degradation, and land use changes.
The Niger River has experienced a significant decrease in its volumetric flow rate throughout its entire course. In the Upper Niger, including the Benue Basin, precipitation has increased again, reaching approximately 5% of pre-1970 levels, enough to end the period of below-average precipitation that persisted in the 1970s and 1980s. However, annual precipitation is still below pre-1970 values in the Upper Niger Basin. This regional disparity is significant because the Upper Niger sub-basin serves as the water tower of the Niger River. The changes observed in the last century can be summarized as follows:
- Upper Niger: The average interannual volumetric flow rate at Koulikoro, which was 1,350 m3/s (1929-1970) or 42 billion m3/year, decreased to 1,039 m3/s (1971-2002), representing a 23% decrease.
- Inland Delta: Significant water losses, mainly through evaporation and infiltration, occur in the inland delta ecosystems, totaling approximately 20 billion m3 in a wet year, 13 billion m3 in an average year, and 11 billion m3 in a dry year. These losses are far from being compensated by local contributions, which are extremely modest (direct precipitation and some minor tributaries). In addition to natural losses, an additional 3.2 billion m3 is used for irrigation (mainly by the Niger Basin Authority). This can represent up to 12% of the volumetric flow rate in a dry year.
- The mean flow rate at Niamey between 1971 and 2002 was only 704 m3/s, compared to 1,062 m3/s between 1929 and 1970, resulting in a 34% decrease. This has been accompanied by significant losses in terms of biodiversity in the middle Niger wetlands up to the coastal delta. During the dry season, flow rates can drop very low, almost to a complete interruption.
- Lower Niger: Similar to the Upper Niger and Middle Niger, the Lower Niger has experienced a significant decrease in flow. The average flow rate between 1929 and 1970 was 6,055 m3/s, compared to 5,066 m3/s between 1971 and 2001. Problems with surface water availability primarily occur in the Upper Benue, where significant water shortages are recorded.
These changes in the hydrological regime of the Niger River have implications for the ecosystem, biodiversity, water supply, and agriculture in the region. It is essential to take measures to sustainably manage water resources, prevent land degradation, and adapt land use practices to mitigate these impacts.
