The Sahelian Paradox in the Niger River Basin

While North Africa faces the depletion of its non-renewable groundwater resources, the Sahel, on the contrary, struggles with too much renewable groundwater.

The paradox in Niamey, Niger

Niger, like many semi-arid countries of the Sahel, experiences constant variations in precipitation. From the 1970s to the late 20th century, rainfall significantly reduced, and droughts became longer. At the same time, quite paradoxically, a constant increase in groundwater level was recorded (Leblanc et al., 2008).

This phenomenon called ‘the Sahelian paradox’ is typical for southwestern Niger, where regular droughts and rapid population growth have driven up the demand for agricultural production (Favreau et al., 2009). Extensive areas of land were cleared in the 1950s, which has led to extensive and long-term losses of vegetation cover in the savannah. Cultivated land reduced the amount of evapotranspiration, and in turn, increased Hortonian runoff. As a consequence, the groundwater table climbed by as much as 40 meters by 2021 since 1961, a change greater than that induced by rainfall deficits.

In Niamey, the water table has been constantly rising (top) (Floodlist News, 2020),

despite reduced rainfall (bottom) (Leblanc et al., 2008)

The rise in the water table has brought numerous challenges. In Niamey, the capital of Niger, the increase has contributed to more frequent flooding events and is exacerbating existing sanitary contamination issues (Halidou et al., 2021). Water recharged from agricultural fields into aquifers was nitrate-rich, and ponds were formed around valley bottoms. Exposed to the air, these ponds have unusually high levels of salinity, affecting soil and groundwater quality, making irrigation and crop growth unsustainable in their proximity.

Office du Niger, a past lesson

Mali - a region I previously examined - has experienced similar challenges much earlier. However, the consequences there are more pronounced and less controlled. The rising water table was less caused by clearing, but rather by a large-scale transition from rain-fed, rotational crops to irrigated rice monoculture (Bertrand, Keita and N’Diaye, 1993). Being one of the oldest irrigation systems in the Sahel, the Niger Office’s drainage system is very inefficient. In some cases, the water table has risen so high that the soil can no longer provide sufficient oxygen for plants. Moreover, after decade-long irrigation with sodium-rich water, salinisation and alkalinisation of the soil are in parts so severe that degradation has become irreversible (Bationo et al., 2007). The soil has lost much of its organic matter and is no longer able to support crop growth.

Highly salinised soil at the Office du Niger, Mali (Bertrand, 1993)

While the devastation in Mali may be alarming, in the case of Niamey, there is yet hope. If managed properly, the abundant groundwater could largely benefit the agricultural sector (Favreau et al., 2009). Downstream catchments have been demonstrated to be productive zones for cultivation. Compared to North Africa, pumping renewable blue water from the ground for irrigation would be favourable, as it can help stabilise the water table while ensuring crop yield under variable rainfall patterns and prolonged droughts brought by climate change.