Long-term rise in a Sahelian water-table: the Continental Terminal in South-West Niger

The Continental Terminal water-table near Niamey (S-W Niger) has been the subject of a dense and detailed survey conducted over a period of almost 15 yr. The continuous rise in the groundwater level was unexpected but manifest and varied between 0.01 and 0.45 m yr−1. As shown by corroborating measurements made throughout the twentieth century, this rise has been taking place for much longer and present levels are the highest ever recorded. Since the beginning of the 1960s, groundwater resources have increased by up to 150% (+15% on median), in spite of the severe droughts of the 1970s and 1980s. Based on isotopic data, infiltration is estimated at around 5 mm yr−1 for median over the long-term. Hydrodynamic observations show that in recent years it has exceeded 20 mm yr−1. The rise acceleration during the past decade is apparent from groundwater level chronicles. Even if fluctuations in rainfall may interfere, this phenomenon is mainly explained by a change in land-use. In this semi-arid area, intense land clearing has modified the hydraulic properties of the top cm of the soil and has consequently increased surface runoff. As runoff concentrates in temporary endoreic ponds and then infiltrates to the water-table, higher runoff implies higher groundwater recharge and a subsequent rise in the water-table. This is one of the best documented examples of a long-term rise of an African water-table in such a semi-arid context.