Water transit time and active recharge in the Sahel inferred by bomb-produced 36Cl
Résumé
the lack of data and suitable methods to quantify regional hydrological processes often hinders sustainable water management and adaptation to climate change in semiarid regions, particularly in the Sahel, which is known for its climatic variability. Here we show that 36 Cl from nuclear tests is a promising method to estimate water transit times and groundwater recharge rates on the catchment scale, and to distinguish water and chloride cycles. 36 Cl was measured in 131 surface and groundwater samples in the Chari-Logone sub-catchment of the emblematic Lake Chad Basin, located in central Sahel. It was found that only 12 ± 8% of the catchment is connected to the main rivers. Groundwater supporting rivers in the upper humid part of the catchment has a mean transit time of 9.5 ± 1 years and a recharge rate of 240 ± 170 mm yr −1. In the lower Sahelian part of the catchment, stream-focused recharge yields recharge rates up to 78 ± 7 mm yr −1 in riparian groundwater against 16 ± 27 mm yr −1 elsewhere. Our estimates suggest that aquifers in the Sahel host a significant amount of renewable water, which could therefore be used as a strategic freshwater resource. Freshwater resources in semi-arid regions are facing a number of stress factors, such as rapid population growth with the associated economic and agricultural developments, and climate change 1. Concerns have been raised that growing pressure on freshwater resources might result in conflicts at sub-national to international levels. Sustainable water management relies on a sound understanding of fundamental hydrological catchment characteristics such as hydrologically active areas, catchment scale water transit times 2-4 or groundwater recharge rates 5,6. However, the assessment of these key parameters of the hydrological cycle remains difficult in semi-arid regions for two main reasons. First concepts are mainly adapted to temperate climates and thus can only be partly applied to these areas 7 , second hydrological and climatological data in many semi-arid and arid regions are scarce, particularly in sub-Saharan Africa 8. The Sahel, a latitudinal belt stretching across the southern edge of Sahara, has been subject to humanitarian crises and social instability, exacerbated by the recurrence of persistent severe droughts since the 1980s 9. Since then, standing debates have arisen on both the driving forces of climate variability in the Sahel 10 and the complex non-linear hydrological responses to climate variability 11. Despite international research programs 12 , Sahel's catchment hydrology remains too poorly understood to support sustainable water governance. In particular, data on aquifer recharge rates and surface water-groundwater connectivity are lacking. Located in the centre of the Sahel, the Lake Chad Basin (LCB) is an endorheic catchment of 2.5 million km 2. As Lake Chad integrates climatic and hydrologic changes over the region, it is a relevant scientific study site of Sahelian hydrology. The LCB is a hotspot of water-related issues in the Sahel, as the escalating conflicts over natural resources in Lake Chad point to the urgent need for a fair and sustainable management of natural resources. Dissolved gas tracers (such as CFCs, 3 H/ 3 He, SF 6 , 85 Kr) provide temporal constraints on the water transit time of surface and subsurface flows within a catchment 13. However, contamination by atmospheric or soil gases 14 often limits their applicability as age tracers to open-water systems. Bomb-produced tritium was widely applied as a young age tracer in the 1960s 6 , but the combined effect of removal by rain and radioactive decay (t 1/2 3 H = 12.3 years) leads to an ambiguous age determination for recharge periods between 1975 and 2010 15,16. 36 Cl appears to be a relevant alternative tracer 17 : chloride is a ubiquitous ion, the production of 36 Cl associated with nuclear
Domaines
Hydrologie
Origine : Publication financée par une institution
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