Estimate of Recharge of a Rising Water Table in Semiarid Niger from ³H and ¹⁴C Modeling

Abstract

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.

Simulated impacts of climate change and land‐clearing on runoff from a small Sahelian catchment

“…Using isotopic analysis and hydrodynamic modelling of the aquifer on a 4000 km 2 zone including the Wankama catchment, Favreau et al (2002a) estimated regional groundwater recharge to have increased by one order of magnitude over the past 50 years, from a 1-5 mm year 1 initial range to 20-50 mm year 1 now. Our hydrological simulations do show an amplification of pond recharge over the period ( Figure 5) but by a factor less than two.…”

Comment on “Improving noble gas based paleoclimate reconstruction and groundwater dating using 20 Ne/ 22 Ne ratios,” by F. Peeters et al. (2003) Geochim. Cosmochim. Acta , 67, 587–600 1 1Associate editor: B. Marty

“…For instance, natural leakages occur between the unconfined aquifer and confined aquifers at discrete places east of 3°E (Daddy Gaoh and Dassargues, 1995;Guéro et al, 2003). Recent investigations of the water table revealed that, following the regional land clearing observed for the past 50 yr, groundwater levels have risen by a few metres in most parts of the basin in response to a recharge increase of about one order of magnitude Favreau et al, 2002;Guéro et al, 2003). These spatio-temporal changes often require both hydrodynamic and geochemical data to be identified, and must be taken into account for selecting an adequate data set.…”

La petite irrigation par les eaux souterraines, une solution durable contre la pauvreté et les crises alimentaires au Niger ?

“…1). Le taux de recharge a été estimé de l'ordre de 5 mm par an, à partir des données isotopiques sur la période 1950-1960(Favreau et al, 2002. Toutefois, la dynamique récente de la nappe suggère que la recharge s'est accélérée ces dernières décennies pour avoisiner les 20 mm par an dans les années 1990.…”

“…Les débits spécifiques des ouvrages peuvent atteindre 15 m 3 /h/ m dans la partie centrale du dallol, tandis qu'au-delà, ils ne dépassent que rarement 3 m 3 /h/m. En considérant un taux de recharge variant entre 5 et 25 mm par an (Favreau et al, 2002), Tableau 1. Description des différents types de systèmes irrigués au Niger.…”