title: Constraining groundwater modeling with magnetic resonance soundings.
abstract: Magnetic resonance sounding (MRS) is a noninvasive geophysical method that allows estimating the free water content and transmissivity of aquifers. In this article, the ability of MRS to improve the reliability of a numerical groundwater model is assessed. Thirty-five sites were investigated by MRS over a ∼5000 km(2)  domain of the sedimentary Continental Terminal aquifer in SW Niger. Time domain electromagnetic soundings were jointly carried out to estimate the aquifer thickness. A groundwater model was previously built for this section of the aquifer and forced by the outputs from a distributed surface hydrology model, to simulate the observed long-term (1992 to 2003) rise in the water table. Uncertainty analysis had shown that independent estimates of the free water content and transmissivity values of the aquifer would facilitate cross-evaluation of the surface-water and groundwater models. MRS results indicate ranges for permeability (K = 1 × 10(-5)  to 3 × 10(-4)  m/s) and for free water content (w = 5% to 23% m(3) /m(3) ) narrowed by two orders of magnitude (K) and by ∼50% (w), respectively, compared to the ranges of permeability and specific yield values previously considered. These shorter parameter ranges result in a reduction in the model's equifinality (whereby multiple combinations of model's parameters are able to represent the same observed piezometric levels), allowing a better constrained estimate to be derived for net aquifer recharge (∼22 mm/year).
authors.affiliation: IRD, Hydrosciences Montpellier, BP 11416, Niamey, Niger. Marie.boucher@ird.fr
authors.forename: Marie
authors.initials: M
authors.lastname: Boucher
authors.forename: Guillaume
authors.initials: G
authors.lastname: Favreau
authors.forename: Yahaya
authors.initials: Y
authors.lastname: Nazoumou
authors.forename: Bernard
authors.initials: B
authors.lastname: Cappelaere
authors.forename: Sylvain
authors.initials: S
authors.lastname: Massuel
authors.forename: Anatoly
authors.initials: A
authors.lastname: Legchenko
mesh_terms: D004784:Environmental Monitoring / Q000379:methods*; D060587:Groundwater / Q000032:analysis*; D009682:Magnetic Resonance Spectroscopy; D008962:Models, Theoretical; D014872:Water Movements