Abstract
Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology inAfrica) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (θMRS) known to be higher than the specific yield. Hence, we scaled the 2-D model of θMRS with a single factor (α). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of α is obtained with aMonte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to α = Sy/θMRS = 0.63 ± 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.
Highlights
Water Storage Changes (WSC) in unsaturated soils and aquifers are a key variable for water resource management, yet still challenging to estimate (Scanlon et al 2002; Healy & Scanlon 2010; Dubus & Dubus 2011)
Absolute gravity data from FG5 monitoring have been compared to modelled gravity variations derived from WSC measurements by neutron probe (NP) and water-table level
Gravity data have been corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology and residuals are associated to local WSC and show seasonal variations of up to 11 μGal
Summary
Water Storage Changes (WSC) in unsaturated soils and aquifers are a key variable for water resource management, yet still challenging to estimate (Scanlon et al 2002; Healy & Scanlon 2010; Dubus & Dubus 2011) This is true in the weathered hard-rock basement of the Sudanian zone in West–Africa, where the total storage volume is low, but shows strong annual variations (MacDonald et al 2012). There, urban development relies on the ability to provide enough fresh water along the year through high yield boreholes These are still difficult to implement and the knowledge of local recharge is important for their sustainability.
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