Abstract
AbstractHard rock aquifers (HRAs) in West Africa (WA) are located within a thick regolith layer. The representation of thick tropical regolith in integrated hydrological models lacks consensus on aquifer geometries and parameter ranges. Our main objective was to determine the knowledge requirements on saturated hydraulic conductivity (Ks) to model the critical zone (CZ) of HRAs in WA. A parametric sensitivity analysis with a focus on the representation of the Ks heterogeneity of the regolith was conducted with a critical zone model (Parflow‐CLM [Community Land Model]) of the Upper Ouémé catchment in Benin (14,000 km2) at a 1‐ × 1‐km2 resolution. The impact of parameter changes in the near subsurface (0.3‐to‐5‐m depth) and in the deeper regolith aquifer (24‐ and 48‐m maximum depth) was assessed in five modeling experiments. Streamflow was largely dependent on Ks and on clay distribution in the near subsurface and less on the properties of the deeper subsurface. Groundwater table depths and amplitudes were controlled by vegetation and topography as observed on instrumented hillslopes and for Ks within the literature range. Experiments with higher Ks suggested a Ks threshold where dynamics become less determined by one‐dimensional vertical and more determined by lateral processes. Such heterogeneity impacts from smaller scales need to be accounted for when hydrological models are upscaled to larger domains (1‐ × 1‐km2 resolution or coarser). Our findings highlight the need for a new conceptual approach to represent clay distribution in order to develop catchment‐scale CZ models of HRAs in WA that capture the observed processes.
Highlights
A large part (∼40%) of sub-Saharan Africa is underlain by crystalline hard rock (Vouillamoz, Lawson, et al, 2015; Vouillamoz, Tossa, et al, 2015) where 46% of the population lives. (BRGM, 2020; CIESIN, 2018)
Core Ideas ∙ Simulated water balance components for a catchment in West Africa were confirmed by observations. ∙ Subsurface (0.3-to-5-m depth) exerts stronger control on streamflow than deeper regolith. ∙ Ks magnitude determines transition from topography- to recharge-controlled water table dynamics. ∙ We identified a limit of Ks where the impact of one-dimensional processes on water table dynamics ceases. ∙ A high-permeability fissured zone at the bottom shows little impact on the simulations
The simulation 1b with the constant geometric mean Ks of KsH3 = 0.015 m h−1 and assuming an amount of 24% clay of model cells spreading from the river cells in Horizon 2 (H2) performed best in both locations with respect to the Kling–Gupta efficiency (KGE) criterion and qualitative inspection of ET and WT depth (WTD) vs. WT fluctuations (WTA) relationships
Summary
A large part (∼40%) of sub-Saharan Africa is underlain by crystalline hard rock (Vouillamoz, Lawson, et al, 2015; Vouillamoz, Tossa, et al, 2015) where 46% of the population lives. (BRGM, 2020; CIESIN, 2018). A large part (∼40%) of sub-Saharan Africa is underlain by crystalline hard rock (Vouillamoz, Lawson, et al, 2015; Vouillamoz, Tossa, et al, 2015) where 46% of the population lives. The crystalline hard rock is overlain by a thick weathering layer, called regolith (Grimaud et al, 2018; Lachassagne et al, 2011). This geological structure has a large impact on the hydrological cycle and water access in sub-Saharan Africa, especially in areas with sub-humid climate (Cuthbert et al, 2019; Kotchoni et al, 2019; MacDonald et al, 2021). Reaching out a wider and sustainable access to drinking water calls for urgently improving the understanding of the characteristics of regolith aquifers, and their connectivity to soil moisture, vegetation, and surface water (SW)
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