Groundwater-atmosphere Interactions as Environmental Drivers of Water Cycle, Energy, and Greenhouse Gas Fluxes in West Africa

Abstract

Reducing discrepancies in the simulation of water and energy fluxes remains a key challenge in accurately representing surface water flux processes, particularly in regions with limited observational data. This study evaluates the sensitivity of the WRF-Hydro model to three parameterization schemes - Free Drainage (FD), TOPMODEL, and MMF - over West Africa. The results show that MMF outperforms the other schemes in representing surface water flux variables, especially in topographic convergence zones, where soil moisture and evapotranspiration in riverbeds increase by 20% with respect to FD. At the catchment scale, soil moisture, evapotranspiration, and groundwater storage are well simulated, with correlation coefficients reaching 0.9. In addition, model calibration for the Donga River gives reliable performance with KGE value up to 0.74. The shrubland, bare soil, and grassland (SBG) in MODIS-IGBP land cover is substituted by the Evergreen Broadleaf Forest (EBF), Savanna (SAV), and Woody Savanna (WS) to mimic the the Great Green Wall (GGW) initiative. At basin scale, the seasonal cycle and inter-annual variability are well captured as there is a strong linear relationship between the observed and simulated values with correlation coefficients from 0.9 to 0.97. The KGE values reaches respectively 0.72, 0.71, and 0.72 in Oueme, Sissili, and Faga catchments. Compared to the current land use (REF) scenario, EBF-VC and WS-VC experiments decrease the mean soil moisture (SM) by 0.2 and 0.1 mm, while the SAV-VC increases it by 0.8 mm in drier conditions in Faga. However, the scenarios reveal a decrease of mean SM by 0.5, 0.6, and 0.1 mm for EBF-VC, SAV-VC, and WS-VC in higher precipitation areas (Oueme). Remarkably, the average ET is increased whatever the climatic condition except for SAV-VC in Sissili where a negative effect is recorded. For instance, EBF-VC, SAV-VC, and WS-VC increases the average ET by 0.25, 0.08, 0.07 mm d-1 in Faga. EBF-VC, SAV-VC and WS-VC experiments reduce streamflow respectively by 24%, 18%, and 21% in Donga and 31%, 26%, and 28% in Oueme. The change in the surface fluxes (e.g., LH, SH, GH, RN, ET) and subsurface dynamics (e.g., water table depth) in response to the variation of the lineaments permeability (K) is evaluated with three experiments namely High, Moderate, and Low K (see section 3.4.2.5). Remarkably, the most significant change in the diurnal cycle of the energy fluxes occurred around noon. Compared to the reference simulation (without lineament), High and Moderate K experiments decrease the outgoing longwave (LW) by -2% and -1% in the dry season. Higher permeability in the fractures results in a decrease of the outgoing longwave radiation. An increase of 1.1 and 1.5 W m-2 of sensible heat is associated with Moderate K and Low K experiments from March to May. The energy balance closure increases significantly by 36.9 and 25.4% for Moderate K and High K experiments from September to November (SON). The average groundwater storage (GWS) of the basin increases with High K and Moderate K experiments by 355.8 and 326.8 million m3. The climate projections of five Global Circulation Models (GCMs) namely GFDL-ESM4, HadGEM3-GC31-LL, IPSL-CM6A-LR, MIROC6, and NorESM2-MM under two different Shared Socioeconomic Pathways (SSP1-2.6, SSP5-8.5) are used to assess the subsurface dynamics’ sensitivity to extreme warming scenarios. Under SSP1-2.6, 3 out the 5 GCMs show an increase of groundwater storage (GWS) by 0.45 to 30.39 million m3 in Donga basin. All the GCMs indicate a decrease of mean surface water storage (SWS) under SSP5-8.5 projection except GFDL-ESM4. According to NorESM2-MM, IPSL-CM6A-LR, MIROC6, and HadGEM3-GC31-LL projections, a decrease of surface water storage (SWS) will occur whatever the warming level by the end of the century. Changes in land use and land management significantly affect the global emissions budget, influencing the climate through biogeochemical processes. This study provides the assessment of soil greenhouse gas GHG emissions in the Sudanian savanna region of West Africa using a chamber-based experimental setup. Our results reveal significant variation in methane (CH₄) fluxes across the sites. However, nitrous oxide (N₂O) fluxes did not vary significantly, likely due to uniformly low nitrogen input across all systems. The highest seasonal CH₄ emissions were recorded in the rainfed rice field (0.69 ± 0.17 and 0.82 ± 0.22 kg C ha-1 season-1, on average), while the forest reserve acted as a net CH₄ sink (−0.019 ± 0.20 and −0.42 ± 0.13 kg C ha-1 season- 1). In contrast, soils across all sites, both managed and natural, were sources of N₂O, with fluxes ranging from 0.01 kg N ha⁻¹ season⁻¹ in the forest reserve to 0.16 kg N ha⁻¹ season⁻¹ in the rice field. This study also analyzed the environmental drivers of GHG fluxes and found that CH₄ variability was significantly influenced by soil water content and soil temperature (partial R² between 0.21 and 0.42). No significant relationship was observed between these variables and N₂O emissions. These results highlight that changes in land cover and land management in the Sudanian can substantially increase CH₄ emissions, while their impact on N₂O fluxes is marginal.