West Africa experiences great climate variability, as shown by the long-lasting drought since the 1970s. The impacts of the drought on surface water resources are well documented but remain less studied regarding groundwater resources. The nexus between climate variability and groundwater level fluctuations is poorly documented in this area. The present study focuses on the large reserve of groundwater held by the Kou catchment, a tributary of Mouhoun river (formerly the Black Volta) in the southwest of Burkina Faso, in the Sudanian region. Analyses were undertaken using climatic time series (1961–2014), two rivers’ hydrometric data (1961–2014), and 21 piezometers’ time series (1995–2014) applying statistical trend (Mann–Kendall) and break (Pettitt) tests, correlation analysis, and principal component analysis. The analyses showed that rainfall in the area underwent a significant break in 1970 with an 11%–16% deficit between the period before the break and the period after the break that resulted in a deficit three times greater for both surface and base flows. This significant deficit in flow results from the combined effect of a decrease in rainfall and an increase in evapotranspiration. The response of the catchment to the slight increase in rainfall after 1990 was highly dependent on hydrological processes. At Samendéni, on the Mouhoun River, the flow increased with a slight delay as compared to rainfall, because of the slow response of the base flow. Whereas at Nasso on the Kou river, the flow steadily decreased. The analysis showed that the groundwater level responds to rainfall with a delay. Its response time to seasonal fluctuations ranges from 1 to 4 months and its response time to interannual variations exceeds the timescale of one year. This response is highly dependent on the local aquifer’s physical characteristics, which could explain the spatial heterogeneity of the groundwater response.

The Sahel Transboundary Taoudéni Basin, covering about 20% of Burkina Faso, hosts vital aquifers critical for water security and development. Effective groundwater monitoring is essential for sustainable resource management. In the Kou sub-basin, groundwater quality assessment is increasingly important. This study integrates hydrochemistry, water stable isotopes (δ¹⁸O, δ²H), GIS, and multivariate statistics to understand subsurface geochemical processes. A total of 48 samples—43 groundwater and 5 surface water—were analyzed for 19 hydrochemical parameters and isotopes. In surface water, δ¹⁸O ranged from −5.96‰ to −5.09‰, and δ²H from −37.65‰ to −29.15‰. In groundwater, δ¹⁸O ranged from −5.93‰ to −4.39‰, and δ²H from −34.62‰ to −25.05‰. The spatial distribution of δ¹⁸O and δ²H was mapped using inverse distance weighted (IDW) interpolation in ArcGIS 10.8. A δ²H vs. δ¹⁸O plot showed groundwater values clustered near the Global Meteoric Water Line, indicating minimal evaporation during recharge. Groundwater chemistry was dominated by Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and HCO₃⁻ > NO₃⁻ > Cl⁻ > SO₄²⁻. Key hydrogeochemical processes include water–rock interaction (leaching, weathering, ion exchange) and anthropogenic pollution. Isotopic signatures reveal heterogeneous recharge sources and aquifer connectivity. These findings enhance the understanding of water sources and geochemical processes in the Kou basin, supporting informed groundwater resource management.