Fowé et al. (2026) Assessing the Future of Droughts Using Relative Standardized Indices: Insights from the Nakanbé River Basin, West Africa

Identification

• Journal: Earth Systems and Environment
• Year: 2026
• Date: 2026-02-17
• Authors: Tazen Fowé, Roland Yonaba, Lawani Adjadi Mounirou, Elias Nkiaka, Harouna Karambiri
• DOI: 10.1007/s41748-026-01057-y

Research Groups

• Laboratoire Eaux, Hydro-Systèmes et Agriculture (LEHSA), Institut International d’Ingénierie de l’Eau et de l’Environnement (2iE), Ouagadougou, Burkina Faso
• Department of Geography, Catchments and Coasts Research Group, University of Lincoln, Lincoln, UK

Short Summary

This study assesses future meteorological and hydrological droughts in the Nakanbé River Basin, Burkina Faso, using relative standardized drought indices and CMIP6 projections. It reveals robust warming and increased evaporative demand will lead to longer, more severe, and more frequent droughts by 2100, particularly under high-emission scenarios, despite uncertain rainfall changes.

Objective

• To assess historical drought conditions in the Nakanbé River Basin (NRB) using relative standardized indices.
• To evaluate projected changes in drought characteristics under multiple climate models and SSP2-4.5 and SSP5-8.5 scenarios.
• To quantify future shifts in drought duration, severity, and intensity, providing actionable insights for water resource management in the basin.

Study Configuration

• Spatial Scale: Nakanbé River Basin upstream of Wayen (NRUW), Burkina Faso, covering approximately 20,800 square kilometers.
• Temporal Scale:
  • Observations: 1979–2020 (monthly).
  • Historical Baseline: 1981–2014 (for index calibration), 1990–2014 (for drought characteristics comparison).
  • Future Projections: 2015–2100, analyzed for mid-term (2036–2065) and long-term (2071–2100) horizons.

Methodology and Data

• Models used:
  • GR2M (Génie Rural à 2 paramètres Mensuel) lumped conceptual rainfall-runoff model.
  • Hargreaves-Samani (H-S) method for Potential Evapotranspiration (PET) estimation (calibrated monthly).
  • MBCn algorithm for multivariate bias correction.
  • Relative Standardized Precipitation Index (rSPI), relative-Standardized Precipitation Evapotranspiration Index (rSPEI), and relative-Standardized Streamflow Index (rSSI) at a 12-month timescale.
• Data sources:
  • Hydro-climatic observations (monthly rainfall, minimum/maximum temperature, potential evapotranspiration, discharge) from two synoptic meteorological stations (Ouagadougou, Ouahigouya) and Wayen hydrometric station (ANAM-BF and DGRE, Burkina Faso) for 1979–2020.
  • NASA Earth Exchange Global Daily Downscaled Projections, CMIP6 version (NEX-GDDP-CMIP6) from 27 Global Climate Models (GCMs) at 0.25° x 0.25° spatial resolution, under SSP2-4.5 and SSP5-8.5 scenarios for 1981–2100.

Main Results

• Warming: Robust warming across all ensembles, with basin-mean temperatures projected to increase by +1.5 to +4.5 °C by 2071–2100. Minimum temperatures increase more rapidly than maximum temperatures, particularly under SSP5-8.5.
• Potential Evapotranspiration (PET): Consistent and significant increases in PET, ranging from 9–10% by mid-century to 12–18% by late century, reflecting enhanced atmospheric water demand.
• Rainfall: Modest but statistically significant annual increases (8–14%), primarily concentrated in July-September, but with large inter-model uncertainty.
• Discharge: Projections show a general decline relative to the historical baseline, with long-term changes being smaller and statistically insignificant due to the interplay of increased rainfall and evaporative demand.
• Drought Characteristics (Dry Models, SSP5-8.5, by 2100 vs. 1990-2014 baseline):
  • Duration: Meteorological and hydrological drought duration extends by 1–2 months (e.g., rSPI extreme duration from 1.2 to 3.0 months).
  • Severity: Intensifies by 15–25% (p < 0.05), with rSPEI extreme severity nearly tripling and rSSI extreme severity quadrupling.
  • Intensity: Increases significantly, with rSPEI and rSSI extreme intensity doubling or quadrupling.
• Extreme Events: The frequency of extreme drought events (≥ 90th percentile) is projected to increase twofold by 2100 under SSP5-8.5.
• Ecohydrological Status: "Dry" trajectory models (lower excess water, higher excess energy) amplify drought trends, while "wet" trajectories offer partial mitigation but fail to avert overall aridification.

Contributions

• Provides a robust and transferable framework for non-stationary drought assessment using relative standardized drought indices (rSPI, rSPEI, rSSI) in semi-arid basins.
• Offers actionable insights for adaptive reservoir operations, drought early-warning systems, and risk management in Burkina Faso and the wider Sahel region.
• First study in Burkina Faso and the West African Sahel to apply relative standardized drought indices to evaluate both past and future drought conditions, addressing limitations of traditional indices under non-stationary climate.
• Explicitly bridges meteorological, evapotranspiration-driven, and hydrological droughts within a unified framework.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Citation

@article{Fowé2026Assessing,
  author = {Fowé, Tazen and Yonaba, Roland and Mounirou, Lawani Adjadi and Nkiaka, Elias and Karambiri, Harouna},
  title = {Assessing the Future of Droughts Using Relative Standardized Indices: Insights from the Nakanbé River Basin, West Africa},
  journal = {Earth Systems and Environment},
  year = {2026},
  doi = {10.1007/s41748-026-01057-y},
  url = {https://doi.org/10.1007/s41748-026-01057-y}
}