# Tracer-aided hydrological modeling in the upper Ouémé basin, Benin

Diego Chavez, Christine Vallet-Coulomb, A. Zapelli, Sylvain Kuppel, Christophe Peugeot, Basile Hector, Maxime Wubda, Théodore Ouani, Simon Afouda

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## To cite this version:

Diego Chavez, Christine Vallet-Coulomb, A. Zapelli, Sylvain Kuppel, Christophe Peugeot, et al. Tracer-aided hydrological modeling in the upper Ouémé basin, Benin. International Symposium on Isotope Hydrology: Sustainable Water Resources in a Changing World, International Atomic Energy Agency (IAEA), Jul 2023, Vienna, Austria. hal-05382936

HAL Id: hal-05382936

https://hal.science/hal-05382936v1

Submitted on 26 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
TRACER-AIDED HYDROLOGICAL MODELING IN THE UPPER
OUÉMÉ BASIN, BENIN

D. CHAVEZ-ESPINOZA,
Aix-Marseille Univ., CNRS - IRD - INRAE – CEREGE, Technopôle de l'Arbois-Méditerranée, Aix-en-Provence, France
E-mail address: chavez@cerege.fr.

C. VALLET-COULOMB, A. ZAPELLI
Aix-Marseille Univ., CNRS - IRD - INRAE - CEREGE, Technopôle de l'Arbois-Méditerranée, Aix-en-Provence, France

S. KUPPEL
Géosciences Environnement Toulouse, Univ. Toulouse, CNRS - IRD - UPS - CNES, Toulouse, France

C. PEUGEOT
HydroSciences Montpellier (IRD - CNRS, Univ. Montpellier, IMT Mines Alès), Montpellier, France

B. HECTOR
Institut des Géosciences de l'Environnement (Univ. Grenoble-Alpes, IRD - CNRS), Grenoble, France

M. WUBDA
Univ. Joseph Ki Zerbo (UFR-SVT/DST), Ouagadougou, Burkina Faso

T. OUANI, S. AFOUDA
Observatoire AMMA-CATCH, Institut de Recherche pour le Développement, Cotonou, Bénin

**Abstract:** In West Africa Sudanian region, the climate is strongly influenced by seasonal and interannual rainfall variability associated with the occurrence of the West African monsoon seasonal cycle (Vischel et al., 2011). Here, rivers flow intermittently with a dry season that lasts at least six months, and annual river floods of highly variable intensity (Le Lay et al., 2008). Moreover, studies along the soil-plant-river-atmosphere continuum in this region have highlighted the important role of vegetation water uptake in shaping regional water pathways. Plants may control recharge through water uptake in the root zone, but also through tapping groundwater during and outside the rainy season (Richard et al., 2013), therefore impacting groundwater-fed river regimes as well. Understanding the complex hydrological mechanisms of such systems and their response to climatic and environmental drivers remains a significant knowledge gap, which is critical in the prospect of enhancing the resilience of the populations and ecosystems that rely on these hydrosystems.

This scientific question is here addressed in the upper Ouémé basin, covering 10,000 km² in northern Benin. At this scale where hillslope dynamics combine with larger-scale organization of water pathways, we explore the application of a process-based, spatially distributed model integrating vegetation dynamics as well as the tracking of stable isotopes (²H, ¹⁸O) along flow paths. Such a combination of features is a key tool to provide information about the processes occurring in hydrological systems using a variety of observational information content, from
the local scale at which evaluation data is measured up to basin-level water budgets. This watershed is a long-term critical zone observatory monitored by the AMMA-CATCH observatory (www.amma-catch.org) since the late 1990s and features long-term hydroclimatic data records (Galle et al, 2018), in addition to measurements of water isotopic composition (δ¹⁸O, δ²H) conducted since 2018. This network provides critical information to assess model performance, such as records of soil moisture, piezometric level, eddy-covariance measurements (including evapotranspiration), and streamflow.

This study thus aims to quantify water transfers and stocks, by deploying the EcH₂O-iso model (Kuppel et al., 2018) with daily simulations performed in combination with a 19-year hydroclimatic record (2002 - 2020) and a 3-year isotopic record (2018 - 2020). Model parameters were manually adjusted until a model-data agreement was reached regarding stream discharge and isotopic records at the basin outlet. The first simulations show encouraging results regarding channel dynamics, for both discharge and isotopic signature. Further calibration is ongoing to obtain a better representation of groundwater depth and relevant output fluxes in this region such as evapotranspiration. This analysis of water partitioning and pathways also relies on a new compartment-tagging feature of the model, which enables tracking of the temporal variations in groundwater contributions to the streamflow along the channel network, expanding the interpretation of isotopes dynamics. In parallel, a thorough, spatially- explicit examination of the contribution of simulated transpiration flux to evapotranspiration is performed.

## REFERENCES

[1] GALLE, S. et al. AMMA-CATCH, a Critical Zone Observatory in West Africa Monitoring a Region in Transition. Vadose Zone J. **17**, 1–24 (2018).

[2] LE LAY, M. et al. Model representation of the Sudanian hydrological processes: Application on the Donga catchment (Benin). J. Hydrol. **363**, 32–41 (2008).

[3] RICHARD, A. et al. Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin). Hydrol. Earth Syst. Sci. **17**, 5079–5096 (2013).

[4] KUPPEL, S., TETZLAFF, D., MANETA, M. P. & SOULSBY, C. EcH2o-iso 1.0: water isotopes and age tracking in a process-based, distributed ecohydrological model. Geosci. Model Dev. **11**, 3045–3069 (2018).

[5] VISCHEL, T. et al. Generation of High-Resolution Rain Fields in West Africa: Evaluation of Dynamic Interpolation Methods. J. Hydrometeorol. **12**, 1465–1482 (2011).