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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 May 2018

Research article | 18 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Hydrological functioning of West-African inland valleys explored with a critical zone model

Basile Hector1, Jean-Martial Cohard1, Luc Séguis2, Sylvie Galle1, and Christophe Peugeot2 Basile Hector et al.
  • 1IGE , Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, F-38000 Grenoble, France
  • 2HSM, Univ. Montpellier, CNRS, IRD, F-34000 Montpellier, France

Abstract. Inland valleys are seasonally waterlogged headwater wetlands, widespread across West-Africa. Their role in the hydrological cycle in the humid, hard rock-dominated Sudanian area is not yet well understood. Thus, while in the region recurrent floods are a major issue, and hydropower has been recognized as an important development pathway, the scientific community lacks a precise knowledge of streamflow (Q) generation processes and how they could be affected by the presence of inland valleys. Furthermore, inland valleys carry an important agronomic potential, and with the strong demographic rates of the region, they are highly subjected to undergo land cover changes. We address both the questions of the hydrological functioning of inland valleys in the Sudanian area of West-Africa and the impact of land cover changes on these systems through deterministic sensitivity experiments using a physically-based critical zone model (ParFlow-CLM) applied on a synthetic catchment which comprises an inland valley. Model forcings are based on 20 years data from the AMMA-CATCH observation service and parameters are evaluated against multiple field data (Q, evapotranspiration – ET –, soil moisture, water table levels, and water storage). The hydrological model applied to the conceptual lithological/pedological model proposed in this study reproduces the main behaviors observed on a highly instrumented elementary catchment. We found that yearly water budgets were highly sensitive to the vegetation distribution: average yearly ET for a tree-covered catchment (944mm) exceeds that of herbaceous-cover (791mm). ET differences between the two covers vary between 12 and 24% of the precipitation of the year for the wettest and driest year, respectively. As a consequence, the tree-covered catchment produces a yearly Q budget 28% lower on average as compared to a herbaceous-covered catchment, ranging from 20% for the wettest year to 47% for a dry year. Trees also buffer interannual variability in ET by 26%. On the other hand, pedological features (presence – or absence – of the low permeability layer commonly found below inland valley, upstream and lateral contributive areas) had limited impact on yearly water budgets but marked consequences on intraseasonal hydrological processes (sustained/unsustained baseflow in the dry season, catchment water storage redistribution ...). Therefore, subsurface features of inland valleys have potentially significant impacts on downstream water-dependent ecosystems and water uses as hydropower generation, and should focus our attention.

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Short summary
The hydrological functioning of West-African headwater wetlands remain poorly understood, despite their potential for small scale farming and their role in streamflow production. We found how vegetation significantly affects water budgets, and pedo-geological features control streamflow in the dry season by conducting virtual experiments with a comprehensive physically-based critical zone model (from the bedrock to the top of canopy) evaluated against a uniquely dense dataset for the region.
The hydrological functioning of West-African headwater wetlands remain poorly understood,...