Abstract
Abstract. Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from a permanent and deep groundwater table to the atmosphere even during the dry season. It should be the case for riparian forests too, as these streambank forests are key landscape elements in Sudanian West Africa. The interplay of riparian forest and groundwater in the local hydrodynamics was investigated, by quantifying their contribution to the water balance. Field observations from a comprehensively instrumented hillslope in northern Benin were used. Particular attention was paid to measurements of actual evapotranspiration, soil water and deep groundwater levels. A vertical 2-D hydrological modelling approach using the Hydrus software was used as a testing tool to understand the interactions between the riparian area and the groundwater. The model was calibrated and evaluated using a multi-criteria approach (reference simulation). A virtual experiment, including three other simulations, was designed (no forest, no groundwater, neither forest nor groundwater). The model correctly simulated the hydrodynamics of the hillslope regarding vadose zone dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics. The virtual experiment showed that the riparian forest transpiration depleted the deep groundwater table level and disconnected it from the river, which is consistent with the observations. The riparian forest and the deep groundwater table actually form an interacting transpiration system: the high transpiration rate in the riparian area was shown to be due to the existence of the water table, supplied by downslope lateral water flows within the hillslope soil layer. The simulated riparian transpiration rate was practically steady all year long, around 7.6 mm d−1. This rate lies within high-end values of similar study results. The riparian forest as simulated here contributes to 37% of the annual hillslope transpiration, and reaches 57% in the dry season, whereas it only covers 5% of the hillslope area.
Highlights
The West African climate is characterised by strong interactions between the atmosphere and the land surfaces (Koster et al, 2004)
Knowing that riparian forests are a key element of the landscape in northern Benin (Natta et al, 2002), this paper addresses the interplay between riparian forests and the underlying groundwater and its impacts on the hydrological processes, which control the local and mesoscale water cycle
A first important result obtained in this study is the ability of the Hydrus 2-D model to correctly reproduce the annual cycle of the evapotranspiration and its corresponding annual total, using in situ observed properties as much as possible (2006 simulation)
Summary
The West African climate is characterised by strong interactions between the atmosphere and the land surfaces (Koster et al, 2004). By taking into account hillslope processes (overland flow and interflow) in an enhanced 1-D SoilVegetation-Atmosphere Transfer (SVAT) model, Giertz et al (2006) correctly simulated the streamflow of the 16.5 km Aguima catchment (Benin). A recent review by Peugeot et al (2011) shows that the hydrological and SVAT models used to compute the mesoscale water cycle over the Ouémé catchment agreed on streamflow simulation but that at the same time they differed significantly in evapotranspiration and water storage terms. The African Monsoon Multidisciplinary Analysis – Coupling the Tropical Atmosphere and the Hydrological Cycle (AMMA-CATCH) observation system (Lebel et al, 2009) and its Sudanian site of the Ouémé catchment located in northern Benin (Fig. 1) provided an unprecedented set of data to explore the interaction between riparian vegetation and the water cycle.
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