Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin)

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.

Energy fluxes and surface characteristics over a cultivated area in Benin: daily and seasonal dynamics

“…They showed that the partition of the available energy reached a constant regime during the wet season. Guyot et al (2009) provided evidence that dry season evapotranspiration was linked to water table discharge; this result was further supported by recent hydrological studies Séguis et al, 2011;Richard et al, 2013).…”

Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry

“…Mean annual reference evapotranspiration [Allen et al, 1998] [S eguis et al, 2011b]. During the wet season, once soils are wet enough, actual evapotranspiration is equal to reference evapotranspiration [Richard et al, 2013], with a constant evaporative fraction [Guyot et al, 2012;Mamadou et al, 2014]. No streamflow is observed during the dry season at the outlet of the Nalohou catchment, in the Ara River, or in the larger Donga (586 km 2 ) basin.…”

Interaction between groundwater and trees in an arid site: Potential impacts of climate variation and groundwater abstraction on trees

“…The S-shaped function (van Genuchten, 1987) was used to describe the water stress response as it has been widely used to represent the water stress response function of trees (Zhu et al, 2009 andRichard et al, 2013):…”

“…Water, including surface water, groundwater, and soil water, is vital for tree growth (Zhu et al, 2009 andRichard et al, 2013), with groundwater being an important source in arid regions as it is relatively constant and stable. Many trees in arid regions are reported to be groundwater-dependent (Bulter et al, 2007 andGribovszki et al, 2010) and they can obtain large amounts of groundwater for transpiration through deep roots, especially during dry seasons (David et al, 2007 andLubczynski, 2009).…”