Groundwater recharge by Sahelian rivers—consequences for agricultural development: example from the lower Komadugu Yobe River (Eastern Niger, Lake Chad Basin)

International audienceIn the Sahel region, temporary rivers and ponds constitute green spaces of welfare where sustainable development requires parsimonious management of water resources. The Komadugu Yobe (KY) valley in Eastern Niger is presented here as an example case of recent agricultural development based on irrigated pepper cropping. Piezometric maps indicate there that the river recharges the upper quaternary aquifer. A simplified numerical model allows to quantify the exchanges between the river on the aquifer, based mainly on exploration geophysics data and three piezometric records, covering between 1 and 3 years of our 4 years modeling period. Recharge at the valley axis is modeled with a linear river coefficient constant for each hydrological year. The main findings are that: (i) during dry years, the river is disconnected from the aquifer and separated from it by a layer of unsaturated medium, (ii) however, this effect can be reversed, such as during the 2010‐2011 wet year or after the Sahelian drought event of the 1980s and 1990s, (iii) the infiltration rate from the river amounts to 30 to 40% of its total discharge and to at least four times the abstraction for pepper irrigation along its 150 km lower course at the Niger‐Nigeria boundary, which implies that neither the aquifer recharge nor the river discharge are at risk due to the present agricultural development. Similar modeling near temporary river axes could provide some help in water resource management in the Sahel. keywords: aquifer recharge, irrigated pepper farming, groundwater modeling, sustainable development, Lake Chad Basin, Komadugu Yobe 2 1. Introduction Water availability is a key factor for the sustainable development of Sahelian societies (Cheo et al. 2013; Babamaajii and Lee 2014). In several instances they can rely on groundwater mainly recharged in the past (Taylor et al. 2009; MacDonald et al. 2012) and from which the present renewal rate can be estimated (Scanlon et al. 2006), generally by geochemical methods (Edmunds 2009). In the northern Sahel, rivers, ponds and lakes constitute green spaces of relative welfare where the production activities rely on both surface water and groundwater (Tarhule and Woo 2002). Local water budgets are needed for estimating the water resource available for sustainable development (Carter and Alkali 1996). Numerical modeling of groundwater/surface water interaction may help to understand the effect of climate changes (Engelhart et al. 2013), to assess the consequences of various management scenarios (Feng et al. 2011) and to solve possible future conflicting uses. Flowing at the eastern part of the boundary between Niger and Nigeria, the Komadugu Yobe (KY) is a sahelian temporary river which drains into Lake Chad (Fig. 1). This region was strongly impacted by the Great African Drought, which began during the 1970s and is considered as one of the most significant climatic events worldwide (Hulme 2001). Local people have achieved strong adaptation to the drought effects almost without external help (Mortimore and Adams 2001; Mortimore 2010). In the KY area they mostly substituted fishing and rainfed agriculture (pearl millet) with sweet pepper cropping irrigated with surface water, however with minor abstraction of groundwater (Luxereau et al. 2011). There are concerns on the sustainability of the development of this area given the limited water resource, the climate variability and the limited size of the pepper market