Underground Water Temperature as a Marker of Infiltration in Sahel Area

In SW Niger land surface changes since the last decades resulted in increased runoff and increased infiltration from endoreic ponds. This resulted in groundwater level rise in the superficial aquifer of the region although rainfall was decreasing, which is known as the Niamey paradox (Leduc et al., J. of Hydrol, 2001, Favreau et al., WRR, 2008). In these areas, rain results mainly from convective events and therefore rainwater temperature is lower than the underground temperature by several °C. Therefore, infiltration of rain produces a cold signal which propagates toward the saturated zone and is advected in groundwater. It results that groundwater temperature cannot be considered as a climatic proxy in these areas. The propagation of the temperature signal in groundwater is controlled by advection and diffusion of heat. As diffusion of heat may operate over time constants of the same order of magnitude of land surface change, groundwater temperature may provide valuable information on this phenomenon. However, the limited precision of temperature sensor as well as uncertainties on the properties of the aquifer as well as on thermal boundary condition may prevent fruitful interpretation of the groundwater signal. Here we present simplified numerical models to assess this effect on two examples of the Sahel area, namely, a temporary pond of the SW Niger and the Nigerian part of the Komadougou Yobé river under influence of dams built upstream during the 70's. Then, a quantitative comparison with water level and water temperature data gathered in SW Niger since more than 10 year, owing the HAPEX, AMMA, and SO AMMA CATCH programs is attempted.