Modeling of the natural groundwater recharge under climate change: Sisseb El Alem Nadhour Saouaf basin (Central Tunisia) case study

In this work, two methods were used to assess the groundwater natural recharge of the Sisseb El Alem Nadhour Saouaf basin (SANS) in Northeastern of Tunisia. In fact, the natural recharge of the SANS aquifer remains a major unknown for hydrogeologists and decision makers. Based on the various databases that include geology, sedimentology, hydrology, geochemical and isotopic data, the groundwater natural recharge was estimated using two different approaches. In a first approach, a historical measurement of tritium, the radioactive isotope, was used to calculate the age of modern groundwater (< 50 years). The application of the tritium radioactivity model shows that in the SANS basin the renewal rate of groundwater ranges between 1.3 and 3%. However, this is difficult to generalize across the entire aquifer, given the large spatial and temporal variations of rain, and the uncertainty affecting the measurement of tritium rates in rain, as well as the uncertainty of the used radioactivity model. In the second approach, a quasi-steady hydrological model called the WetSpass model (Water and Energy Transfer between Soil, Plants and Atmosphere under quasi-steady state) was used to assess the spatiotemporal variability of the recharge. The results show that the average annual recharge between 1971 and 2016 is 10 Mm³/year. A value that remains minimal and that characterizes this kind of arid region. The spatial distribution maps of the recharge show that the maximum recharge occurs in the northwest edges of the basin, west and in the southern part, at the level of the outcrop permeable formations of the aquifers. Finally, the two methods used show that the groundwater natural recharge is very heterogeneous and limited in time and space. However, these results remain a further step to ameliorate the water budget of the SANS basin to perform a realistic hydrogeological model.