The Lake Chad transboundary aquifer. Estimation of groundwater fluxes through international borders from regional numerical modeling

Africa. The Lake Chad transboundary aquifer To understand transboundary groundwater hydrodynamics and estimate quantitative groundwater fluxes values between aquifer-sharing countries. To enable estimations, we developed an updated 3D ‘quasi steady-state’ regional groundwater flow model of the Chad Formation based on integrating extensive available information and reflecting the best current conceptual understanding to date. The conceptual model was tentatively assessed by a steady-state numerical model based on MODFLOW. This model simulates lateral groundwater flows between neighboring countries, and also provides insights into the large-scale flow pattern and flows among hydroestratigraphic units. Modeling indicated that groundwater fluxes through international borders exist between Basin-sharing countries, except between Central African Republic and Cameroon, where a buffer area was considered for modeling purposes, leading to more uncertain results. From 14ºN parallel to further north, data are scanty and outcomes should be carefully considered. Forecasting transboundary impacts indicated that changes in recharge rates were more sensitive that changes in groundwater abstraction. To date, abstraction represents a small part of the water balance, but if it increases, it can become a driving factor in the future. Land use change and water use in the source areas (southern area) will have the strongest impact on transboundary groundwater flows due to changes in recharge, which will lead to quantitative changes in groundwater levels, artesian conditions, or even water quality.