Groundwater flow modelling of the Central Main Ethiopian Rift lakes basin

A three dimensional steady-state finite difference numerical groundwater flow model (Modflow) was used to quantify the groundwater fluxes and to study the hydrogeological system of the central Main Ethiopian Rift lakes. Two additional simple groundwater flow codes were also used to develop the conceptual view of the entire basin along with hydrochemical and isotope (H-2 and O-18) techniques. The groundwater flux estimated from the model is compared with the net groundwater fluxes estimated by conventional water balance approach. The result from the two methods agrees reasonably. Despite the geographic proximity of the lakes the groundwater flux is highly variable, mainly owing to geologic factors. The net groundwater flux ranges from 7011 m(3)/day for lake Abiyata to 651022 m(3)/day for lake Shala. The most alkaline and isotopically enriched lakes were found to be terminal from both surface water and groundwater point of view. The groundwater flow converges from all sides of the basin to the center of the rift. Local, intermediate and regional flow systems were identified. Major flow systems are controlled by the rift faults. The direction and magnitude of the faults have strong impact in the occurrence of groundwater and the flux of the lakes than the volcanic stratigraphy.