Three-dimensional groundwater flow modeling to assess the impacts of the increase in abstraction and recharge reduction on the groundwater, groundwater availability and groundwater-surface waters interaction: A case of the rib catchment in the Lake Tana sub-basin of the Upper Blue Nile River, Ethiopia

Study region: Rib catchment in the Lake Tana sub-basin, Upper Blue Nile River, Ethiopia. Study focus: This paper aimed to assess the impacts of future increase in abstraction and recharge reduction on the groundwater, groundwater availability, and groundwater-surface waters interaction based on a three-dimensional groundwater flow modeling. Calibration was made under the steady state condition. Scenario analysis performed for 1) increase in abstraction, 2) decrease in recharge, 3) the worst-case scenario that combined the aforementioned two scenarios and with additional extraction for irrigation, and 4) for the optimal-case scenario, which considers 5% recharge increase for the worst-case scenario model. New hydrological insights for the region: It is found that the groundwater flows from uplands toward the Tana Lake. The total inflow to and outflow from the system in the calibrated model are 1733480 m(3)/d and 1840451 m(3)/d, respectively. Groundwater level drop, reduction in base flows to surface waters, and in evapotranspiration flux compared to the calibrated values encountered for all scenarios, which are significant (mean 38.4 m, 28.5-100 %, and 97.8 %, respectively) for the worst-case scenario. On the other hand, an increase in groundwater level (mean 9.8 m), base flows (0-14.4 %), and evapotranspiration flux (29.5 %) observed for the optimal scenario when compared to the worst-case scenario results. Results suggest that groundwater management measures should be implemented to mitigate the impacts.