Investigation of interbasin groundwater flow using multiple approaches: the case of the Tana and Beles basins, Ethiopia.

In most water balance studies, basins are considered closed systems; however, interbasin groundwater flow (IGF), which is an unaccounted hydrological component in most studies, could occur at basin boundaries where there is a hydraulic gradient and appropriate permeability of the geologic formations. IGF is a complex hydrogeological process and hidden from direct observation, thus identifying and characterizing it is difficult. This study investigated IGF in the case of the contact between the Tana and Beles basins, located at the northwestern margin of the Ethiopian Highlands, using a multimethod approach: (1) morphotectonic analysis; (2) lithostratigraphy and geological structure analysis; (3) regional groundwater potentiometric-surface map; (4) presence of large-volume springs; (5) hydrochemistry and isotope tracing; and (6) water budget imbalances. Primary data (geological investigation, water point inventory, and water sampling), satellite images and gravity data, and other secondary data, were collected and analyzed. The results illustrate the extension of the fractured basaltic aquifer, high topographic contrast, and high lineament density along the boundary between the Tana and Beles basins. The connection of the hydro-stratigraphic units, fracture distribution and orientation, high hydraulic gradient, spatial distribution of high-flux springs, and water budget imbalance of Tana Basin revealed that there is IGF from Tana Basin to Beles Basin, despite poor indications from results for hydrochemistry and stable isotopes. These findings will have practical implications for groundwater and water balance studies of the two basins. The study demonstrates the importance of a multimethod approach to investigate IGF where a single approach is imprecise.