Characterizing regional groundwater flow in the Ethiopian Rift: A multimodel approach applied to Gidabo River Basin

Abstract

Being located in the tectonically active Ethiopian Rift, the hydrogeology of the Gidabo River Basin is complex due to the disruption of lithologies by faults and the variability and lateral discontinuity of the aquifers. Hydrogeochemical and isotopic data suggest that the aquifers within the rift floor receive a relevant contribution of groundwater recharged in the highland. However, the incomplete knowledge about the aquifer properties, the hydraulic behavior of the faults, and the boundary conditions cause uncertainties in this conceptual hydrogeological model. To account for these uncertainties fourteen different numerical models with a stepwise increase from 7 to 40 adjustable parameters were developed, calibrated against the same hydraulic head observations, and ranked according to the Akaike information criteria (AIC and AICc) and Bayesian information criterion (BIC). Based on the information criteria five plausible models were identified, all of which were successfully verified against the river baseflow. The highest likelihood is attributed to a model with eleven adjustable parameters that does not explicitly account for the fault zones; other plausible models considering faults as semi-barriers achieve a slight improvement in model fit but have lower likelihood due to the increased number of calibration parameters. Thus, the effect of faults on groundwater flow needs further investigation, particularly at a local scale.On a regional scale, the hydraulic head distributions of the plausible models agree reasonably well with the equipotential map interpolated from well observations. The estimated transmissivity values range between 30 m 2 / day and 1350 m 2 / day and generally increase from the mountains towards the rift floor. The water budget shows that 75 % of the groundwater recharge supplies baseflow to the rivers. The remaining water infiltrates to the deeper aquifers where less than 1 % is abstracted by pumping wells and the rest flows towards Lake Abaya. Within the rift floor, the majority of inflow to the aquifers is from direct recharge; nevertheless, 35 % of the inflow is contributed by mountain block recharge (lateral groundwater flow from the escarpment and highland). The results of this study strongly advocate the idea to incorporate alternative plausible models instead of relying on single models in the practice of groundwater modeling especially in areas of complex hydrogeology and limited data availability.Die Hydrogeologie des im tektonisch aktiven Äthiopischen Graben gelegenen Gidabo-Flusseinzugsgebiets ist aufgrund der störungs bedingten Unterbrechungen der Lithologien sowie der Variabilität und lateralen Diskontinuität der Aquifere komplex. Hydro geochemische und Isotopen-Daten weisen darauf hin, dass in den Aquiferen der Grabensohle ein relevanter Beitrag von Grundwässern vorliegt, die im Hochland neugebildet werden. Die unvollständige Kenntnis der Aquifereigenschaften, der hydraulischen Wirkung der Störungen und der Randbedingungen führen jedoch zu Unsicherheiten in diesem ko...

Mountain‐Block Recharge: A Review of Current Understanding

“…They then applied this MBR as a boundary condition to a groundwater flow model of the adjacent basin-fill aquifer in the Tobacco Root Basin, MT, and found that MBR accounts for 36% of basin-fill aquifer recharge. Similarly, Mechal et al (2016) estimated recharge to the Gidabo River Basin in the Ethiopian rift valley using a semidistributed soil water model and then applied those values as recharge to a groundwater flow model. They estimate MBR composes 35% of recharge to the rift basin and found that including faults acting as both flow barriers and conduits improved model fit.…”

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

“…Close to Lake Abaya however, the equipotential lines appear to indicate groundwater flow toward an area south of where the river flows into the lake. Mechal et al (2016) therefore assumed that the groundwater basin does not coincide with the surface-water divide in this area but extends further southward along the lake. Groundwater flow in this part of the catchment appears to be affected by the faults (see geological structures in Fig.…”

“…The results of a highly simplified 2D groundwater model of Gidabo River Basin (Mechal et al 2016) also suggest that the rift floor aquifers receive a significant contribution of groundwater flow from the highland and escarpment (mountain-block recharge).…”

“…Only recently, Mechal et al (2015) employed a hydrological model to assess the spatial and temporal recharge distribution within Gidabo River Basin. Using a highly simplifying twodimensional (2D) groundwater flow model, Mechal et al (2016) showed that measured groundwater levels and the knowledge about the general trends of the hydraulic properties within the basin are consistent with the spatial recharge distribution from the hydrological model; yet, very little is known about the detailed hydrogeology of the area.…”

Characteristics of hydraulic conductivity in mountain block systems and its effects on mountain block recharge: Insights from field investigation and numerical modeling

“…Consequently, the F11 fault zone partially impedes groundwater flow from flowing south, likely acting as a combined "conduit-barrier." Similar flow behavior and discontinuity in permeability of faults have been examined with numerical modelling (Mechal et al, 2016) and field investigation (Caine et al, 2017). The large discontinuity of the fault in the eastern Española Basin, Rio Grande rift, New Mexico, indicated that it was unlikely to systematically impede groundwater flow (Caine et al, 2017).…”