Hydrogeology of the Ethiopian Rift: The example of Gidabo River Basin

Steffen Birk1, Abraham Mechal1, Thomas Wagner1, Gerfried Winkler1, Aberra Mogessie1, Martin Dietzel2, Albrecht Leis3, Seifu Kebede4
1 Institute of Earth Sciences, NAWI Graz, University of Graz, Austria
2 Institute of Applied Geosciences, NAWI Graz, Graz University of Technology, Austria
3 Institute of Water, Energy and Sustainability, Joanneum Research Forschungsgesellschaft, Graz, Austria
4 Department of Earth Sciences, Addis Ababa University, Ethiopia

O 2.3 in Aquifer systems in Europe and beyond

14.04.2016, 14:45-15:00, Audimax B, Geb. 30.95



The availability of groundwater in the Ethiopian Rift is believed to be high (MacDonald et al., 2001). The complex hydrogeologic setting, however, complicates the development of groundwater resources within this region. Using the example of Gidabo River Basin this work aims at improving the knowledge of the hydrogeology of the Ethiopian Rift. For this purpose, an approach combining hydrogeological as well as hydrogeochemical and isotopic field investigations with hydrological and groundwater modelling is employed.

Gidabo River drains a catchment area of 3302 km2 extending from the highland through the escarpment to the rift floor, where the river terminates in Lake Abaya. The climate ranges from humid in the highland and escarpment to semi-arid in the rift floor. To characterize the temporal and spatial variability of groundwater recharge the semi-distributed hydrological model SWAT was calibrated and validated using river discharge records from three gauging stations (Mechal at al., 2015). The results reveal a decrease of the average annual recharge from 410 mm in the highland to 25 mm in the rift floor. The time period from 1998 to 2010 exhibits six years without any recharge in the rift floor, suggesting a threshold of approximately 800 mm annual rainfall needed to exceed the high evapotranspiration within this part of the catchment.

To characterize the hydrogeologic properties and the water budget of the aquifer system a two-dimensional steady-state groundwater flow model was designed using the recharge obtained from the hydrological model as input. The model was calibrated to hydraulic heads measured in 72 wells. A multi-model approach was employed to account for the incomplete knowledge of the aquifer system and the resulting uncertainty in the conceptual model. Several calibrated models differing in the number of transmissivity zones as well as in the implementation of fault zones and rivers were compared using information criteria. Finally, the models were validated using the observed river baseflow. The transmissivity estimates roughly range from 0.01 m2/s to 0.001 m2/s within the rift floor, but are found to be lower by approximately one order of magnitude in the highland. Direct recharge provides the majority of inflow to the aquifers within the rift floor, but 35% are contributed by lateral groundwater flow from the escarpment and highland (mountain block recharge).

Hydrochemical and isotope data generally support the finding of a flow connection from the recharge areas in the highland to the aquifers in the rift floor. The deeply circulating groundwater recharged in the highland is found to be mixed with shallow groundwater in the rift floor, particularly near fault zones. The flow pattern within the northern part of the catchment, where flow appears to be diverted and channeled by a series of horst graben structeres, suggests that the faults act as combined conduit-barrier systems.

MacDonald, A. M., Calow, R. C., Nicol, A. L., Hope, B., Robins, N. S. (2001): Ethiopia: water security and drought. - British Geological Survey Technical Report WC/01/02.

Mechal, A., Wagner, T., Birk, S. (2015): Recharge variability and sensitivity to climate: The example of Gidabo River Basin, Main Ethiopian Rift. - Journal of Hydrology: Regional Studies. http://dx.doi.org/10.1016/j.ejrh.2015.09.001

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