Hydrogeology of Mehoni Sub-Basin and Lake Ashange Catchment in the Rayavally (Northenr Ethiopia)

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Addis Ababa University


The study area is found in northern Ethiopia at the northern portion of the Raya valley, which forms its own trough at the western shoulder of the Afar Depression. Its total aerial coverage is about 1302 krn2 . It is largely covered by the Mehoni Sub-basin, which is surrounded by volcanic hills with sediment filled depression at the center. The Lake Ashange basin is also located on the south-western highlands of the study area. T~e main objective of this work was to study the overall hydrogeological and hydrochemical setup of the study area. The overall water balance of the study area was computed with an aim of estimating the amount of annual recharge to the groundwater. Accordingly, the mean annual rainfall as the only water input into the study area was estimated to be 722.42 mm1year. On the out put side; the actual annual evapotranspiration was estimated to be 687.24mm1year, the annual surface water out flow was estimated to be 52.3 MCM, the annual lake surface evaporation was estimated to be 16.73 MCM and the annual bareland evaporation was assumed to be half the lake surface evaporation. Finally, a annual recharge to the groundwater of the study area is approximated to be 60.9 MCM. The study area comprises two major hydrolithologic units. They are the alluvial aquifers 'and the basaltic aquifers where the alluvial aquifers especially on the western flank of the valley floor and along large rivers were found to be good groundwater potential zones and wells sunk into these aquifers have fairly good efficiencies and productiVities. Generally, the groundwater has good chemical quality with the exception high EC (TDS) and N03 values in some areas. The Lake Ashange water does not have similar chemistry to that of the springs that emanate from the western highlands and the groundwater in the valley floor of the Mehoni sub-basin. Therefore, it is concluded that the lake may not have significant sub-surface outflow towards the sub basin. Lowering of the static water level by about 2m within 2 years time was detected in a monitoring well located at the groundwater irrigated field around Werabaye. This could be attributed to the large-scale pumping from the groundwater and intensive diversion activities in the upstream areas. Generally, two ground water flow systems were detected. One is in the Mehoni sub-basin where the groundwater flows from all directions towards the central valley floor with probable groundwater out flow at the southeastern part of the study area. And the other is in the lake Ashange basin where groundwater flows from all dirrections towards the lake.