"Hydrogeology of Mehoni sub basin and lake Ashange catchment in the Raya Valley Northern Ethiopia"
Hydrogeology of Mehoni sub basin and lake Ashange catchment in the Raya Valley (Northern Ethiopia) Ermias Hagos Relief Society of Tigray, P.O.Box 20, Mekelle, Tigray 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 km2. 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. The main objective of this work was to study the overall hydrogeological and hydrochemical setup of the study area. The 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 with a good chemical quality of the ground water except high EC (TDS) and NO3 values in some areas. The hydrochemistry of the Lake Ashange seems different from that of the springs that emanate from the western highlands and the groundwater in the valley floor of the Mehoni sub-basin which may indicate that the lake may not have significant sub-surface outflow towards the sub basin. Lowering of static water level by about 2m within 2 years time was detected in a monitoring well located at the groundwater irrigated field around Werabaye which can be due to large-scale pumping and intensive diversion activities in the upstream areas. 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 directions towards the lake.