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UNESCO-IHE Institute for Water Education, Delft (2007)

Groundwater assessment and modelling of the Aynalem basin

Abrha, L.T.

Titre : Groundwater assessment and modelling of the Aynalem basin

Auteur : Abrha, L.T.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2007

The study area is part of the Ethiopian central plateau just to the west of the Ethiopian-Afar Rift valley, about 770 km north of Addis Ababa. The basin covers a surface area of420 Km2. The area is covered with Mesozoic sedimentary rocks ; the dominant lithological units are Quaternary sediment deposits, dolerite, sandstone, limestone-marlshale intercalation and well-bedded limestone. From aerial photograph interpretation and field observation two fault systems (WNW-ESE and NNE-SSW) and different joint sets (NNE, NE, NW and WNW) with non-systematic sets were traced. The study involved hydro-geological assessment of the Aynalem basin with reference to its potential for suitable water supply steady state modelling was conducted using hydro-meteorological, geological, hydro- geological, hydro-chemical and geophysical techniques.The area has a mean annual rainfall of 570 mm/year with actual and potential evapotranspiration of 476 mm/year and 1050 mm/year respectively. The mean annual surface runoff that leaves the basin is 61mm/year of rainfall. The amount of annual groundwater recharge is around 50 mm/year. The hydrostratigraphic units were recognized in the catchments based on well logs and cross- section with additional qualitative interpretation such as degree of weathering and fracturing extent and frequency.The hydro-chemical study indicates the presence of two major water types within the Aynalem basin. The up-stream side of the Aynalem basin is characterized by a calcium bicarbonatedominated type water where as the down-stream side is dominated byCaSO4 type. This reflects the difference in lithology. Water samples from the Aynalembasin are basically within acceptable limits for irrigation use. The high concentration ofcalcium and sulphate that create a high TDS and hardness, limits their domestic usewithout treatment. From the geophysical data the presence of faults were traced, the different drawdown time curves from the catchments, the emergence of springs at the middle of uniform elevation area and their highly variable aquifers and aquitard thickness and depth in the Aynalem basin shows high disturbance of area with geotectonic activity. Groundwater modelling calibration was subsequently applied to verify the recharge. The model was calibrated using measured hydraulic heads and scenarios are analysed tosee the effect of particular pumping rates on the hydrological system.

Sujets  : groundwater modelling groundwater quality water balance catchment areas Ethiopia

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