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Regional groundwater flow modeling of the Geba Basin, Northern Ethiopia

maandag, 28 september, 2009 - 14:00
Campus: Brussels Humanities, Sciences & Engineering campus
auditorium P. Janssens
Tesfamichael Gebreyohannes Tewolde

The Geba basin is located in a semi-arid mountainous region in the northern part of Ethiopia, in
the Tigray Regional state. It is one of the most water-stressed areas of the country, with only a
short rainy period from mid-June to mid-September. Because rainfall in this region has been
consistently erratic in last few decades, both in time and space, rain-fed agriculture has become
problematic. Hence, in order to supplement rain-fed agriculture with small to large scale
irrigation, a detailed understanding of local and regional groundwater resources is important.
However, information on the availability and distribution of groundwater resources is scarce as
there has never been a systematic and detailed investigation in this region. Hence, the main
objective of this study is to study the groundwater system of the Geba basin, using techniques as
groundwater modeling and other tools, and to identify and quantify potential groundwater
resources for exploitation.

An intensive field campaign has been undertaken from 2004 to 2008 to collect relevant
information regarding geology, hydrology, and hydrogeology throughout the Geba basin. An
inventory was made of groundwater levels, wells, discharge and water quality of springs, base
flow of perennial streams, and interaction with reservoirs. Land-use was also investigated during
field visits to be used as ground truth for supervised classification of satellite images. An
extensive field campaign was also conducted for geological mapping.

Water samples collected were analyzed in the hydrochemistry laboratory of Mekelle University
and results were compared with Ethiopian drinking water standards. For most of the chemical
constituents, water samples are within the safety standards, except for SO4 and NO3 of which
15% and 19% of the samples fail to meet the standards. Further analysis of the water samples
using multivariate statistical analysis enable to cluster the water samples in two groups and five
subgroups, which reflect differences in lithology and/or recharge-discharge conditions.

From the geological survey, it follows that the Geba basin is covered by metamorphic,
sedimentary and volcanic rocks, most of which have been affected by different types of
geological structures as folding, faulting, fracturing, and/or shearing by successive episodes of
geological disturbances. A digital geological map of the Geba basin was prepared, which
includes eighteen different types of lithological units as well as all major geological structures in
the basin. This information, together with borehole lithological logs and pump test data, form the
basis for building a regional groundwater flow model of the basin.

The WetSpass model was applied to simulate the hydrological water balance of the Geba basin.
Seasonal and annual evapotranspiration, surface runoff, and groundwater recharge are the main
outputs of the model. Accordingly, about 76% of the precipitation in the basin is lost through
evapotranspiration, 18% becomes surface runoff, and only the remaining 6% is groundwater
recharge. Results of the WetSpass model were calibrated against river flow measurements, which
show that the predictions are within an acceptable range. From the WetSpass results additional
maps can be derived as accumulated surface runoff, safe yield for groundwater abstraction, and
water deficit for crop growth. Comparison of existing reservoirs with the accumulated runoff
map shows that many reservoirs have failed because their design capacity is much higher than
the actual inflow. Comparison of the safe yield map with the crop water deficit map shows that
in most areas groundwater can be safely abstracted to supplement the water deficit for crop
growth during the wet season. However, the crop water deficit in the dry season is too high to be
supplemented by sustainable groundwater abstraction, except in some small parts of the basin

A groundwater flow model of the Geba basin was developed which accurately predicts the
observed groundwater levels, and yields a groundwater balance with a small error (0.43%). A
calibration graph using 340 observation points show a good fit with a normalized RMS of 0.8%.
The obtained hydraulic conductivity values after model calibration range from 6 × 10-5 m/d for
basement rocks to 0.023 m/d for Adigrat and Enticho sandstones. Other derived results of the
model are a groundwater table map for the entire basin and piezometric maps for each formation,
which can be used for groundwater assessment. Comparison of the groundwater table map with
topography indicates areas where the groundwater connects to the surface, and, hence, where
discharge areas are located. These areas can be potential sites for groundwater abstraction by
hand dug wells or shallow boreholes.