Numerical Groundwater flow Modelling for Planning and Management of the Resource in Bacho Plain Upper Awash Basin, Central Ethiopia
No Thumbnail Available
Date
2019-05-09
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
The Bacho catchment, model area, is located in upper Awash River basin. The model domain has
an area of 4553km2.The area dominantly covered with volcanic rocks. The upper cover is a thin
alluvial deposit at Bacho plain. Pyroclatic flows, ash, ignimbrite and basaltic rocks interlayered
forming the top aquifer layer. Below these units massive and welded ignimbrite comes with
extensive and recognizable thickness with low permeability and eventually the last model domain
layer is a very thick scoraceous basalt that cover wide area with relatively better groundwater
potential and permeability.
A groundwater flow model was developed for the Bacho Catchment in the upper Awash basin to
analyze the groundwater flow characteristics and determine the aquifer parameters for future
groundwater management and planning. This model will assist the decision makers to develop
long term strategies to ensure environmentally responsible and economically sustainable use of
this valuable natural resource.
The numerical model simulates groundwater flow within three hydrostratigraphic units; the top
150m unit comprises basaltic and pyroclastic rocks; the middle is 100m thick ignimbrite which act
as confining layer; and the third is a unit with 300m thick scoraceous basalt layers which is the
major aquifer in the model domain. Most water boreholes were developed in first and third
hydrostratigraphic layers for domestic, industrial and irrigation purposes.
Conceptually developed model of research area was represented to numerical model by means of
computer code_ processing modflow version 8.0.15. The area was discretized to three layers and
a uniform grid size of 200m by 200m active model cells. Total perimeter boundary is about 492km
of which 75% is no flow and about 25% is head dependent flow boundaries. The model was
simulated with a steady state condition. Analytically determined aquifer parameters were encoded
and five flow package namely, drain, head dependent boundary flow, recharge, river leakage and
well packages were simulated.
Conceptual model was developed from the hydrological, geological and hydrogeological historical
data and field observations. The conceptual model was translated in to mathematical expression
using processing modflow code. The model was calibrated by trial and error method by comparing
the observed and measured hydraulic head and water balance.
iv
Recharge components to the aquifers are from rainfall, inter aquifer flow from Abay basin across
the northern boundary and Awash River at upstream part. Discharge is predominantly through
groundwater leakage to rivers, under aquifer outflow across the southern boundary to Ada’a plain
and withdrawal by pumping. Groundwater is dominantly flow from north to south in general. In
the upper layer groundwater flow is towards center of model area and dominantly towards Awash
River at lower topography.
The model was most sensitive to recharge, horizontal hydraulic head, and river hydraulic
conductance. It is less affected by change in vertical hydraulic conductivity and other aquifer
parameters. The calibrated model recharge volume is estimated to be 1.07Mm3/day. In flow from
river leakage is approximated 0.330Mm3/day and general head dependent inflow is about
0.683Mm3/day. The final calibrated model total outflows is calculate to be 2.09Mm3/day. At final
model calibration resulted in the spatial recharge distribution ranges from 5.4x10-5m/d to1.99x10-
4 m/d; horizontal hydraulic conductivity varies from 0.44m/d to 9.2m/d and 0.07m/d to 45m/d in
the top and bottom aquifer layers, respectively.
Recharge reduction by 50% and groundwater withdrawals scenarios were formulated to examine
the aquifer response to such stresses. Accordingly, occurrence of both stresses simultaneously will
cause surface water to lose 23.4% amount of water to sub-surface system and water level dropped
with an average of 20.6m in the research boundary. However, increase of recharge through
injection well will only rise water level by 2.65m and discharges much of the recharge water to
Awash River. Introduction of 100 productive wells at expected potential area like center of Bacho
and Holeta resulted in 3.8 m overall decline of head in the model area and less drawdown at Holeta
water wells.
Most rivers and stream channels are losing type in the upper higher topographic locations while at
lower Bacho flat land almost all river gains about 1.78Mm3/day of groundwater as baseflow.
Hence, river _ groundwater interaction is very recognizable flow process in catchment.
Description
Keywords
Numerical Groundwater, Modelling, Planning, Management, Resource in Bacho, Plain Upper Awash Basin, Central Ethiopia