Integrated Hydrogeological Investigation of Upper Bilate River Catchment: South Western Escarpment of Main Ethiopian Rift
No Thumbnail Available
Date
2009-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa Universty
Abstract
The study was conducted at Upper Bilate River Catchment 230km, South West of Addis
Ababa and East of Hossana to characterize aquifer and assess hydrogeological system of
Upper Bilate catchment by giving particular emphasis to groundwater potential
investigation, mechanism of recharge and groundwater surface water interaction, in an
integrated manner. Data has been collected from the respective institutes and
organizations and processed using hydrogeological and GIS and remote sensing software.
Integrated approaches of hydrogeological investigations are adopted by paying particular
attention to geology, recharge estimation, hydrogeological system and hydrogeochemistry
in the assessment of water resources potential of the catchment. Areal depth of
precipitation of the area was estimated by categorizing the area in to unimodal and
bimodal zones. Potential evapotranspiration (PET) of the area and the water body was
estimated using Thornthwaite Mather and Penman Combination methods and Pan
Evaporation method respectively. Actual Evapotranspiration (AET) was estimated using
Turc and Soil moisture balance methods. Base flow separation and conventional water
balance approaches were used to estimate recharge. To characterize the aquifer system of
the area pumping test data, well completion reports, well logs and geology of the area
were analyzed. Piper plots, Collins bar diagram and Hierarchical cluster Analysis (HCA)
were used to classify the water chemistry. Maps, cross-sections and well-logs are used to
show groundwater flow system and recharge discharge conditions. The estimated values of
PET are 1222.93mm and 842.4mm using Penman and Thorhthwaite method respectively.
The results indicate that, the highland which is unimodal rainfall pattern zone receives
more precipitation (1284.65mm) than the lowland (1216.6mm). Pan Evaporation method
resulted about 2.57mm for the open water evaporation which was insignificant as
compared to the area. AET was estimated to be 800.92mm and 1019.8mm using Turc and
Soil moisture balance methods respectively. The estimated recharges of the area are 129
and 96.18mm using base flow separation and water balance approaches respectively. The
average of the two values is taken as the annual recharge of the area which is 9.2% of the
total precipitation. The pumping test analysis results indicate that the hydraulic
conductivity of the area ranges from 0.17 – 8.8 m/d and the mean and median were 2.17
m/d and 1.12 m/d respectively. There was variation in water type from Low TDS Na-Ca-
HCO3 to high TDS and EC Na-HCO3 water types. Accordingly, the western and northern
part of the area is recharging zone and the central and southern part is discharge zone
while the area just southeast of Boyo plain is categorized under deep groundwater zone
due to the damming effect of Ambericho fault. Thus, detailed hydrogeological especially
remote sensing and geophysical investigation should be conducted to delineate major
water bearing zones, aquifer thickness and aquifer depth in the volcanic and alluvial
formation particularly in the northern part of the study area.
Key words: Upper Bilate, western escarpment, recharge estimation, aquifer characterization,
hydrochemistry, recharge/discharge zone.
Description
Keywords
Upper Bilate, Western escarpment, Recharge estimation, Aquifer characterization, Hydrochemistry, Recharge/discharge zone