Baseflow Analysis of Rivers in Lake Tana Sub Basin
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Date
2010-02
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Addis Ababa University
Abstract
The Lake Tana sub basin is situated on the northwestern plateau of Ethiopia at the headwaters of
the Blue-Nile basin, west of the Afar depression. The drainage area of the lake is approximately
15,000 square kilometers, of which 3062 is the lake area. Topographic high surrounding the
basin forces the major River systems to drain toward Lake Tana. The altitude of the basin ranges
between 1765m to 3000 m a.m.s.l. The climate of the region is 'tropical highland monsoon' with
one rainy season between June and September. The rainfall data for 23 long-term records of
rainfall stations lying within or around the Tana sub Basin are used for the computation of
rainfall. The Tana sub Basin receives an average arulUal rainfall of 1329 mm. The air
temperature shows small seasonal changes with an annual average of 20°C. The major geological
formations that outcrops in Lake Tana sub-basin and its adjacent areas are Tertiary and
Quaternary volcanic rocks and alluvial along the major tributaries of the Lake Tana sub-basin.
Alluvial sediments have limited distribution within Lake Tana sub-basin dominant at the eastern
and not1hern side of the Lake. The understanding and knowledge of hydrology and
hydrogeologic systems is very crucial for every activities involving economic development in the
Lake Tana sub basin. Groundwater recharge is one of the most important factors governing the
sustainable yield of grou ndwater and surface water exploitation. The recharge estimation of the
Lake Tana sub basin was carried out based on the principle of base flow separation using HMS SMA
for gauged catchments on daily bases. The hydrological and meteorological data used for
the period of 1992 to 2006. StatistiXL Multiple linear regression was used to estimate the runoff
for ungauged catchments. A level of significance of 0.05 was used for the forward and
backwards stepwise regressions. Climate and physical characteristics of the catchments were
used in multiple regression to predict the flow characteristics of ungauged catchments. The
amount of rainfall, topographic setting and geology are the main controlling factors of climate
and physical characteristics of the catchments. The grow1dwater contribution from gauged
catchments is about 161.17mm/yr or 12. I % of the total rainfall of the basin. The ungauged
catclm1ents contribute a total of28. 18 mm/yr or 2.28% of the total rainfall of the basin.
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Keywords
Rivers in Lake Tana Sub Basin Base flow Analysis