Baseflow Analysis Of Rivers In Lake Tana Sub Basin

The Lake Tana sub basin is situated on the northwestern plateau of Ethiopia at the headwaters ofrnthe Blue-Nile basin, west of the Afar depression. The drainage area of the lake is approximatelyrn15,000 square kilometers, of which 3062 is the lake area. Topographic high surrounding thernbasin forces the major River systems to drain toward Lake Tana. The altitude of the basin rangesrnbetween 1765m to 3000 m a.m.s.l. The climate of the region is ‘tropical highland monsoon’ withrnone rainy season between June and September. The rainfall data for 23 long-term records ofrnrainfall stations lying within or around the Tana sub Basin are used for the computation ofrnrainfall. The Tana sub Basin receives an average annual rainfall of 1329 mm. The airrntemperature shows small seasonal changes with an annual average of 20˚C. The major geologicalrnformations that outcrops in Lake Tana sub-basin and its adjacent areas are Tertiary andrnQuaternary volcanic rocks and alluvial along the major tributaries of the Lake Tana sub-basin.rnAlluvial sediments have limited distribution within Lake Tana sub-basin dominant at the easternrnand northern side of the Lake. The understanding and knowledge of hydrology andrnhydrogeologic systems is very crucial for every activities involving economic development in thernLake Tana sub basin. Groundwater recharge is one of the most important factors governing thernsustainable yield of groundwater and surface water exploitation. The recharge estimation of thernLake Tana sub basin was carried out based on the principle of base flow separation using HMS –rnSMA for gauged catchments on daily bases. The hydrological and meteorological data used forrnthe period of 1992 to 2006. StatistiXL Multiple linear regression was used to estimate the runoffrnfor ungauged catchments. A level of significance of 0.05 was used for the forward andrnbackwards stepwise regressions. Climate and physical characteristics of the catchments werernused in multiple regression to predict the flow characteristics of ungauged catchments. Thernamount of rainfall, topographic setting and geology are the main controlling factors of climaternand physical characteristics of the catchments. The groundwater contribution from gaugedrncatchments is about 161.17mm/yr or 12.1% of the total rainfall of the basin. The ungaugedrncatchments contribute a total of 28.18 mm/yr or 2.28% of the total rainfall of the basin.

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