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Analytical, Recursive Digital Filtering, Drought vulnerability, Recharge


Estimation of groundwater recharge and drought vulnerability assessment was done by comparing Recursive Digital Filtering (TIMESPLOT) and Graphical Hydrograph Separation (HYSEP) methods. In both methods, long term daily stream flow data (1991 - 2003) was used from seven river discharge gauging stations in three physiographic settings (rift, escarpment and highland) and corresponding variable climatic conditions. The results from both methods have similar trend and show general agreement on annual basis with a correlation coefficient of 0.994. Moreover, long-term rainfall-discharge relationship of the sub-basin shows 0.75 correlation. The long term annual weighted catchment recharges are 94.9 mm and 111.9 mm for TIMESPLOT and HYSEP methods, respectively. There is cyclic recurrence of drought roughly every three years. The amount of annual rainfall in drought times is approximately half of the rainfall in good seasons. Moreover, due to the ever-increasing usage of water for different consumptions, even the stream flows in good seasons are highly stretched. The yields of springs are dwindling due to pumping of water around spring sources. This is in spite of the relatively high long-term average annual base flow indices for TIMESPLOT and HYSEP methods which are 0.58 and 0.69, respectively. Those vulnerable to drought are the areas closer to the rift floor. The long-term annual and monthly averages of recharge and base flow indices are more dependent on slope and the hydrogeology of the catchments. Recharge clearly increase with elevation; however, base flow indices increase towards flat-lying alluvial deposited areas.