Recharge Estimation of Andassa River Catchment, North-Western Ethiopia

Estimating the recharge amount of a given area is essential for sustainable management of groundwater resource. Andassa catchment which has area coverage of 609.68 square kilometer is located in north western Ethiopia. The area has flat, moderately slope and cliff topography. The rivers which have dendritic drainage pattern general flow south to north direction. Geological-geomorphological-structural feature suggest that the Andassa plain is a potential source of groundwater. The catchment has oligocene to miocene flood basalt with some deposit intercalation. There are tertiary and quaternary formations in the study area. The objective of this study was to estimate groundwater recharge of Andassa catchment using chloride mass balance, base flow separation, soil moisture balance, water balance methods were used to estimate the recharge value of the catchment. Meteorological, river discharge, pump tests, boreholes, springs and hand dug wells data together with well completion reports were used to estimate the recharge value. PET for the area was calculated using Penman gives annual potential evapo-transpiration value of 1335.26mm/year. Actual evapotranspiration (AET) for the area calculated using Turc and Thornthwaite Mather gives a value of 908.486mm/year and 1012.4 mm/year respectively. The recharge value is 370.68mm/y, 321.5 mm/y, 282.1mm/y and 255mm/year for base flow separation, soil moisture balance, water balance and chloride mass balance approaches respectively and the average recharge of the area is 307.3mm/year. The groundwater-bearing formation of the area is grouped as very low, low, moderate, high and very high aquifers based on transmissivity and yield of the borehole or springs. Very high productive aquifer in the low lands is recharged from regional groundwater flow and the middle elevated areas aquifer mainly shows local recharge.