Mehoni sub-basin which is found in the Raya Valley is located in the southern part of TigrayrnRegional State of the Northern Ethiopia. The area covers 60% of the Raya valley with arearncoverage of 1183km2.The sub-basin water resource is used for irrigation and domestic waterrnsupply and is covered by tertiary volcanic in western and eastern highlands and alluvial depositrnin the valley fill. The western rocks are affected by tectonic movement .The N-S faulting isrnresponsible for tilting of the rock formations with a general strike NE-SW and dipping to the SErndirection.rnAs numerical groundwater flow models represent the simplification of complex natural systems,rndifferent parameters were assembled into conceptual model to represent the complex naturalrnsystem in a simplified form. The conceptual model was put into the numeric model to examinernsystem response.rnNumerical groundwater flow was simulated in the model by the finite-difference method usingrnMODFLOW, 1996 (McDonald and Harabaugh, 1988). The finite difference grid consisted of 1rnlayer, 116 rows and 111 columns. Two dimensional profile model was developed considering thernsystem to be under steady state condition and assuming flow system view point. Four scenariosrnIncrease withdrawals by three fold of the average existing withdrawals for whole sub-basin andrnlocal areas, assigned 25 wells with daily discharge 1000m3/d and decrease recharge by 50% werernused. Model calibration was carried out by trial and error calibration method using groundwaterrncontours constructed from heads collected in 40 observation points. The calibration showed thatrnabout 97.5%of simulated heads were within the calibration target and the overall root meanrnsquare error for simulated hydraulic heads is about 8.52m. The poor fit at some points was due tornnumerous limitations associated with the model.rnModel sensitivity analysis was conducted by considering the horizontal hydraulic conductivityrnand recharge because they are sensitive. A change in hydraulic conductivity by -55%,-45%,-25%rnand 55%, 45% 25% resulted in root mean square (RMS) 27.51, 19.293, 12.948, 8.499, 11.86 and 13.356 respectively. A change in recharge by the same amount as the horizontal hydraulicrnconductivity RMS head changes by 9.553 ,9.254, 8.854,8.290, 8.385, and 8.571 respectively.rnThe calibrated model was used to simulate the possible effect of increased groundwater pumping.rnIncrease of three fold of withdrawals from average existing withdrawals in the whole sub-basinrnhas resulted in the maximum and minimum decline in head by 2.33m and 0.56m respectively.rnLocal increase of withdrawal by three fold of an average existing withdrawal indicated that therntwo (volcanic and alluvial) aquifer systems are not interconnected.