Simulation of large basins (over 1,000 km2) is needed for largescale water resources planning and management. Due to increased volume of calculations and increased heterogeneity, basin simulation is challenging. In areas with severe withdrawal of water resources, simulating and having the ability to predict future changes is important, while severe withdrawals complicate issues and simulation. The aim of this study is to evaluate the ability of Hydro Geosphere, a fully integrated hydrologic model, to simulate a large basin (Hamadan-Bahar Catchment, Iran) with an area of 2,456 km2 and severe groundwater withdrawals. In this study, fully-integrated surface/ subsurface flow modeling was done using the Hydro Geosphere model. Simultaneous solution of surface and groundwater flow equations and calculation of actual evapotranspiration as a function of soil moisture in each unit of evaporation zone improves the simulation of interdependent processes such as aquifer recharge and drainage, which is one of the difficult issues in modeling. To obtain initial conditions, the model was used in steady state mode using 20-year average of rainfall data and withdrawal from the aquifer. Then, to apply the model in unsteady state and evaluate its performance in daily stresses, the model was used for the period of 19922005 and parameters were calibrated. Validation was done for the period of 2006-2010 and results showed satisfactory hydrologic simulation of the study area.
