For optimal management of water resources in a catchment it is important to estimate the amount of water within a catchment by hydrological modeling. Hydrological modeling is very crucial in arid and semi arid areas because of limited water resources in these areas. In this study, the problems of hydrological modeling in these areas have been investigated and particularly is focused on effective rainfall. Then, a method is proposed to calculate effective rainfall and hortonian flow in a catchment. A hydrological distributed model, BTOPMC, is utilized and its performance is compared before and after applying modified hortonian flow formulation. Nash-Sutcliffe and volume error are the criteria used to calculate the model performance. Results have shown that applying modified equation for hortonian flow in BTOPMC have improved Nash-Sutcliffe and volume error about 12% and 10%, respectively that reflects its favorable impact on estimation water amount in a catchment by using BTOPMC.