In this study, a mathematical modeling program called Hydrological Simulation Program-FORTRAN (HSPF) developed by the United States Environmental Protection Agency (EPA) was used for the hydrological modeling of the Middle Seydi Suyu Watershed in Turkey for the 1991-1994 water years. The model was calibrated using the water years 1991-1993, and validated with data from water year 1994. The response of the watershed to various scenarios was determined and compared to base simulations conducted for the 1991-1994 water years. In the scenarios, the effects of an increase in temperature due to climate change, and the effects of maximum and minimum watershed cover modifications were examined. The results indicate that an annual mean temperature increase of 3 degreesC due to climate change will decrease the watershed outflows by 21%. The existence of the deep rooted vegetation covering the whole of the watershed is observed to cause a decrease in the total stream outflow by 37% compared to base model results. In the case of no deep rooted vegetation the total stream outflow increases by 40% compared to the current vegetation distribution. (C) 2003 Elsevier B.V. All rights reserved.