In this study it has been aimed to predict the effects of climate change on watersheds by using a comprehensive modeling tool, HSPF (Hydrologic Simulation Program- Fortran) which can simulate both the hydrological cycle and processes affecting water quality in a watershed. An agricultural watershed has been modeled by HSPF. Base simulations have been conducted to assess the present hydrological conditions prevailing on the watershed. Subsequently, by reflecting the climate change likely to occur in the Mediterranean countries in the 2 Is' century into the relevant input meteorological time series, the simulations have been repeated to compute the watershed outflows and streamflows emanating from the watershed. Among other results, it has been found out that the climate change will affect streamflows and water yields profoundly. The results obtained from the scenarios prepared constitute an invaluable aid in the planning of the future management of the watershed.