Zahabiyoun - Assistant professor, Iran University of Science and Technology,Narmak, Tehran, I.R. of Iran

Since General Circulation Models (GCMs) do nor provide useable results on a time scale such a day on a spatial scale such as a catchnent of about 1000 km2 , a methodology is required to predict the changes which may occur in the climate inputs of a catchment , and the resulting impactes on water resource.. the approach developed here foe construction of rainfall scenario (as the main indication of clanate change scenario) cmployed a stochastic model for rainfall (Neyman – Scolt Rectaugular Pulses , WERP, model) to generale daily rainfall . this model has been valiated using historic records for the study catchment . the transient GCM climate scenarios are used as the staring point for assessing climate change impacts. Regression relationships are derived between almostpheric circulation variables and rainfall statistics used in fitling the NSRP model foe present climate conditions and then used predict the rainfall statistics for fulure conditions using GCM outputs. That is , the scenarios of a climate model calehment scale. Generaled railfall scenario may be used as input to a catchment response model to generate daily streamflow data to assess water resource impacts.