Estimating the runoff coefficient that is influenced by morphometric, geologic and hydro climatologically factors are the most important issues in hydrology and information of its role in the planning and management of water resources is more important. In this research, twenty hydrometric stations with common period from 1974 to 1999were selected. Physiographic parameters of the catchments from the GIS environment were extracted. Run off coefficient was calculated and  then base flow and related index were extracted from daily stream flow data using one parameter recursive digital filters. Lithological units using digital geological map, with the scale of 1: 250,000, based on expert opinion divided on two classes and area covered by each unit in each catchment were calculated. Factor analysis using 15 parameters were conducted and then the regional equations using linear regression at 1% significant level were determined. To compare and evaluate the accuracy and efficiency of the models, independence errors, colinerity and normal distribution of error were tested. Also the accuracy of the models and their estimation error using coefficient of determination, the standard error and the mean absolute error, were examined. Overall results showed that the estimated error rate for first homogeneous area 17.97 percent and for the second area 27.81 percent obtained

Keywords: Base flow, Digital filters, Lithological units, Regional models, Run off Coefficient

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