SWAT2005 (Soil and Water Assessment Tool) was used to simulate runoff and investigate the effect of various rain-gauge stations on the results of the model in Tamer watershed in northern Iran. The calibrated model will be used to predict the impact of different management operations on the runoff, sediment, and nutrient loads in the 1524 km2 watershed. SUFI2 version 2.1.5 was used to calibrate the model and perform uncertainty analysis. The watershed studied included two climate and rain gauge stations Tamer and Golidagh. Tamer is located at the very end of the watershed and Golidagh is located at the top of the watershed. The data for tamer rain gauge station in watershed was available from 1990 to 2005 and also for Golidagh rain gauge station from 1999 to 2005. Considering available data the model at first was run with both Tamer and Golidagh rain gauge station. The calibration was done in this period. However, calibration results were not satisfactory. To improve the model results also to show the effect of different rain gauge stations on model, the model was run using only Golidagh rain gauge station for calibration period (1999-2005) and Tamer rain gauge station for validation period (1990-1993). With this adjustment the results were improved significantly. Four factors were considered in judging the model performance of runoff: P-factor, R-factor, R2 and NS. The respective values of each were: 0.65, 1.2, 0.55 and 0.55 for calibration and 0.56, 1.2, 0.77 and 0.7 for validation.

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