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This article investigates the effect of different lengths on prediction accuracy. For this purpose, 732 monthly data of streamflow of the Kortian gauging station at the Kortian Stream Watershed were used. To study the impact of the type of data in terms of monthly or seasonal observation data on the accuracy of modeling results, monthly data were converted into seasonal data and the results of monthly and seasonal modeling were compared. Therefore, multiplicative ARIMA models were performed for the monthly and seasonal modeling. Compared with the seasonal modeling, the monthly modeling presented more precise results that the sum of the squares errors of monthly and seasonal modeling were 0.9408 and 2.5 respectively. For the monthly modeling, 5 different lengths of data were used. C1 model used the last 60 data, C2 used the last 120 recorded observations, C3 used the last 240 data, C4 used the last 480 observations and C5 used the last 708 data. To test the precision of models, 24 observations were held out. Among C1 to C5 models, the C4 model presented the best results in predicting 2 years ahead and C1 had the worst results.