The effect of time series length on runoff characteristics analysis

The length of time series is of great importance, which is an uncertain factor in runoff characteristics analysis, especially in the analysis of trend and period. This paper investigates the tendency and period using runoff observation data of 107 years from 1909 to 2015 at Monticello hydrological station in the Upper Sangamon River in USA. The data series of 30 years, 40 years, 50 years and so on until 100 years and 107 years are selected to test their trend and period, respectively, and then are compared with each other. The results show that, with the increase of time series length, the tendency and period tend to be stable, and the annual runoff begins to show a significant increase trend once the length of time series is more than 80 years. The period tends to a dynamic stability value of 22 years when the time series length is more than 70 years. When the series length is larger than 70 years, the results of runoff characteristics analysis are much more reliable.