The study of rainfall characteristics changes is critical for agricultural, environmental and water resources projects. Generally, trend test performs for evaluation of direction and magnitude of changes, but the test is not able to demonstrate abrupt change in time series if it occurs. In this research work, recent changes of rainfall characteristics of the Karun and Dez watershed, located in southwest of Iran, were evaluated. For determination of change points and trends of rainfall characteristics of varrious time series, Pettitt and Man-Kendall tests were applied to analyse these time series. A forty-eight (48)-time series from 12 meteorological stations in the aforementioned watershed were investigated. Results of the study showed that the change points in 5-time series were statistically significant at 95% confidence level. Change points produced considerable errors in evaluation of magnitude and direction of trends of the time series. Therefore, for prevention of false conclusion, it is suggested that change points of the time series must be known before analysis.
Key words: Trend, precipitation characteristics, Pettitt test, Mann-Kendall test.
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