آشکارسازی روند سری‌های زمانی هواشناسی در حوزه آبخیز گرگان رود در سه دهه اخیر

  Due to global warming, and its resulting changes in the frequency and amount of precipitation and temperature, there has been a great concern on climate change impacts by the scientific community. This study focuses on trend analysis of annual and seasonal rainfall (P), maximum temperature (Tmax) and minimum temperature (Tmin) time series from seven meteorological stations in Gorgranrood river basin during 1982-2011. Three statistical tests including, Mann-Kendall (MK), the Modified Mann-Kendall (MMK) test and Man Wintney (MW) were examined at 10% significance level. To quantify the slope we used Sen’s non-parametric estimator of slope. Both positive and negative trends were identified by all the tests. However, all negative trends were insignificant and only some positive trends were significant. Trend tests reveal that the median of slopes of seasonal series of Tmax, Tmin and P are located above zero line which is revealed that increasing trend is observed in most of the station Steepest positive slope of trend line of the Tmax is found in the spring then in summer time. Trend analysis has revealed strong seasonal variability of Tmin series. The highest number of stations with significant trends occurred in the Tmin while less significant trends were detected in the Tmax and P. There is a non-significant increasing rainfall trend in the basin. With regard to seasonality P, it is remarkable that positive significant trends were detected in few stations at both annual and seasonally time scale. In order to remove serial correlation, the trend-free pre-whitening procedure has been applied and less number of positive trends has been again observed, highlighting that, in this case, serial correlation affect the results of Mann-Kendall trend test.