Long-Term Trend Analysis of Climate and Hydrological Series using Removal of the Autocorrelation Statistical Methods in the Eastern Gorganrood Basin, Golestan Province

Given the importance of climate change issue, consideration of the climate and hydrological data trend has become very important. This study aimed to evaluate the effectiveness of different methods to remove the effect of autocorrelation in assessing the trend of parameters such as temperature, precipitation and discharge of the Eastern Gorganrood Basin, Golestan Province. For this purpose, in addition to conventional Mann-Kendall test, nonparametric tests like MK-PW, MK-TFPW, MK-VCA, MK-SA and MK-HSA were used for trend analysis. As well as determining the slope of the trend line and identifying the jump points, San slope estimator and Mann-Kendall sequence (SQMK) were used. The results showed that the values of temperature and precipitation at all stations uptrend and this was significant for temperature in Lazoreh stations and Ramian. Also, discharge flow in Arazkuseh station had significant descending trend. Using nonparametric tests decreased Z statistic. In most stations, MK-PW method had the minimum significance and MK-TFPW and MK-VCA methods showed better performance and reduced the risk of Type I error. In the station where there was no significance in the first and higher order autocorrelation, MK-VCA and MK-PW methods provided similar results. MK-TFPW showed a different behavior due to calculating the autocorrelation after the removal process. Considering of turning points in Mann-Kendall sequence test indicated that at Arazkuseh, Ramiyan, Nodeh, Tamar and Ghazaghli stations, the begining point of trend was related to the years 2000, 1987, 1988, 2001 and 2001 and only in 2 stations including Tamar and Ghazaghli, it was significant in the years 2006 and 2005 (at 5%). In all of the stations, temperature had an abrupt change point of different trend and the time of increasing temperature was equal to the point jump of discharge decreasing trend in 1993 in Arazkuseh station.