A Study of Annual and Seasonal Precipitation Trends in Iran Using a Nonparametric Method)Sen's Slope Estimator (

IntroductionPrecipitation has always been under investigation as the most important climatic element from both temporal and spatial perspectives. One of the methods that contribute to the review of the course of precipitation in the past and present is the 'time series trend analysis'. So far, numerous statistical methods have been provided for the time series trend analysis, which can be categorized into two parametric and nonparametric methods. The nonparametric methods are based on the differences between observation data; these methods are independent from statistical distribution and are more appropriate for the series with more skewness and kurtosis. Therefore, the objective of this study is to investigate the precipitation time series trend over Iran in seasonal and annual scales using the Sen’s slope estimator nonparametric method.Materials and MethodsIn order to examine precipitation changes in Iran and make a proper pattern for this field, the data related to total monthly precipitation of 44 synoptic stations in a 33-year statistical period (1976-2008) were received from Iran Meteorological Organization.In order to investigate precipitation time series trend of Iran on seasonal and annual scales, the Sen’s slope estimator nonparametric method was applied. This method is based on a mean slope for the time series and judgment of the significance of the obtained slope in different confidence levels.Results and Discussion The implementation of the Sen’s nonparametric method on in seasonal and annual precipitation time series of 44 synoptic stations indicated that despite the decreasing (28 stations) and incremental (14 stations) signs which were observed in pring precipitations of Iran, no trend was significant at the probability level of 0.05. Also, in autumn, considering that 33 stations had a decreasing trend and 2 stations experienced an incremental one, only Sanandaj station indicated a significant decreasing trend.In summer, due to subtropical high pressure system in Iran, most parts of this vast country lacked any kind of precipitation. However, significant incremental trends were observed at the probability level of 0.05 in Hamedan’s-Nozhe, Saghez, and Zanjan stations.However, winter precipitation had the highest frequency in this trend compared with other seasons since five stations of Gorgan, Kerman, Khoy, Sanandaj, and Zanjan had significant decreasing trends at the probability level of 0.05 in this season. Therefore, because the largest amount of Iran’s precipitations occurs in the winter, any changes in the precipitation trend of this season could be a warning for water domain planners.The annual precipitation trend also indicated a significant decreasing trend at the probability level of 0.05 in 11 stations out of total 44 studied stations. The accumulation of stations with the decreasing trend in the west, northwest and east of Iran might signify changes in precipitation conditions in these parts of Iran. Although of a significant trend in the precipitation time series cannot guarantee the occurrence of climate change in a region, it might enhance the possibility of its occurrence. Therefore, it is essential to consider the occurred changes in the westest and northwest of Iran, especially in Sanandaj station which experienced autumn, winter and annual trend changes.ConclusionUsing this method, the results indicate that there was no significant trend at the probability level of 0.05 in total spring precipitation and autumn precipitation of Iran and; only Sanandaj station experienced a significant decreasing trend. In summer, Hamedan’s-Nozhe, Saghez and Zanjan stations showed significant incremental trends. However, winter precipitation had the maximum trend changes compared with other seasons because Gorgan, Kerman, Khoy, Sanandaj, and Zanjan stations showed a significant decreasing trend in this season which was at the probability level of 0.05. The annual precipitation trend signified a significant decreasing trend at the probability level of 0.05 in 11 stations out of 44 studied stations. The accumulation of stations with decreasing trend in the west, northwest and east of Iran signified changes in precipitation conditions in these parts of Iran. Therefore, special attention should be paid to changes which occurred in west and northwest of Iran.