Rainfall Trend Analysis of Mazandaran Province Using Regional Mann-Kendall Test

Changes in the rainfall trends can be a sign of the global climate change. The study on the hydroclimate trends can be conducted at both station and regional scales. Fewer studies have however paid attention to this subject at the regional scale. In this study, existence of a trend in both mean annual rainfall amounts and maximum 24-hour rainfall amounts over a 30-year period for 35 raingauge stations located in Mazandaran Province, northern Iran, is investigated with nonparametric Mann-Kendall test at both local and regional scales. The Trend Free Pre-Whitening approach (TFPW) was used to remove the effects of serial correlation in the time series on Mann-Kendall test. Removal of auto correlation in the time series is also performed by the same approach. The results of the regional Mann-Kendall test showed that if Mazandaran Province is considered as a unit region, no trend in the time series of maximum 24-hour rainfall is observed in 5% significance level. But if Mazandaran Province is divided into hydrologic regions using Fuzzy cluster analysis, it is observed that a significant trend exists in the western part of the province at 5% significance level. The investigation of the mean annual rainfall time series indicates an upward trend at 1% significance level for the whole province.