The analysis of precipitation variation and quantiles in Iran

Daily precipitation data from 38 stations in Iran are analyzed for the period of 1960-2001 to evaluate the possible long term trend in annual and seasonal precipitation. The Mann-Kendall method was used to test the existence of trends, and the least square method to estimate the values of trend. The results show that the annual precipitation had a decreasing trend at stations in the west, northwest and southeast and an increasing trend at most of the stations in the other parts of the country. Most of the stations have shown a reduced precipitation during the spring. There is a significantly positive trend in the total number of rainy days during the year and for each season, while the mean daily precipitation over the rainy days has decreased at most of the stations. The analysis of trend has also performed for different classes of intensities of daily precipitation. To do this, daily precipitation series for each station were sorted according to their magnitudes and categorized into 10 class intervals. The intensity thresholds for each class are defined separately for the year and for each season in a way that the sum of precipitation falling in that class equals 10% of the total annual or seasonal precipitation over the period 1960-2001. The results show that the type of variations of the total precipitation is mainly determined by the trend in heavy precipitation. For instance, for the stations (seasons) where total precipitation has a positive, large and often significant trend heavy precipitation shows an increasing trend and light precipitation shows a decreasing trend. For some other stations (seasons), where total precipitation has a large negative trend, there is negative trend in the upper intensity quantiles and a positive trend in lower intensity quantiles.