Analysis of trends in annual and summer rainfall of Golestan Province

Rainfall, as the most important climatology element is the most variable element and has a variety of area and time difference in different spatial-time scales. Due to the important role of precipitation in the determining physical and geographical characteristics of a region, the element has a significant importance in natural sciences. Considering the location of Iran in arid and semiarid regions of the world, changes in rainfall amounts in water resources management issues makes it even much more important. This study was conducted to examine the annual and summer rainfall trends using 9 rain gauge stations data in Golestan province and in a 41-year period (1971-2011). The non-parametric Mann-Kendall was tested in seasonal and annual time scales. Using statistical distributions, the days with heavy and widespread summer rainfall were selected and studied. According to the results, in the 41-year period, annual rainfall in most stations showed no descending or ascending trends (static) but in Bahlake Dashli station ascending trend was observed. Descending trend in summer rainfall amounts in most stations was observed while number of days with heavy summer rainfall was increasing. Using data geopotential levels of 500 and 850 hPa, omega 500 hPa, the average long-term geopotential 500 hPa and anomaly data from NOAA, synoptic patterns of the region of 41-year (2011-1970) and in the days with heavy and widespread rainfall were studied and compared. The results showed that the North East region of Iran, due to the changes in pressure patterns had experienced more changes than other parts of the country and July had the greatest changes amount in summer months. These changes lead to changes in rainfall amounts and increasing the number of days with heavy rainfalls.