Long term trend detection of annual precipitation over Iran in relation with changes in frequency of daily extremes precipitation

In current study, we underlined the trends of annual precipitation over Iran in relation with changes in high and low extremes and normal values of precipitation from spatial analysis point of view. To this end, the third version of Asfazzari national database with 10 10 km spatial resolution and daily temporal resolution for 46 years (1970/3/21 to 2016/3/19) is adopted. Trend analysis depicted that in the major parts of the country (approximately 80.9%) the annual precipitation has experienced decreasing trends which tend to reducing about 1.5 billions cubic meters of input water in the hydrologic system of the country. Our finding showed that the decreasing trends of precipitation prompted by large scale and global systems. However, the local factors (longitude, latitude, elevation, slop) are culprits for changing the effects of abovementioned factors. These factors cause the decreasing trends to be statistically significant solely in a part of 33.53% of Iran territory (northeast, parts of eastern of Iran, parts of inner slops of Zagros mountain chain, an area from Kerman to Isfahan, Charmahal-o-Bakhtiari, Kohgiloyeh-oBoyerAhmad, from west of Shiraz, and north of Bushehr to Sanandaj in the west of Iran). Furthermore, the results of our study illustrated that the decreasing trends are due to decreasing in high values of precipitation, particularly decreasing in the 75th percentiles and above of precipitation values. The areas which include increasing trends covered spatially spread areas of about 2.93% of the country. This area has not been able to compensate for the lack of precipitation due to decreasing trends in the area associated with statistically significant decrease in precipitation.