Trend Analysis of annual rainfall over Iran

Identification of heavy rainfall events are important in the design of water-related structures, agriculture, weather modification, policy making and planning and in monitoring climate change. Generally the change in rainfall in the world was examined in both time and space. Hence, much attention has been paid to different methods of analysis of extreme precipitations during recent years.Karl and Knight (1998), studied the twentieth century trends of precipitation are examined by a variety of methods to more fully describe how precipitation has changed or varied. They believed that since 1910, precipitation has increased by about 10% across the contiguous United States. The increase in precipitation is reflected primarily in the heavy and extreme daily precipitation events. For example, over half 53% of the total increase of precipitation is due to positive trends in the upper 10 percentiles of the precipitation distribution.