Spatial patterns of Importance of Iranian Rainfall Persistency

Iran is located in dry zone of mid-latitude in which the mean rainfall is 250 mm and it has dramatic tempo-spatial changes. Rainfalls with short persistency are of characteristics of dry regions and it is also tangible in Iran. However، Iran’s rainfalls persistency ranges from 1 to 45 days and have dramatic tempo-spatial changes، but the maximum amount and days of rainfalls are supplied by rainfalls with short persistency. So، the phenomenon of rainfalls with long persistency is considered as an extreme event which has extreme variability. With regard to the importance of rainfall، especially in supplying Iran’s water resources، the investigation of the role of different rainfall persistence''s in supplying such resources is necessary. Therefore، understanding the fact that each case of persistency of rainfall produces which share of Iran’s rain days and rainfalls as a whole is necessary. To investigate the spatial patterns of Importance of Iranian Rainfall Persistency، a rainfall database with time-place order has been used. In this database، daily rainfall observations exist from 1961/3/20 till 2004/12/30 with 15-kilometer distances on the 7187 cells. First، different persistence''s، rainfall amounts and raining days in the time series of every cell were evaluated and in addition to that، different persistence''s share in the supply of rainfall days and rainfall amount of each cell were also calculated. The proportion of these two estimates gives the importance of rainfall persistency in various regions of Iran. Results showed that Importance of Iranian Rainfall Persistence''s have various spatial patterns. The cases of rainfall persistence from 1 day till 7 days exist approximately in all parts of Iran and larger cases of rainfall persistence are only present in some parts of Iran. The share of rainfall persistency sharply reduced with the increase of raining length. In contrast، Increasing length of persistency does not reduce raining in all parts of Iran. One-day cases of rainfall persistency have the greatest role in the formation of raining days and the production of Iran’s rainfalls whose role is tangible in Central، Western and Southwestern parts (mid-east). In contrast، increasing length of persistency of the rains، rain days and rainfall reduced with different spatial patterns. As the persistency of precipitations increases، the share of precipitation in the Central and Southwestern Iran decrease severely، but in Western and Northern Iran، vice versa is the case. In some of high precipitation regions of Iran’s Western and Northern half، the decrease of rainfall persistency is together with the decrease of the share of precipitation supply and in other regions (high precipitation regions)، and the decrease of the share of precipitation supply is gradual. Therefore، it is a various spatial patterns of share of the supply of rainfall and rainy days by means of different rainfall persistence''s that implies the various spatial patterns of Importance of Iranian Rainfall Persistency. The importance of one-day rainfalls did not show any specific spatial pattern. Therefore، most of the precipitation of persistency is considered the least important of them. The greater the length of persistency rainfall، most of them is important and creates different spatial patterns. The importance of short persistence''s (1 to 3 days) in the eastern half and long persistence''s (more than six days) in the western half of Iran is maximum.