بررسي تغييرات فضايي سهم بارش‌‌هاي يك‌روزه در تامين روزهاي بارشي و مقدار بارش ايران

An investigation of the spatial changes of one-day (24-Hour) precipitation in the supply of Iran precipitation days and amount

ايران در كمربند خشك عرض ميانه قرار گرفته كه متوسط بارش آن 250 ميلي‌متر است و تغييرات زماني و مكاني شديدي دارد. رويدادهاي بارشي با دوام كوتاه‌مدت يكي از مشخصه‌‌هاي مناطق خشك است كه در ايران نيز ملموس است. هرچند بارش ايران دوامي بين 1 تا 45 روز دارد و داراي تغييرات زماني‌ مكاني شديدي نيز هست اما بيشينه بارش و روزهاي بارشي ايران، از بارش‌‌هاي كوتاه‌دوام تامين مي‌شود. بنابراين رخدادِ بارش‌‌هاي طولاني‌دوام يك رويداد فرين تلقي مي‌شود كه تغييرپذيري شديدي دارد. بارش‌‌هاي با دوام يك تا هفت روزه تقريباً در كل گستره ايران و بارش‌‌هاي با دوام بزرگ‌تر فقط در برخي از قسمت‌‌هاي ايران رخ داده‌اند. با افزايش دوام بارش‌ها، نقش آنها در تامين روزهاي بارشي ايران، به‌شدت كاهش مي‌‌يابد. برخلاف آن، افزايش دوام بارش‌‌ها، سبب كاهش تامين بارش كل گستره‌‌ ايران نمي‌شود. بارش‌‌هاي يك‌روزه، بيشينه‌ روزهاي بارشي كل ايران و بيشينه‌ بارش بخش بزرگي از ايران را تشكيل مي‌‌دهند و اهميت چنداني در ايران ندارند. سهم بارش‌‌هاي يك‌روزه در تامين روزهاي بارشي و مقدار بارش ايران در گذر زمان دچار تغييراتي شده كه براي ارزيابي اين تغييرات از پايگاه داده بارش ايران استفاده شده است. نتايج ارزيابي تغييرات سالانه سهم آنها در تامين روزهاي بارشي و مقدار بارش ايران، نشان مي‌‌دهد كه در يك‌چهارم از گستره ايران از سهم آنها در تامين روزهاي بارشي كاسته شده و فقط در 3 درصد از گستره‌ ايران بر سهم آنها افزوده شده است. علاوه بر آن در يك‌‌‌پنجم از گستره ايران، سهم آنها در تامين مقدار بارش ايران كاهش يافته و فقط در 6 درصد از گستره ايران سهم آنها در تامين بارش ايران افزوده شده است. از طرف ديگر، ارزيابي تغييرات ماهانه سهم بارش‌‌هاي يك‌روزه در تامين بارش ايران، نشان مي‌‌دهد كه گستره روند منفي آنها در ماه‌‌هاي باراني سال بزرگ‌تر از گستره روند مثبت آنها است.

In different regions, precipitation takes place with different persistencies and every persistency supplies a share of rainfall days and precipitation. Therefore, the importance of rainfall persistence could be evaluated in all places. Iran is located in Mid-Latitude of an arid region, in which the mean rainfall is 250 mm and it has dramatic tempo-spatial changes. Rainfalls with short persistence are of characteristics of arid regions and it is also tangible in Iran. However, Iran’s rainfalls persistence 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. As the persistence of precipitations increases, their role in generating Iran’s rainfall days decreases severely in such a way that the maximum rainfall days of Iran is supplied by one-day rainfalls. However, the share of one-day rainfalls in the supply of precipitation days of Iran’s Western half is more accentuated. In contrast, the increase in the persistence of rainfalls does not have an identical role in decreasing the supply of Iran’s precipitation. As the persistence 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 heavy precipitation regions of Iran’s Western half, the decrease of precipitation persistence is associated with the decrease of the share of precipitation supply and in other regions; the decrease of the share of precipitation supply is gradual. Therefore, in every space, some of the persistent rainfalls supply the great share of precipitation days and precipitation amount and are considered important. However, it is possible that this precipitation persistency do not have such importance in those areas. Every kind of variability and change in the role of precipitation persistence in every space will be considerable. Spatial changes of one-day precipitation’s share in the supply of Iran’s precipitation days and precipitation amount could be evaluated from this angle. To evaluate the changes in one-day precipitation’ share in the supply of precipitation days and precipitation amount, the daily observations of precipitations in 1437 stations of throughout Iran was used. Drawing upon Kriging method, the observations of the stations were generalized in a regular network by 15*15 km dimensions and Iran’s isotheral digital maps were developed from 1961/03/21 till 2004/12/30. These digital maps include daily time series (15991 days) of precipitation amount for 7187 cells. Precipitation persistence in the time series of every cell was evaluated and in addition to that, their share in the supply of precipitation days and precipitation amount of each cell were also calculated. Then, the most important persistence of Iran’s precipitations (one-day persistence) was identified and their importance was investigated. Yearly and monthly time series of one-day precipitation’ share in the supply of precipitation days and precipitation amount were entered in a trend analysis for evaluating and understanding its changes and its results were considered. In spatial analyses including identification of climatologically variables trend, more confident way is that firstly, spatial interpolation is done; then, an appropriate trend test is performed on the data on the nodes. The results obtained from such analyses not only enjoy higher degree of spatial attribution, but based on closeness principle, spatial order of points themselves provide intuitional reason for accepting or rejecting trend analysis. One-day precipitations supply more share of Iran’s precipitation days compared to remaining precipitation persistencies in such a manner that it may be noted that in all regions of Iran, the frequency of one-day precipitations is maximum compared to remaining precipitation persistence. In contrast, Iran’s precipitation is provided by different persistencies and the share of one-day precipitations in precipitation supply is maximum only in Western half (Central and Southeastern parts). However, although one-day precipitations do not have much importance throughout Iran, the degree of their importance in Eastern half is maximum compared to Western half. The share of on-day precipitations in the supply of Iran’s precipitation days and precipitation amount has changed with time. The results of yearly changes of share of on-day precipitations in the supply of Iran’s precipitation days and precipitation amount indicate that their share in the supply of precipitation days decreases in one quarter of Iran’s area and only in 3% of Iran’s area, their share increases. Given that Western and Central Iran’s maximum precipitation days are provided by one-day precipitations, precipitation days of Eastern Iran have decreased. In addition, their share in the supply of precipitation days decreases in 1/5 of Iran’s area and only in 6% of Iran’s area, their share has increased. On the other hand, Given that Central Iran’s maximum precipitation days are provided by one-day precipitations, their share in the supply of precipitation days has decreased; just in discrete regions and along with Zagros and Alborz unevennesses, their share increases. The results of yearly changes of share of one-day precipitations in the supply of Iran’s precipitation indicate that their negative trend in all rainfall months is greater than their positive trend. Looking more generally into the share of monthly changes of one-day precipitations in the supply of Iran’s precipitation, the aspects of Iran’s precipitation concentration becomes evident, especially in Eastern and Central Iran.