چکیده:
در پژوهش حاضر روند بارش سالانۀ ایرانزمین با توجه به روند تغییرات فراوانی نمایههای فرین بالا و پایین و نیز نمایههای بهنجار بارش، به روش تحلیل پهنهای در معرض توجه قرار گرفت. برای انجام پژوهش حاضر از نسخۀ سوم پایگاه دادهای اسفزاری با تفکیک مکانی 10 کیلومتر و برای بازه زمانی 01/01/1349 تا 29/12/1394 استفاده شد. روند بارش سالانه در کشور نشان داد که تغییرات بارش در حدود 9/80 درصد مساحت کشور کاهشی بودهاست و تنها حدود 19 درصد از مساحت کشور روند افزایشی را تجربه کردهاست. روند کاهشی بارش باعث کاهش آب حاصل از نزولات جوی در کشور به میزان حدود 5/1 میلیارد متر مکعب شدهاست. اگرچه استنباطهای آماری نشان از عملکرد عوامل بزرگ مقیاس در شکلگیری روند کاهشی در بخش عمدهای از کشور است، امّا عوامل محلّی باعث شدهاست که تنها 53/33 درصد پهنههای توأم با روند کاهشی ( شمالشرق و بخشهایی از شرق ایران، پیشکوههای داخلی زاگرس و پهنهای ممتد از استان کرمان تا استانهای اصفهان، چهارمحال و بختیاری و کهگیلویه بویراحمد، از غرب شیراز و شمال بوشهر در امتداد مرز غربی کشور تا شمال سنندج، هستههای پراکنده در بخشهای جنوبی و شرقی ناحیۀ شمالغرب و شمال ایران) به لحاظ آماری معنیدار بودهاست. نواحی توأم با روند کاهشی معنیدار تحت تاثیر کاهش بارندگیهای پرمقدار (عمدتاً صدک 75 و بیشتر) بودهاند. نواحی توأم با روند مثبت و بهلحاظ آماری معنیدار حدود 93/2 درصد از کشور را دربر میگیرند. این نواحی با گسستگی مکانی و بهصورت هستههایی بعضاً در درون نواحی توأم با روند منفی دیده میشوند.
Introduction Trend analysis includes three primary components; trend detection, trend estimation, and trend modeling. Graphical and analytical techniques are common pieces of the instrument which used in trend detection, as our concern in the current study. Trend detection has caught attentions of a major part of experts in the field. Correspondingly, trend detection of precipitation, as a climatic element which is sensitive to climate change, is analyzed by many climatologists in Iran and experts around the world. In current study, we underlined the trends of annual precipitation (TAP) over Iran in relation with changes in high and low extreme and normal values of precipitation from spatial analysis point of view. To this end, the third version of Asfazzari database with 10 km spatial resolution and daily temporal resolution for 46 years (1970/3/21 to 2016/3/19) was adopted. Methods and Material The third version of Asfazari database was adopted in the current study. The database resulted from interpolating 2188 synoptic, climatologic, and rain gauge stations. This database characteristic of 10 km spatial resolution and daily temporal resolution for 46 years (1970/3/21 to 2016/3/19). According to Iran's territory, 25-40 N and 44-64 E, and based on spatial resolution, and also due to the 16801 under investigation days, the dataset matrix includes a 167×205×16801 dimensions. With reference to above mentioned dataset the annual total precipitation calculated in order to provide a general picture of spatial distribution of annual precipitation. The annual total precipitation also calculated for each map's pixel and for every individual year. Afterward, the annual changes and its P-value calculated for each and every pixel. Consequently, annual mean and trends of the following factors are calculated: 1) The numbers of days containing low amounts of precipitation. These days, including dry days, days with precipitation equal or lower than 10th and 25th percentiles. 2) The number of days, including high amount of precipitation. These days, including days with precipitation equal or higher than 70th and 90th percentiles. 3) The number of normal amount of precipitation, days with precipitation between 25th and 75th percentiles, and rainy days. Results and Discussion - Long term of TAP According to the estimation of trends of precipitation of Iran for the understudy period (1970-1916), the declining trends are common in a majority (80.9% of Iranian territory) parts of the country. However, only 33.58% of these areas showed statistically significant decreasing in precipitation. The areas to increase in precipitation include 19% of the understudy area. The area with statistical significance, nevertheless, includes 2.93% of Iran's territory. Geographical distribution of areas covered by decreasing trends is as follows: Region 1: This region is the most widespread in the country which predominated on the northeast and the eastern part of Iran. Region 2: This region includes the inside slope of Zagros mountain chain from the Kerman province to the Isfahan, the Chahar-mahal-o Bakhtiari and the Kohgiloyeh- BoyerAhmad. In addition, a discontinued areas from north of Isfahan to Hamedan are among of this region. Region 3: This region started in the west of Shiraz and the north of Bushehr and continues between western border of the country and outside slop of Zagros mountain in the west to north of Sanandaj. Region 4: This region comprise a set of discontinued small areas in the northwest and north of Iran. Geographic distribution of positive trends cells can be seen in three main regions as follow: North region: Part of this region includes middle to the eastern part of the northwest of Iran. Another part is scattered in the east of Rasht, Lahijan, Gorgan, Bojnoord, and south of Bojnoord . Inner Slope of Zagros: This scattered small region started from the west of Ghazvin to by- stander of Yazd High Elevation region in the west: This region coincident with highest parts of the Zagros and includes the Iran - Turkey borderline from Oroomieh to Sanandaj. Apart from above-mentioned parts of Iran, the rest of the country comprise non significant trends. Thus, we consider these parts as a region with stationary behavior in precipitation. The role of STF on the temporal TAp It is important to distinguish the (internal or external) origins of the different trends (increasing, decreasing, and stationary) which is in relation with local and large scale events. According to our finding, the relationship between the trends in annual precipitation and STF (regardless of significance in correlation, and ordinary or partial correlation) is noticeably low. Discriminating negative and positive trends illustrated the effects of longitude on TAP, according to which the increasing trends decreased toward east and decreasing trends emboldened. Elevation is among of topographic factors which shows a small amount of positive relation with trends in annual precipitation. These finding depicted that the local factors have small impacts on the trends. The strong relationship between the trend and annual precipitation amount proves the finding. The same results are true for the area with significant trends. Changes in water input of hydrological cycle of the country The calculation of the change of the amount of water the country received, exhibited that in a vast area (26.8% of the country) experienced decreases in precipitation by 1-4 mm / year. This decreasing trend dwindles the hydrological input up to 1.5 billion cubic meters per year, which does not compensate for the region with positive trends. General features and trends in extreme precipitation in Iran The average and trends of a number of days with high and low extreme also the normal values are calculated in order to discover the areas including the trends in those values. The correlation coefficients between the precipitation amounts and the dry days (-0.73) is negative and for the other extreme factors is positive. The trends in values equal or lower than 10th and 25th percentile of precipitation, possibly due to local geographical condition, do not show any predictable pattern. Accordingly, it could not infer any logical relationship between these factors and TAP. A study of TAP in relation with understudy extreme indices showed that the areas with negative TAP experienced these decreasing in precipitation due to declining in values of precipitation equal to or more than 75 percentile. Conclusion The local and synoptic - global factors control the tempo - spatial distribution of annual precipitation in Iran. The small changes in these factors may lead to extreme climatic anomalies. In the current study, the TAP, and local-circulation factors in relation to TAP, and effects of extreme values on TAP are investigated. To this end, the Asfazari database with daily temporal resolution and 10 km spatial resolution is used. The prevalent TAP feature over Iran is recognized with declining TAP (at least 1mm/year and primarily 1-4 mm/year). This TAP prompts a declining of water input to the 1.5 billion of cubic meters. This TAP results from many geographical factors which lead to tempo -spatial variety in TAP. To sum up, the decreasing changes were due to decreasing in high values of precipitation. Based on our finding, Iran will face many more intense anomalous dry years. It is vital to consider this condition in water management, trend of precipitation mitigation, and water resources selection. Keywords: Trend detection, Iran Annual precipitation, Precipitation trends, Extreme precipitation
خلاصه ماشینی:
مطالعات در این زمینه را میتوان به دو گروه تقسـیم نمـود؛ اول مطالعـاتی کـه بـه رونـد بارش در نقاط منفرد توجه داشته اند (برای مثال مقیمـی، ١٣٧٨؛ عسـاکره ، ١٣٨٢؛ کاویـانی و عسـاکره ، ١٣٨٤، بیـنش و همکاران ، ١٣٩٦) و دوم مطالعات متکی بر پایگاه داده ای یا تحلیل هـای پهنه ایکـه اخیـرا رواج زیـادی داشـته اسـت و آشکارسازی روند بارش را برای یک ناحیه از کشور (برای نمونه میرعباسـی نجف آبـادی و دین پـژوه ، ١٣٩١؛ دارنـد و همکاران ، ١٣٩٣؛ اکبـری و نـودهی، ١٣٩٤؛ خـوش رو و همکـاران ، ١٣٩٦)، یـا کـل کشـور (بـه عنوان نمونـه غیـور و مسعودیان ، ١٣٧٥؛ عساکره ، ١٣٨٦؛ کتیرایی، ١٣٨٦؛ محمدی، ١٣٩٠؛ اختصاصی و همکاران ، ١٣٩٤؛ نـاظری تهـرودی و همکاران ، ١٣٩٥) مورد آزمون قرار میداده اند.
(رجوع شود به تصویر صفحه) شکل ٤- توزیع مکانی روند تغییرات بارش سالانۀ ایران (میلیمتر در سال ) برای دورٔە ١٣٩٤-١٣٤٩ و نواحی توأم با تغییرات معنیدار و فاقد معنی آماری تهیه و ترسیم : نگارندگان میتوان دید که روند تغییرات در بخش عمده ای (حدود ٨٠/٩ درصد) از مسـاحت کشـور کاهشـی بـوده اسـت و تنها بخش محدودی از مساحت کشور (حدود ١٩درصد) روند افزایشی را تجربه کرده است .