پايش وضعيت‌هاي رطوبتي و روند آن‌ها بر مبناي شاخص بارش- تبخير و تعرق استاندارد شده (SPEI) در نواحي مختلف آب و هوايي ايران

Assessment of Humidity Conditions and Trends Based on Standardized Precipitation Evapotranspiration Index (SEPI) Over Different Climatic Regions of Iran

خشكسالی یك ویژگی طبیعی تكرار شونده از تغییرات اقلیمی می‌باشد كه اساسا در ارتباط با كمبود بارش تعریف می‌گردد. امروزه همگان پذیرفته‌اند كه خشكسالی یك پدیده‌ی چند متغیره می‌باشد و نوسانات هر یك از متغیرهای جوی و اقلیمی می‌توانند سبب افزایش یا كاهش شدت این پدیده گردد. شاخص نسبتاً جدید بارش- تبخیر و تعرق استاندارد شده (SPEI) از داده‌های بارش و تبخیر و تعرق در قالب یك مدل بیلان آبی بر پایه‌ی مفاهیم شاخص بارش استاندارد شده (SPI) استفاده می‌كند و قادر به در نظر گرفتن اثرات ناشی از گرمایش جهانی می‌باشد. در این تحقیق با بررسی ارتباط بین شاخص SPEI با شاخص‌های SPI و شناسائی خشكسالی (RDI)، از شاخص SPEI برای پایش خشكسالی 11 ایستگاه واقع در 6 منطقه‌ی مختلف آب و هوائی ایران در مقیاس‌های سالانه، كوتاه‌مدت (1، 3 و 6 ماهه) و بلندمدت (9، 12، 18و 24 ماهه) استفاده شده است. هم‌چنین به منظور بررسی روند وضعیت‌های رطوبتی طی 51 سال اخیر (2010- 1960) در كشور از روش گرافیكی LOWESS و آزمون ناپارامتری من– كندال به ترتیب در مقیاس سالانه و ماهانه استفاده شده است. مطابق نتایج بدست آمده بیشترین همبستگی شاخص ‌SPEI با شاخص¬های SPI و RDI مربوط به ایستگاه‌های واقع در نواحی مرطوب ساحلی می‌باشد و با كم‌شدن مقدار رطوبت، همبستگی آن‌ها با یكدیگر كاهش می‌یابد. مقایسه‌ی نتایج شاخص‌های SPI و SPEI نشان داد كه شاخص SPI ضمن داشتن نتایج مناسب در مناطق مرطوب، در مناطق خشك دقت كمتری دارد. هم‌چنین بر اساس نتایج شاخص SPEI در مقیاس‌های بلندمدت، سه دوره‌ی مشخص رطوبتی در طول 51 سال گذشته قابل تشخیص می‌باشد كه شروع وقایع خشكسالی دوره‌ی سوم از سال 1997 می‌باشد. نتایج بررسی روند سالانه وضعیت‌های رطوبتی نیز نشان دهنده‌ی تغییرات افزایشی وقوع خشكسالی‌ها در سال‌های منتهی به سال 2010 می‌باشد. از طرفی بررسی روند وضعیت‌های رطوبتی بر اساس شاخص SPEI یك ماهه نشان می‌دهد كه روند مشاهد شده برای بیشتر ماه‌های سال (اكتبر تا دسامبر و ژانویه تا ژوئن) در ایستگاه‌های مورد بررسی به جز تهران و شیراز بیشتر از نوع كاهشی می‌باشد كه نشانگر افزایش كمبودهای رطوبتی طی سال زراعی می‌باشد.

Introduction: Drought is a recurrent feature of climate that caused by deficiency of precipitation over time. Due to the rise in water demand and alarming climate change, recent year’s observer much focus on drought and drought conditions. A multiple types of deficits and relevant temporal scales can be achieved through the construction of a joint indicator that draws on information from multiple sources and will therefore enable better assessment of drought characteristics including return period, persistent and severity. The Standardized Precipitation Evapotranspiration Index (SPEI) combines information from precipitation and temperature in the form of water surplus or deficit according to Standardized Precipitation Index (SPI). Rainfall over some regions of Iran during some resent year was below average while mean and maximum temperatures were very high during this period, as was evaporation. This would suggest that drought conditions were worse than in previous recent periods with similarly low rainfall. The main objective of this study is to assess the influences of humidity on the SPEI index and investigate its relation with SPI and Reconnaissance Drought Index (RDI) over six different climatic regions in Iran. Materials and Methods: Iran has different climatic conditions which vary from desert in central part to costal wet near the Caspian Sea. In this study the selection of stations was done based on Alijani et al (2008) climatic classification. We chose 11 synoptic stations from six different climatic classes including costal wet (Rasht and Babolsar), semi mountains (Mashhad and Tabriz), mountains (Shiraz and Khoram Abad), semi-arid (Tehran and Semnan), arid (Kerman and Yazd) and costal desert (Bandar Abas). The Meteorological datasets for the aforementioned stations were obtained from the Iran Meteorological Organization (IRIMO) for the period 1960-2010. The compiled data included average monthly values of precipitation, minimum and maximum air temperature, mean relative humidity, sunshine hours) and wind speed at 2 m height. A probability-based overall water deficit assessment was achieved from multiple drought-related indices (i.e. SPEI, SPI and RDI). The humidity conditions were monitored for given stations based on each index during annual, short term (1, 3 and 6 months) and long term (9 , 12, 18 and 24 months) periods. This research further examine the Locally Weighted Scatter plot Smoothing (LOWESS) graphical method and nonparametric Man- Kendal test to evaluate the trends associated with humidity deficiency in annual and monthly time scales during 51 years period (i.e. 1960-2010). Results and Discussion: Our results revealed that the maximum correlation between SPEI index with indices of SPI and RDI was achieved in the coastal wet region and with a declining trend in relative humidity condition in the rest of the regions, this correlation is down over both short- and long-term periods. A comparison between SPI and SPEI also performed that the SPI index was able to reflect prolonged drought over the costal wet region where it showed significant inconstancy in desert and semi desert regions. SPEI result suggested substantial deficiencies in relative humidity at the beginning of 1997 during long term period which indicated an increasing trend of drought statues during last decades. Overall, according to the results of SPEI index in 1month periods monthly drought assessment showed a declining trend in drought magnitude during autumn, winter and spring season months (October to June) at investigated stations excepts Tehran and Shiraz stations and with a potential deficiency in relative humidity conditions. Unlikely, annual trend showed increasing trends in drought frequency and persistent over last decade. Conclusion: Our results can be summarized as below: Focusing on various types of deficits, the result of humidity based deficiencies indicated that for semi-mountains, mountains, semi-arid, arid and costal desert regions the period of 1997 to 2010 has a large total moisture shortage over all climatic regions. Most of the climate stations showed moisture deficits (decline trends) during October to June (9-month) at many stations expect Tehran and Shiraz stations which revealed a significant increasing over 51 years. We recommend using SPEI index for arid and semi-arid regions because it includes temperature variability in drought model so it reflects drought conditions better than other indices. Furthermore, three drought indices (i.e. SPEI, SPI and RDI) have similar sensitivity to water deficits over wet climatic regions; therefore, each of those indices can be used.