الگوي توزيعي جهت و شدت روند تغييرات خشكسالي ايران به كمك نمايه توزيعي شدت خشكسالي پالمر

Spatial pattern of droughts intensity and direction variations using trend analysis of Distributed Self-Calibrated Palmer Drought Severity Index in Iran

خشكسالي در طول تاريخ يكي از تهديدهاي اصلي ساكنان و تمدن هاي كشور ايران بوده است. نمايه هاي زيادي بر مبناي داده هاي هوانشناسي، ماهواره اي و مدل هاي شبيه سازي به منظور تعيين شدت خشكسالي توسعه داده شده است. در اين تحقيق توزيع مكاني نمايه شدت خشكسالي پالمر براي يك دوره 36 ساله (1975 تا 2010) با استفاده از داده هاي شبكه اي شده خاك و هواشناسي (متشكل از 296 ايستگاه سينوپتيك و بيش از 1500 ايستگاه بارانسنجي) با تفكيك مكاني 4 كيلومتر و تفكيك زماني ماهانه، توسط كد نويسي در نرم افزار مطلب در مقياس كشوري محاسبه و استخراج شد. براي مشاهده الگوي شدت و روند خشكسالي از رونديابي با روش رگرسيون خطي با گام زماني يك ماهه و بصورت توزيع مكاني استفاده شد. نتيجه محاسبات نشان مي دهد كه بيش از 60 درصد از سطح مورد مطالعه داراي روند منفي (افزايش خشكسالي) بوده و كمتر از 4 درصد از آن روندي مثبت (بهبود ترسالي) دارد (در سطح 5 درصد معني دار). شيب روند نيز بين 12/0 تا 22/0- متغير است.

Spatial pattern of droughts intensity and direction variations using trend analysis of Distributed Self-Calibrated Palmer Drought Severity Index in Iran Mohammad reza Keshavarz: PhD student of Irrigation and Remediation, College of Agriculture, Tehran University Majid Vazifedoust: Assistant Professor of Water Engineering, College of Agriculture, Guilan University Ebrahim Fattahi: Associate Professor of Atmospheric Science and Meteorological Research Center Mohammad Bagher Behyar: Assistant Professor of Atmospheric Science and Meteorological Research Center Abstract Drought has been always known as one of the most important threats for civilizations in Iran. Several indices based on meteorological data, satellite data and simulation model has been introduced for detection of drought extension. In this study, spatial distributions of PDSI (Palmer Drought Severity Index) patterns were derived for a 36-year-period (1975-2010) using gridded soil properties and meteorological data consists of 296 synoptic stations and more than 1500 rain gage stations with 4 km spatial resolution. The calculations were performed using an automated interface program developed in MATLAB. To see the drought pattern intensity and its direction spatially, a regression trend analysis was performed in monthly and general time steps pixel by pixel. The results indicated that during the 36-year-period more than 60 percent of the area had a downward trend (increasing droughts) and less than its 4 percent shows upward trend. The slope of trends was variable between -0.22 and 0.12. Keywords: SC-PDSI, PDSI, DSC-PDSI, Drought routing.