چکیده:
خشک سالی یک ناهنجاری اقلیمی است که با کمبود (نبود) بارش و مهیایی منـابع آب در پهنـه جغرافیـایی بزرگ پیوند داشته و در طول دوره قابل توجهی از زمان گسترش مییابد که میتواند بخش های مختلـف از زندگی و محیط زیست را متاثر سازد. نمایه های زیادی برای پایش شرایط خشـک سـالی توسـعه یافتـه انـد. گسترش فضایی، زمانی و شدت (سختی) خشک سالی با استفاده از این شاخص ها مشخص مـی گـردد. در این بررسی، یک شاخص ترکیبی برای ارزیابی خشک سالیهای تلفیقی (آب - هواشناختی) در جنوب شرق ایران توسعه داده شده است . شاخص ترکیبی به طور جامع همه اشکال فیزیکی خشک سـالی (هواشـناختی، آب شناختی و کشاورزی) را از طریق انتخاب متغیرهای مرتبط به هر نوع خشک سالی شامل میشود. در این بررسی، ارزش های روزانه شاخص خشک سالی موثر و شاخص خشک سالی جریان رودخانه ای تحت یک تحلیل مولفه مبنا قرارگرفته و بر اساس مولفه نخست آن ، ارزش شاخص ترکیبی واحد به دست آمده است . دوره مورد بررسی از سال آبی ٦١-١٣٦٠ تا ٩٠- ١٣٨٩ شمسی و سرآغاز سال آبی نیـز از مـاه مهـر تعیـین گردیده است . بر اساس نتایج شاخص ترکیبی، یک دوره بزرگ خشک سـالی از سـال آبـی ٧٩-١٣٧٨ الـی ٨٥-١٣٨٤ در جنوب شرق ایران حاکم بوده است که شدت آن در سال آبی ٨٣-١٣٨٢ فوق العاده زیاد بـوده است . توزیع فضایی بارش ایران در این هنگام نیز آشکار میسازد که در فصل های پـاییز و زمسـتان مقـدار دریافتی بارش در منطقه جنـوب شـرق ایـران بسـیار نـاچیز بـوده اسـت . روش شناسـی شـاخص ترکیبـی خشک سالی، یک رویکرد عینی و واضح برای توصیف شدت خشک سالیهـای تلفیقـی فـراهم مـیکنـد. شاخص مزبور به خوبی قادر به نمایش رفتار خشک سالیهای آب - هواشناختی محدوده مورد مطالعه بـوده و به عنوان شاخص ترکیبی جدید برای پایش و ارزیابی خشک سالی منطقه ای توصیه میگردد
Introduction : A drought is a prolonged period of water deficit and usually occurs when an area does not receive significant precipitation for a sustained period of time، say several months (Chen، Kuo، & Yu، 2009; Linsely، Kohlerm، & Paulhus، 1959). The effects of drought often accumulate slowly over a considerable period of time that may linger for several years even after the termination of drought. Therefore، some authors have called it a creeping phenomenon (Wilhite، 2000). It is difficult to precisely determine the onset and end of a drought event. A drought can be short، lasting for a just a few months، or it may persist for years before climatic conditions return to normal. Drought considered as the most complex، but the least understandable phenomenon of all the natural hazards affecting more people than any other hazards (Mishra & Desai، 2005). Droughts can be classified into four categories as meteorological، hydrological، agricultural and socioeconomic (American Meteorological Society، 1997; Palmer، 1965; White & Walcott، 2009). The preparedness and planning for a drought depend on the information about its areal extent، severity and duration (Mishra & Singh، 2011). This information can be obtained through drought monitoring that is usually done with the use of drought indices (DIs) which provide information to decision makers about drought characteristics. Thus، these indices can be used to initiate drought action plans. Prediction of droughts is useful for early warning that may reduce the response time and consequently the impact of a drought. For many regions، especially semi-arid regions، limited knowledge is available about the diurnal and seasonal cycles of land surface interactions. Semi-arid areas pose a challenge due to large contrasts between dry and wet conditions within a temporal cycle (Schuttemeyer، 2005). Meteorological drought is generally an indicator of other drought types with below normal precipitation، and usually occurs first before other drought types do. The seasonality and climatological conditions vary by location. Drought severity may differ from site to site under different climatic conditions، hence، as many as applications of DIs and their comparisons are beneficial for specific regions in the world. The objective of this study is to develop an aggregate drought index (ADI) based on principal component analysis (PCA) to assess the severity of Hydro-Meteorological drought in the southeast of Iran. Therefore، Stream flow drought index (SDI) and effective drought index (EDI) is determined for several different monthly time steps. The ADI is calculated after a principal component analysis to determine aggregated index value.
Study Area : Southeast area of Iran is confined to Sistan and Baluchestan province and located southeast of the intersection of between 29ºN and 57ºE. In this area، precipitation mostly falls in winter and summer seasons. Summer rainfall related to the monsoon mass effect، which comes from Indian Ocean. Climate is arid and semi-arid. Bahouklat is the most important river in area. The origin of this river is the Pir Abad Mountains in the southeastern of Iranshahr city and the passing of Sarbaz، Bahoukalat، Pishin and Dashtyari regions eventually lead to Govare Bay. This river has various names in different regions and called Sarbaz River from Sarbaz to Rask and passes of Pishin dam. Discharge of this river (Sarbaz) is very important in this area.
Material and Methods : Hydrological and meteorological drought values are necessary for ADI computation over a long period. Therefore، streamflow drought (SDI) and effective drought (EDI) Indicators were used to estimate the hydrological and meteorological droughts. The input variables include the daily values of stream flow of Sarbaz River in the Pirdan hydrometric station and rainfall during the 21 March 1985 to 20 March 2011 common period. Variables were arranged according to water year (also water year، flow year or discharge year)، which begins in October in Iran. Daily timestep of SDI and EDI were estimated respectively according to Nalbantis (2008) and Byun and Wilhite (1999) and then ADI were calculated based on principal component analysis according to Keyantash and Dracup (2004).
Results and Discussion : Drought is a very common phenomenon in Iran and particularly in the south east and it has become a recurrent phenomenon in this area in the last few decades .The hydrometeorological drought index was applied for the identification of drought severity in south east of Iran and drought occurrences were monitored during the experimental years of 1985/86 to 2010/11. Results based on the aggregate drought index (ADI) revealed that a long period of hydro-meteorological drought occurred from 1994/95 to 1998/99. Also، a prolonged wet period has started immediately after the end of drought and has continued to June 2005. In addition، according to drought severity، 1997/98 and 2003/04 water years respectively are extreme drought and extreme wet years. Finally، we have compared the performance of EDI (Meteorological drought index) and SDI (Hydrological drought index) with aggregate drought index. Based on this comparison، aggregate drought index (ADI) could be better highlighting the drought conditions and its characteristics.
Conclusion : Aggregate drought index (ADI) is methodology presented for replacing the unidimensional analysis of drought phenomena which can assist authorities and stakeholders to take rational decisions for combating droughts. Using this approach، a more effective way for assessing the severity of drought is obtained. On the other hand، ADI shows capability to detect dry and wet years. Moreover، it has capability to detect historical drought. Overall، by considering the results، the ADI is more reliable than EDI and SDI for drought monitoring in the study area. In this study، ADI time series were compared against the EDI and SDI to describe an important and long drought event in south east of Iran، the 2003-2004 event from a hydro-meteorological perspective. The ADI methodology provides a clear، objective approach for describing the intensity of drought and can be readily adapted to characterize drought on each area.
خلاصه ماشینی:
"جغرافیا و مخاطرات محیطی، شماره پانزدهم ، پاییز ١٣٩٤ صص ١١١-٩١ توسعه یک شاخص ترکیبی چندمتغیره بر پایه تحلیل مؤلفه مبنا برای ارزیابی خشک سالیهای آب – هواشناختی در جنوب شرق ایران (مطالعه موردی: حوضه سد پیشین ) حمید نظری پور١- استادیار آب و هواشناسی، گروه محیط زیست ، پژوهشکده علوم محیطی، پژوهشگاه علوم و تکنولوژی پیشرفته و علوم محیطی، دانشگاه تحصیلات تکمیلی صنعتی و فناوری پیشرفته ، کرمان ، ایران تاریخ دریافت : ١٣٩٣/١١/٧ تاریخ تصویب : ١٣٩٤/٤/٩ چکیده خشک سالی یک ناهنجاری اقلیمی است که با کمبود (نبود) بارش و مهیایی منـابع آب در پهنـه جغرافیـایی بزرگ پیوند داشته و در طول دوره قابل توجهی از زمان گسترش مییابد که میتواند بخش های مختلـف از زندگی و محیط زیست را متأثر سازد.
بنابراین لازم است ابتدا ارزش نمایه های خشک سالی آب شناختی و هواشناختی محاسبه و سپس شاخص 1 Stream flow volume(River discharge) 2 Aggregate Drought Index(ADI) 3 Hydro-Meteorological ترکیبی بر اساس روش شناسی پیشنهادی ایجاد گردد.
/ / شکل ٩ توزیع فضایی مجموع بارش روزانه ایران در دوره مرجع نخست (شکل سمت راست ) و دوره مرجع دوم (شکل سمت چپ ) در سال ١٣٨٢ 1 Domain 2 PERSIANN: Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks 3 National Center for Atmospheric Research(NCAR) 4 University Corporation for Atmospheric Research(UCAR) 5 Hsu et al 6 Sorooshian et al 7 Mehran and AghaKouchak 8 AghaKouchak et al مجموع بارش روزانه ایران در هنگام ترسالی ٧٧- ١٣٧٦ نیز در شکل های ١٠ آورده شده است ."