پهنه‌بندی خصوصیات خشکسالی تحت اثر تغییر اقلیم با استفاده از روش توابع مفصل در حوضه زاینده‌رود

خشکسالی یکی از پدیده‌های حدی است که اثرات آن به‌طور تدریجی و در طول زمان، وسعت زیادی را در بر می‌گیرد. بررسی اثرات تغییر اقلیم بر خشکسالی  برای مدیریت منابع آب اهمیت به‌سزایی است. تاکنون شاخص‌های خشکسالی متعددی برای تعیین و کمی کردن خشکسالی مورد استفاده قرار گرفته‌اند که در بین آن‌ها شاخص spi نتایج بسیار مناسبی به‌دست داده است. در این مطالعه، از شاخص spi برای محاسبه مدت و شدت خشکسالی در حوضه زاینده‌رود برای دوره تاریخی (2008-1979) و آیندۀ دور (2099-2058) با استفاده از داده‌های 15 مدل gcm حاصله از سناریوهای پنجمین گزارش ارزیابی icpp(ar5)، استفاده شد. یکی از شدیدترین خشکسالی‌های دوره تاریخی، با مدت شش ماه و شدت برابر با 4.39، به‌عنوان خشکسالی معیار انتخاب شد. با توجه به مرور سوابق به‌دلیل همبستگی زیاد بین خصوصیات خشکسالی چند تابع مفصل از خانواده‌های ارشمیدسی و بیضوی،  برای ایجاد توزیع توام و ارزیابی دوره‌های بازگشت خشکسالی در حوضه، استفاده شدند. نتایج حاکی از آن بود که دوره بازگشت خشکسالی شدید معیار در دوره تاریخی حدود پنج سال بوده و این مقدار به حدود 25 سال در آینده افزایش خواهد یافت.

Zoning Map of Drought Characteristics under Climate Change Scenario using Copula Method in the Zayandeh Roud River Catchment

Drought is one of the extreme events that can impact vast areas gradually over time. Also understanding the implications of climate change on drought is important for water resources management in order to manage the available water resources in the basin appropriately. Having better understanding of drought condition, drought indices were developed. Several drought indices are used for identifying and quantifying droughts that among them the standardized precipitation index (SPI) provides proper results. Based on each drought indices, drought characteristics can be calculated namely drought duration and drought severity. Drought characteristics are highly correlated to each other. Trusting on one of the drought characteristics for managing the water resources may lead to inappropriate understanding of drought condition. Therefore, it is important to notice all characteristics together by using a joint distribution function that among them copula function is prevalently used in hydrology studies. Several studies were examined the impact of climate change on the drought conditions by using different drought indices in many basins in the word and Iran (Bazrafshan et al., 2015, Kouchaki ei al. 2007, Mahsafar, 2011, Eghtedarnejad et al., 2016, Naserzadeh and Ahmadi, 2012, Hoffman et al., 2009, Kirono et al., 2011, Selvaraju and. Baas, 2007, Lee et al., 2013, Serinaldi et al., 2009, Mirabbasi et al., 2013). There have been many studies which using copula function in order to compute the return period of the drought (Abbasian et al., 2014, Golian, 2010, Serinaldi et al., 2009, Mirabbasi et al., 2016, Maddadgar and Moradkhani, 2011, Chen et al., 2011). Therefore, in this study drought condition was analyzed by using copula under climate change condition to have a better understanding of future drought situation and the return periods of drought events in the future. The SPI was used to extract the drought duration and drought severity in the ZayandehRoud River basin for a historical period (19792008), and the far future (20582099) by using 15 GCM models from the IPCC Fifth Assessment Report (AR5) scenarios. A significant past drought event in the basin was used as a benchmark with severity of 4.39 and duration of 6 months. The Archimedean and Elliptical families of copula functions were used to construct the joint distribution functions for evaluating the drought return periods in the basin. Results from historical analysis show that the return period of significant past drought is about 5 years and this period will increase to about 25 years in the future.