كليدواژه :
تئوري مقادير حدي , تغيير اقليم , خشكسالي , مدت سختي
چكيده فارسي :
در اين پژوهش، به منظور ارزيابي ويژگي‌هاي آتي خشكسالي ابتدا ميزان توانمندي سه مدل اقليمي در شبيه‌سازي دما و بارش براي دوره پايه (2005-1976) براي چهار نمونه اقليمي ايران (اهواز، بندر انزلي، شهر كرد و كرمان) بررسي شد و در ادامه رويدادهاي حدي خشكسالي (SPEI≤-1) از سري ماهانه براي دو دوره پايه و 2050-2021 تحت سناريوهاي خط سير گازهاي گلخانه‌اي (RCP)، شناسايي و ويژگي‌هاي شدت، سختي و مدت استخراج شد. نتايج نشان داد مدل‌هاي اقليمي بجز بندر انزلي، در ساير ايستگاه‌ها، از مهارت مناسبي در شبيه‌سازي متغيرهاي دما، بارش، فراواني طبقات SPEI-1 برخوردار هستند. بر اساس پيش‌نگري انجام شده، در اهواز تحت سناريوي RCP4.5، فراواني طبقه متوسط (64 رويداد) و تحت سناريوي RCP8.5، فراواني طبقه شديد SPEI-1(3 رويداد) بيش از دوره پايه (به ترتيب 58 و 1 رويداد) خواهد بود. به ازاي دوره‌هاي بازگشت يكسان، مقادير شدت خشكسالي‌هاي حدي تحت سناريويRCP8.5 با دوره پايه تفاوت قابل ملاحظه‌اي خواهد داشت. براي دوره بازگشت 50 سال، شدت اين بليه تحت سناريوي RCP8.5، 4/3 خواهد بود در حاليكه دوره پايه داراي شدت 6/2 مي‌باشد. تحت سناريوي RCP4.5، شدت (4/2 براي دوره بازگشت 50 سال) نسبت به دوره پايه اندكي كاهش نشان مي‌دهد. براي شهركرد، فراواني طبقه متوسط نمايه خشكسالي تحت دو سناريو نسبت به دوره پايه كاهش و در مقابل فراواني انواع شديد آن افزايش نشان مي‌دهد. با بررسي تغييرات تابع چگالي احتمال (PDF) توزيع مقادير حدي تعميم يافته (GEV) براي ايستگاه شهركرد نشان داده شد كه ميانگين و تغييرپذيري شدت مقادير حدي اين پديده تحت دو سناريوي افزايش مي‌يابد. در ايستگاه كرمان، فراواني مقادير شديد SPEI-1 تحت سناريوي RCP4.5 نسبت به دوره پايه افزايش نشان داد. تحت سناريوي RCP8.5، به ازاي دوره‌هاي بازگشت يكسان، انتظار بروز خشكسالي‌هاي حدي ضعيف‌تر نسبت به دوره پايه وجود دارد.
چكيده لاتين :
Introduction
The Intergovernmental Panel on Climate Change (IPCC) fifth assessment report has pointed out that global warming is intensifying, and the frequency and intensity of extreme climate events such as high temperature and heavy rainfall will increase significantly (IPCC 2013). The weather and climate disasters have also increased and had a very serious impact on social stability, economic development and people’s lives . Probabilistic analysis of drought events plays an important role for an appropriate planning and management of water resources systems, especially in arid or semi-arid regions characterized by low annual or seasonal precipitation. In particular, estimation of drought return periods can provide useful information for a proper water use under drought conditions. As the changes in the frequency and intensity of extreme drought events bring great threats to natural and social systems, studies on the drought events, especially on the analysis of statistical characteristics of extreme drought events, have attracted the attention of an increasing number of scholars in the recent decades.
This study aimed to project future SPEI using RCP8.5 and RCP4.5 projection data. The drought characteristics by the threshold level to the projected SPEI were identified. This study also projected the drought risk of each station (representing four climate zones of Iran) in the 21st century by fitting the drought characteristics to the Generalized Extreme Value (GEV) distribution.
In this study, the analysis of extreme values of standardized precipitation-evapotranspiration index (SPEI) was performed to evaluate the potential future changes in drought characteristics in Keramn, Sharekord, Ahvaz and Bandar Anzali stations which are representing the warm and arid, cold and semi-arid, warm and semi-arid and humid climate respectively. The capability of three global climate models in simulating temperature and precipitation during the base period (1976-2005) for four climate zones of Iran (Ahvaz, Bandar Anzali, Shahrekord and Kerman) was investigated. Drought extreme events (SPEI≤-1) were identified from the monthly series for the base period and the period 2021-2051 under the RCP scenarios, and the characteristics of intensity, severity and duration were extracted.
Materials and methods
As it is difficult to define when the drought started and ended, previous researches assessed the risk of drought in an indirect way by conducting the frequency analysis of the drought indexes.
SPEI can reflect the effect of not only the variability in precipitation but also the variability of evapotranspiration. Thus, this study used SPEI. SPEI is the difference between the random month 𝑖 and PET obtained by using the precipitation and the Hargreaves & Samani (1982) equation, as shown in
D= P_i- 〖PET〗_i (1)
Which is synthesized in each time scale like
D_n^k=∑_(i=0)^(k-1)▒〖P_(n-i)-〖PET〗_(n-i) 〗 (2)
Here, 𝑘 is the time scale of synthesis, and 𝑛 is the month used for calculation.
The severity and duration and drought intensity were calculated using SPEI. Negative SPEIs mean the dry condition; a drought event is defined when the SPEI is continuously negative and reaches a value of “−1.0” or less. Thus, it is assumed that “−1.0” is the threshold level and that the drought starts in the level lower than “−1.0” in monthly SPEI. The aggregate of SPEI while one drought event lasts was defined as the severity of drought.
Results
The results showed that GCMs in the stations under study (except Bandar Anzali), have good skill in simulating the variables of temperature, precipitation, frequency and drought classes. In Ahvaz under RCP4.5 scenario, the frequency of moderate events and under RCP8.5 scenario, the frequency of severe class will likely be more than the base period. For the same return periods, the extreme drought intensity values under RCP8.5 scenario with the base period will be significantly different. Under the opposite scenario, the intensity decreases slightly compared to the baseline period. For Shahrekord, the frequency of the middle class of this phenomenon under two scenarios projected to decrease compared to the 1975-2005 priod and in contrast to the frequency of severe types projected to increase. Changes in Probability Density Function (PDF) of GEV for Shahrekord station showed that the mean and variability of the intensity of extreme values of this phenomenon will likely increase under two scenarios. At Kerman station, the frequency of severe drought under RCP4.5 scenario increase compared to the base period. Under the RCP8.5 scenario, droughts projected to be reduced.
