توسعه شاخص تركيبي خشكسالي منطقه‌اي و ارائه منحني‌هاي دوره بازگشت با استفاده از تابع كاپولا

خشكسالي يك جزء جدايي ناپذير بلاياي طبيعي محسوب مي­شود. معمولاً خشكسالي بدون هيچ گونه هشدار قبلي و به تدريج آغاز مي­گردد. علاوه بر اين اثر، اين پديده معمولا در طول زمان ايجاد شده و به سرعت از بين نمي­رود. ايران در يك منطقه خشك و نيمه خشك واقع شده كه معمولا پديده خشكسالي را تجربه مي­كند. شاخص­هاي زيادي براي اندازه­گيري شدت خشكسالي پيشنهاد شده است كه از جمله مهم­ترين آنها مي­توان به شاخص بارش استاندارد شده (SPI)، شاخص پالمر (PDSI) و شاخص ذخيره آب­هاي سطحي (SWSI) اشاره نمود. هر كدام از اين شاخص­ها داراي مزيت­ها و معايب مخصوص به خود هستند و براي انواع خاصي از خشكسالي مورد استفاده قرار مي­گيرند. پارامترهاي مختلفي در اندازه­گيري اين شاخص­ها به كار مي­رود كه از جمله آن مي­توان به بارش، ذخيره مخزن، دبي، دما و تبخير و تعرق پتانسيل اشاره نمود. در اين مقاله ابتدا سه شاخص اصلي اشاره شده براي حوضه آبريز اهرچاي، واقع شده در آذربايجان شرقي، محاسبه شده، سپس بر اساس تركيب اين شاخص‌ها به همراه دو پارامتر مهم ديگر، سطح آب زيرزميني و انرژي خورشيدي، يك شاخص تركيبي خشكسالي (CDI) براي منطقه تهيه و محاسبه شد. اين شاخص تركيبي جنبه­هاي مختلف آب و هوايي، هيدرولوژي و مشخصه­هاي كشاورزي منطقه را نشان مي­دهد. در گام بعد تحليل توام دو متغير شدت و مدت خشكسالي با استفاده از تابع مفصل (كاپولا) صورت گرفته و منحني‌هاي مختلف دوره بازگشت به ازاي دو حالت عطفي و فصلي رسم شد. نتايج نشان داد كه شديدترين خشكسالي در اين حوضه در ژوئن (تير ماه) 2004 رخ داده است. همچنين در تحليل دو متغيره دوره بازگشت‌هاي روابط عطفي بيشتر از فصلي به دست آمد. از نتايج اين تحقيق در پايش و آماده سازي براي خشكسالي استفاده مي­شود.

Drought is an integral part of natural hazards. It usually occurs gradually and without any warning. Moreover, this phenomenon is usually created over time and does not disappear quickly. Recently, some factors such as climate variability and the impact of climate change have influenced drought frequency and intensity in many parts of the world. Various definitions have been provided for drought but in general the lack of water resources in a specific period in a geographical area is considered as drought which implies this phenomenon as a regional hazard. IRAN is located in an arid and semi-arid region in which it experiences drought frequently. There are different types of drought such as meteorological, hydrological, agricultural and socialeconomic. These types are differentiated based on the influential factors which are rainfall, river flows, soil moisture, and social-economic consequences. There are many indices proposed for measuring drought severity; among them Standardized Precipitation Index (SPI), Palmer Drought Severity Index (PDSI) and Surface Water Supply Index (SWSI) could be mentioned. Each of these indices has its own pros and cons and is suitable for a particular type of drought. Therefore, knowing the types of drought can provide a better understanding of shortages and their characteristics. Various factors are utilized for measuring these indices including precipitation, reservoir storage, discharge, temperature and potential evapotranspiration. In this study the three main aforementioned indices were first calculated for Aharchay watershed, located in East Azerbaijan province. Next based on combining these three indices with another two important parameters, groundwater level and solar radiation, a combined drought index is developed and calculated for the studied region. Considering the fact that the aforementioned parameters and indices have different level of importance in combined index, different weights based on the expert opinions (subjective approach) and the level of variation (objective approach) are assigned to the parameters considering how critical each parameter is in the overall drought analysis. This combined index demonstrated various climatic, hydrological and agricultural aspect of the region. In the next step, bivariate analysis of the two variables, intensity and duration, is carried out using copula. This is done by first checking the dependency between intensity and duration using Pearson, Spearman, and Kendall correlation coefficients. Second, various copula functions were fitted such as Gaussian, T, Clayton and Gumbel functions. Third, based on the Ordinary Least Square (OLS) and test, the best copula functions were used. Lastly, based on the chosen copula the joint probability distributions were obtained. Two cases named “OR” and “AND” were defined for joint probability of the two variables and different return period curves were drawn. The results showed that the most severe drought in this watershed occurred in June 2004. Moreover, by assessing correlation coefficient between the considered indices it is shown that analysis of the drought in a region based solely on one index would neglect other imperative aspects in drought determination which necessitates a more integrated indicator. Furthermore, in bivariate analysis, return periods of “AND” cases were more than “OR” case. The results of this study could be utilized in preparedness and monitoring drought.