تاثير شكست سري زماني داده هاي بارش بر تغييرات مشخصه هاي خشكسالي (مطالعه موردي شهرهاي تبريز و اراك)

The Impact of Breakpoints in Rainfall Time Series on Drought Characteristics Changes (Case Study: Tabriz and Arak Stations, Iran)

خشكسالي يك مخاطره اقليمي است كه در همه‌ي مناطق آب و هوايي رخ مي‌دهد. متغيرهاي مختلف هواشناسي از جمله بارندگي، دما و رطوبت‌نسبي نقش مهمي در وقوع، شدت و تداوم خشكسالي دارند. تغيير اقليم و گرمايش جهاني مي‌تواند سبب بروز ناهمگني و ايجاد شكست در داده‌هاي سري زماني متغيرهاي هواشناسي شود. اين ناهمگني مي‌تواند سبب تغييراتي در مشخصه‌هاي خشكسالي شود. هدف اصلي اين پژوهش بررسي همگني سري زماني بارش به‌عنوان مهم‌ترين متغير موثر بر خشكسالي هواشناسي، در تعدادي ايستگاه سينوپتيك در طول دوره آماري 1957 الي 2016 در مناطق مختلف كشور و بررسي تغييرات مشخصه‌هاي خشكسالي در دوره‌هاي قبل و بعد از نقطه شكست مي‌باشد. پس از بررسي سري‌هاي زماني مقادير بارش سالانه مشخص گرديد كه فقط ايستگاه‌هاي تبريز و اراك داراي نقطه شكست مي‌باشند. از اين‌رو در ادامه، وضعيت خشكسالي بر اساس شاخص‌ها‌ي SPI، SPEI، RDI و eRDI در ايستگاه‌هاي مذكور در دو دوره‌ي قبل و بعد از نقطه شكست پايش و مشخصه‌هاي خشكسالي بر مبناي مدل زنجيره ماركف و ماتريس احتمال انتقال و هم‌چنين ويژگي‌هاي دوره‌هاي خشكسالي بر مبناي تيوري ران تعيين گرديد. نتايج حاكي از كمتر شدن اطمينان‌پذيري و بيشتر شدن آسيب‌پذيري نسبت به خشكسالي در دوره دوم در مقايسه با دوره اول است. ضمن آن‌كه، متوسط و حداكثر تداوم دوره‌هاي خشكسالي نيز در همه‌ي موارد در دوره دوم نسبت به دوره اول بيشتر شده است. به‌طور خلاصه تمامي مشخصه‌هاي خشكسالي در دوره دوم نسبت به دوره اول داراي شدت، مدت و فراواني بالاتري هستند. به‌طور كلي نتايج تغييرات مشخصه‌هاي خشكسالي در دوره بعد از شكست نسبت به دوره قبل از آن مي‌تواند ناشي از گرمايش جهاني و در نتيجه افزايش تبخير-تعرق و ايجاد و يا تشديد اثرات خشكسالي باشد. ضمن آن كه بر اساس نتايج حاصل از شاخص eRDI در هر دو ايستگاه و در هر دو مقياس زماني، شرايط رطوبتي نسبت به ساير شاخص‌ها كمي خشك‌تر شده است. به عبارت ديگر مي‌توان بيان نمود كه تا حدودي بارش موثر در سال‌هاي اخير نسبت به سال‌هاي اوليه دوره مورد بررسي كاهش يافته است. پيشنهاد مي‌شود مشابه با اين پژوهش در مورد همه ايستگاه‌هاي سينوپتيك كشور كه داراي دوره آماري طولاني مي‌باشند، انجام شده تا اثر گرمايش جهاني و در نتيجه تغييرات بارش و دما بر وقوع خشكسالي نمايان‌تر شود.

Introduction: Drought is a climatic phenomenon and an integral part of climate fluctuations that occurs periodically and intermittently throughout the world and across all climates. However, the magnitude of this natural hazard in arid and semi-arid regions, such as most parts of Iran, is more acute due to the high sensitivity and weakness of these areas, and its effects may persist for years after the occurrence of drought. Drought is a multifaceted phenomenon as precipitation, temperature, evaporation, wind and relative humidity play important roles in the drought characteristics such as occurrence, severity, and magnitude. Climate change and global warming, and in some cases displacement of meteorological stations cause heterogeneity in time series of meteorological data. Therefore, the purpose of this study was to investigate the homogeneity and break point in precipitation time series data and the effects of a break point in drought characteristics in some synoptic stations in Iran. Materials and Methods: In this study, homogeneity of rainfall time series data at two time scales of annual (water year) and plant growth periods in some selected synoptic stations of Iran with different climatic conditions was investigated. For this purpose, four tests including Standardized Normal Homogeneity test (SNH), Buishand’s Range test (BHR), Buishand’s U test (BUR) and Petite’s test were applied and the break points were determined. Then, at the stations with break points in the precipitation data series, the drought severity values were determined using four indices of SPI, SPEI, RDI and eRDI, for two periods, (before and after of the break points). Then drought characteristics based on Markov Chain Model and Transition probability matrix including vulnerability, reliability, reversibility and stationary of three condition of droughts (dry, normal and/or wet condition) were determined for the two time scales periods (annual and plant growth periods). Then, the differences between the characteristics for the two periods were investigated. Also, the characteristics of drought-free time intervals for the two periods based on Run’s theory were determined and compared. Results and Discussion: Based on the homogeneity tests, precipitation data of Arak and Tabriz stations for two scales of annual and plant growth periods have break points. According to the results, in the most cases, the second period's reversibility was lower than the first period. Reliability and vulnerability also decreased and increased in all cases in the second period, respectively, compared with the first period. In most cases, there was an increase in stationary of drought in the second period relative to the first period. The rate of change in the probability of survival of the normal and wet condition in both periods was increasing and in some cases decreasing. Regarding the results of Run’s theory at the growth periods scale, the average and maximum duration of drought periods increased in all cases in the second period. The minimum, average and maximum severity of drought periods also increased in most cases in the second period. The minimum, average, and maximum values increased in most cases in the second period. On an annual basis, the number of drought periods in most cases has increased in the second period. The average and maximum duration of drought periods increased in all cases in the second period. The minimum, average, and maximum severity of drought periods also increased almost in all cases in the second period. Minimum, average, and maximum of drought magnitude values increased in most cases in the second period with respect to the first one. The minimum, average and maximum values of the drought-free durations (interval time without drought conditions) in most cases were lower in the second period. At the annual scale, the minimum duration of drought was one year in all cases and no change was found between the time slices. The average duration in most cases was lower in the second period. Conclusion: The results show that the rainfall data of Arak and Tabriz stations have break points in the time scales of plant growth period and annual periods. The reliability was decreasing while the vulnerability of drought was increasing in the second period, indicating an increase in drought occurrence in recent decades. Moreover, the probability of drought stability (stationary) in the second period increased in most cases. The average and maximum duration of drought periods also increased in the second period. The minimum, average, and maximum drought severity, and the minimum, average, and maximum of magnitude of drought periods were higher during the second period. In most cases, the minimum, average, and maximum of severity and magnitude of drought-free time intervals were lower in the second period. In general, difference in the characteristics of drought before and after of precipitation break point can be due to increased evapotranspiration, as a result of global warming, intensifying the effects of drought. Moreover, based on the results of the eRDI index, the climatic conditions became drier in both stations and time periods. In other words, it can be stated that the effective rainfall has decreased to some extent in recent years compared to the early years of the study period. Further studies are needed to assess the changes in drought characteristics in all synoptic stations in the country having long-term data.