تحليل فضايي خشكسالي‎هاي بلندمدت ايران

Spatial Analysis of Long Duration Droughts in Iran

هدف از انجام اين پژوهش، پايش و پهنه‎بندي خشكسالي‎هاي بلندمدت ايران زمين است. از آنجاكه شاخص بارش استانداردشده (SPI) به تحليل‎گر امكان مي‎دهد تا تعداد وقوع خشكسالي را در مقياس‎هاي زماني مختلف تعيين كند و با برازش خشكسالي‎ها بر تابع چگالي احتمال، تحليل فراواني را روي آن انجام دهد، مزيت بهتري براي انتخاب و اجرا دارد. در اين پژوهش از داده‎هاي آماري هفتاد ايستگاهي استفاد شد كه دوره آماري سي سال به بالا داشتند و با به‎كارگيري نمايه SPI در بازه‎هاي زماني دوازده و بيست‎وچهارماهه، داده‎ها مورد بررسي و تجزيه‎وتحليل قرار گرفتند. براي تحليل فضايي با استفاده از روش‎هاي زمين‎آمار، نقشه‎هاي پهنه‎بندي خشكسالي‎ها در محيط ArcGis ترسيم شدند. نتايج پژوهش نشان مي‎دهد كه در بازه دوازده‎ماهه قسمت‎هاي شرق، جنوب غرب، غرب و مركز كشور، خشكسالي‎ها بسيار شديد و شديد است، اما در مناطق شمال شرق، شمال غرب و شمال كشور خشكسالي‎ها متوسط و ملايم است. در بازه بيست‎وچهارماهه غرب، شرق، جنوب شرق، جنوب و مركز كشور خشكسالي‎ها بسيار شديد و شديد است.

Introduction Drought is one of the devastating hazards of human history. It can occur in all times and any places. Drought is a regional event that its occurrence in many world zones cannot be avoided. It is more important than other natural misfortunes due to loss of life and property and social factors. This event is more display in arid zones such as Iran for the sake of regional characteristics like small precipitation, intensity in oscillations of rainfall in different years, inappropriate distribution of the rainfall in a period of years and the great fluctuations inplace and time of the rainfall. For this reason this research has studied the severity and spatial organization of these droughts over the country. Methodology Important parameters that influences drought are temperature and precipitation. In this research, in order to make a spatial analysis of the long duration droughts in Iran, the precipitation data were used. For this reason the monthly precipitation of 70 long period stations during 1976-2005 were obtained from the Meteorological Organization of Iran. Researchers all over the world offer different methods for study of the droughts. One of these is Standardized Precipitation Index (SPI). This method was developed by McKee, Doesken, and Kleist (1993) to analyze the impact of precipitation deficit on groundwater, reservoir storage, soil moisture, snowpack, and stream flow in different time scales for defining and monitoring the phenomenon. These time scales reflect the impact of drought on the availability of different water resources. Soil moisture conditions respond to precipitation anomalies in a relatively short time scale. Groundwater, stream flow, and reservoir storage reflect the longer-period precipitation anomalies. For these reasons, McKee et al. (1993) originally calculated the SPI for 3, 6, 12, 24, and 48 months time scales. The SPI computes the temporal frequency and anomalies of droughts. It can also be used to determine periods of anomalously wet events. The SPI is recognized as a better quality model compared to other models. The advantage of this index is that allows the analyzer to determine the number of occurrences of droughts in different stages of time. In the other words, it is, because of the non distance of this index that we could use of it to compare information of different zones and make carefully anextent of drought maps. The other advantage of this index is that it is measuring the drought function of probable density so that you can perform more analysis about that. The study has used the SPI method for a period of12 and 24 months during the 1976-2005. The spatial distribution of the drought was interpolatedvia ordinary kringingin the ArcGIS environment. Results and Discussion In this study SPI time series were calculated in time scales of 12 and 24 months. Then, the frequency and spatial distribution of droughts were prepared and analyzed using the Geostatistics methods (Ordinary Kringing). These results have showed that the intensive 12-months period droughts were frequent in the east, southwest, central, and western parts of the country ( for example Birjand Station 8%, Ramhormoze Station 6% Ilam Station 5% and Sirjan Station 5%), while the southeast and northern parts experienced moderate and weak droughts. But the 24- months droughts were intensive in the eastern parts of the country(for example, Tabass Station 6%,Ramhormoze Station 6%,Iranshahr Station 6%, and Sirjan Station 5%). This means that the longer droughts are common in the east, where they affect the underground water resources. Conclusion Drought is one of the devastating hazards inhuman histories. The occurrence of the phenomenon is possible in any time and places. Iran as an arid country with fragile climate is prone to frequent droughts. For this reason this research has studied the severity and spatial organization of these droughts over the country. The study has used SPI method with the 12 and 24 months scales during the 1976-2005 periods. The spatial distribution of the drought was performed via Ordinary Kringingin the Arc GIS software. The results have indicated that the 12-month period droughts were intensive in the east, southwest, central, and western parts of the country while the southeast and northern parts experienced moderate and weak droughts. But, the 24- months scale droughts were intensive on the eastern parts of the country. This indicates that the longer droughts are common in the east, where they affect the underground water resources.