عنوان مقاله [English]
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.
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
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
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.