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The Islamic Republic of Iran (Figure 1) has an area of 1,648,000 km2 and a population of 65 million people (1995 estimate). The country has arid and semiarid climates and the occurrence of rainfall is unreliable, with a coefficient of variation as high as 70%. The average annual precipitation over the country is around 250 mm. Two mountain ridges, the Alborz and Zagros (Figure 1), which run east and southeast from the northwest corner of the country, play an influential role in determining the amount and spatial distribution of rainfall. The peaks of Alborz and Zagros are about 5,700 m and 4,000 m, respectively.
Rainfall generally occurs from October to March (winter), with extreme events during January and February. Annual rainfall over the northern sides of the Alborz range may reach 1,800 mm, but for the central and eastern deserts, the yearly total is around 50 mm. Droughts and floods are common, and the severity and hardships of these natural disasters frequently hit both rural and urban societies. Drought limits dryland farming and affects the productivity of irrigated lands. Moreover, due to massive overgrazing, large-scale soil erosion occurs during dry spells. Atmospheric and climatic incidents (i.e., floods, droughts, and lightning) account for about 97% of all natural disaster costs. Concern about water resources is currently realized as one of the most important issues for most of the Iranian scientific and management communities. Most parts of the Islamic Republic of Iran recently experienced an exceptional drought that lasted more than 2 years (1998–2000). In some areas, drought has also extended into winter 2001. The 1998–2000 drought inflicted $3.5 billion in damages, killing 800,000 head of livestock and drying up major reservoirs and internal lakes (Pagano et al., 2001).
Nazemosadat and Cordery (2000a) and Nazemosadat (1999) have recently revealed that the autumn rainfall in Iran is negatively correlated with the Southern Oscillation Index (SOI). The relationships were found to be strong and consistent over the southern foothills of the Alborz Mountains, northwestern districts, and central areas. Since winter rainfall contributes a major portion of Iranian water resources, the shortage of rainfall during this season is the most important cause of drought in Iran. Nazemosadat and Cordery (2000b) have therefore focused on the impact of ENSO on winter precipitation in Iran. The present study outlines some key results of the aforementioned studies.