The Islamic Republic of Iran is situated in the eastern portion of the northern hemisphere, in the southwest of Asia, and in the west of the Palaearctic region. With a total surface area of 1,648,000 km2, Iran ranks 18th in size among the countries of the world, placed in the Middle East and surrounded by the Armenia, Azerbaijan, Caspian Sea, Turkmenistan on the north, Afghanistan and Pakistan on the east, Oman Sea and Persian Gulf on the south, and Iraq and Turkey on the west. The country is both a meeting point for many cultures as well as for many types of climate, land, water, and biodiversity of flora, fauna, and people (Collins, 2001). Population of Iran is about 73 million and the growth rate estimated as 1.3% and it has 31 provinces (FAO, 2005). Various environmental conditions with respect to wide latitude and longitude range; 44° 05’ to 63° 18’ E longitude, 25° 03’ to 39° 47’ N latitude, topographic diversity; altitude varies from −28 to 5774 m (the Caspian coastal area is below sea-level (−28 m) while Damavand peak in the Alborz reaches 5774 m above mean sea level) and high geological and geomorphologic diversity and also very variable from the stand point of climate producing variable ecological diversity and habitats (Sharifi, 2011). Iran is situated where three climatic zones meet the Mediterranean, the arid or semi-arid West Asian and the temperate humid/semihumid Caspian zone. Of the total land area, about 55% are rangelands; 12% are forests and 33% are deserts including bare salty lands. Being dominantly in an arid environmental zone, approximately 85% of Iran’s agricultural lands are located in arid and semi-arid areas (Misra, 2009). Iran is well-known as one of the world’s major centers of biodiversity and natural heritage, because of the junction of major plant geographical regions. As a result, most rivers are seasonal and their flows depend heavily upon the amount of rainfall (Ansari, 2000; Calabrese et al., 2008). The climate is extremely continental with hot and dry summer and very cold winter particularly in inland areas. Apart from the coastal areas, the temperature in Iran is characterized by relatively large annual range about 22°C to 26°C. The 166rainy period in most of the country is from November to May followed by dry period between May and October with rare precipitation. The average annual rainfall of the country is about 240 mm (less than a third of world average precipitation) with maximum amounts in the Caspian Sea plains, Alborz and Zagros slopes with up to more than 2,000 and 480 mm, respectively. Going inland at the central and eastern plains, the ranges of precipitation decreases to less than 100 mm annually depending on the location. From the synoptic aspects, the climate of most part of Iran is dominated by subtropical high in most part of the year. This phenomenon causes hot and dry climate in summer. The rainfall in the country is produced by Mediterranean synoptic systems, which move eastward along with westerly winds in cold season. Synoptic systems and year-to-year variation in the number of passing cyclones cause high variability in annual rainfall. Frontal Mediterranean cyclones associated with the westerly air flows produce most of precipitation in the whole country in late autumn and particularly in winter. In addition to the frontal Mediterranean cyclones, rainfall bearing systems called Sudanian cyclones which come from the southwest make an important contribution to increase annual rainfall amount of the west and southwest of the country (Raziei et al., 2005). In northwest mountainous regions, convective and frontal thunderstorms are important atmospheric process responsible for rainfall in spring and early summer. These rainfalls bearing systems are restricted to the west portion of the country and have no more energy and moisture to pass far to the east. These systems sometimes may reach central and east dry regions of the country when there will be no potential to produce rainfall due to the long trajectory and loss of moisture. This region is the most drought-prone area in the country due to high inter-and intra-annual irregularity in rainfall and high coefficients of variation. This region that accounts for over half of Iran’s land area is surrounded by Alborz mountain range from the north and Zagros range from the northwest to southwest. These mountains play an important role in determining the non-uniform spatial and temporal distribution of precipitation in the whole country. The eastern part consists mostly of desert basins such as the Dashte-Kavir, Iran’s largest desert, in the north-central portion of the country, and the Dasht-e-Lut, in the east, as well as some salt lakes. This is because the mountain ranges are too high for rain clouds to reach these regions (Kazemi, 2014). The area within the mentioned mountain ranges is the high plateau with its own secondary ranges and gradually slopes down to become desert which continues into southern part of Afghanistan and near the Pakistan border. The Zagros range prevents Mediterranean moisture 167bearing systems to pass through to the east. This phenomenon gives rise to high irregularity in rainfall in the center of Iran. Lack of rainfall in May to October compounded with high temperature leads to high evapotranspiration and water deficit in this region. Iran is situated within the dry belt of Asia (Breckle, 2002). As a result, the central and eastern parts of Iran are arid and semi-arid regions. Since Iran is in the arid zone, approximately 85% has arid, semi-arid or hyper-arid environment. The specific features and location of Iran cause it to receive less than a third of the world average precipitation. Prolonged drought in this area and availability of moisture in other parts of Iran has led to the formation of different ecological zones (Heshmati, 2007). Iranians depends almost entirely on water resources in the form of spring rains or melting snow descending from the mountains or underground water resources. Traditionally, in most parts of the country, the rich underground water resources in the higher altitudes are transferred by the man-made underground water channels systems to the lower and dryer agricultural and settled areas which are known as Qanat or Karez. In contrast to Afghanistan, where the mountains form a ‘backbone’ of the country, in Iran, the mountains are surrounding the desert lowlands of the central part (Noroozi et al., 2008).