Indigenous Plant Species from the Drylands of Iran, Distribution and Potential for Habitat Maintenance and Repair - Combating Desertification in Asia, Africa and the Middle East - page 343

to May followed by dry period between May and October with rare precipitation.
The average annual rainfall of the country is about 240 mm with maximum amounts
in the Caspian Sea plains, Alborz and Zagros slopes with more than 1,800 and
480 mm, respectively.
Going inland at the central and eastern plains, 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 for most 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 the 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
the 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 processes responsible for rainfall in spring and early summer.
These rainfall bearing systems are active only in the west portion of the country and
have no more energy and moisture to pass farther east. These systems sometimes
may reach central and eastern dry regions of the country but have no potential to
produce rainfall due to 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 the country area, is surrounded by Alborz mountain range from the north
and Zagros range from the northwest to southwest. Zagros range acts as a wall to
prevent Mediterranean moisture bearing systems to pass through to the east. Hence
these two ranges prevent the arid and semi arid regions of the central and eastern
part of Iran from access to moisture. 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 evapo-transpiration and water deficit in this region.

2 Geobotanical Features

Most parts of Iran are desert. Iran has a serious shortage of water and wood
resources. Just the northern part of Iran, is green. Further south, i.e. the north beach
of the Persian Gulf and Oman Sea, is desert because of the rain shadow effect of the
Alborz mountain range. The humidity of Caspian Sea is blocked and this makes the
North green (Fig. 17.2 ).
Since Iran is in the arid zone, some 65 % of its territory is arid or hyper arid, and
approximately 90 % has an arid, semi-arid or hyper arid environment the specific
features and location of Iran causes it to receive less than a third of the world average
precipitation (Table 17.1 ).