Climatic parameters Aridity indices Reference Evapotranspiration Statistical distribuation Iran

Arid regions are very diverse in terms of their land forms, soils, fauna, flora, and water resources. However, aridity is the one binding element to all arid lands and is the best indicators for definiation of the climate state in each region. Management of natural resources in an ecologically sustainable manner, especially in arid lands, require to identify and which areas are experiencing drought conditions and how severe the conditions. Linear momentum and cluster analysis techniques are useful in the process of determining of climatic homogeneous regions. The main objects of this research is to determine climatic homogeneous regions using UNEP aridity index applying cluster analysis and linear momentum techniques in eastern Iran. First, by using the meteorological data of 20 synoptic stations in the eastern part of the country (Iran), reference Evapo-Transpiration were calculated using the FAO-Penman Montith method for the period of 36 years. Figure 1 shows distribution map of the selected synoptic stations in Iran. Then, an useful "representation" of aridity i.e. UNEP aridity index were calculated in each stations. The UNEP aridity index employs a ratio of annual precipitation to potential evapotranspiration (P/PET). This aridity index includes evaporation from the soil and transpiration from the vegetation from a specific region in a given time interval. Under the UNEP classification, drylands are characterized by a P/PET of between 0.05 and 0.65 as shown in Table 1. In the next step, drought homogeneous stations were selected using cluster analysis technique. Then, the most important parameters which has affects on UNEP aridity index, were determined using factor analysis techniques. Finally, statistical distribution and probability of occurrence and homogeneous of stations were characterized in R software.Results show that combining the linear momentum and cluster analysis techniques are efficiency to identify homogeneous regions. It can be concluded that it is appropriated to determine homogeneous regions using cluster analysis technique initially, and then, it can be approved and accepted by using linear momentum technique.