فصلنامه پژوهش های جغرافیای طبیعی
پیاپی 80 (تابستان 1391)

When a river reaches from high gradient mountains to a low gradient plain, aggradations phenomenon is occurred and alluvial fans are built. At least five factors influence fan processes, including catchment bedrock lithology, catchment shape, neighboring environments, climate, and tectonics. Rocks of differing lithology yield contrasting sediment suites and volumes due to their variable response to weathering. Bedrock in desert settings is optimal for fan development, especially tectonically maintained mountain fronts, yields sediment in varying size.thevolume depends on: (a) the style of fracturing in proximity to faults, (b) the presence or absence of internal discontinuities such as bedding planes or foliation planes, and (c) the reaction to chemical weathering and non-tectonic types of physical weathering. Aeolian, fluvial, volcanic, lacustrine, or marine environments that border alluvial fans can impact fan processes by modifying the conditions of deposition. Fluvial environments, usually in the form of longitudinally oriented rivers, may affect fans by eroding their distal margins. Climate and its fluctuations affect water supply and vegetation cover. Without continued tectonics, fans may become minor or short-lived features. In this study, the evidence and impacts of the active faults are investigated in 16 alluvial fans morphometry and morphology located at the north part of the city of Damghan. The method is based on the obtained qualitative and quantitative data. The quantitative data includes satellite image interpretation and digital elevation models, alluvial fan morphometry, channel displacement and rate of sediment uplift. Super imposed profiles techniques, longitudinal and cross profile and gradient analysis were used to interpret the active fault effects on alluvial fans. Topography maps (1:25000), aerial photos (1:55000), ETM satellite image with 30 m spatial resolution, 8 bands, geology maps (1:100000) and digital elevation models (50m pixel resolution) were utilized in this study. Also, field work for investigation of evidence of faults activities were performed in two stages. This area is located between two geo-structural zones. Alborz zone in the north and central Iran zone in the south. There are two main faults in this area. Tazareh fault is located at Alborz mountain front and affects the apex of alluvial fans, whereas Damghan fault is located far from mountain front. Distance between these faults is about 5 km. Tazareh fault has caused the displacement of the main channel in fan apex, and therefore has changed the sedimentation position; while evidence of Damghan fault activities are more, and has caused uplifting surfaces, derelict of fan surface, change of intersection point, uplift of fan sediment and lateral change of fan surface channels. The slope of most alluvial fans is 2-5 degree, except for fan No. 7 that is less than 2 degree. This fan is the largest fan in the study area. The lower gradient may be contributed to its large drainage basin. Damghan fault activities have caused alluvial fan deposits uplifted about 10 m. The findings showed that Tazareh and Damghan faults were active in quaternary epoch and position of alluvial fans was affected by activities of these faults. Faults have had either lateral or vertical displacements. Findings showed that the slope of all alluvialfans (except for fan number 7) increased suddenly in the apex of fans. Therefore the longitudinal profiles of alluvial fans are in a concaved shape. This means that tectonic activities affect the evolution of alluvial fans in quaternary epoch. Investigations show that there is no statistical correlation among the variables that affect the alluvial fans. Generally, tectonic activities disrupt natural evolution of alluvial fans. Each fault has a different effect on alluvial fan evolution. Tazareh fault has caused the displacement of the main channel to the east of fan apex and has increased slope of this part. Damghan fault has caused uplift of fan deposits, change the intersection point and reconstruction of new alluvial fans in the lower of this point. Accommodation space of alluvial fans from Siahkooh to Roodbar has affected by Damghn fault activities. Indeed, uplift of this part of fault has limited accommodation space for alluvial fans growth. There are a correlation between number of uplifted surface of alluvial fans, number of uplifted playa sediments (clay and silt) and number of channel displacements; this means that Damghan fault experienced three separate activities in quaternary epoch.

The rapid reduction of fossil fuels, global warming, green house effects, environmental pollution and falling of acid rains have increased the need of attention to renewable energies especially wind energy as a clear interminable and free energy source (Rahman at el, 2003). Therefore, in order to exploit the potential energy of wind, assessment in different areas should be considered. This study assessed the potential of wind energy at Kermanshah province. Wind energy potential based on statistics of three hours wind direction and speed of synoptic stations of province including Kermanshah, Eslamabade Gharb, Ravansar, Kangavar and Sarpolzahab, Was assessed from 1997 to 2006. For showing wind speed and direction of province stations, the Wrplot software was used as well us the Weibull mathematical model for befitting of data possibilities distribution. Distribution of wind speed is a feature of wind that contains important data not only for environmental and structural design, but also as the potential for wind energy and wind energy conservation system. During the past two decades many researchers have spent much time to develop an appropriate statistical model for the distribution of wind speed. Application of two-parameter Weibull probability distribution is one of the best ways to describe the distribution of fluctuating wind speed and has been used to investigate wind power in many different countries (Li, 2005). Al-Nassar et al (2005) investigated the potential for wind energy production in Kuwait. Belu and Koracin (2009) studied wind characteristics and wind energy potential in Western Nevada. Similar research has been done in Iran. Salahi et al (2004) evaluated wind energy using the probability distribution function Weibull for Ardebil province. Jehangiri et al (2005) calculated the wind energy with two parameters Weibull distribution for… province. As part of the US Department of Energy’s Federal Wind Energy Program, developed a wind power classification scheme. Areas are classified on the basis of wind power, ranging from 1 (lowest) to 7 (highest). Each class represents a range of wind power density (W/m2) or a range of equivalent mean wind speeds (m/s) at specified heights above ground level. Typically, grid blocks designated as Class 4 or greater are considered to be suitable for most wind turbine applications. Class 3 areas are suitable for wind energy development using taller wind turbine towers. Class 2 areas are considered marginal for wind power development and Class 1 areas are unsuitable (Ilinca et al., 2003). In this research using Weibull probability distribution function, Kermanshah province was zoning for developing wind power potential Weibull function is a case of Gama distribution. This distribution is usually used to plot the wind distributions for specific locations in the determined monthly and annual time period (Zhou et al., 2006). This distribution is defined as follows: (1) In this formula, (K) is a dimensionless parameter known as the "form factor" and (C) is a parameter called "scale factor" that the unit is meters per second (Ahmad-shata and Hanitsch, 2006). After calculating the components of Weibull function, wind power density at a height of 10 meters can be obtained from the following relation: (2) Where Γ is Gama function, C and K are parameters of Weibull function and r is density of air. Density of air at standard conditions of temperature and pressure is equal to 1.225 kg per cubic meter. Studies have shown that the height of most commercial wind turbines is about 30 to 80 meters from ground level (Jahangiri et al, 1384). Here 2 and 50 meter values were selected. To estimate the wind speed, 1/7 power low model was used according to the following equation: (3) In this Equation z is height of desirable wind speed, V(z) is the wind speed that must be estimate, and z0 and V(z0) are reference Height and wind speed, respectively (Annaser et al., 2005). Weibull continuous probability values (Pw) in Kermanshah station is between 2 and 6, the Eslamabad Gharb between 3 and 8, Kangvar and Sarpol zehab between 4 and 9, and Ravansar between 5 and 10. The amount of annual wind power density at heights of 10, 20 and 50 meters, is the lowest and highest value belong to Kermanshah and Ravansar station respectively. Sarpol zehab, Islamabad Gharb and Kangavar Wether stations have higher amount of wind power respectively after Kermanshah. The value of annual potential wind energy at four named stations at the level of 10 meter is 254, 223, 214, 146 and 82 watt on square meter, respectively. In general, the results show that Ravansar, Kangavar and Sarpolzahab stations are suitable for using wind energy; EslamabadeGharb is suitable provided that high elevation wind turbines are used and Kermanshah station is known unsuitable for using wind energy.

The Caspian Sea as the largest land-locked water body is susceptible to environmental changes. The Caspian Sea has experienced different sea-level fluctuations with an impact on the rivers mouth and displacement of the mouth, changes in depositional regime, deformation of delta or renewed excavations of channel in mouth location. The south-eastern Caspian sea, as the study area is one of the most appropriate regions for studying and reconstructing the changes in coastal line, changes in river courses, delta displacement, lagoons and coastal barriers. Each of them is important as a result of geomorphologic evidence of the Caspian Sea level changes. The aim of this study is the identification of geomorphologic evidences of Caspian Sea level changes in Gorganroud River confine during the past thousand years. For this purpose, library descriptive method were done to examine theoretical and research background as well as field method to match the data of maps and find geomorphologic evidence, sample sediments and fossils, note-taking from the height coordinates of points. Furthermore, the research used experimental studies to determine the age of marine terraces and examine the sediment of sections under study, and finally achieves an analytical method. Topographic maps, geological maps, aerial photographs, Landsat and IRS imagery were used. Considering the purpose of this research to achieve the desired results, four marine terraces were found in the region. The location and height of the terraces were noted by an accurate method of measuring height and using Geodetic GPS with (Cm) precision. To identify the age of the terraces, the Carbon 14 method was used and three samples of Bivalve fossils were sent to the Shirakawa laboratory in Japan. To examine Paleontological studies, different samples of marine terrace fossils of the study area were collected and sent to Moscow University to identify their type and living environment. Also, from two sections of Kooresou with the water level -23 meters and Ghomishtappeh with the water level -24 meters, the sediments were sampled. The percentage of organic matter and calcium carbonate were investigated using Walkley and Black and Titration methods respectively. In addition, the granulometry of the particle size distribution was determined using hydrometer method. To assess the spatial and temporal variations in Gorgan Bay and Gorganroud delta, LANDSAT satellite imagery, sensors of MSS October 1975, images TM, June 1987, images of ETM, April 2001 and May 2005 and aerial photos of 1345 were utilized. To examine the changes of the old path of Gorganroud River, field visits, IRS satellite images with the resolution of 5/2 meters and historical resources were used. The research technique is the identification and analysis of geomorphologic evidences of Caspian Sea level changes in Gorganroud River confine during the past thousand and compare changes in Gorgan Bay and Gorganroud delta during 31 years. Generally, The form of coasts and geomorphologic of the study area depends on the Caspian Sea level changes. Geomorphologic evidence of the Caspian Sea level changes in the study area can be divided in to three categories as follows: - Geomorphologic evidence resulting from the Caspian Sea Level rise in the late Quaternary and the present age, for example: lagoons and sand spit. Lagoon environments are formed as a result of fluctuations of sea water level. Increase of water level before 940±24 and 653±24 years ago led to the formation of an open lagoon in the study area. - Geomorphologic evidence resulting from the Caspian Sea Level fall in the late Quaternary and present age, for example: marine terraces. Marine terraces are excellent morphological markers and important elements of coastal geomorphology as have been used world-wide to recognize past sea-level changes. The main factor in the formation of marine terraces in Khazar plain is recent tectonic movements and climate changes - Sea level changes and its impact on Gorganroud delta and river. Hydrodynamic forces (waves & currents) in Caspian Sea are gradually changing, which affects the low slope of the region, Gorganroud River mouth and its delta; and results in their deformation and displacement. Totally it can be concluded that Gorganroud River mouth morphology and its delta are affected by Caspian Sea water level fluctuations. Also Gorganroud River changed its path during the Quaternary. South East of Caspian Sea is located on very gentle slope to be strongly sensitive to the fluctuation of the Caspian Sea. The results of satellite image studies indicate that four marine terraces were formed in the height levels of -21,-23,-24 and -26 meters and with the ages of 653±24, 940±24,478±23 and 32 years respectively. The study of the data from sediments and fossils in two sections of Kooresou and Gomishtapeh corresponding with water levels of -23 and -24 meter indicates that an increase in the water level before 940±24 and 653±24 years ago caused sand spit growth in the study area. Due to the low slope of this area, lagoon is formed behind it. This lagoon, however, is dried in the offshore of the sand spit because of the sea water level fall. Moreover, some changes like changes in the depositional regime, and displacement of delta took place in Gorganroud River during the late quaternary. Also, through the last hundred years, there has been different evidence of changes of water level of the Caspian Sea. Satellite images indicate that Gorganroud delta had a progress of one-kilometer toward the sea from 1975 to 2005. Gorganroud delta is only formed during sea level fall and its morphology during sea level rise will be changed. Furthermore, Gorgan bay area was 330 square kilometers in 1975 and, in 2006, this areaincreased to 458 square kilometers, due to an increase in the water level of the sea.

Synoptic systems control precipitation patterns. One of the synoptic systems aspect that could determines the rainfall amount and spatial distribution, is geographical distribution of synoptic patterns. Moreover, changes in synoptic patterns in each region could explain daily weather conditions. Identifying synoptic systems in each region could provide a clear picture of that region climate. Climate as one of the basic structure of the environment has direct interaction with human life and activity. Identification of atmospheric circulation in each region determines the dominant weather patterns that help to understand the biological conditions in that region. In present study, the behavior of synoptic patterns corresponding? with precipitation during September months among 30 years (1976 – 2005) in Guilan province is addressed. Guilan is located in north of Iran. It is one of the wettest regions in the country. Where the most precipitation occurs at the second half of September. In order to achieve a comprehensive view of daily synoptic conditions with heavy precipitation in Guilan, lowest levels including Sea level pressure, wind advection, humidity advection in 850 and 700 (hPa) heights are analyzed. The framework of atmospheric investigation has been chosen between 0 to 70 degree in Eastern hemisphere and 10 to 60 of Northern hemisphere. This area includes all possible systems that might affect southern Caspian coast. This area has the dimensions of 29 × 21 pixels and has been covered by 609 cells. The period under investigation is the months of September during 1976-2005. The desired atmospheric data have been acquired from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP / NCAR) in 6 - hours (00: 00, 06: 00, 12: 00, 18: 00 GTM) and 2.5 × 2.5 degree resolution. Daily precipitation data from 23 stations have been chosen from IRIMO (Islamic Republic of Iran Meteorological Organization) and MOP (Ministry of Power). Percentile method has been used for extraction of days with heavy rainfall in September. The relationship between synoptic patterns and heavy rainfall in September has been calculated based on Pearson correlation coefficient. Based on Lund correlation method, circulation patterns of sea level pressure patterns have been classified. 72 days out of 77 days of heavy rains during September were classified. The remained 5 days were unclassified, because they were abnormal in compare with the period under investigation. The patterns of daily mean sea level pressure, 850 and 700 hPa height and advection convection, humidity advection and daily precipitation maps were drawn for each pattern. The result of this paper showed that the systems during heavy rainfall occurrence in Guilan are high pressure on the North-West of Black Sea, Northern Russia, and North West Europe as well as on the Norwegian Sea, North Sea and West Black Sea - Mediterranean Sea. Humidity advection shows that the increasing in humidity in all patterns occurred all over Guilan. In all patterns, presence of deep trough and its front axle is appeared. Thus, convection conditions at the surface, could justify instability as well as heavy rainfall in the region. In some patterns, the wind advection was cross Caspian Sea through Black Sea. Therefore, in addition to Caspian Sea, Black Sea was involved in supply of heavy precipitation of Guilan. In all under investigated patterns, most of the produced precipitation among the coast is notecible in compare to the southern regions. The results of the five patterns showed that high-pressure systems (North West Black Sea, Northern Russia – sub polar, North West Europe - the Norwegian Sea, North Sea and West Black Sea - Mediterranean Sea) are appeared when heavy precipitation took place. The trough formation in upper levels, fluxes high-latitude cold air toward Caspian Sea and Black Sea as well as front trough axis location over Guilan, provided a convection conditions on the surface made suitable conditions for instability and occurrence of heavy precipitation. In some patterns, in addition to the moisture of the Caspian Sea, Black Sea was involved in the heavy precipitation.

Studies have shown that Earth's climate is not stable and any changes in climate can be result of climatic system variability and external factors that can be assessed at various scales monthly, quarterly and annually. Teleconnection such as ENSO, including measures are able to assess the quantity, quality and power of a broad synoptic pattern, in the form of numerical criteria are. ENSO is one of the atmospheric phenomena with warm phase (El Nino) and cold phase (La Nina) that occurs in East of Darwin and Tahiti and West of Pacific. In recent years researchers have paid particular attention to the effectiveness of temperature, as one of the basic elements of climate formation and its role in global climate stability, ENSO phenomenon. In the present study to investigate ENSO phenomenon with the parameters of temperature, a broad sector of the country was considered and Southern Oscillation Index and Trans-Nino Index as role models for large-scale atmospheric - oceanic components on changes in average temperature in the southern half of the country was studied. In this study the relationship between the ENSO phenomenon with a mean monthly and seasonal air temperature of the country half south over a period of 55 years (1951-2005) was considered. For this purpose, temperature data of 12 synoptic stations gathered from Meteorological Organization and SOI and TNI indexes data were obtained from the Australian Meteorological Center. One of the powerful tools in defining the relationship between patterns is the correlation method. In the present study, to analyze data and explore relationships, the correlation method was used. First Pearson correlation between SOI and TNI indexes with temperature in both seasonal and monthly time step analysis was performed. Also, the effective temperature of the warm phases (El Nino) and cold (La Nina) phenomenon was considered to help index percentage changes of temperature. The results showed that in both the time scale (monthly and seasonal), the most significant correlations were negative. This means that with increasing amounts of ENSO indexes, temperature decreases. Comparison of results of both indexes showed that the number of stations with significant correlation temperature with TNI index was far more than the SOI index. Based on the TNI Index, the air temperature in January and based on the SOI index, air temperature in October were the effective months from ENSO. The seasonal scale, in the summer in most stations had a significant correlation with the ENSO index. Also the results showed that in this season, the negative correlation of seasons will turn to positive. Amplitude of the correlation between the values of SOI and the temperature was varied in all the months of minimum -0.549 in August to maximum +0.463 in October and for the TNI index from -0.458 in August and September to +0.512 in September. In seasonal scale for SOI index from -0.365 in spring season to +0.459 in autumn and for TNI index from -0.530 to +0.384 in summer season were. Based on TNI index, Percentage changes of stations temperature in each of the phases were showed in seasonal scale; La Nina phase increased 4 percent winter temperature and in monthly scale La Nina phase increased 8 percent in December temperature. Also, higher air temperature in the phase La Nina to El Nino phase was observed in all months and seasons of the study area. This study investigated the relationship between ENSO phenomena and mean air temperature in monthly and seasonal scales in southern half of the country over a period of 55 years (1951-2005).Some reviews and studies on the temperature of 12 selected stations in the southern part of Iran were done. It was shown that the temperature of 12 selected stations had a significant relationship with SOI and TNI indexes. But many more stations had significant correlation with the TNI index. In Both monthly and seasonal scale, the effect of La Nina phase was more than El Nino phase. So that in the seasonal scale, La Nina phase increased 4 percent winter temperature and in monthly scale La Nina phase increased 8 percent temperature in December. The overall results confirm considerable effective temperature of the ENSO phenomenon that explain the region temperature fluctuations as an important role.

Ilam, Kermanshah and Kordestan are three provinces in Western Iran regarded as the second agriculture realm with 65 rainy days per year. It is proved that the dynamic high pressure is prevailing system over study area during warm season. It is known by the name of Azores high but we find a different situation during cold season. The tongues of Siberian high and slow-migrant ones are alternately dominant and vanished. This alternation happened in the context of westerly winds with their ridges and troughs as western waves. Western Iran is located in the front of western waves and so, precipitable systems that increase their water vapor from Black Sea at the northwestern side of Iran to Aden Gulf and even Persian Gulf at southwestern and southern sides. This potential in comparison with northern realm including Gilan, Mazanderan and Golestan Provinces with 111 rainy days, Central realm with 20 rainy days is very important for environment managers. Heavy and widespread rainfalls feed aquifers as principal sources of water for the numerous rain-fed farms in Western Iran and of course cause financial and human problems during floods. However, we can understand the temporal and spatial considerations about precipitable systems to design synoptic models in order to optimize our utilizations. Our study area is located over the west of Iran from 32 to 37 degrees in northern latitudes and 45 to 39 degrees in eastern longitudes. At first we set the tables of rainfall data for six weather stations over Western Iran to include 30 millimeters per day and more. These tables availed us to knowledge the most Heavy Rain Waves (HRW) during 1997 to 2006. Secondly, the daily weather charts of HRWs at sea and 500 HPA levels were analyzed to determine the precipitable systems including cyclones at sea level and troughs at 500 HPA level. Finally, the horizontal flux charts of specific humidity helped us to recognize principal water bodies around Iran as humidity sources for HRWs. The study area is mountainous and includes gentle and steep slopes, deep, confined and laterally unconfined valleys and closed basins that have agriculture potential and so they are flood-prone. We selected nine weather stations that seven of them were in the area as principal ones and two of them were out of it as control stations. Heavy rainfall was defined as daily rainfall equals 30 millimeters or more that was happened at least in three weather stations during two consecutive days. There is evidence that most of pastures are overgrazed, oak forests are sparse and so, their soils have been eroded in the study area. We didnt find evidence that the mean annual precipitation of study area had increased but there is an alarming increase in the frequency of floods during the recent decades. In fact, heavy rainfalls are as the environmental consequences that are the result of interaction between the atmospheric systems and so, mountainous basins. These atmospheric hazards forming disastrous flows toward downstream areas especially in closed and wide basins. The primitive classification of data showed that Ilam City is the most hazardous point with 11 HRWs and Hamedan City in the west of study area is the least one with 2 HRWs. So, dates such as Jan. 16, 1997, Apr. 18, 2002 and Jan. 8, 1999 were the most rainy days over Western Iran. Whenever a high pressure system such as Siberian tongue or slow-migrant high was prevailed over the central part or eastern part of Iran, especially on the axis of Zagros Mountains or the eastern lands of Central Iran with northwestern-southeastern direction; the perceptible low pressures were slowly appeared and replaced toward study area. So, in these conditions HRWs were heavier and more disastrous. We found these conditions in 10 all HRWs including five events that happened over Iran, four over the Eastern and Northeastern of Iran plateau and one over Turkey. This means that perceptible lows have had enough time for water vapor reservation from water bodies including Red and Mediterranean Seas. The location of western troughs was more dispersed especially over wide and warm seas than eastern ones as interior and arid lands. It means precipitable systems have converged toward the Iran plateau and caused heavy rain falls. Water bodies are locating in the west of Iran plateau from 10 to 50 degrees. The eastern part of Mediterranean Sea, middle and southern parts of Red Sea and so, Aden Gulf are two recognized water bodies that feed water vapor to precipitable systems toward Western Iran. It seems southern water bodies as warm seas can cause liquid precipitation and so more heavy rainfalls than northern seas. The formation and strengthen of humidity feeder centers on Southern Red Sea and Aden Gulf based on the horizontal flux charts of specific humidity confirmed our results.

Dust storm is one of the most important natural phenomena and a kind of severe natural disaster that begins and diffuses under the influence of atmospheric systems. It occurs frequently in deserts and their surrounding areas in arid and semi arid regions. So, the major and most persistent sources for dust scattering in the Northern Hemisphere are located in the ‘‘dust belt’’ which extends from 20°N to 30°N and are developed under the subtropical high-pressure subsidence (Kalderon et al., 2009). In the recent years, there has been an increase in the trend of dust storms in the west and southwest of Iran, especially in spring and summer. Now it is going to change into a persistent environmental problem in Iran and the Middle East region. Dust storms have an impact on many aspects of society, such as the quality of the inhabitants` lives, transportation, air pollution, communication systems and consequent crisis such as, eco-social and biological problems. Additionally, dust can play multiple roles in mediating physical and biogeochemical exchanges among the atmosphere, land and water. So the dust storms affect many aspects of Iranian inhabitants especially who live in the western region. There are many reasons for dust scattering in the atmosphere, including the composition and moisture content of soils, wind velocity and distribution of pressure at the surface and atmospheric levels. The aim of this study is to analyze the different synoptic patterns of springtime dust occurrence in the west and southwest of Iran. In this research, for determining the synoptic patterns of dust occurrence in west and southwest of Iran, three types of data were used including: 1) hourly data of dust phenomena and horizontal visibility for 2000-2011 years for 45 meteorological stations in west and southwest of Iran. 2) In order to detect dust, the data of Moderate Resolution Imaging Spectroradiometer (MODIS) sensor from NASA’s Aqua and Terra satellites were used. Since in (MODIS) sensor, bands31 and 32 (11 and 12 micrometer) are within the thermal infrared range, so the brightness temperature difference between11 and 12micronbandsinthespectrumcanbe used for the detection of dust storms from other climatic phenomenon such as clouds.3) Six-hourly global data analysis with 2.5°×2.5° resolution from the NCEP/NCAR reanalysis, including air temperature, sea level pressure, geo-potential height, U-wind and V-wind components, relative and specific humidity and omega from 1000hPa to 200hPa, were used for the preparation of maps and identify the synoptic patterns. The method of the research was based on the synoptic approach. After clarification of the most patterns of dust occurrences, we identified 15 widespread storms in 11-years period. In this paper, by investigating the synoptic patterns of the dust storms, we identified three main synoptic patterns including: Dynamic pattern: This pattern is the main pattern for creation of dust storm in the Middle East and division to westerly trough, coupling block and omega block. When, the emigrant systems of westerly waves are dominant atmospheric phenomena in Iran and its neighbors, the formation of a trough or blocking in the waves in east Mediterranean cause ascendant weather in Iraq or north of Arabian Peninsula which also cause the formation of a cyclone under the upper divergence part in the surface. Increased pressure gradient and formation of the cold or warm front which cause an increase in wind speed in Iraq and north eastern Arabian Peninsula, are basic sources for generating dust in west and southwest of Iran. Finally the lack of humidity in this region will cause dust storm in these regions. Dynamic-Thermal pattern: In this pattern, westerly waves and the emigrant systems caused creation of dust in higher latitude (Iraq), similar to dynamic pattern, but at the same time in lower latitude, in Arabian Peninsula, Sub Tropical high-pressured are dominated and cause stable atmosphere in this region. So, the pattern of dust creation is different from upper latitudes. In this region, dust creation is under the influence of the increase of air temperature and reduction of relative humidity in the surface in east and northeast of Arabian Peninsula. Thus, it causes the development of Persian Gulf low pressure and an increase in wind`s speed and creation of dust. The dusts generated with wind stream line in 700 or 850 HP scatter in southwest on Iran. Thermal pattern: When sub tropical high-pressured are dominated in the Middle East, the increase of air temperature and reduction of relative humidity in the surface in Iraq and Arabian Peninsula cause the development of Persian Gulf low pressure and increase of wind` s speed in this region that is suitable for dust creation In recent years dust occurrence is an important natural hazard in west and southwest of Iran. The annual dust cycle in the Middle east are associated with seasonal occupation changes in westerly winds, rainfall and soil moisture, downward flowing jet stream in cold period and thermal cyclone in warm period. This paper includes the following a) When westerly waves and their emigrant systems are dominant atmospheric phenomenon in low latitudes (Iraq and Arabian Peninsula), these ascendants cause the creation of dust in west and southwest of Iran. b) At the same time westerly waves and their ascendant in upper latitudes cause dust occurrence in Iraq. But in Arabian Peninsula, Sub Tropical high-pressure is dominant and creation of dust is affected by thermal surface low pressure. c) At the end of spring, the Middle East is dominated by subtropical high pressure and subtropical jets stream, but the circulation of the atmosphere during dust storm, shows that a low pressure extends over Persian Gulf and south of Turkey. So, the surface low pressure is the cause of dust occurrence in Iraq or Arabian Peninsula. Basic sources for scattering of dust in west and southwest of Iran are Iraq, desert lands in north and northeast of Arabian Peninsula, East and southeast of Syria, also in some times is Sahara desert.

Generally, yearly variability of precipitation make rangelands unsuitable for crop production, and livestock grazing presents a sustainable means of food and fiber production. Droughts which often occur within a long period, may drastically affect plant community composition and may make rangelands more susceptible to diseases, insect pests, weed invasions, and overgrazing. Therefore, continuous monitoring of rangeland status would help decision makers to do necessary interventions and proper reactions against drought spells. In this regard, current ground-based methods seem to be not suitable for rangeland monitoring and regional assessment of drought impacts on its vegetation cover and seasonality. These methods always consume a plenty of time and expense. Conversely, satellite remote sensing (RS) techniques can provide useful information about changes of rangeland status due to changes in climatic attributes of the region. This technique has significant promise for development of more reliable and economically feasible measures of vegetation status over large areas. This technique can be used to monitor vegetation cover status both in time and space. In fact, monitoring of large areas at low cost and time is one of the benefits of remote sensing technique. The current study intends to assess the impact of climatic conditions on rangeland vegetation cover in Yazd-Ardakan plain in Central Iran. Total area of Yazd-Ardakan plain is 11630 square kilometers with elevations between 3240 and 990 m above sea level. For the assessment, an indicator namely Index of Drought Impact (IDI) was introduced. This indicator uses a remotely sensed Vegetation Index (VI) as a proxy for vegetation cover and relates the selected VI to a suite of climatic data to define relationships. In this study 36 cloud free images were taken for derivation of ten well-known vegetation indices. These images were acquired from the MODIS sensor of Terra satellite in first decade of new century (2000 to 2009). Additionally, a Landsat ETM+ image was used for preparation of land use map and also interpretations due to its good spatial resolution (30 m). Identifying relationships between climatic attributes and rangeland vegetation status was then accomplished. In this stage, records of eight meteorological stations were collected for 2000-2009 time period. Precipitation (P), temperature (T), relative humidity (H), wind speed (W) and standardized precipitation index (SPI) were selected as the main climatic indicators of the region. Correlations between yearly values of these climatic parameters and different vegetation indices were established using R2 correlation matrix. Through this procedure, fair correlations were investigated and therefore, the best vegetation index was identified. This index was then used for monitoring of drought impact on Yazd-Ardakan rangeland vegetation. Digital Elevation Model (DEM) was also employed as ancillary data for generation of temperature and precipitation maps. In this study, SAVI was found as the most preferred MODIS-derived vegetation index and therefore combined with precipitation and temperature, as two main climatic parameters of the region to generate the IDI map. Based on the rationale relationship in the IDI, regions with high values of IDI would be more susceptible for degradation of their resources (vegetation cover) as direct impacts of main climatic attributes in the region. In this case study, IDI map has been generated and classified into three different levels (low, medium and high) and then interpreted, visually. Results show that in the northern areas of the studied region which are also the plain outlet have the maximum IDI values compared with other areas. These areas can be found as more susceptible regions to degradation of their resources (vegetation cover) due to climate inconsistencies and droughts in the recent decade. Lower part of Yazd-Ardakan plain (outlet) is a region with minimum agricultural and pastoral activities due to its degraded soil and water resources, salinization, harsh climatic conditions and poor natural vegetations. In contrast, lower values for “IDI” belong to border areas. These areas are high elevated regions with relatively cold winters and moderate summers. This indicator was also classified into three degradation classes to show the spatial extent of climatic impacts on rangeland vegetation cover. Results show that about 23%, 59% and 18% of this area belongs to low, medium and high IDI values, respectively. Results indicate that the IDI provides an approach to connect the climate conditions with vegetation cover to assess vegetation degradation in such arid environment. This index can be used by researchers; decision makers and stakeholders for doing necessary interventions and for the purpose of sustainable rangeland management. It can also be interpreted and used based on other related aspects such as grazing pattern, distance to water and population centers. Considering results of this case study, the introduced indicator was found suitable for monitoring of drought impact on rangeland vegetation and its classification. Repeatability and relatively low cost and time consumption are the major advantages of the introduced indicator.