جستجوی مقالات مرتبط با کلیدواژه « گرادیان چرا » در نشریات گروه « آبخیزداری، بیابان، محیط زیست، مرتع »

The southeastern region of Iran is faced with dust phenomenon, which is primarily caused by 120-day Sistan winds and topographic conditions. Moreover, considering the great influence of the winds on the occurrence of dust in the study area, fluctuations of wind speed and dust are of great importance. Therefore, this study sought to investigate and analyze the cycles governing dust and wind speed in Hirmand watershed.As a climatic event, dust phenomenon may occur in all weather conditions. However, the phenomenon disturbs human activities and damages social infrastructures such as agriculture, transportation, and industries in arid and semi-arid regions.Dust storms frequently occur in some parts of the world, especially the Middle East. However, the occurrence of successive dust storms has so increased in the region during the last decade that it is also observed in the cold and rainy months. In this regard, studies conducted on dust storms in Iran suggest that in addition to natural factors that create the mass of fine dust, environmental or human factors are also involved in the occurrence of such a phenomenon and its long-term stability and continuity. 

The current study set out to examine and analyze the cycles governing dust and wind speed in the Hirmand watershed. To this end, the required data concerning wind speed and dust data were collected from 2000 to 2018 (18-year period) from seven synoptic stations located within the Hirmand watershed, including Zabul, Zahak Hamon, Nusratabad, Zahedan, Dermian, Deh Salam, Nehbandan, and Birjand stations, using the information published by the National Meteorological Organization (Figure 1). After extraction and quality control of the data, the code of phenomena related to dust has been used to extract the dust. After preparing the annual wind speed and dust data, spectral analysis has been used to analyze the cycles. 

Fluctuations (cycle), probability, and variance of dust were calculated for Hirmand watershed stations. The high variance indicates the importance of the dominant fluctuation of the cycle. In Birjand station, the time series of dust showed two short-term cycles of two and six years, and the probability of the occurred cycles were 0.2 and 0.5, respectively. Therefore, as the variance of the two-year cycle was found to be higher, it could be said that the cycle in the dust time series of Birjand station is very important.However, only one dominant cycle of two years was observed in Nehbandan station, whose occurrence probability was reported to be 0.5. Moreover, the variance of the two-year cycle in Nehbandan station was 529, indicating the significance of the cycle in the time series of Nehbandan dust.As for the Zabul and Zahak stations (located in eastern half of Hirmand watershed), the dust was found to have six and two-year fluctuations. On the other hand, an 18-year cycle was observed in the dust time series of Nusrat Abad station. Moreover, the examination of wind speed fluctuations in Hirmand watershed stations revealed that wind speed was also dominated by short-term fluctuations (just like what was found for the dust). However, various cycles of six and eleven years were observed in some stations. For instance, an eleven-year cycle was found to have prevailed in the drainage station. Therefore, it can be argued that the fluctuations governing dust and wind speed are similar, indicating the relationship between these two parameters in the southeast region of Iran. 

Sistan basin is known as one of the most active sources of dust and windy desert environments in the world. Although dust activity in Sistan reaches its maximum during summer, the region may be hit by rare severe dust storms in winter. Many studies conducted on precipitation have already used spectral analysis method and the extraction of ruling cycles. However, the technique has not been used in the study of dust and wind.The analysis of dust cycles showed that the major fluctuations governing the dusts of the Hirmand watershed occur in a short-term (two to four years). Moreover, the status of cycles governing wind speed revealed that short-term fluctuations of two to four years prevailed in most stations (the same as what occurred for the dust). Therefore, the fluctuations governing these two parameters in Hirmand watershed vary from two to four years. A similarity is also observed in the dominant cycles of wind speed and dust, indicating the fact that the dominant cycles of this part of Iran are the same under the influence of the macro-scale oceanic atmospheric system.

Wind erosion and dust phenomenon are considered as one of the important processes of land degradation and a serious challenge in Iran. This phenomenon occurs due to the interaction between weather and terrestrial processes. The purpose of this research is to investigate and analyze the trend of temporal and spatial changes in the number of dusty days in Iran and to investigate the role of climate in its spread.

In this research, after receiving hourly and daily statistics of dust events (through 148 synoptic and climatology stations) and parameters of precipitation, evaporation, temperature as key climatic elements (from 171 stations) in 22 provinces located in arid and semi-arid regions of Iran, The spread of dust was investigated and the correlation between climate and dust was determined. After calculating the number of dust days in Iran, the information was called in the GIS environment and a map of the spatial changes of the parameter of the number of dust days in the three decades of 1987-1996, 1997-2006, 2007-2016 was prepared. In the following, the spatial analysis of this parameter and its expansion over three decades were investigated. Next, the climate parameter that had the highest correlation with the number of dusty days was selected and after preparing a map of its spatial changes in the GIS software environment, the spatial compatibility of the desired parameter with the expansion of the number of dusty days was investigated. In the next step, after performing the homogeneity test on the dust occurrences, the partial square regression model was used to determine the contribution of the three mentioned climatic elements as independent variables and the number of dust days as a dependent variable.

The results showed that the number of dusty days and the area of its occurrence have increased in Iran, so that in the last decade under study (2007-2016) compared to the previous two decades, the dustiest event occurred in Iran. This increase has been more intense especially in the western and southwestern regions of the country, which were also affected by extra-local dust events. These dust events have spread to the central regions of the country with greater frequency. The homogeneity test in many provinces showed a common change point (year 2007-2008) in the number of dusty days, which could be related to the beginning of severe droughts and climate changes from this year in Iran. The study of climatic parameters (temperature, precipitation and evaporation) on changes in the number of dust days in Iran showed that during recent years, temperature had the highest significant correlation level (0.58) with the occurrence of dust. The temperature gradient map in the last decade (2007-2016) compared to the previous decade (1997-2006) also showed the increasing trend of temperature, especially in the western and southwestern regions as well as the southeastern regions of Iran. Modeling the effect of climatic parameters of temperature, precipitation and evaporation on the parameter of the number of dusty days showed that the contribution of the studied climatic parameters to the occurrence of dusty days is about 33% and other effects can be related to things such as human factors or soil characteristics, type of vegetation, solar radiation, etc.

In general, the results of examining the trend of changes in the number of dust days and climatic indicators for nearly 3 decades indicate a significant increase in the number of dust days in most parts of Iran and the conditions for this increase have been provided and if the trend of changing these indicators in the coming years is according to the current trend, the risk of environmental problems in the country can be expected to increase.

Aim of this study was to introduce of the functional threshold by ecological process changes along grazing gradient to determine destroyed area. In order to prove the existence of a threshold, two linear models and three nonlinear models (exponential curve, a piecewise regression and a sigmoid logistic) were fitted on stability, infiltration and nutrient cycling data. Two nonlinear models (exponential curve and a piecewise regression) provided a much better fit to the data than the linear models. It is the evidence of threshold along grazing gradient. Results also showed that ecosystem function descended near the water point that it needs special attention of the managers. The passing of the structural threshold, increase of invasive vegetation and animal trampling were identified as the main factors in the decline of ecosystem function near the water point and occurrence of functional threshold. According to the results, the recognition of the threshold is suggested for rangeland ecosystem because defining the threshold will help land managers to prevent the occurrence of undesirable states and promote for sustainable management of rangeland ecosystems.