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排序方式： 共有191条查询结果，搜索用时 15 毫秒
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In order to investigate the development of forest soils formed on loess, six representative modern soil pedons were selected along a precipitation gradient extending from eastern Golestan(mean annual precipitation, MAP = 500 mm)to eastern Mazandaran Provinces(MAP = 800 mm).Physiochemical, micromorphological and magnetic properties, as well as clay mineralogy of soils were studied using standard methods. Soils are mainly classified as Alfisols and Mollisols. Downward decalcification and the subsequent clay illuviation were the main criteria of soil development in all study areas. Pedogenic magnetic susceptibility of pedons studied varied systematically across the precipitation gradient in Northern Iran, increasing from 14.66 ×10~(-8) m~3 kg~(-1) at the eastern part to 83.75 × 10~(-8) m~3 kg~(-1) at the western margin of this transect. The frequencydependent magnetic susceptibility showed an increasing trend with rainfall as well. The micromorphological study of soils indicated that there is a positive relationship between climate gradient (increasing rainfall) and the micromorphological index of soil development(MISECA). The area and thickness of clay coatings showed an increasing trend with rainfall. Grain size analysis indicates that pedogenic processes are responsible for changing original grain size distribution of loess in our soils.The correlation achieved among modern soil properties and precipitation could be applied to the buried paleosols in the whole study area to refer degree of paleosol development and to reconstruct the paleoclimate. 相似文献
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Monitoring and prediction of land use/land cover changes using CA-Markov model: a case study of Ravansar County in Iran 总被引：1，自引：0，他引：1
Hazhir Karimi Javad Jafarnezhad Jabbar Khaledi Parisa Ahmadi 《Arabian Journal of Geosciences》2018,11(19):592
Human‐induced land use/land cover (LULC) changes are among the most important processes that shape the dynamics of the earth’s surface. This phenomenon, which is occurring at an astonishing rate, and its consequential environmental impacts have become an important area of research for scientists.Therefore, a wide range of methods and models have been developed to detect and predict these alterations, among which cellular automata (CA) models such as the CA‐Markov model, due to their affinity to geographic information system (GIS) and remote sensing (RS), are appropriate for detailed resolution modelling and simulating dynamic spatial processes. In Iran, the district of Ravansar has undergone severe LULC changes recently, thus to take the necessary precautions, decision‐makers need to predict and determine the extent of these changes. In this study, using spatial analysis methods the LULC changes in Ravansar were investigated from 1992 to 2015. Subsequently, the CA‐Markov model was applied to simulate the spatial pattern changes of LULC until 2030. Our results indicated that from 1992 to 2015, this region has witnessed a noticeable increase in the areas of the built‐up and agricultural lands (both aquatic and non‐aquatic), resulting in the decrease of the gardens, range, and bare lands. The simulated LULC map showed that this trend will continue due to more urbanization and development of agricultural areas. 相似文献
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Understanding the impacts of land‐use changes on hydrology at the watershed scale can facilitate development of sustainable water resource strategies. This paper investigates the hydrological effects of land‐use change in Zanjanrood basin, Iran. The water balance was simulated using the Soil and Water Assessment Tool (AVSWAT2000). Model calibration and uncertainty analysis were performed with sequential uncertainty fitting (SUFI‐2). Simulation results from January 1998 to December 2002 were used for parameter calibration, and then the model was validated for the period of January 2003 to December 2004. The predicted monthly streamflow matched the observed values: during calibration the correlation coefficient was 0·86 and the Nash–Sutcliffe coefficient 0·79, compared with 0·80 and 0·79, respectively, during validation. The model was used to simulate the main components of the hydrological cycle, in order to study the effects of land‐use changes in 1967, 1994 and 2007. The study reveals that during 1967 a 34·5% decrease of grassland with concurrent increases of shrubland (13·9%), rain‐fed agriculture (12·1%), bare ground (5·5%) irrigated agriculture (2·2%), and urban area (0·7%) led to a 33% increase in the amount of surface runoff and a 22% decrease in the groundwater recharge. Furthermore, the area of sub‐basins that was influenced by high runoff (14–28 mm) increased. The results indicate that the hydrological response to overgrazing and the replacing of rangelands (grassland and shrubland) with rain‐fed agriculture and bare ground (badlands) is nonlinear and exhibits a threshold effect. The runoff rises dramatically when more than 60% of the rangeland is removed. For groundwater this threshold lies at an 80% decrease in rangeland. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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Ambient response measurements were made on an eighteen-storey building at three different stages of construction to detect any changes in the frequencies, mode shapes and stiffness with construction. The first nine frequencies and corresponding mode shapes for each stage of construction are found. A comparison is made among these mode shapes and frequencies and with the mode shapes and frequencies of an analytical model incorporating beams, columns, shear walls, panels and diagonal elements. The added effects, on frequencies and mode shapes, of non-structural elements such as stairs, elevators, claddings and partition walls are studied. Using Improved Statistical Structural Identification, an attempt is made to study the stiffening effect of non-structural elements by updating the stiffness matrix of the building. 相似文献
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Development and testing of a serially multiplexed fiber optic sensor system is described. The sensor differs from conventional
fiber optic acoustic systems, as it is capable of sensing AE emissions at several points along the length of a single fiber.
Multiplexing provides for single channel detection of cracks and their locations in large structural systems. An algorithm
was developed for signal recognition and tagging of the AE waveforms for detection of crack locations. Laboratory experiments
on plain concrete beams and post-tensioned FRP tendons were performed to evaluate the crack detection capability of the sensor
system. The acoustic emission sensor was able to detect initiation, growth and location of the cracks in concrete as well
as in the FRP tendons. The AE system is potentially suitable for applications involving health monitoring of structures following
an earthquake
Supported by: National Science Foundation, Grant number CMS-9900338 相似文献
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In order to study the effects of different land vegetative covers on soil quality attributes, a loess hill slope was selected in eastern Golestan Province, Ghapan watershed, Iran. Four profiles in four land uses, including Quercus natural forest; Pinus artificial forest; Cupressus artificial forest and a cultivated land, were studied. Results showed that MWD was significantly different in the studied land uses, and it varied between 1.6 mm in Quercus natural forest and o.31 mm in cultivated land use. The lowest CEC, microbial respiration rate and organic carbon were 28.4 cmol·kg^1, 177 μgCO2·g^-1·day^-1 and 1.32 % found in cultivated land use, respectively. The organic matter was considerably higher content in the forest areas than that of cultivated land use. The studies on soil profile development revealed that the natural forest soils were highly developed. The soils of the Quercus natural forest were classified as Calcic Haploxeralfs with a well developed argillie horizon unlike the cultivated soils which showed the minimum development and classified as Typic Xerorthents. The soils of the artificial forests had both mollic epipedons and were classified as Typic Calcixerolls with moderate profile development. Micromorphological studies revealed that argillic horizons had speckled and partly crystallitic b-fabric in the natural forest indicating the high landscape stability. In contrast, the crystallitic b-fabric of other land uses shows the absence of enough leaching of carbonate and the subsequent migration of clay particles indicating the unstable conditions and high soil erosion. Intense erosion of the surface horizons of cultivated land use has resulted in the outcropping of the subsurface carbonate rich horizons preventing soil development. 相似文献
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