全文获取类型
收费全文 | 372篇 |
免费 | 22篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 23篇 |
地球物理 | 125篇 |
地质学 | 187篇 |
海洋学 | 12篇 |
天文学 | 17篇 |
综合类 | 5篇 |
自然地理 | 20篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 13篇 |
2020年 | 30篇 |
2019年 | 20篇 |
2018年 | 40篇 |
2017年 | 37篇 |
2016年 | 45篇 |
2015年 | 24篇 |
2014年 | 26篇 |
2013年 | 30篇 |
2012年 | 31篇 |
2011年 | 26篇 |
2010年 | 16篇 |
2009年 | 8篇 |
2008年 | 7篇 |
2007年 | 6篇 |
2006年 | 8篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1996年 | 2篇 |
1993年 | 1篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1965年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有407条查询结果,搜索用时 15 毫秒
11.
12.
The influence of physical and biological processes on the ichthyoplankton communities in the Gulf of Sirte (Southern Mediterranean Sea)
下载免费PDF全文
![点击此处可从《Marine Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Angela Cuttitta Sergio Bonomo Salem Zgozi Angelo Bonanno Bernardo Patti Enza Maria Quinci Marco Torri Mohamed Hamza Abdul Fatah Daw Haddoud Akram El Turki Abdul Bari Ramadan Simona Genovese Salvatore Mazzola 《Marine Ecology》2016,37(4):831-844
This paper reports on the first study on the ichthyoplankton community structure in the Gulf of Sirte and its spatial distribution in relation to environmental conditions and zooplankton abundance. The results make an important contribution to the future management of fisheries in this unexplored, but much exploited, area. Biological samples were collected in July 2008 using a Bongo40 net. In total, 1914 larvae were found and 1652 of these were identified. In particular, bathypelagic taxa were the most abundant, followed by demersal, mesopelagic, pelagic and epipelagic taxa. The ichthyoplankton community had a patchwork distribution influenced by oligotrophic conditions, the bottom depth and oceanographical features. The results suggest that environmental forcings were able to transport the ichthyoplankton to productive areas. Indeed, maximum fish egg densities were found in coastal stations in correspondence with the Atlantic Tunisian Current inflow, whereas larvae were mainly concentrated in the east side of the Gulf, probably as a result of advection by the anticyclonic circulation. Additionally, the distribution patterns of the total larvae density and the different assemblages were well matched with the abundance of the zooplankton, probably determining final larval survival, growth and recruitment. 相似文献
13.
14.
Rahim Barzegar Asghar Asghari Moghaddam Amir Hossein Nazemi Jan Adamowski 《Environmental Earth Sciences》2018,77(16):597
Rapid population growth, industrialization, and agricultural expansion in the Khoy area (northwestern Iran) have led to its dependence on groundwater and degradation of groundwater quality. This study attempts to decipher the major processes and factors that degrade the groundwater quality of the Khoy plain. For this purpose, 54 groundwater samples from unconfined and confined aquifers of the plain were collected in July 2017 and analyzed for major cations and anions (Na, K, Ca, Mg, HCO3, SO4, and Cl), minor ions (NO3 and F), and Al. Magnesium and bicarbonate were identified as the dominant cation and anion, respectively. Several ionic ratios and geochemical modeling using PHREEQC indicated that the most important hydrogeochemical processes to affect groundwater quality in the plain were weathering and dissolution of evaporitic and silicate minerals, mixing, and ion exchange. There were smaller effects from evaporation and anthropogenic factors (e.g., industries). Results showed that the high salinity of the groundwater in the northeast area of the plain was due to the high solubility of the evaporitic minerals, e.g., halite and gypsum. Reverse ion exchange and the contribution of mineral dissolution were more significant than ion exchange in the northeastern part of the plain. Elevated salinity of the groundwater in the southeast was attributed mostly to reverse ion exchange and somewhat to evaporation. 相似文献
15.
Ahmad Jonidi Jafari Majid Kermani Roshanak Rezaei Kalantary Hossein Arfaeinia 《Environmental Earth Sciences》2018,77(2):38
Due to the explosive industrialization and rapid expansion of the population in many parts of the world, heavy metals are released into the environment continuously and pose a great risk on human health. Street dust and surface soil samples from very heavy, heavy, medium and low traffic areas and a natural site in Tehran, Iran, were analyzed for some physicochemical features, total and chemical fractionating of selected metals (Zn, Al, Sr, Pb, Cu, Cr, Cd, Co, Ni and V) to investigate the influence of traffic on their mobility and accumulation in the environment. The pH, electrical conductivity (EC), carbonates and organic carbon contents were similar in soil and dust samples from the areas with same traffic. The traffic increases EC contents in dust/soil matrixes, but has no effect on concentrations of metals in soil samples. Rises in metal levels with traffic were found in dust samples. Moreover, the traffic increases the percentage of both acid-soluble and reducible fractions, which are related to Pb and Zn. The mobilization of Cu, Zn, Pb, Cr in dust samples was easier than in soil. The speciation of metals except Cd is mainly affected by physicochemical features in soil, though total metals affected the speciation in dust samples (except chromium and nickel). 相似文献
16.
Precipitation is a major climatic element with high spatial variations. Temporal and spatial variations may differ in large and small scales. It is, therefore, of utmost importance to study areas with similar gradients in terms of precipitation patterns in order to shed light on the complexities of precipitation variations. In the present study, attempts were made to identify areas with similar gradients experiencing the same precipitation pattern over a 50-year period (1964–2013). To this end, data were collected from synoptic stations in Iran in two phases (i.e., 1434 stations in the first phase and 673 stations in the second one). Alexanderson’s technique was adopted to examine sudden changes in precipitation patterns. The results showed that five regions with similar gradients could be identified in terms of precipitation patterns: negative and high variations, negative and moderate variations, positive and high variations, positive and moderate variations, and little or no variations. The distribution of such regions indicated that the regions with positive trends experienced more annual variations and had further spatial distribution. Furthermore, the findings revealed that the regions with negative precipitation patterns experienced more sudden changes in comparison with those with positive precipitation patterns. Additionally, more variations were observed in the precipitation patterns in recent years. 相似文献
17.
Hossein Akbarzadeh Kasani Naser Hamidzadeh 《Geotechnical and Geological Engineering》2018,36(1):179-192
Presence of aggressive chemicals in ground and groundwater can deteriorate concrete foundations or its reinforcements. Concrete codes require preventive measures to assure durability of concrete foundations which is based on geochemistry information of the site. Construction in the city of Rasht, in the Iranian Caspian coast, is fast growing often without sufficient geochemistry data for residential buildings. The later may pose risk of chemical attacks on concrete foundations. To resolve this shortcoming, this study aimed at investigating geochemistry of ground and groundwater from the upper 10 m which can serve as a preliminary guideline for shallow foundation constructions in the city. The database for this study included previous geochemical investigations from 50 boreholes in the city as well as boring eleven test pits at various locations along with soil and groundwater geochemistry tests. The geochemistry experiments included measurement of sulfate, chloride, organic matter, as well as pH in soil and groundwater. Geochemistry of ground and groundwater in the city mostly fell within permissible limits set by a local concrete code. Based on current study, service environment for shallow concrete foundations in the city was categorized in a moderate class; hence, durability requirements from the same local concrete code were emphasized. 相似文献
18.
Taherynia Mohammad Hossein Koopialipoor Mohammadreza Azizzadeh Abbas Fahimifar Ahmad Ghyasvand Soheil 《Geotechnical and Geological Engineering》2021,39(7):5119-5133
Geotechnical and Geological Engineering - Small variation in shear strength parameters results in remarkable changes in the safety factor (SF) of a rock slope. In this regard, rock mass strength of... 相似文献
19.
One important step in binary modeling of environmental problems is the generation of absence-datasets that are traditionally generated by random sampling and can undermine the quality of outputs.To solve this problem,this study develops the Absence Point Generation(APG)toolbox which is a Python-based ArcGIS toolbox for automated construction of absence-datasets for geospatial studies.The APG employs a frequency ratio analysis of four commonly used and important driving factors such as altitude,slope degree,topographic wetness index,and distance from rivers,and considers the presence locations buffer and density layers to define the low potential or susceptibility zones where absence-datasets are gener-ated.To test the APG toolbox,we applied two benchmark algorithms of random forest(RF)and boosted regression trees(BRT)in a case study to investigate groundwater potential using three absence datasets i.e.,the APG,random,and selection of absence samples(SAS)toolbox.The BRT-APG and RF-APG had the area under receiver operating curve(AUC)values of 0.947 and 0.942,while BRT and RF had weaker per-formances with the SAS and Random datasets.This effect resulted in AUC improvements for BRT and RF by 7.2,and 9.7%from the Random dataset,and AUC improvements for BRT and RF by 6.1,and 5.4%from the SAS dataset,respectively.The APG also impacted the importance of the input factors and the pattern of the groundwater potential maps,which proves the importance of absence points in environmental bin-ary issues.The proposed APG toolbox could be easily applied in other environmental hazards such as landslides,floods,and gully erosion,and land subsidence. 相似文献
20.
Asiyeh SHEIKHZADEH Hossein BASHARI Mostafa TARKESH ESFAHANI SayedHamid MATINKHAH Mohsen SOLEIMANI 《山地科学学报》2019,(6)
This study aimed to identify indicator species and explore the most important environmental and management variables contributing to vegetation distribution in a hilly upper dam landscape in Zagros Mountain chain, Iran. A stratified random sampling method was used to collect topographic, edaphic, management and vegetation data. The density and cover percentage of perennial species were measured quantitatively. Indicator species were identified using the two-way indicator species analysis. Besides calculating physiognomic factors in sample sites, 24 soil samples were collected from 0 to 30 cm of soil depth and analyzed in terms of gravel percentage, texture, saturation moisture, organic matter, pH and electrical conductivity in saturation extract, lime percentage, soluble calcium and magnesium, available phosphorus, Cation Exchange Capacity(CEC) and soluble sodium and potassium. Multivariate techniques including Canonical Correspondence Analysis and Multi-Dimensional Scaling were used to explore the relationships of species with environmental and management variables. Seven plants were identified as indicator species due to being significantly correlated with management(grazing or non-grazing) and edaphic variables such as CEC, soil texture, pH, CaCO_3 percentage and physiographic variable including slope, elevation, and convex and concave formations(p 0.05). Overall, overgrazing and its subsequent effects on soil characteristics, loss of vegetation cover and trampling were found as the major causes of deterioration. Sustainable and integrated management practices such as the implementation of appropriate grazing systems were suggested to enhance soil quality and reduce the accelerated erosion in upper dam zones. 相似文献