首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   267篇
  免费   18篇
  国内免费   8篇
测绘学   7篇
大气科学   20篇
地球物理   81篇
地质学   102篇
海洋学   15篇
天文学   35篇
综合类   18篇
自然地理   15篇
  2023年   2篇
  2022年   4篇
  2021年   5篇
  2020年   2篇
  2019年   8篇
  2018年   17篇
  2017年   12篇
  2016年   23篇
  2015年   22篇
  2014年   19篇
  2013年   30篇
  2012年   15篇
  2011年   16篇
  2010年   12篇
  2009年   14篇
  2008年   6篇
  2007年   10篇
  2006年   9篇
  2005年   6篇
  2004年   12篇
  2003年   9篇
  2002年   5篇
  2001年   4篇
  2000年   1篇
  1999年   1篇
  1998年   2篇
  1997年   2篇
  1996年   1篇
  1995年   2篇
  1994年   2篇
  1993年   4篇
  1992年   3篇
  1991年   2篇
  1990年   3篇
  1989年   1篇
  1988年   1篇
  1987年   2篇
  1982年   1篇
  1979年   1篇
  1976年   1篇
  1975年   1篇
排序方式: 共有293条查询结果,搜索用时 671 毫秒
51.
52.
In this paper, we discuss cosmological application of holographic Dark Energy (HDE) in the framework of f(G) modified gravity. For this purpose, we construct f(G) model with the inclusion of HDE and a well-known power law form of the scale factor a(t). The reconstructed f(G) is found to satisfy a sufficient condition for a realistic modified gravity model. We find quintessence behavior of effective equation of state (EoS) parameter ω DE through energy conditions in this context. Moreover, we observe that the squared speed of sound $v_{s}^{2}$ remains negative, which indicates the instability of HDE f(G) model.  相似文献   
53.
The aim of this study was to assess trace metal contamination of drinking water in the Pearl Valley, Azad Jammu and Kashmir (Pakistan). The objectives were to determine physical properties and the dissolved concentration of five trace metals, i. e., lead, copper, nickel, zinc, and manganese, in drinking water samples collected from various sites of municipal water supply, natural water springs and wells in the valley. Concentrations of the metals in the water samples were determined by flame atomic absorption spectrometry. Results showed physical parameters, i. e., appearance, taste and odor within acceptable limits and pH was between 5.5 and 7.0. The observed concentrations of the metals varied between sources of water samples and between sampling sites. Maximum dissolved concentration observed was 4.7 mg/L for Pb and Mn, 4.6 mg/L for Zn, 2.9 mg/L for Ni and 2.8 mg/L for Cu. The observed concentrations of the metals were compared with the World Health Organization's guideline values for drinking water. Overall, the quality of water samples taken from the water springs at Mutyal Mara and Bonjosa was good; however, the water quality was unsuitable for drinking in Kiraki, Kharick, and Pothi Bala localities particularly. Finally, the authors discuss possible causes for increased concentrations of the trace metals in drinking water in the study area.  相似文献   
54.
This paper discusses patterns of annual and monthly precipitation variability at seven weather stations in east central Europe (1851–2007). Precipitation patterns were compared to three simple regional indices of atmospheric circulation, i.e., western circulation, southern circulation and the cyclonicity (C) index and a relationship between precipitation and the North Atlantic Oscillation index was identified. Correlations of the monthly records and multiple regression, using a principal components’ analysis, helped determine the statistical significance of the dependence of precipitation on the circulation indices. The Mann–Kendall test revealed no trend to change in any of the precipitation series, but a certain spatial regularity could be discerned in the phase of the annual periodic component. A common feature of the variability in central European annual precipitation is the dry period identified in the 1980s and the first half of the 1990s. In the northern part of the region, above-average precipitation was noted from the 1960s through to the mid-1970s as a result of the frequent prevalence of depressions. South of the divide, the wettest period was recorded at the turn of 1930s/1940s. After a number of very wet years in the last decade of the twentieth century and the beginning of the twenty-first century, precipitation began to fall at all of the region’s weather stations. The C index is the strongest circulation-linked factor influencing precipitation in central Europe and it accounts for more than 40% of the variance in spatially averaged wintertime precipitation.  相似文献   
55.
利用MODIS大气廓线产品计算空气相对湿度的方法研究   总被引:2,自引:1,他引:2  
森林火灾遍及全球,损失巨大。火险天气指数(FWI)是森林火灾预警和管理的有效工具。相对湿度(RH)是计算天气火险指数一个非常重要的参数。然而,由插值方法得到的RH在远离气象站点的地方往往存在很大的误差。因此,尝试利用MODIS遥感数据计算基于像元的高分辨率RH。MODIS的MOD07大气廓线产品,可以提供大气可降水量和地表气温数据。基于地面水汽压和大气可降水量的经验关系,利用MODIS数据计算了地表空气的相对湿度,平均绝对误差小于5%。结果表明,利用MODIS数据能简单而有效地计算地表空气的相对湿度,拓展了遥感技术在森林火险管理中的应用。  相似文献   
56.
The Domingos Petrolini Swamp, located on the Southern Coastal Plain of Rio Grande do Sul, near the town of Rio Grande, constitutes an ample holocene lagoonal deposit. Events related to the holocene transgression and their diverse phases of glacial-eustatic ocillations were found through palynological analysis. Bases on these palynological data, four distinct phases to the paleoenvironmental evolution of the ecosystem were identified. It has also shown a transgressive restricted marine sequence on the base , which suggests the existence of a coastal brackish lagoon or a marisme type of environment. Later, there was a gradation for continentality conditions along with a scarce pollinical record. The third phase is characterized by the presence of marine components and halophyte vegetation, showing another transgressive event in that place. Lastly, the end of direct marine influence over the swamp was recorded and also the vegetation development as it appears these days.  相似文献   
57.
This paper focuses on the process of Jewish settlement in the West Bank and its economic consequences in the period from 1967 to the outbreak of the Intifada late in 1987. It attempts to show that this process is the practical application of Israeli objectives which are based on expansion and occupation of neighbouring Arab lands, facilitated by the fashioning of a dual or bifurcate economy there.After the war of June 1967, Israeli occupation authorities started to draw up plans with a view to settling Jews in the West Bank. There has been an increase in the numbers of settlements and settlers, estimated at 122 and 52,000 respectively in 1987. The pattern of settlement distribution is randomly dispersed, although it is concentrated in a region located to the NW of Jerusalem. The settlement began in the Jordan valley and extended gradually westward in the highlands.Jewish settlement has affected economic development of the West Bank, where there were many constraints on Arab agriculture and industry. Inequality is evident between settlers and Palestinians in an economy that has been structurally bifurcated: although settlers represent about 3% of total population of the West Bank, their economic activity constitutes at least 35% of the GDP of the West Bank.  相似文献   
58.
Aqueous Surfactant Washing of Residual Oil Contamination from Sandy Soil   总被引:2,自引:0,他引:2  
A laboratory study was conducted to determine the efficiency of different aqueous concentrations of an alcohol ethoxylate surfactant in washing residual levels of an oil [automatic transmission fluid (ATF)] from sandy soil. Five glass columns packed with the soil were prepared in a manner that simulated conditions leading to residual saturation in an actual oil leak. Each of four columns was washed continuously with 28 pore volumes of solution by pumping either 0.0 percent (water), 0.5 percent, 1.0 percent, or 2.0 percent aqueous surfactant solutions through the columns. The fifth column was washed intermittently with 28 pore volumes of a 1.0 percent surfactant solution. Water washed only 25.5 percent of the ATF from the column soil, while the 0.5 percent, 1 percent, and 2 percent surfactant solutions washed 55 percent, 60 percent, and 72.8 percent of the ATF, respectively. The distribution of the ATF remaining in the column after washing showed that the ATF removed by water was mainly from the outlet side of the column, while the ATF removed by the 2.0 percent surfactant solution was mainly from the inlet side of the column. This observation indicated that different mechanisms were involved; namely, the displacement of oil through the soil-pore space, the dispersion of oil due to reduced surface tension, and the solubilization of oil by surfactant micelles. In the case of water, the displacement of oil was the main washing mechanism, while all three mechanisms were operative during surfactant washing. ATF dispersion and solubilization were improved at higher surfactant concentrations. The column that was washed intermittently to pulse ATF from dead end pores did not show a significant improvement over the column that was continuously washed with the same 1.0 percent surfactant solution. The results show promising potential for application in the field and will be further investigated in a two-dimensional model aquifer.  相似文献   
59.
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号