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   17899篇
  免费   2145篇
  国内免费   3277篇
测绘学   1210篇
大气科学   2582篇
地球物理   3503篇
地质学   8876篇
海洋学   2551篇
天文学   1892篇
综合类   1069篇
自然地理   1638篇
  2024年   122篇
  2023年   339篇
  2022年   781篇
  2021年   1025篇
  2020年   895篇
  2019年   932篇
  2018年   1139篇
  2017年   1075篇
  2016年   1061篇
  2015年   828篇
  2014年   1036篇
  2013年   1102篇
  2012年   1091篇
  2011年   1189篇
  2010年   1078篇
  2009年   1018篇
  2008年   949篇
  2007年   996篇
  2006年   907篇
  2005年   514篇
  2004年   427篇
  2003年   456篇
  2002年   521篇
  2001年   468篇
  2000年   367篇
  1999年   370篇
  1998年   285篇
  1997年   310篇
  1996年   262篇
  1995年   257篇
  1994年   234篇
  1993年   182篇
  1992年   164篇
  1991年   110篇
  1990年   95篇
  1989年   111篇
  1988年   79篇
  1987年   64篇
  1986年   58篇
  1985年   46篇
  1984年   42篇
  1983年   42篇
  1982年   41篇
  1981年   26篇
  1980年   24篇
  1979年   29篇
  1978年   17篇
  1977年   17篇
  1975年   22篇
  1973年   17篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Unimpeded trade is one of the cooperation priorities in the Belt and Road Initiative proposed by China. On 15 May 2017, the Joint Communique of the Leaders Roundtable of the Belt and Road Forum for International Cooperation reaffirmed the participants’ shared commitment to build an open economy and ensure free and inclusive trade. The Belt and Road Initiative (BRI) is not only China's new action to drive its open and global development, but also a platform for an increasing number of countries to explore free and inclusive trade and promote a universal, rule-based, open, non-discriminatory, and equitable multilateral trade system. It is therefore important to examine the topological relationship between the BRI and global trade networks. More specifically, this article first analyzes the community structure of trade networks using a community detection algorithm, and then estimates the topological relationship between different trade communities. The findings of this article are as follows. First, this research identified three trade communities and two sub-communities in the BRI trade network, in which China is the core, Russia is the sub-core of the biggest trade community, and India, United Arab Emirates, and Saudi Arabia are cores of the second trade community (South Asia-West Asia). Second, it identified five trade communities in the global trade network, centred on China, USA, Russia, India-United Arab Emirates, and Germany- Netherlands-France-Britain and other European developed countries. Third, the topological analysis indicated that in the global trade network, most BRI countries are attracted by the core nodes of the BRI regions, such as the China, Russia and India-United Arab Emirates core nodes, and have strong trade contacts with BRI countries. Most Central-East European countries are mainly attracted by Germany-Netherlands-France-Britain and other developed European countries with a low penetration of BRI trade. Although some Southeast Asian countries are incorporated into the Asia-Australia-South Africa community with China as the core, they still need to strengthen trade linkages with BRI countries.  相似文献   
52.
Zhao  Kang  Zhou  Yun  Yu  Xiang  Yan  Yajing  Song  Yufeng  Wang  Junqiang  Suo  Tianyuan  Guo  Xiao 《Natural Resources Research》2021,30(5):3839-3852
Natural Resources Research - Depletion of shallow mineral resources caused by deep mining has become an inevitable trend, and deep mining can increase safety accidents and geological hazards....  相似文献   
53.
The transport of bromide (Br) under matric heads of 0, ?2, ?5, and ?10 cm using undisturbed soil columns was investigated for understanding the solute transport in arid soils. Undisturbed soil cores were collected at ground surface, directly below where tension infiltrometer measurements were made in the Amargosa Desert, Nevada, United States. Laboratory experiments were conducted by introducing water containing Br tracer into a soil column maintained at steady‐state conditions. The observed data of breakthrough curves (BTC) were well fitted to an one‐region model, except for the cores at saturation, and a core at the matric head of ?5 cm, from which the observed data were better fitted to a two‐region model. Fitted pore water velocities with the one‐region model ranged from 1.2 to 56.6 cm/h, and fitted dispersion coefficients (D) ranged from 2.2 to 100 cm2/h. Results for the core analyzed with the two‐region model indicated that D ranged from 27.6 to 70.9 cm2/h at saturation, and 25.7 cm2/h at the matric head of ?5 cm; fraction of mobile water (β) ranged from 0.18 to 0.65, and mass transfer coefficient (ω) ranged from 0.006 to 0.03. In summary, the water fluxes and Br dispersion coefficients at investigated matric heads were very high due to the coarseness of the soils and possibly due to preferential flow pathways. These high water fluxes and Br dispersion coefficients would lead to a higher risk of deeper leaching accumulating nitrate nitrogen to the groundwater, and have significant effects on the desert ecosystem.  相似文献   
54.
油松树轮记录的过去134年伏牛山5-7月平均最高温度   总被引:2,自引:0,他引:2  
利用豫西伏牛山的两组油松树轮宽度年表,重建了该区域1874年以来5-7月的平均最高温度.并用Jackknife和Bootstrap等方法进行了方程稳定性检验,统计检验参数表明重建序列与实测序列吻合较好,且方程稳定可信,重建方程的解释方差为40%(调整自由度后为39%).重建显示,在过去134年中,豫西伏牛山区5-7月平均最高气温经历了4次冷期和5次暖期.其中,20世纪20-30年代末是最显著的暖期,之后开始降温,至50年代降到谷底,温度小幅回升后,在70-80年代中期又显著下降,80年代末以后开始增温.该重建温度序列与秦岭中部南五台地区温度序列有较好的一致性.  相似文献   
55.
广州市环境噪声总体水平分析及其控制:以越秀区为例   总被引:2,自引:0,他引:2  
余汉豪 《热带地理》1995,15(2):103-108
随着广州市改革开放的深入发展,作为广州市中心区的越秀区环境噪声总体水平居高不下,而且还有上升趋势。本文分析了越秀区环境噪声现状和发展,对环境噪声总体水平的控制提出建议和对策。  相似文献   
56.
The frequency-dependent amplification for rock (NEHRP-class B) sites was studied using earthquake ground-motion database collected in Taiwan during implementation of the Taiwan Strong Motion Instrumentation Program. The database used includes several hundred records from earthquakes of ML 4.0–7.3 occurred between 1993 and 2004. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical Fourier spectral ratio (H/V) of the S-wave phase [Lermo J, Chavez-Garcia FJ. Site effect evaluation using spectral ratios with only one station. Bull Seism Soc Am 1993;83:1574–94]. The study allows us to analyze peculiarities of rock sites amplification in Northern and Eastern Taiwan. It was suggested to divide the NEHRP-class B site amplification into four types based on frequency of maximum amplification and the shape of amplification function. The applicability of the technique was also checked for a few stiff and soft soil sites (NEHRP-classes D and E).  相似文献   
57.
定量评价流域径流变化特征并开展其归因识别研究是应对气候变化实现水资源合理开发利用的基础.本研究以泾河流域为研究区,采用M-K检验、滑动t检验、小波分析等方法分析流域近70 a的水文气象要素变化特征,并基于Budyko假设评价气候变化和人类活动对径流变化的贡献率.结果表明:(1)泾河流域年径流深以0.41 mm·a-1的...  相似文献   
58.
Based on TM image data and other survey materials, this paper analyzed the spatiotemporal patterns of land use change in the Bohai Rim during 1985–2005. The findings of this study are summarized as follows: (1) Land use pattern changed dramatically during 1985–2005. Industrial and residential land in urban and rural areas increased by 643,946 hm2, of which urban construction land had the largest and fastest increase of 294,953 hm2 at an annual rate of 3.72%. (2) The outward migration of rural population did not prevent the expansion of residential land in rural areas by 184,869 hm2. This increase reveals that construction of rural residences makes seriously wasteful and inefficient use of land. (3) Arable land, woodland and grassland decreased at a rate of −0.02%, −0.12% and −1.32% annually, while unused land shrank by 157,444 hm2 at an annual rate of −1.69%. (4) The change of land use types showed marked fluctuations over the two stages (1985–1995 and 1995–2005). In particular, arable land, woodland and unused land experienced an inversed trend of change. (5) There was a significant interaction between arable land and woodland. Industrial construction land in urban and rural areas showed a net trend of increase during the earlier period, but only adjustment to its internal structure during the second period. The loss of arable land to the construction of factories, mines and residences took place mainly in the fringe areas of large and medium-sized cities, along the routes of major roads, as well as in the economically developed coastal areas in the east. Such changes are closely related to the spatial differentiation of the level of urbanization and industrialization in the region.  相似文献   
59.
According to observations, drops freely falling in the air under the action of gravity are deformed and oscillate in a wide range of frequencies and scales. A technique for calculating surface axisymmetric oscillations of a deformed droplet in the linear approximation under the assumption that the amplitude and wavelength are small when compared to the droplet diameter is proposed. The basic form of an axisymmetric droplet is chosen from observations. The calculation results for surface oscillations agree with recorded data on the varying shape of water droplets falling in the air.  相似文献   
60.
The solar sources of the magnetic storms of November 8 and 10, 2004, are analyzed. The preliminary results of such an analysis [Yermolaev et al., 2005] are critically compared with the results of the paper [Tsurutani et al., 2008], where solar flares were put in correspondence with these magnetic storms. The method for determining solar sources that cause powerful magnetospheric storms is analyzed. It has been indicated that an optimal approach consists in considering coronal mass ejections (CMEs) as storm sources and accompanying flares as additional information about the location of CME origination.  相似文献   
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