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   49406篇
  免费   640篇
  国内免费   324篇
测绘学   1334篇
大气科学   3830篇
地球物理   9288篇
地质学   16262篇
海洋学   4172篇
天文学   12170篇
综合类   108篇
自然地理   3206篇
  2020年   339篇
  2019年   337篇
  2018年   828篇
  2017年   810篇
  2016年   1052篇
  2015年   696篇
  2014年   1170篇
  2013年   2443篇
  2012年   1110篇
  2011年   1544篇
  2010年   1436篇
  2009年   1930篇
  2008年   1774篇
  2007年   1776篇
  2006年   1681篇
  2005年   1544篇
  2004年   1509篇
  2003年   1415篇
  2002年   1359篇
  2001年   1230篇
  2000年   1155篇
  1999年   1121篇
  1998年   1056篇
  1997年   1062篇
  1996年   844篇
  1995年   836篇
  1994年   794篇
  1993年   738篇
  1992年   709篇
  1991年   680篇
  1990年   776篇
  1989年   674篇
  1988年   649篇
  1987年   741篇
  1986年   620篇
  1985年   824篇
  1984年   947篇
  1983年   912篇
  1982年   861篇
  1981年   819篇
  1980年   722篇
  1979年   698篇
  1978年   692篇
  1977年   627篇
  1976年   596篇
  1975年   517篇
  1974年   597篇
  1973年   586篇
  1972年   362篇
  1971年   334篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
952.
953.
954.
Mini-sosie consists in using a vibration-rammer as seismic source and changing the striking rate by varying the engine speed, resulting in a random impulse transmission. The recording instruments are made up of two seismic traces, two constant gain amplifiers and a two-channel sosie processor which performs the decoding in real time by using the actual transmission times supplied by a captor located on top of the vibration-rammer's plate. An idea of the possible penetration is given by the recording of a velocity survey. Other results obtained in seismic reflection and refraction are given.  相似文献   
955.
Bioassay of a No. 2 fuel oil dispersion with shrimp in a continuous flow system using measured waterborne oil as the indicator of oil concentrations reveals a treatment more definable than those previously described in terms of volume ratios and produces lower lethal concentrations. Shrimp 96-h LC50 was 0.8 mg/l in this study as compared to values from 1.5 to 50 mg/l reported for other methods. Mean concentrations in tests do not give significant differences in concentration with respect to day of the test or spatial distribution in the exposure tanks.  相似文献   
956.
Water Resources - The forked channel is a rare and poorly known type of branching channel, the distribution of flow in which is of key importance for water transport, economic activities, and water...  相似文献   
957.
Flow and transport processes in porous media occur on different spatial and temporal scales and may also be locally different. Additionally, the structure of the porous medium itself generally shows a high dependence on the spatial scale.  相似文献   
958.
In the present study an attempt has been made to understand the variation of surface energy fluxes such as net radiation, sensible, latent and soil heat during different epochs of thunderstorm activity at Kharagpur. The study also focuses in delineating the difference in the surface energy budget from the days of thunderstorm activity to fair weather days in the pre-monsoon months (April and May) which is locally known as thunderstorm season. For this purpose, experimental data obtained from the Severe Thunderstorms- Observations and Regional Modeling (STORM) programme during pre-monsoon months of 2007, 2009 and 2010 at Kharagpur (22°30′N, 87°20′E), West Bengal, India are used. The present study reveals quick response, in the order of a few days, in the variations of transport of energy fluxes at soil-atmosphere interface to the upper atmosphere vis-à-vis to the occurrence of thunderstorm activity. Rise of surface sensible heat flux to the level of surface latent heat flux a day or two before the occurrence of a thunderstorm has been identified as a precursor signal for the thunderstorm occurrence over Kharagpur. Distinguishable differences are found in the partitioning of the surface energy fluxes to that of net radiation between thunderstorm and non-thunderstorm days. The present study reveals more Bowen’s ratio during thunderstorm days to that of nonthunderstorm days. These results are useful in validating mesoscale model simulations of thunderstorm activity.  相似文献   
959.
Cerro Pinto is a Pleistocene rhyolite tuff ring-dome complex located in the eastern Trans-Mexican Volcanic Belt. The complex is composed of four tuff rings and four domes that were emplaced in three eruptive stages marked by changes in vent location and eruptive character. During Stage I, vent clearing produced a 1.5-km-diameter tuff ring that was then followed by emplacement of two domes of approximately 0.2 km3 each. With no apparent hiatus in activity, Stage II began with the explosive formation of a tuff ring ~2 km in diameter adjacent to and north of the earlier ring. Subsequent Stage II eruptions produced two smaller tuff rings within the northern tuff ring as well as a small dome that was mostly destroyed by explosions during its growth. Stage III involved the emplacement of a 0.04 km3 dome within the southern tuff ring. Cerro Pinto’s eruptive history includes sequences that follow simple rhyolite-dome models, in which a pyroclastic phase is followed immediately by effusive dome emplacement. Some aspects of the eruption, however, such as the explosive reactivation of the system and explosive dome destruction, are more complex. These events are commonly associated with polygenetic structures, such as stratovolcanoes or calderas, in which multiple pulses of magma initiate reactivation. A comparison of major and trace element geochemistry with nearby Pleistocene silicic centers does not show indication of any co-genetic relationship, suggesting that Cerro Pinto was produced by a small, isolated magma chamber. The compositional variation of the erupted material at Cerro Pinto is minimal, suggesting that there were not multiple pulses of magma responsible for the complex behavior of the volcano and that the volcanic system was formed in a short time period. The variety of eruptive style observed at Cerro Pinto reflects the influence of quickly exhaustible water sources on a short-lived eruption. The rising magma encountered small amounts of groundwater that initiated eruption phases. Once a critical magma:water ratio was exceeded, the eruptions became dry and sub-plinian to plinian. The primary characteristic of Cerro Pinto is the predominance of fall deposits, suggesting that the level at which rising magma encountered water was deep enough to allow substantial fragmentation after the water source was exhausted. Isolated rhyolite domes are rare and are not currently viewed as prominent volcanic hazards, but the evolution of Cerro Pinto demonstrates that individual domes may have complex cycles, and such complexity must be taken into account when making hazard risk assessments.  相似文献   
960.
The seasonal nature of Australia’s tropical rivers means that connected groundwater aquifers are an important source of both consumptive and non-consumptive water, particularly during the dry season. The management of these common pool groundwater resources is one of the predominant water issues facing northern Australia. A national program of water reform stipulates the expansion of water trading as a key instrument for water allocation. The effectiveness of new institutional arrangements such as water markets will be determined mostly by how well they coordinate with local environmental requirements, local institutions and local norms. This paper describes a novel application of combined field work, institutional analysis, experimental economics and agent-based modeling to the analysis of a potential water market in the Katherine region of the Northern Territory, Australia. The effectiveness of different versions of the policy instrument is assessed in light of local conditions. Instruments that enable personal relationships and local institutions and norms to play a role in water management are found to be more effective in terms of both farming income and environmental impact.  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