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   33183篇
  免费   694篇
  国内免费   346篇
测绘学   688篇
大气科学   2698篇
地球物理   6867篇
地质学   11399篇
海洋学   3108篇
天文学   7034篇
综合类   102篇
自然地理   2327篇
  2021年   251篇
  2020年   295篇
  2019年   336篇
  2018年   694篇
  2017年   650篇
  2016年   790篇
  2015年   623篇
  2014年   874篇
  2013年   1657篇
  2012年   999篇
  2011年   1415篇
  2010年   1230篇
  2009年   1649篇
  2008年   1384篇
  2007年   1386篇
  2006年   1265篇
  2005年   1075篇
  2004年   1081篇
  2003年   1035篇
  2002年   970篇
  2001年   847篇
  2000年   801篇
  1999年   709篇
  1998年   699篇
  1997年   670篇
  1996年   531篇
  1995年   508篇
  1994年   450篇
  1993年   401篇
  1992年   378篇
  1991年   352篇
  1990年   368篇
  1989年   341篇
  1988年   287篇
  1987年   392篇
  1986年   315篇
  1985年   415篇
  1984年   480篇
  1983年   454篇
  1982年   404篇
  1981年   389篇
  1980年   388篇
  1979年   335篇
  1978年   376篇
  1977年   316篇
  1976年   318篇
  1975年   322篇
  1974年   276篇
  1973年   275篇
  1971年   185篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
961.
Bjorn Lomborg's critique of environmentalism has been praised by The Economist as ‘right’ on its main points and ‘just’ in its criticism of much green activism, and damned by Scientific American as a ‘failure’ in its purpose of describing the state of the world. Most of the debate has centred on Lomborg's treatment of environmental issues. But the book is about more than the environment, and its flaws extend beyond his use and interpretation of the data. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
962.
A probabilistic technique is developed for assessing water level in the mouth reach of the Ural River in the course of interaction between the river runoff and irregular rises in the Caspian Sea level (positive setups) at the predicted background sea levels of –26.5 and –26.0 m abs.  相似文献   
963.
--The earth's tectonic plates are strong, viscoelastic shells which make up the outermost part of a thermally convecting, predominantly viscous layer. Brittle failure of the lithosphere occurs when stresses are high. In order to build a realistic simulation of the planet's evolution, the complete viscoelastic/brittle convection system needs to be considered. A particle-in-cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain-dependent yield stress. This is applied to a plate-deformation problem. Numerical accuracy is demonstrated relative to analytic benchmarks, and the characteristics of the method are discussed.  相似文献   
964.
Recent nitrate, chloride, and bromide concentrations were studied in the Ogallala Aquifer of northwest Texas. The study included 361 wells with a median depth of 92 m in a rural area dominated by agricultural activity and oil and gas production. Only five observations surpassed the 44.3 mg/L standard for nitrate (10 mg/L NO3-N). Four other observations, and one from the preceding set, exceeded the secondary standard of 250 mg/L for chloride. Maximum concentrations were 91.2 mg/L, 1530 mg/L, and 0.70 mg/L for nitrate, chloride, and bromide, respectively. Chloride/bromide ratios covered a broad range, from 30.4 to 10930, but medians were < 160 for each of two years analyzed. There were statistically significant correlations between nitrate and chloride, and chloride and well depth. Results of this study suggest that agricultural activity has locally impacted ground water in north-west Texas. Regionally, low aquifer recharge rates have curtailed ground water contamination from potentially adverse land uses.  相似文献   
965.
v--vThis second paper (Part 2) pertaining to optimized site-specific threshold monitoring addresses the application of the method to regions covered by a teleseismic or a combined regional-teleseismic network. In the first paper (Part 1) we developed the method for the general case, and demonstrated its application to an area well-covered by a regional network (the Novaya Zemlya nuclear test site). In the present paper, we apply the method to the Indian and Pakistani nuclear test sites, and show results during the periods of nuclear testing by these two countries in May 1998. Since the coverage by regional stations in these areas is poor, an optimized approach requires the use of selected, high-quality stations at teleseismic distances.¶To optimize the threshold monitoring of these test sites, we use as calibration events either one of the nuclear explosions or a nearby earthquake. From analysis of the calibration events we derive values for array beamforming steering delays, filter bands, short-term averages (STA) lengths, phase travel times (P waves), and amplitude-magnitude relationships for each station. By applying these parameters, we obtain a monitoring capability of both test sites ranging from mb 2.8-3.0 using teleseismic stations only. When including the nearby Nilore station to monitor the Indian tests, we show that the threshold can be reduced by about 0.4 magnitude units. In particular, we demonstrate that the Indian tests on 13 May, 1998, which were not detected by any known seismic station, must have corresponded to a magnitude (mb) of less than 2.4.¶We also discuss the effect of a nearby aftershock sequence on the monitoring capability for the Pakistani test sites. Such an aftershock sequence occurred in fact on the day of the last Pakistani test (30 May, 1998), following a large (mb 5.5) earthquake in Afghanistan located about 1100 km from the test site. We show that the threshold monitoring technique has sufficient resolution to suppress the signals from these interfering aftershocks without significantly affecting the true peak of the nuclear explosion on the threshold trace.  相似文献   
966.
967.
On the basis of growing evidence thatstrong earthquakes are preceded by a periodof accelerating seismicity of moderatemagnitude earthquakes, an attempt is madeto search for such seismicity pattern in NWAegean area. Accelerating seismic crustaldeformation has been identified in the areaof southern Albanides mountain range(border region between Greece, formerYugoslavia and Albania). Based on certainproperties of this activity and on itssimilarity with accelerating seismicdeformation observed before a strongearthquake which occurred in the sameregion on 26 May 1960 (M = 6.5), we canconclude that a similar earthquake may begenerated in the same region during thenext few years. This conclusion is inagreement with independent results whichhave been derived on the basis of the timepredictable model.  相似文献   
968.
A paleolimnological evaluation of cladoceran microfossils was initiated to study limnological changes in Lake Apopka, a large (125 km2), shallow (mean depth = 1.6 m), warm, polymictic lake in central Florida. The lake switched from macrophyte to algal dominance in the late 1940s, creating a Sediment Discontinuity Layer (SDL) that can be visually used to separate sediments derived from macrophytes and phytoplankton. Cladoceran microfossils were enumerated as a means of corroborating extant eutrophication data from the sediment record. Inferences about the timing and trajectory of eutrophication were made using the cladoceran-based paleo-reconstruction. The cladoceran community of Lake Apopka began to change abruptly in both total abundance and relative percent abundance just before the lake shifted from macrophyte to algal dominance. Alona affinis, a mud-vegetation associated cladoceran, disappeared before the SDL was formed. Planktonic and benthic species also began to increase below the SDL, indicating an increase in production of both planktonic and benthic species. Chydorus cf. sphaericus, an indicator of nutrient loading, increased relative to all other cladocerans beginning in the layer below the SDL and continuing upcore. Changes in the transitional sediment layer formed before the lake switched to phytoplankton dominance, including an increase in total phosphorus concentration, suggest a more gradual eutrophication process than previously reported. Data from this study supported conclusions from other paleolimnological studies that suggested anthropogenic phosphorus loading was the key factor in the hypereutrophication of Lake Apopka.  相似文献   
969.
970.
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