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   917篇
  免费   47篇
  国内免费   6篇
测绘学   57篇
大气科学   74篇
地球物理   284篇
地质学   296篇
海洋学   82篇
天文学   94篇
综合类   4篇
自然地理   79篇
  2023年   3篇
  2022年   11篇
  2021年   11篇
  2020年   22篇
  2019年   21篇
  2018年   23篇
  2017年   22篇
  2016年   36篇
  2015年   25篇
  2014年   30篇
  2013年   67篇
  2012年   44篇
  2011年   41篇
  2010年   47篇
  2009年   62篇
  2008年   45篇
  2007年   46篇
  2006年   37篇
  2005年   36篇
  2004年   26篇
  2003年   31篇
  2002年   26篇
  2001年   25篇
  2000年   18篇
  1999年   14篇
  1998年   15篇
  1997年   17篇
  1996年   12篇
  1995年   12篇
  1994年   12篇
  1993年   10篇
  1992年   11篇
  1991年   8篇
  1990年   9篇
  1989年   7篇
  1988年   10篇
  1987年   9篇
  1986年   6篇
  1985年   7篇
  1984年   7篇
  1983年   10篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1978年   2篇
  1977年   5篇
  1976年   3篇
  1975年   3篇
  1974年   6篇
  1970年   2篇
排序方式: 共有970条查询结果,搜索用时 46 毫秒
51.
A sedimentological and geochemical study of the Lago Enol sequence (Cantabrian Mountains, northern Spain), together with detailed geomorphological mapping, provides a first record of glacier evolution and climate change over the last 40 ka in the Picos de Europa National Park. The Enol glacier retreated from its maximum extent prior to 40 ka BP as demonstrated by the onset of proglacial lacustrine sedimentation in two glaciated depressions: the Comella hollow to the north (before 40 ka BP) and the Lago Enol (before 38 ka BP). These results support previous evidence that the maximum extent of southern European glaciers occurred earlier than in northern Europe. Alternation of homogeneous and laminated proglacial sediments during the glacier retreat illustrate a dynamic glacial evolution during the Marine Isotope Stage (MIS) 3 (40–26 ka BP). A slight warming is detected at 26 ka ago with the change from proglacial sediments (in a lake located in contact to the glacier) to glaciolacustrine sedimentation (in a non‐contact or distal lake). Finally, the onset of organic‐rich sediments took place at 18 ka ago. This last transition occurred in two phases, similarly to the North Atlantic Last Termination, suggesting a link between North Atlantic Deep Water formation oscillations and palaeohydrological variability in the Cantabrian Mountains. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
52.
Natural wetlands perform significant sewage treatment, but artificial reedbeds are likely to be more reliable and effective. The processes responsible for purification in wetlands and reedbeds are described in this paper, together with an outline of the experimental reedbeds which have been established in the UK up to 1987. Major research needs are outlined.  相似文献   
53.
Ten years after the last effusive eruption and at least 15 years of seismic quiescence, volcanic seismic activity started at Colima volcano on 14 February 1991, with a seismic crisis which reached counts of more than 100 per day and showed a diversity of earthquake types. Four other distinct seismic crises followed, before a mild effusive eruption in April 1991. The second crisis preceded the extrusion of an andesitic scoriaceous lava lobe, first reported on 1 March; during this crisis an interesting temporary concentration of seismic foci below the crater was observed shortly before the extrusion was detected. The third crisis was constituted by shallow seismicity, featuring possible mild degassing explosion-induced activity in the form of hiccups (episodes of simple wavelets that repeat with diminishing amplitude), and accompanied by increased fumarolic activity. The growth of the new lava dome was accompanied by changing seismicity. On 16 April during the fifth crisis which consisted of some relatively large, shallow, volcanic earthquakes and numerous avalanches of older dome material, part of the newly extruded dome, which had grown towards the edge of the old dome, collapsed, producing the largest avalanches and ash flows. Afterwards, block lava began to flow slowly along the SW flank of the volcano, generating frequent small incandescent avalanches. The seismicity associated with the stages of this eruptive activity shows some interesting features: most earthquake foci were located north of the summit, some of them relatively deep (7–11 km below the summit level), underneath the saddle between the Colima and the older Nevado volcanoes. An apparently seismic quiet region appears between 4 and 7 km below the summit level. In June, harmonic tremors were detected for the first time, but no changes in the eruptive activity could be correlated with them. After June, the seismicity decreasing trend was established, and the effusive activity stopped on September 1991.  相似文献   
54.
55.
56.
Jim Whitman 《GeoJournal》1994,34(2):167-175
In the Post-Cold War environment, the role of the military has been extended to include intra-state peace enforcement on humanitarian grounds. This raises issues of legality. In this paper, four arguments are advanced that a right of states to intervene in humanitarian emergencies now exists. Problems contingent upon such intervention are then examined.  相似文献   
57.
Summary Data on thermal radiation from clear skies obtained at Granada from May to November 1983 were utilized to carry out a detailed comparison between measured and calculated values of longwave radiation with several analytical and empirical equations. The goal of this work is to check the aplicability of these equations to Granada conditions. The results obtained for all the equations are quite satisfactory. Nevertheless, Idso's formula significantly overestimates our experimental results, probably as a consequence of the differences in atmospheric dust conditions between our test area and the location for which that equation was originally derived. A reduction in the independent coefficient, in accordance with previous results of other authors, provides a better fit to our data set and clearly shows the climatological dependence of the constants in the estimation equations.
Zusammenfassung Die von Mai bis November 1983 in Granada gesammelten Daten über Wärmestrahlung bei klarem Himmel wurden dazu verwendet, um einen detaillierten Vergleich zwischen gemessenen und errechneten Werten der langwelligen Strahlung aus verschiedenen analytischen und empirischen Gleichungen durchzuführen. Das Ziel dieser Arbeit ist es, die Anwendbarkeit dieser Gleichungen auf die Verhältnisse in Granada zu überprüfen. Die gewonnenen Ergebnisse sind für alle diese Gleichungen ganz zufriedenstellend. Idsos Formel überschätzt jedoch deutlich unsere experimentellen Ergebnisse, möglicherweise als Folge der unterschiedlichen atmosphärischen Staubbedingungen in unserem Testgebiet und dem Standort, für den die Gleichung ursprünglich abgeleitet wurde. Eine Reduktion des unabhängigen Koeffizienten, in Übereinstimmung mit früheren Ergebnissen anderer Autoren, bietet eine bessere Anpassung an unseren Datensatz und zeigt deutlich die klimatologische Abhängigkeit der Konstanten in Schätzungsgleichungen.


With 3 Figures  相似文献   
58.
The Resource Conservation and Recovery Act (PL 94–580) and related federal and state legislation have mandated routine monitoring of ground water quality at regulated facilities. The objective of the required monitoring activities is detection of adverse changes in ground water quality caused by the facilities.
Both failure to detect pollution and an incorrect determination of pollution can be very expensive. It is crucial, therefore, that monitoring programs be designed and operated to provide statistically sound information. It is equally important that users of ground water quality data understand the nature and limitations of information from monitoring.
To address the preceding issues, the authors present a general approach to analyzing ground water quality data in light of the stated monitoring objective. The suggested approach accounts for "natural" variation in background water quality through pairing of observations. The limitations of quarterly sampling for detecting small changes in quality over a short time frame are discussed.  相似文献   
59.
60.
Although microorganisms are certainly present in swamp and marsh peats, their geochemical role has not been demonstrated, in spite of their importance in determining the input to coalification processes. In order to obtain information on the gross level of microbial activity in the natural habitats, a broad-spectrum test for a type of enzyme important in the respiration of all organisms has been applied to cores of peat from a number of different environments in the Florida Everglades. In most cases, the respiratory activity in the upper layer is comparable to that in a fertile mineral soil, and this level of activity is found also at greater depths in several cores. In addition, some properties of the peat that tend to define their character as habitats for microorganisms were determined. pH values close to neutrality were observed at all sites. Chlorinity and total ionic strength naturally were high in the coastal swamps; seasonal variations in profiles in the brackish areas somewhat away from the coast suggested that some bulk flow of water through the peat into the porous limestone bedrock occurs in the rainy season. Eh profiles, regarded as empirical characterizations of environments, did differ considerably between saline and fresh water peats, though on a graphite indicator electrode were more positive than might have been expected in systems in which sulfate reduction was active. However, in these non-equilibrium systems, the potential recorded apparently depends on the nature of the electrode material used.  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