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   747篇
  免费   137篇
  国内免费   238篇
测绘学   18篇
大气科学   57篇
地球物理   114篇
地质学   698篇
海洋学   80篇
天文学   4篇
综合类   45篇
自然地理   106篇
  2024年   3篇
  2023年   13篇
  2022年   17篇
  2021年   30篇
  2020年   26篇
  2019年   52篇
  2018年   26篇
  2017年   22篇
  2016年   34篇
  2015年   36篇
  2014年   38篇
  2013年   69篇
  2012年   31篇
  2011年   53篇
  2010年   47篇
  2009年   47篇
  2008年   48篇
  2007年   33篇
  2006年   53篇
  2005年   33篇
  2004年   46篇
  2003年   28篇
  2002年   38篇
  2001年   25篇
  2000年   35篇
  1999年   33篇
  1998年   31篇
  1997年   34篇
  1996年   28篇
  1995年   22篇
  1994年   15篇
  1993年   16篇
  1992年   16篇
  1991年   11篇
  1990年   14篇
  1989年   9篇
  1988年   5篇
  1987年   3篇
  1986年   1篇
  1985年   1篇
排序方式: 共有1122条查询结果,搜索用时 107 毫秒
71.
The preproposal stage of the rulemaking process is notoriously understudied, but enormously important in determining regulatory outputs. Recently, Rinfret (2011c Rinfret , S. R. 2011c . Frames of influence: U.S. environmental rulemaking case studies . Rev. Policy Res. 28 ( 3 ): 231245 .[Crossref], [Web of Science ®] [Google Scholar]) analyzed the U.S. Fish and Wildlife Service (USFWS) rulemaking process and developed a frame analysis model to interpret stakeholder influence during the preproposal stage. Rinfret argues that stakeholders use three frames to influence agency rulemaking, including an expertise, a fiscal feasibility, and an information frame. This article tests this model to determine whether it is applicable to other federal agencies such as the National Park Service (NPS). Through an analysis of stakeholder framing within the 2012 NPS Yellowstone Winter Use Rule, this research confirms that Rinfret's model is indeed applicable to the NPS process. Therefore, this research suggests that this theoretical model examining stakeholder influence is applicable to other public land agencies, and arguably can be applied across the bureaucracy.  相似文献   
72.
赵良民 《水文》2014,34(2):84-86
讨论了固态存储水位数据精简摘录存在问题及所需满足的要求。介绍了曲线矢量压缩的Douglas-Peucker算法,对它做了改进并应用于水位过程线的精简摘录。应用实例表明该法能够以精简的数据反应出水位变化完整过程,是使用计算机完成固态存储水位摘录的有效方法。  相似文献   
73.
推进城乡一体化协调发展是事关海南自由贸易港建设成效的重点任务之一,新的发展环境及政策框架正促使海南省城乡关系快速转型。采用变异系数法、耦合协调度模型、回归模型,从城乡空间、经济和社会等维度,分析了建省办特区以来海南省城乡关系演化特征及驱动机制。结果表明:①建省办特区以来,海南省城乡协调综合指数呈现明显的先降后升态势,由1988年的0.319 9到1993年的0.153 9,2019年达0.851 9;②海南省城乡各子系统耦合度和耦合协调度均呈波动上升态势,耦合度主要处于磨合和高水平耦合阶段,耦合协调度由轻度失调转变为初级协调,最后达到中级协调,城乡各子系统间相互作用增强;③海南省城乡关系演化经历城乡二元加剧、城乡二元固化、城乡关系趋缓和城乡一体化初期四个阶段,低水平城乡均衡发展是重要特征,高质量推进城乡一体化协调发展是关键;④经济发展对城乡协调发展影响最为明显,但呈负相关,其次为市场消费、产业结构、乡村发展、交通通达性、城镇发展和政府行为,对外开放对城乡关系演化影响相对较弱;⑤制度创新、对外合作、产业优化、城镇建设和乡村振兴是推进城乡融合发展的重要路径。  相似文献   
74.
Uncertainty in discharge data must be critically assessed before data can be used in, e.g. water resources estimation or hydrological modelling. In the alluvial Choluteca River in Honduras, the river‐bed characteristics change over time as fill, scour and other processes occur in the channel, leading to a non‐stationary stage‐discharge relationship and difficulties in deriving consistent rating curves. Few studies have investigated the uncertainties related to non‐stationarity in the stage‐discharge relationship. We calculated discharge and the associated uncertainty with a weighted fuzzy regression of rating curves applied within a moving time window, based on estimated uncertainties in the observed rating data. An 18‐year‐long dataset with unusually frequent ratings (1268 in total) was the basis of this study. A large temporal variability in the stage‐discharge relationship was found especially for low flows. The time‐variable rating curve resulted in discharge estimate differences of ? 60 to + 90% for low flows and ± 20% for medium to high flows when compared to a constant rating curve. The final estimated uncertainty in discharge was substantial and the uncertainty limits varied between ? 43 to + 73% of the best discharge estimate. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
75.
具有硅酸盐尘埃特征碳星的新的候选者   总被引:1,自引:1,他引:0  
陈培生  高衡 《天文学报》1994,35(4):443-447
本文根据红外天文卫星的宽波段测光及低分辨率光谱资料,给出了两颗新的可能具有硅酸尘埃特征的碳星及4颗可能的候选者。  相似文献   
76.
尾亚韧性剪切带形成于石炭纪晚期.根据其宏观变形构造、显微变形构造等变形特征,说明其经历了早期的推覆剪切和后期的右行推覆剪切两个主要演化阶段.对该区的构造演化及指导找矿增添了一点新的内容.  相似文献   
77.
High‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ18O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ18O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ18O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
78.
雄安新区雾迷山组地热资源丰富,研究其岩溶热储特征及其形成机理对于雄安新区清洁地热资源的开发利用具有重要的理论意义和实用价值。前人已经对该区雾迷山组油气储集层的分布、古岩溶、成岩作用、储集空间等特征做过一定的研究,但对于该区雾迷山组地热资源的成储机制、特别是3期岩溶等成岩特征尚缺乏系统性的研究。充分利用野外露头、岩心、薄片和测录井等地质及地球物理资料,并结合区域地质背景的分析,对雄安新区雾迷山组的岩石学特征、成岩作用、岩溶热储的成储机制进行了深入研究,并建立了雄安新区雾迷山组岩溶热储的发育模式。结果表明:(1)雄安新区雾迷山组主要岩性为白云岩,主要的储集空间为次生孔隙和构造缝—构造溶蚀缝,热储孔渗变化较大;(2)雄安新区最主要的建设性成岩作用是3种溶蚀作用,包括同生—准同生溶蚀、表生溶蚀和埋藏溶蚀,主要破坏性成岩作用是压实作用和胶结作用;(3)根据构造演化、岩心及测井资料,将表生溶蚀作用划分为3期,分别是芹峪期、印支期和燕山—喜山期。芹峪期雾迷山组局部出露遭受淋滤,形成的溶孔经后期改造后不易识别;印支期大部分区域抬升并遭受淋滤,形成高孔渗储集层,后期遭受了进一步改造;燕山—喜山期是雄安新...  相似文献   
79.
张晓  张旭 《地学前缘》2022,29(5):322-333
千年尺度气候突变事件是第四纪冰期普遍存在的气候现象。这些事件可以被分为两类,一类为Dansgaard-Oeschger Event (DO事件),另一类为海因里希(Heinrich Stadial,HS)事件,后者有时也被认为是一种特殊的DO事件,因此也被称为 HS-DO事件。HS事件期间北大西洋冰架的融化一般对应DO振荡的冷相位,这与通常认为的冰架在较冷气候下体积增加并不相符。这两类事件在北大西洋重建数据中表现得最为明显,但其气候影响具有全球性。由于没有显著的外强迫驱动这两类气候突变事件,自20世纪90年代首次被确认以来,HS-DO事件一直是古气候界关注的重点,且人们对其触发机理仍没有定论。本文基于目前对这两类千年事件的研究现状,集中总结了目前已有的可反映千年事件的重建数据,利用已有的模拟工作,重点回顾了现有的机制理论,评述了目前研究DO事件以及HS-DO事件的局限性,并对其后续工作,尤其是模拟部分,进行了展望。  相似文献   
80.
We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan.  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