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   389篇
  免费   48篇
  2021年   4篇
  2020年   15篇
  2019年   18篇
  2018年   25篇
  2017年   15篇
  2016年   28篇
  2015年   17篇
  2014年   10篇
  2013年   54篇
  2012年   16篇
  2011年   21篇
  2010年   22篇
  2009年   22篇
  2008年   11篇
  2007年   14篇
  2006年   20篇
  2005年   8篇
  2004年   9篇
  2003年   3篇
  2002年   7篇
  2001年   3篇
  2000年   5篇
  1999年   9篇
  1998年   4篇
  1996年   3篇
  1995年   3篇
  1994年   5篇
  1993年   7篇
  1992年   9篇
  1991年   4篇
  1989年   4篇
  1988年   3篇
  1987年   2篇
  1986年   2篇
  1985年   3篇
  1984年   2篇
  1983年   3篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1978年   2篇
  1976年   4篇
  1974年   2篇
  1972年   1篇
  1970年   2篇
  1968年   1篇
  1967年   1篇
  1966年   1篇
  1963年   1篇
  1950年   1篇
排序方式: 共有437条查询结果,搜索用时 15 毫秒
41.
This article describes the main features of a Land Use Cellular Automata Model (LUCAM) developed within the scope a Ph.D. thesis by using both physical and socio‐economic data and on the basis of the relationships between the smallest cellular function and other functions that vary according to the fractal structure of settlements. An assessment of the research findings is also included. The article has five sections: Following the introduction, the evolution of land use models, in particular Cellular Automata (CA) based land use models, is described. Then, the main features of LUCAM are presented, and the basics of the criteria used in determining the model parameters is described. The model developed, which can easily be used in computer environments, aims to minimize the calculation time in growth estimation processes, and thus become a widely used tool. In this article, the model was applied to the settlement of Bursa (Turkey), and the results obtained were assessed. The conclusion section offers general guidelines on the use of the model and discusses the advantages of employing the model in CA based studies that are likely to emerge in the future.  相似文献   
42.
抗战初期的南调命令是国民党在发展和维护它的华中敌后利益,尤其是大别山根据地利益的过程中提出来的.由于抗战的新形势,国民党对在失去大举"剿共"手段之后的策略选择和反共部署,其实并没有多少及时而深入的考虑,尤其因为共产国际五月指示以来共产党在华中采取攻势策略,国民党更显缺乏心理和策略准备,南调命令最终在国民党内外重重阻力下不了了之.但是,这并不意味着它在国共关系史上没有产生实际的影响.在皖南,国共都因为南调命令而发生了策略变化,两党由此不可避免地开始积累矛盾,直至事变发生.  相似文献   
43.
本文主要目的为重新对元代初期之书画鉴赏家及收藏家汤生平之考证及与其同时文人往来之情形。因此,本文除了藉用地方志、文集、诗集及书画题跋等为佐证之外,同时也将汤父亲汤炳龙之背景,与其他宋末元初文人、画家之交往,及对汤仕途之影响作进一步研究。本论文之结论则定汤约生于1250年代中期,而卒于1310年代中期;乃确定汤活跃于元代初期,且旅居杭州多年,最后移居且卒于北京。汤撰有二书,现仅存《画鉴》一书。此书对了解元初绘画发展及收藏品味具有重要贡献。  相似文献   
44.
Recent zooarchaeological studies on water buffalo (Bubalus sp.) remains from China and south Asia question the traditional view that water buffalo were first domesticated in Neolithic China over 7000 years ago. The results from several recent population genetic studies of modern domesticated buffalo (Bubalus bubalis) are not consistent with each other, placing the original center of buffalo's domestication in south Asia, southeast Asia, or China. This paper reports a study using an ancient DNA approach to analyze water buffalo remains from Neolithic sites in north China to investigate their affinities with modern domesticated water buffalo, and to shed light on the origin of modern domesticated water buffalo in China.A 169 bp fragment of D-loop mitochondrial DNA was successfully amplified and verified for 13 of 24 bone samples obtained from seven archaeological sites along the Wei River valley in Shaanxi Province, China. The bone samples which yielded positive DNA can be dated to 8000–3600 cal. BP. The phylogenetic analysis of the obtained DNA sequences along with modern water buffalo sequences indicated that the ancient water buffalos were not the direct ancestor of modern domesticated water buffalo. However, the phylogenetic analysis, along with BLAST searches of these ancient DNA sequences, did demonstrate their relatedness to water buffalo more so than to any other bovid species, confirming the existence of indigenous wild (but now extinct) water buffalo species (B. mephistopheles) in ancient China.The DNA analysis of these ancient remains failed to establish direct links between modern domesticated water buffalo (B. bubalis) and indigenous water buffalo (B. mephistopheles) from ancient China. If further DNA studies of more ancient remains from other regions of China confirm the observation of solely indigenous water buffalo species in ancient China, it would suggest modern water buffalo might not have been first domesticated in China.  相似文献   
45.
The aim of this paper is to provide a summary of structural or surface modifications of vertebrate bone remains in continental palaeoenvironments that may have repercussions on their chemical composition. Both before and after burial, a different set of physical, chemical and biological agencies may produce modifications of the bone morphological structure and/or bone chemical composition. Several of these morphological modifications are diagnostic of particular agents, which otherwise may not be noticed or identified in a fossil association. In order to understand diagenesis, those events that occurred before final burial have to be considered, as they may strongly modify the bone morphological structure and influence post–burial changes.  相似文献   
46.
47.
48.
49.
RECENT CHANGES IN THE PATTERN OF FARMING IN ICELAND   总被引:1,自引:0,他引:1  
  相似文献   
50.
We used hydrologic models to explore the potential linkages between oil‐field brine reinjection and increases in earthquake frequency (up to Md 3.26) in southeastern New Mexico and to assess different injection management scenarios aimed at reducing the risk of triggered seismicity. Our analysis focuses on saline water reinjection into the basal Ellenburger Group beneath the Dagger Draw Oil field, Permian Basin. Increased seismic frequency (>Md 2) began in 2001, 5 years after peak injection, at an average depth of 11 km within the basement 15 km to the west of the reinjection wells. We considered several scenarios including assigning an effective or bulk permeability value to the crystalline basement, including a conductive fault zone surrounded by tighter crystalline basement rocks, and allowing permeability to decay with depth. We initially adopted a 7 m (0.07 MPa) head increase as the threshold for triggered seismicity. Only two scenarios produced excess heads of 7m five years after peak injection. In the first, a hydraulic diffusivity of 0.1 m2 s?1 was assigned to the crystalline basement. In the second, a hydraulic diffusivity of 0.3 m2 s?1 was assigned to a conductive fault zone. If we had considered a wider range of threshold excess heads to be between 1 and 60 m, then the range of acceptable hydraulic diffusivities would have increased (between 0.1–0.01 m2 s?1 and 1–0.1 m2 s?1 for the bulk and fault zone scenarios, respectively). A permeability–depth decay model would have also satisfied the 5‐year time lag criterion. We also tested several injection management scenarios including redistributing injection volumes between various wells and lowering the total volume of injected fluids. Scenarios that reduced computed excess heads by over 50% within the crystalline basement resulted from reducing the total volume of reinjected fluids by a factor of 2 or more.  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