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   2770篇
  免费   192篇
  国内免费   425篇
测绘学   59篇
大气科学   29篇
地球物理   501篇
地质学   234篇
海洋学   1668篇
天文学   2篇
综合类   204篇
自然地理   690篇
  2024年   12篇
  2023年   59篇
  2022年   123篇
  2021年   135篇
  2020年   133篇
  2019年   135篇
  2018年   105篇
  2017年   139篇
  2016年   120篇
  2015年   126篇
  2014年   153篇
  2013年   230篇
  2012年   125篇
  2011年   153篇
  2010年   127篇
  2009年   141篇
  2008年   142篇
  2007年   158篇
  2006年   130篇
  2005年   125篇
  2004年   96篇
  2003年   85篇
  2002年   68篇
  2001年   62篇
  2000年   72篇
  1999年   75篇
  1998年   47篇
  1997年   53篇
  1996年   44篇
  1995年   34篇
  1994年   33篇
  1993年   34篇
  1992年   23篇
  1991年   13篇
  1990年   16篇
  1989年   8篇
  1988年   8篇
  1987年   4篇
  1986年   2篇
  1985年   6篇
  1984年   10篇
  1983年   6篇
  1982年   7篇
  1981年   7篇
  1980年   3篇
排序方式: 共有3387条查询结果,搜索用时 31 毫秒
991.
In many highly dense urban environments, the urgent needs of residents for increasing green space, improving the quality of the community environment and reconstructing the relationships among residents have given birth to the new space type of community gardens. However, China still lacks this relevant experience. In contrast, New York City’s community gardens had a relatively early start, and they now have rich experience in space construction, operation and maintenance. Given their level of experience, they can be used as references for the development of community gardens in China. This paper adopts a bibliometric research method, identifies 201 periodical literature sources published between 2000 and 2020 from the core library of the Web of Science as the object of study, and finally assesses the research hotspot for transferring from macro-research to space-type construction method, social impact, and so on, through CiteSpace software analysis. By virtue of the research process analysis and the results of field surveys and interviews, this paper probes the development status of space construction and social organization construction of the community gardens in New York City, and summarizes that area’s effective experience of development. Based on the current development situation of China’s community gardens, it is proposed that the development of community gardens should be directed by ensuring the land for development, giving full play to social benefits, and mobilizing social organizations, so as to effectively realize urban space construction and social governance.  相似文献   
992.
孙向楠  胡晓珂  王慧 《海洋与湖沼》2015,46(6):1304-1311
应用分子生态学方法, 研究在不同芳香族化合物胁迫下海洋沉积物中细菌群落结构的演替 规律。在实验室建立驯化系统, 在海洋沉积物中分别添加芘和苯甲酸钠, 采用Illumina MiSeq 高通量 测序技术分析实验组和对照组细菌群落16S rRNA 基因V3-V4 可变区数量的组成及变化, 研究沉积物 中添加芘、苯甲酸钠后, 微生物群落多样性和群落结构的变化。实验结果表明: 芘、苯甲酸钠极大地 降低了海洋沉积物中菌群丰度和多样性; 在添加苯甲酸钠的实验组, 变形杆菌门中海杆菌属 (Marinobacter)、假海源菌属(Pseudidiomarina)的细菌显著富集; 在添加芘实验组中拟杆菌门中的 Balneola 属、碳酸噬胞菌属(Aequorivita)极大富集。初步分析了在海洋沉积物环境中不同多环芳烃胁迫 下细菌群落结构的演替特征, 实验结果为海洋环境中针对不同芳香烃的微生物修复提供理论依据。  相似文献   
993.
2009年春季胶州湾浮游植物群落结构特征   总被引:2,自引:4,他引:2  
本文研究了2009年春季胶州湾23个大面站浮游植物的群落结构特征。共检出浮游植物3门47属74种,平均细胞丰度为827.77cell/m L,其中硅藻在物种数量和细胞丰度上占绝对优势。主要优势种为丹麦细柱藻Leptocylindrus danicus和中肋骨条藻Skeletonema costatum。调查期内浮游植物表层细胞丰度的平面分布趋势为北部高于南部,湾内高于湾口,浮游植物的垂直分布则为各水层之间细胞丰度相差不大,没有明显的差异。表层浮游植物的Shannon-Wiener指数和均匀度指数的分布显示胶州湾东北部浮游植物群落较西南部更加均匀稳定。与环境因子的相关分析表明浮游植物细胞丰度与温度呈正相关。  相似文献   
994.
基于2014年秋季(11月)在黄海海域所获取的大型底栖动物及环境数据,研究了黄海海域大型底栖甲壳类动物的种类组成、丰度、生物量、多样性以及群落结构特征等。调查海域共发现71种大型底栖甲壳类动物,隶属于32科45属,其平均丰度和生物量分别为278inds./m2和2.33g/m2。IRI值最高的物种为太平洋方甲涟虫Eudorella pacifica。在23%的相似度水平上,可以将大型底栖甲壳类划分为六个群落。Pearson相关性结果显示,太平洋方甲涟虫Eudorella pacifica丰度与水深呈极显著正相关,与底盐呈显著正相关,与底温呈显著负相关。大型底栖甲壳类动物的物种数占比在黄海大型底栖动物类群组成中变化不大,都在30%左右。BOPA指数表明,调查海域底栖生态环境整体良好,仅黄海中部和南部部分站位可能存在中度扰动;BOPA指数能较好地评价研究海域的环境质量状况。  相似文献   
995.
薛晓轩 《东北测绘》2013,(4):98-100
数字城市建设风起云涌,智慧城市引领发展潮流。地理信息技术从2维发展到3维把城市的抽象化变为可视化。智能社区只是把智慧城市微缩到社区,变宏观管理为微观管理,变粗犷管理为精细管理。这是未来社区管理的发展趋势,智能化将成为衡量居民幸福指数的重要标志。  相似文献   
996.
There exist major challenges in accelerating the spatial data infrastructure (SDI) planning process in the developing countries as well as advocating for politicians to support the development of SDI, due to the high complexity of SDI, lack of knowledge and experience, and limited insight in the benefits. To address these challenges, a methodology for SDI planning in Tanzania, based on the system dynamics technique and the communities of practice concept, was adopted and applied within a community consisting of experts from stakeholder organizations. The groups gathered to develop an SDI plan, while they shared their knowledge and discussed their ideas that helped their understanding of SDI. By running the system dynamics model, the development of SDI over time could be simulated that gave the planning community an insight about the future effects of today’s plans and decisions. Finally, an optimum model could be developed by refinements and improvements done with the consensus of the SDI stakeholders. This model included the components and policies that are essential for a successful SDI implementation in Tanzania and can be used as a basis for SDI planning and help to gain political support. Lessons learnt from this research were promising regarding the usability of the methodology for SDI planning in comparable countries.  相似文献   
997.
Mapping dominant vegetation communities is important work for vegetation scientists. It is very difficult to map dominant vegetation communities using multispectral remote sensing data only, especially in mountain areas. However plant community data contain useful information about the relationships between plant communities and their environment. In this paper, plant community data are linked with remote sensing to map vegetation communities. The Bayesian soft classifier was used to produce posterior probability images for each class. These images were used to calculate the prior probabilities. One hundred and eighty plant plots at Meili Snow Mountain, Yunnan Province, China were used to characterize the vegetation distribution for each class along altitude gradients. Then, the frequencies were used to modify the prior probabilities of each class. After stratification in a vegetation part and a non-vegetation part, a maximum-likelihood classification with equal prior probabilities was conducted, yielding an overall accuracy of 82.1% and a kappa accuracy of 0.797. Maximum-likelihood classification with modified prior probabilities in the vegetation part, conducted with a conventional maximum-likelihood classification for the non-vegetation part, yielded an overall accuracy of 87.7%, and a kappa accuracy of 0.861.  相似文献   
998.
The aim of this study was to quantitatively assess vegetable production and consumption in an urban fringe area in Metro Manila, the Philippines. We conducted field investigations into vegetable production at a farm within a subdivision, monitored customers for 1 week at the farm's vegetable stand, interviewed customers of the stand, and conducted household monitoring and interviews to identify purchasing behavior. We also conducted an object-based WorldView-2 image analysis of a larger area to identify current and potential future vegetable farming plots and a spatial analysis using GIS to visualize the vegetable production/consumption ratio. We found that about one-third of current vegetable demand could be supplied by greater promotion of vegetable farming in vacant lots and by setting up more vegetable stands at spatial intervals less than 1-km. Subdivision regulations also influenced vegetable farming because of the level of social acceptance of caretakers of vacant lots. The current planning and institutional framework does not consider social contexts in land use and planning. We suggested that a new framework is necessary; it should consider balancing the formation of social capital and formalize planning and institutional procedures to support sustainable local vegetable production.  相似文献   
999.
Plant communities in dryland riparian ecosystems are influenced by flood disturbance and water availability. As global climate change alters stream flow regimes, there is increasing need to understand which traits allow plant species to persist under increased stress and disturbance. Small seed mass is part of a strategy that adapts for disturbance, but the role of seed mass as an adaptation for drought is less well documented. For dryland riparian plant communities, we asked, does seed mass vary with water availability and flood frequency? We compared community seed mass between sites that vary in flow permanence (longitudinal water gradient) and between hydrogeomorphic surfaces within sites (lateral gradients of moisture and disturbance). Using data from four rivers in Arizona, we contrasted seed mass between plant groups. We found community seed mass to be greater at sites with ephemeral than perennial flow, and to increase laterally from wet, frequently-flooded channel edges to dry, less disturbed terraces. Seed mass varied by moisture group (smallest for hydroriparian species) and by disturbance group (smallest for disturbance species), and showed a trend for being greater in introduced species. We conclude that small seed mass is independently associated with wet and disturbed conditions in dryland riparian ecosystems.  相似文献   
1000.
Luke Drake 《Urban geography》2013,34(2):177-196
Community-produced spaces such as community gardens are attracting widespread scholarly interest for the potential of not only food production, but also for social, environmental, and educational benefits. Yet community gardens have also been scrutinized as sites of governmentality that produce neoliberal subjects. In this article, six case studies are analyzed as representative of three ways to organize and manage gardens—grassroots, externally-organized, and active nonprofit management. I use performativity theory to examine how definitions and enactments of community can be used to include, exclude, or bridge difference. The analysis highlights some of the specific moments in garden organizing and management that influence participation or resistance to community-oriented urban food production.  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