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   866篇
  免费   60篇
  国内免费   87篇
测绘学   115篇
大气科学   135篇
地球物理   103篇
地质学   228篇
海洋学   58篇
天文学   4篇
综合类   70篇
自然地理   300篇
  2024年   7篇
  2023年   11篇
  2022年   26篇
  2021年   38篇
  2020年   38篇
  2019年   52篇
  2018年   41篇
  2017年   45篇
  2016年   47篇
  2015年   32篇
  2014年   58篇
  2013年   112篇
  2012年   50篇
  2011年   56篇
  2010年   42篇
  2009年   31篇
  2008年   50篇
  2007年   35篇
  2006年   29篇
  2005年   30篇
  2004年   21篇
  2003年   26篇
  2002年   18篇
  2001年   23篇
  2000年   18篇
  1999年   9篇
  1998年   12篇
  1997年   11篇
  1996年   6篇
  1995年   7篇
  1994年   4篇
  1993年   3篇
  1992年   3篇
  1991年   4篇
  1990年   5篇
  1987年   2篇
  1986年   1篇
  1985年   1篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1979年   1篇
  1978年   2篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
排序方式: 共有1013条查询结果,搜索用时 15 毫秒
31.
国家湿地公园的旅游环境容量问题多来源于园区内部属性和游客行为之间的差异,国家湿地公园面积大、生态系统独特,园区空间布局、管理制度、主题商品与设施建设、服务方面的不合理、不完善导致了局部时段性的超载现象,宜采取以游客行为为导向的容量调控模式(TBCC),从加强园区自身建设和管理入手调控游客空间流动,从内部提升视角解决容量超载问题,实现游客体验、环境保护、科普教育等多功能为一体的湿地公园建设目标。  相似文献   
32.
《Urban geography》2013,34(4):297-329
This paper examines recent proposals for memorials and monuments on the National Mall in Washington, DC. Demands for memorials are increasingly at odds with the Mall's important role as open green space and public protest space. This article analyzes the broader issues embodied in the competing purposes of this highly visible public space by considering the recent controversy over the World War II Memorial. The controversy focused primarily on the location of the memorial. Opponents contended the World War II Memorial would interrupt or destroy the iconography of the National Mall. Supporters argued that the location selected for the memorial was consistent with its importance in U.S. history and that it deserved such a prized location in the central axis between the Lincoln Memorial and the Washington Monument. The article then analyzes the politics behind the site selection, and examines the debate about the impact of the memorial's location. It concludes that the location of the memorial was not accidental but intentional, and thereby reveals a purposeful re-writing of the Mall's symbolic space to suit a specific vision of the war and its meaning in American history. The article also contends that the location of the memorial represents an emerging social-political agenda that is prioritizing the Mall as a place of commemoration at the expense of open space, and, perhaps, at the expense of public protest space.  相似文献   
33.
Oligotrophic bacteria have been isolated from many habitats, yet environmental regulation of their distributions in soil has not been elucidated. To address the issue of environmental influence upon oligotrophic distributions, Chihuahuan Desert soils were sampled from five sites along an elevational and vegetational gradient within Big Bend National Park during January and August of 2002 and 2003. Soils were diluted and plated on oligotrophic media, and plates were incubated at 15, 25, 35, 45 and 60 °C. Additionally, measurements of soil organic matter, pH, moisture, extractable nitrate, extractable ammonium and microbial biomass carbon were collected for each sample to relate oligotrophic bacterial distributions to soil nutrient and edaphic characteristics. Analysis of variance indicated significant site, season, incubation-temperature and interaction effects on total oligotroph numbers. Canonical correspondence analysis and multiple regressions indicated that all soil-chemistry variables significantly influenced discrete morphologies of oligotrophs. Oligotroph distributions were most congruent with soil-chemistry variation in three sites, whereas oligotrophic diversity in two sites did not adhere closely to measured environmental variables. While vegetation type may structure oligotroph communities at the two mid-elevational sites, abiotic constraints are drivers in low-desert sites.  相似文献   
34.
Glaciers are one of the most important land covers in alpine regions and especially sensitive to global climate change. Remote sensing has proved to be the best method of investigating the extent of glacial variations in remote mountainous areas. Using Landsat thematic mapping (TM) and multi-spectral-scanner (MSS) images from Mt. Qomolangma (Everest) National Nature Preserve (QNNP), central high Himalayas for 1976, 1988 and 2006, we derived glacial extent for these three periods. A combination of object-oriented image interpretation methods, expert knowledge rules and field surveys were employed. Results showed that (1) the glacial area in 2006 was 2710.17 ± 0.011 km2 (about 7.41% of the whole study area), and located mainly to the south and between 4700 m to 6800 m above sea level; (2) from 1976 to 2006, glaciers reduced by 501.91 ± 0.035 km2 and glacial lakes expanded by 36.88 ± 0.035 km2; the rate of glacier retreat was higher in sub-basins on the southern slopes (16.79%) of the Himalayas than on the northern slopes (14.40%); most glaciers retreated, and mainly occurred at an elevation of 4700–6400 m, and the estimated upper limit of the retreat zone is between 6600 m and 6700 m; (3) increase in temperature and decrease in precipitation over the study period are the key factors driving retreat.  相似文献   
35.
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   
36.
The quantitative probabilistic assessment of the undiscovered mineral resources of the 17.1-million-acre Tongass National Forest (the largest in the United States) and its adjacent lands is a nonaggregated, mineral-resource-tract-oriented assessment designed for land-planning purposes. As such, it includes the renewed use of gross-in-place values (GIPV's) in dollars of the estimated amounts of metal contained in the undiscovered resources as a measure for land-use planning.Southeastern Alaska is geologically complex and contains a wide variety of known mineral deposits, some of which have produced important amounts of metals during the past 100 years. Regional geological, economic geological, geochemical, geophysical, and mineral exploration history information for the region was integrated to define 124 tracts likely to contain undiscovered mineral resources. Some tracts were judged to contain more than one type of mineral deposit. Each type of deposit may contain one or more metallic elements of economic interest. For tracts where information was sufficient, the minimum number of as-yet-undiscovered deposits of each type was estimated at probability levels of 0.95, 0.90, 0.50, 0.10, and 0.05.The undiscovered mineral resources of the individual tracts were estimated using the U.S. Geological Survey's MARK3 mineral-resource endowment simulator; those estimates were used to calculate GIPV's for the individual tracts. Those GIPV's were aggregated to estimate the value of the undiscovered mineral resources of southeastern Alaska. The aggregated GIPV of the estimates is $40.9 billion.Analysis of this study indicates that (1) there is only a crude positive correlation between the size of individual tracts and their mean GIPV's: and (2) the number of mineral-deposit types in a tract does not dominate the GIPV's of the tracts, but the inferred presence of synorogenic-synvolcanic nickel-copper, porphyry copper skarn-related, iron skarn, and porphyry copper-molybdenum deposits does. The influence of this study on the U.S. Forest Service planning process is yet to be determined.  相似文献   
37.
陕西西安翠华山地质景观遗迹成因探析   总被引:4,自引:1,他引:4  
翠华山国家地质公园位于西安市南郊,包括水湫池、甘湫池和大坪三个景区。新的实地调查发现,甘湫池景区是一座罕见的古基岩滑坡,具有古滑坡可观察的景观形态特征,甘湫池不足堰塞湖,而是古滑坡形成的拉张洼地;水湫池堰塞坝的沉积物具有典型的“泥砾”结构,在形态特征上属于粘性重力流的碎屑流堆积;其坝体的形成与景区现有崩塌堆积无关;从空间分布和层序关系来看,景区内现有的崩塌砾石均堆积在古滑坡和泥石流堆积之上,由于各处具有不同的形态和结构特征,应具有成因的多样性和时间的多期性。  相似文献   
38.
梅州市名城文化景观的保护与开发研究   总被引:5,自引:1,他引:5  
从名城文化内涵出发,着眼于作为客家人文中心和名城文化景观的集中的梅州市区,全面分析了其中丰富多样的文化景观资源的特征及其文化内蕴。揭示了梅州名城文化景观保护与开发过程中的成绩与存在问题。并从文化地理学的视觉,结合旅游开发和城市规划,论证了科学保护与开发梅州名城文化景观的下述对策与途径的必要性与可行性:提升名城办的职能并与相关部门共营名城文化氛围;强化名城轴线与标志性文化景观;抢救与保护“老字号”等一批文化景观景区;适度利用,以积极的开发促其效的保护。  相似文献   
39.
How are large companies responding to the challenges of reducing their greenhouse gas (GHG) emissions? An analysis of the published climate change policies and performance of 125 large European companies is presented. The results suggest that most large European companies have now developed the management systems and processes necessary for them to effectively manage their GHG emissions and related business risks. However, there is a significant disconnect between the targets that companies set for themselves and the more ambitious targets being set by the European Union (which has committed to a 20% reduction in its emissions by 2020 against a 1990 baseline). Of the companies surveyed, just over one-third had stabilized or reduced their total GHG emissions over the period 2002–2007, and fewer than one-third expected their emissions to stabilize or reduce in the coming years. The relationship between the quality of corporate policies and performance outcomes (in terms of GHG emissions) suggests that while companies with stronger policies are likely to have relatively better performance, only a minority of those companies with the highest-quality policies are committing to absolute reductions in their GHG emissions.  相似文献   
40.
"973"计划资源环境领域数据汇交进展与数据分析   总被引:3,自引:0,他引:3  
国家科技计划项目数据汇交对于促进科学数据的整合集成,增强国家科技投入的效益,提高我国科技计划项目管理水平具有重要意义。经过充分研究,科技部基础司选择国家重点基础研究发展计划(973计划)资源环境领域作为试点,于2008年启动数据汇交工作,并专门成立数据汇交中心。2008年,按照数据汇交计划制定、数据汇交准备、数据实体汇交、数据共享服务4个阶段的工作流程,分"已结题"、"即将于2008年结题"和"在研"3种类型开展汇交工作,完成了24个结题项目的数据汇交,汇交数据总量超过620 GB。为了保证数据汇交的可操作性和规范化,数据汇交中心制定了包括核心元数据规范、数据文档格式等7项标准规范;研制了数据汇交软件工具;建立了海量数据存储环境;设计开发了数据汇交共享服务网络平台。面向数据服务进一步的需要,分析了已汇交数据情况,具体分为5种类型,即定点长期监测和野外定点调查类、区域调查和统计分析类、模型计算类、试验化验分析类、客观或主观描述类,并提出了相应的数据展示策略。  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