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   3616篇
  免费   292篇
  国内免费   279篇
测绘学   179篇
大气科学   226篇
地球物理   288篇
地质学   605篇
海洋学   317篇
天文学   24篇
综合类   168篇
自然地理   2380篇
  2024年   12篇
  2023年   54篇
  2022年   170篇
  2021年   188篇
  2020年   193篇
  2019年   208篇
  2018年   142篇
  2017年   154篇
  2016年   176篇
  2015年   167篇
  2014年   195篇
  2013年   187篇
  2012年   211篇
  2011年   239篇
  2010年   167篇
  2009年   183篇
  2008年   199篇
  2007年   182篇
  2006年   189篇
  2005年   156篇
  2004年   136篇
  2003年   106篇
  2002年   120篇
  2001年   74篇
  2000年   98篇
  1999年   76篇
  1998年   53篇
  1997年   29篇
  1996年   31篇
  1995年   21篇
  1994年   15篇
  1993年   14篇
  1992年   6篇
  1991年   6篇
  1990年   3篇
  1989年   7篇
  1988年   2篇
  1987年   3篇
  1986年   3篇
  1985年   2篇
  1984年   2篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1977年   2篇
  1971年   1篇
  1954年   1篇
排序方式: 共有4187条查询结果,搜索用时 15 毫秒
71.
为了防止商业综合体建设过剩,形成商业综合体之间的恶性竞争。本文基于微信易出行插件里面的城市热力图数据,以杭州市5个商业综合体为例,获取可以量化的热力值数据,对不同类型的商业综合体活力影响因素进行对比分析研究。研究发现:① 城市商业综合体布局密度、周边用地用途、销售模式、内部功能布局以及内部空间活力5个要素对商业综合体活力的影响最明显;② 商业综合体的高密度布局对其活力存在抑制作用,通过对比各个服务半径内的商业综合体的活力变化,发现商业综合体合理的服务半径应该大于1.0 km。商业综合体周边的居住用地、广场和公园用地对其活力具有增强作用,办公用地对商业综合体活力的影响较弱;③ 针对不同消费对象制定的不同销售模式,也会对商业综合体活力变化产生显著影响;④ 在公共活动空间相对缺乏的区域内,以零售为主且兼具城市公共活动空间的商业综合体比以主力店为主的商业综合体的活力更高,空间利用率越高的商业综合体活力也越高,商业综合体承担部分城市公共活动空间的职能对商业综合体的活力起到了提升作用。  相似文献   
72.
INTRODUCTIONInmoststudies,growthrateoffishwasfoundtodecreasewithincreasingdensity ,especiallywhenthedensitywasaboveacriticalpoint (RefstieandKittlesin ,1 976;Refstie,1 977;Trzebia towskietal.,1 981 ;VijayanandLeatherland ,1 988) .However,inacold watercarnivorousfish ,theA…  相似文献   
73.
针对综导系统卡尔曼滤波器的特点,深入研究了灰关联分析法在滤波器精度效能评估中的应用。给出卡尔曼滤波器进行灰关联分析的方法和步骤,并对某一类型的滤波器进行精度效能评估。仿真结果表明了该方法的可行性和有效性。  相似文献   
74.
江西省万安水库对氮、磷营养盐的滞留效应   总被引:6,自引:0,他引:6  
以江西省万安水库为研究对象,于2007-2009年丰、枯水期对水库进行了人库、出库水样采集,研究了万安水库对氮、磷营养盐的滞留效应.结果表明,万安水库对氮、磷营养盐存在显著滞留作用.DIN浓度和输送通量整体均表现为入库>出库,受径流量季节变化的影响.不同时期DIN的滞留情况整体表现为枯水期<丰水期;TP浓度和输送通量整...  相似文献   
75.
《国际泥沙研究》2020,35(5):431-443
In recent years, the damage caused to human settlements in Japan by large woody debris (LWD) has been increasing. For example, the 2013 Izu Oshima typhoon resulted in a large number of fatalities and missing persons, and the Kagoshima Typhoon Disaster and Northern Kyusyu torrential downpour caused vast infrastructure damage due to the associated LWD. Current countermeasures for preventing LWD are insufficient to maintain the safety of residential areas. One type of protective barrier, the open sabo dam, has been constructed in Japan during the past 30 years. The primary function of open sabo dams is to block the flow of boulders, thereby also reducing sediment flow by reducing the gap size. However, because Japanese open sabo dams are designed specifically for boulder-trapping, the ability of these dams to trap LWD remains uncertain. In particular, many problems have been reported with respect to sediment trapping by driftwood with roots in an open sabo dam setting. The objective of this study was to examine the trapping efficiency of open sabo dams for LWD and sediment. The experimental approach clarified the influence of driftwood, without and with roots, on sediment trapping for a straight-channel flume. The flexible roots of the driftwood were shown to have a significant effect on the sediment trapping efficiency of the dam.  相似文献   
76.
The paper summarises the history of human settlement in the Sudetes from the Bronze Age. A more intensive stage was reached in the Middle Ages when settlers from the west established new villages deep in the mountain valleys. By the beginning of the 20th century the Sudetes were already overpopulated but after a near-complete ethnic replacement from a German to a Polish population in 1945–8, population continued to grow until the 1980s. However, the economic system was hardly sustainable by this time. There were many polluting industries in the region and massive transboundary pollution from adjacent areas of Czechoslovakia and Germany had a devastating effect on the forests. Moreover, subsidised state agriculture placed heavy pressure on vulnerable mountain grazings. Since 1989 there has been a process of deindustrialisation in the Sudetes and surrounding areas and, with the disappearance of the state farms and the reduction in subsidies, agriculture is now better adjusted to the natural potential and is complemented by a promising start with agrotourism. The population of the region is growing relatively slowly (0.8% per annum 1956–1999), with the rural areas now in decline. A sustainable future for the region is now a possibility, but while there is a consensus for continued environmental reconstruction, supported by alternative economies, this will have to be carefully managed in the years ahead when EU accession may bring heavier development pressures than those evident at the moment. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
77.
The establishment of local self-government was a key part of the post-1989 transformation in East and Central Europe. Local government in both Western and East and Central Europe has increasingly been expected to play a role in local economic development (LED). Local government is one important agent in the complex processes of building 'institutional thickness' to ensure the development of local economies and the quality of life of inhabitants. This paper presents the results of a national postal questionnaire survey of the LED role of the lowest level of local self-government in Poland, the gmina or commune. The paper establishes a baseline of knowledge regarding: the local economic problems faced by communes; their attitudinal, strategic and organisational responses; and the main factors which are hindering the communes' LED role. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
78.
空气潜孔锤在水源钻井中的应用   总被引:8,自引:0,他引:8  
空气潜孔锤钻进是以压缩空气作为循环介质来洗孔、冷却钻头,将岩屑从孔底带出孔外的钻进方法。它具有钻井效率高、施工周期短和施工用水少等优点。通过两个不同情况钻孔的实际施工,介绍了施工背景、地层结构、配套设备及施工过程。从应用效果来看,该方法的钻效是传统泥浆钻进的3-4倍。根据水源井施工经验,总结出在干旱缺水和漏失层地区采用空气(泡沫)潜孔锤钻进的操作方法,并对其施工工艺进行了分析,指出了实际施工中的注意事项及应关注的问题。  相似文献   
79.
This study employed Slack-based measure (SBM), Meta-frontier analysis, and Malmquist–Luenberger index (MLI) approaches to measure energy efficiency (EE), production technology heterogeneity, and energy productivity variation and its main determinants in developed and developing G20 countries for the period 1995–2020. The study's findings are: (i) Average EE for G20 countries is 0.8577, but still has an improvement potential of 14.23 percent. (ii) Developed countries have a higher average EE than developing (0.8927 > 0.8290). Results further revealed that energy efficiency scores in Argentina, Australia, Brazil, France, Saudi Arabia, South Africa, the US, the UK, and Korea are>1. (iii) Technology gap ratio (TGR) value in developed countries is higher than in developing (0.9869 > 0.6801), indicating that developed countries have advanced energy technologies superiority. The average MLI is less than one, showing a decline in energy productivity mainly due to a decline in technical efficiency, as EC < TC. (iv) Developed G20 countries get 0.64 percent aggregate growth in energy productivity as their MLI score is 1.0064, mainly due to technological growth (TC = 1.0161). Developing G20 countries, on the other hand, witnessed a decline in their energy productivity during 1995–2020, as MLI = 0.9925 > 1, designating that, on average, there is a 0.75 % decline in MLI.  相似文献   
80.
为查清山西省阳泉地区旅游地质资源分布状况,方便当地政府掌握区内旅游地质资源现状,为地质公园开发、保护与发展规划提供决策依据,山西省自然资源厅(原山西省国土资源厅)于2018年开展了“山西省阳泉地区旅游地质资源调查”项目,基于三维可视化遥感影像,采用目视解译方法,并结合已有地质资料和野外调查数据,获取了阳泉旅游地质资源分布信息。在阳泉中东部圈定了一条呈月牙形的旅游地质资源密集分布带,该分布带西起盂县藏山,向南东过平定玉皇洞,终至平定县娘子关,分布在碳酸盐岩区,集山地地貌景观、岩溶地貌景观和泉域地貌景观于一体,是我国北方地区规模较大、较为典型的碳酸盐岩地貌、岩溶地貌和泉域地貌出露区,在区域上连片分布,具有较高的开发潜力。建议在“藏山旅游区—岩溶地貌景区—娘子关旅游区”范围申请建立北方碳酸盐岩景观多成因类型地质公园。  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