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   8962篇
  免费   781篇
  国内免费   917篇
测绘学   614篇
大气科学   477篇
地球物理   501篇
地质学   2780篇
海洋学   1001篇
天文学   14篇
综合类   799篇
自然地理   4474篇
  2024年   27篇
  2023年   86篇
  2022年   339篇
  2021年   368篇
  2020年   337篇
  2019年   341篇
  2018年   277篇
  2017年   296篇
  2016年   333篇
  2015年   328篇
  2014年   404篇
  2013年   542篇
  2012年   497篇
  2011年   472篇
  2010年   393篇
  2009年   427篇
  2008年   418篇
  2007年   460篇
  2006年   502篇
  2005年   439篇
  2004年   437篇
  2003年   374篇
  2002年   385篇
  2001年   389篇
  2000年   411篇
  1999年   326篇
  1998年   244篇
  1997年   208篇
  1996年   142篇
  1995年   95篇
  1994年   79篇
  1993年   73篇
  1992年   48篇
  1991年   40篇
  1990年   26篇
  1989年   19篇
  1988年   19篇
  1987年   17篇
  1986年   12篇
  1985年   7篇
  1984年   8篇
  1983年   3篇
  1982年   8篇
  1981年   1篇
  1980年   2篇
  1977年   1篇
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
171.
A field experiment was conducted to study the effect of different crop sequences on water use, growth and yield of green gram (Vigna radiata (L) Wilezek) during 1992-93 and 1993-94 under rainfed upland condition in Indo-Gangetic plains of West Bengal. Green gram sown in the month of March as pre-rainy (summer) season crop, as and when the winter crops vacated the land, produced highest dry matter of 372 gm-2 which was significantly highest in black gram-yellow sarson sequence.The results of the field experimentation revealed that green gram gave highest grain yield to the extent of 10.80 q/ha when sown after black gram (rainy season) followed by yellow sarson (winter season) while the crop produced 10.63 q/ha under sesame-yellow sarson sequence. Highest water use of 267mm was achieved in green gram under black gram-yellow sarson sequence and the crop gave water use efficiency of 4.07 kg ha-1mm-1 under black gram-yellow sarson sequences.  相似文献   
172.
The soil conservation campaigns that have been a prominent feature of Jamaican agricultural policy since the 1950s are frequently presented as having failed to ameliorate the problem of soil erosion in hillside agriculture. A case study of a small farming community in the Blue Mountains explores the development of the soil conservation practices currently employed by farmers. The use of trash barriers in carrot farming is described and the origins of this technology and its subsequent adaptation are considered. The study concludes that extension interventions and government policy have influenced the development of current soil conservation practices; practices which have their roots in indigenous techniques. Other factors such as the adaptation of trash barriers to suit local conditions and their importance in soil fertility management also play a role in their widespread use in the study area. The study demonstrates that to understand the process of technical change in farming communities, it is necessary to consider a range of factors, external and internal, technical and social, that have over time influenced farmers' decision making.  相似文献   
173.
A field experiment was conducted to study the effect of different crop sequences on water use, growth and yield of green gram (Vigna radiata (L) Wilezek) during 1992-93 and 1993-94 under rainfed upland condition in Indo-Gangetic plains of West Bengal. Green gram sown in the month of March as pre-rainy (summer) season crop, as and when the winter crops vacated the land, produced highest dry matter of 372 gm-2 which was significantly highest in black gram-yellow sarson sequence. The results of the field experimentation revealed that green gram gave highest grain yield to the extent of 10.80 q/ha when sown after black gram (rainy season) followed by yellow sarson (winter season) while the crop produced 10.63 q/ha under sesame-yellow sarson sequence. Highest water use of 267 mm was achieved in green gram under black gram-yellow sarson sequence and the crop gave water use efficiency of 4.07 kg ha-1mm-1 under black gram-yellow sarson sequences.  相似文献   
174.
Luxury Tourism and Regional Economic Development in Mexico   总被引:1,自引:0,他引:1  
This article argues that little attention has been paid to the territorial and socioeconomic impact of consumer–service globalization on Third World countries. It specifically examines the economic role of tourism in Mexico and its limited effect on the country's regional development. Despite governmental support in order to enhance economic growth, currency receipts, and employment, tourism contributes less than 5 percent to the gross domestic product, and the majority of tourism–related jobs are precarious and low–skilled. The promotion of luxury resorts in coastal areas has led to highly concentrated and fast–growing enclaves of mainly foreign investment. However, this strategy has failed to stimulate productive links between tourist centers and their hinterlands and has led to large–scale urbanization characterized by a considerable lack of basic services for the resident population.  相似文献   
175.
信息产业发展对宁波城市内部结构的影响   总被引:2,自引:1,他引:2  
该文从信息产业的概念和分类人手,着重分析信息产业发展对城市产业结构、社会结构和空间结构的影响。对宁波市信息产业与城市产业结构的内在关系进行了量化研究;探讨信息产业对城市内部社会结构分异的变化趋势;提出了信息产业对城市内部空间结构功能置换的两种模式和地域扩张的促动机制。  相似文献   
176.
生态交界带特征与动态变化机理分析   总被引:2,自引:0,他引:2  
文章论述了生态交界带是两个以上生态系统交接的过渡带,它具有特有的生态系统结构,生态交界带生态系统的结构,功能和物质,能是以及信息的交流与相邻生态系统有密切的联系,但又表现出明显的异质性,其动态变化机理显示生态交界带内物质,能量和信息的均衡状况。  相似文献   
177.
我国钾肥需求量逐年增加,80%需要进口,但同时又有大量的低品位非水溶性钾矿资源没有利用,利用非水溶性钾矿生产长效钾肥既能缓解钾肥短缺的矛盾,又可以解决普通钾肥对环境的污染和破坏。长效钾肥原料分布广,生产和施用技术简单,成本低,易于推广普及形成产业化,其对农业可持续发展将会产生重要的促进作用。  相似文献   
178.
区域经济整合发展研究--以上海市南汇区为例   总被引:3,自引:0,他引:3  
文章从区域经济一体化和经济全球化的角度出发,分析了区域经济整合发展的动力来源、基本过程及遵循的原则,并以实例阐述了区域经济整合的主要内容为:城镇体系、产业结构、市政和基础设施及区域经济运行环境等,归纳出了5种典型的整合类型,即领土主权变动、国家重大政策推动、行政归属调整、重大工程启动及城市扩展整合等,最后通过上海市南汇区的实例进行了相应的论证。  相似文献   
179.
浙江原发性产业集群的形成机制与持续发展   总被引:10,自引:0,他引:10  
原发性产业集群是浙江省产业发展的主要模式,是在开放的环境中自发形成的。独特的区域“文化基因”,产业特定性知识,产业氛围与地方政府的促进作用是原发性产业集群形成的主要因素;小企业大群落,区内企业的分工协作与竞争,产品的多样化与差别化及集体学习是其主要特点;生产要素的培育和创造,区域品牌树立,学习型企业群落创建及特色制造业基地的培育是产业集群区持续发展的主要措施。  相似文献   
180.
塔里木河下游输水与生态恢复监测初报   总被引:24,自引:12,他引:24  
根据近三年塔里木河下游生态输水前后地下水位、水质、土壤、植被等项内容的监测资料,分析了塔里木河下游输水对地下水位、水质的影响,揭示了输水与地表生态的响应过程,探讨了地下水位与天然植被生长、恢复的相互关系,阐述了植被退化过程及相关因子,确定了维系塔河下游生态安全的最低生态需水量、最佳生态水位。  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