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   5589篇
  免费   634篇
  国内免费   349篇
测绘学   1039篇
大气科学   283篇
地球物理   870篇
地质学   1541篇
海洋学   1441篇
天文学   12篇
综合类   486篇
自然地理   900篇
  2024年   5篇
  2023年   22篇
  2022年   95篇
  2021年   145篇
  2020年   174篇
  2019年   166篇
  2018年   143篇
  2017年   270篇
  2016年   223篇
  2015年   253篇
  2014年   310篇
  2013年   419篇
  2012年   335篇
  2011年   378篇
  2010年   327篇
  2009年   351篇
  2008年   393篇
  2007年   373篇
  2006年   362篇
  2005年   291篇
  2004年   254篇
  2003年   186篇
  2002年   209篇
  2001年   167篇
  2000年   119篇
  1999年   142篇
  1998年   86篇
  1997年   78篇
  1996年   39篇
  1995年   56篇
  1994年   34篇
  1993年   34篇
  1992年   20篇
  1991年   17篇
  1990年   16篇
  1989年   13篇
  1988年   17篇
  1987年   10篇
  1986年   5篇
  1985年   10篇
  1984年   11篇
  1983年   4篇
  1982年   6篇
  1981年   2篇
  1977年   1篇
  1971年   1篇
排序方式: 共有6572条查询结果,搜索用时 15 毫秒
91.
从实现和推广农村土地集约化经营出发,以成武县为例,对农村土地集约化经营中容易出现的问题、制约因素及前景进行了分析,提出提高农村土地集约化经营效益是土地集约化经营成功的关键。  相似文献   
92.
莱芜市钢城区地理位置优越,交通便利,矿藏丰富。随着经济的高速发展,新农村建设及城市建设速度的加快,涉及土地征用、征地补偿、权属纠纷等方面的矛盾相对突出,涉及土地方面的信访事件相对增多。重点针对土地管理信访问题进行了调查分析,并就如何处置信访事件提出了意见和建议。  相似文献   
93.
国外海洋渔业准入制度的实践分析   总被引:1,自引:0,他引:1  
捕捞生产的准入是进行海洋渔业资源有效管理的必要条件。综合相关国家渔业管理发展过程,准入制度可能有三种类型:初步准入制度、投入型准入制度和产出型准入制度。准入的对象涉及渔船、渔获及渔具等。从实践经验看,在渔船数量扩展前,建立有效的准入制度的成功概率相对较大;如果现有的捕捞能力已经远超过渔业资源的承载能力时,投入型准入制度虽然还是十分重要,但在削减捕捞能力方面作用有限;产出型准入制度在一些国家的渔业实践中取得了一定的成功,但存在着诸如总可捕量控制、监测与执法等方面的问题。总体上,捕捞权利与捕捞许可证流转等是实施准入制度过程中应给予关注的两个事项。我国可在现行的捕捞许可证与船网工具指标制度的基础上完善我国投入型准入制度,将渔具纳入管理;在条件成熟的情况下,可适当过渡实施产出型准入制度。  相似文献   
94.
随着2007年《公安机关海上执法工作规定》的实施,公安海警部队已承担起海上刑事案件的侦查工作。现场勘查是侦破刑事案件的基础。为促进公安海警部队海上犯罪现场勘查工作的科学化、现代化,更好地为侦查破案服务,根据海上犯罪现场勘查的实践,分析了海上犯罪现场勘查指挥处置、现场保护、实地勘验和证据管理各个环节存在的问题,提出了解决方法和途径。  相似文献   
95.
Activity patterns, social behaviour, and reproductive success of Arabian oryx were monitored in a reintroduced population in Mahazat as-Sayd Protected Area, Saudi Arabia. During the first year of the study, precipitation was 38% lower than the long-term average, whereas rainfall in the following year resulted in precipitation that was 92.8% of the long-term average. These dramatically different rainfall conditions corresponded with distinct patterns in various environmental parameters (air and soil temperature, humidity, wind speed, solar radiation, air pressure). Daily activity patterns, the frequency of social behaviours, and foraging activity were significantly reduced during the drought period. The frequency of reproductive behaviour was significantly related to daytime, air temperature and radiation, with a pronounced reduction of reproductive activities during the drought. Monthly rates of conceptions were considerably lower during the drought. Our results substantiate the idea that extended dry periods affect the population development of Oryx, but also raise questions about habitat suitability and carrying capacity. Future management of Arabian oryx should consider extreme climatic events as factors influencing various aspects of the ecology and behaviour of this species. This aspect may become even more important in the face of climate change, including a future increase of extreme climatic events.  相似文献   
96.
于吉涛  陈子燊 《热带地理》2011,31(1):107-112
海滩状态是一个地形与水动力相互作用的三维地形动力问题.国外对海滩状态的地形动力分类已开展了大量卓有成效的研究工作,既包括波控、小潮、开阔海岸海滩状态的研究,也包括波控、不同潮差海岸海滩类型的研究和岬间海岸海滩类型的研究.文中回顾并概括了自20世纪70年代至今国外在砂质海滩地形动力分类方面代表性的研究工作,这些对于我国的...  相似文献   
97.
谢麟 《福建地理》2011,(4):85-89
以湿润亚热带山地生态国家重点实验室培育基地为例,介绍了实验室平台、人才队伍、及管理现状.从团队创新、实验室文化、知识管理等方面入手,探讨了加强实验室管理促进实验室建设的思路和方法.  相似文献   
98.
Bjørn Sletto 《Geoforum》2011,42(2):197-210
Conflicts surrounding protected area management often emerge from contested processes of boundary-making. Such productions of bounded conservation spaces are contingent in part on processes of identity formation, where some social groups are legitimized as belonging to conservation units, while others are constructed as out-of-place. This article draws on the literature in postmodern geopolitics and the political ecology of fire to interrogate processes of boundary-making and identity formation in the savanna landscape of Canaima National Park, Venezuela. The institutional culture of the environmental management agency EDELCA is in part premised on narratives of history and indigeneity coupled with a desire for an imagined, forested landscape. Because of the social constructions shaping this institutional culture, the agency maintains an approach to fire management that emphasizes fire suppression. However, an ecological field study suggests that fire suppression is leading to increased fuel loads, especially in ecologically significant boundaries between grasslands and forest. Although these boundary zones are the focus of indigenous practices of prescribed burning, they fall in-between the state management categories of forest and ecotone. As a result, these interstitial spaces become theaters for performances of domination and resistance, leading to contradictory and inconsistent approaches to fire management that place gallery forests at risk.  相似文献   
99.
王烨 《岩矿测试》2011,30(3):281-284
应用6σ质量管理方法评价X射线荧光光谱法测定多目标地球化学样品分析中4个日常监控土壤标准物质中C l、S、总碳、N、Na2O、MgO、A l2O3、SiO2、P、K2O、CaO、Ba、Ti、V、Cr、Mn、TFe2O3、Zn、Ga、Br、Pb、Th、Rb、Sr、Y、Zr、Nb等27个项目检测质量水平。研究结果显示,随着高精度、自动化仪器的应用,多目标样品分析多个项目的精密度、准确度都得到大幅度的提高,部分检测项目的性能已经达到6σ质量水平;但有少数项目如N、总碳、Cr等低于3σ质量水平,其精密度准确度均需改进,分析性能有待进一步提高,建议更换检测方法。用6σ质量管理方法评价地质实验分析测试质量水平与地质实验室传统质量控制方法得出的结果具有一致性,其评价方法简便、量化、直观、具有一定的应用价值。  相似文献   
100.
Climate change is identified as a major threat to wetlands. Altered hydrology and rising temperature can change the biogeochemistry and function of a wetland to the degree that some important services might be turned into disservices. This means that they will, for example, no longer provide a water purification service and adversely they may start to decompose and release nutrients to the surface water. Moreover, a higher rate of decomposition than primary production (photosynthesis) may lead to a shift of their function from being a sink of carbon to a source. This review paper assesses the potential response of natural wetlands (peatlands) and constructed wetlands to climate change in terms of gas emission and nutrients release. In addition, the impact of key climatic factors such as temperature and water availability on wetlands has been reviewed. The authors identified the methodological gaps and weaknesses in the literature and then introduced a new framework for conducting a comprehensive mesocosm experiment to address the existing gaps in literature to support future climate change research on wetland ecosystems. In the future, higher temperatures resulting in drought might shift the role of both constructed wetland and peatland from a sink to a source of carbon. However, higher temperatures accompanied by more precipitation can promote photosynthesis to a degree that might exceed the respiration and maintain the carbon sink role of the wetland. There might be a critical water level at which the wetland can preserve most of its services. In order to find that level, a study of the key factors of climate change and their interactions using an appropriate experimental method is necessary. Some contradictory results of past experiments can be associated with different methodologies, designs, time periods, climates, and natural variability. Hence a long-term simulation of climate change for wetlands according to the proposed framework is recommended. This framework provides relatively more accurate and realistic simulations, valid comparative results, comprehensive understanding and supports coordination between researchers. This can help to find a sustainable management strategy for wetlands to be resilient to climate change.  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