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   14840篇
  免费   408篇
  国内免费   68篇
财政金融   2346篇
工业经济   1062篇
计划管理   2160篇
经济学   2777篇
综合类   396篇
运输经济   84篇
旅游经济   156篇
贸易经济   1950篇
农业经济   794篇
经济概况   1865篇
水利工程   1710篇
信息产业经济   2篇
邮电经济   14篇
  2021年   138篇
  2020年   150篇
  2019年   190篇
  2018年   247篇
  2017年   244篇
  2016年   237篇
  2015年   222篇
  2014年   311篇
  2013年   1313篇
  2012年   467篇
  2011年   519篇
  2010年   431篇
  2009年   480篇
  2008年   485篇
  2007年   456篇
  2006年   462篇
  2005年   469篇
  2004年   359篇
  2003年   302篇
  2002年   254篇
  2001年   278篇
  2000年   258篇
  1999年   236篇
  1998年   229篇
  1997年   242篇
  1996年   256篇
  1995年   237篇
  1994年   230篇
  1993年   192篇
  1992年   218篇
  1991年   236篇
  1990年   202篇
  1989年   169篇
  1988年   167篇
  1987年   140篇
  1986年   188篇
  1985年   258篇
  1984年   263篇
  1983年   261篇
  1982年   226篇
  1981年   236篇
  1980年   202篇
  1979年   217篇
  1978年   188篇
  1977年   161篇
  1976年   146篇
  1975年   134篇
  1974年   130篇
  1973年   101篇
  1972年   102篇
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
31.
Using bald eagles to indicate the health of the Great Lakes' environment   总被引:1,自引:0,他引:1  
The bald eagle (Haliaeetus leucocephalus) is one of the most studied birds of North America, and a great amount of natural life‐history information, including the response of various stressors on the eagles' ability to reproduce, are well known. In Michigan, the eagle has been chosen to track the trends of bioaccumulative compounds of concern across watersheds in the state. The state has been divided into major watersheds, and 20% of these are surveyed each year. A control area in northern Minnesota, Voyageurs National Park, is also sampled annually. We report here on the methods used, the preliminary results of the 1999 field season, and how differences in mercury concentrations varied over a 10‐year period. Mercury in feathers of nestling eagles declined over time only in Lakes Michigan and Huron, but have not decreased among other subpopulations in Michigan. Concentrations of polychlorinated biphenyls (PCBs) and 4,4′‐DDE in blood plasma from nestling eagles have declined over time for most subpopulations; however, they remain greater for breeding areas associated with the Great Lakes' food web. Sea eagles of the genus Haliaeetus are a good sentinel species to track trends in bioaccumulative compounds in aquatic systems.  相似文献   
32.
33.
Rapport de la Commission chargée d'étudier l'organisation du corps consulaire français. (Mémorial diplomatique van 24 en 31Januari, 7 en 14Februari 1885).  相似文献   
34.
35.
36.
37.
As a major pollutant source to urban receiving waters, the non-point source pollution from urban runoff needs to be well studied and effectively controlled. Based on monitoring data from urban runoff pollutant sources, this article describes a systematic estimation of total pollutant loads from the urban areas of Beijing. A numerical model was developed to quantify main pollutant loads of urban runoff in Beijing. A sub-procedure is involved in this method, in which the flush process influences both the quantity and quality of stormwater runoff. A statistics-based method was applied in computing the annual pollutant load as an output of the runoff. The proportions of pollutant from point-source and non-point sources were compared. This provides a scientific basis for proper environmental input assessment of urban stormwater pollution to receiving waters, improvement of infrastructure performance, implementation of urban stormwater management, and utilization of stormwater.  相似文献   
38.
Continuous simulation is performed using the US Environmental Protection Agency (USEPA) Storm Water Management Model (SWMM) to evaluate regional differences around the United States in hydrologic and water quality performance of wet-weather controls. Controls are characterised as being limited by peak inflow rate (i.e. any device with little or no storage, such as screens, filters and some proprietary devices) or by storage capacity (e.g., ponds, tanks). For flow-limited devices, results are presented in the form of percentage of annual runoff volume captured (passing through the device) for a given inflow capacity. For storage-limited devices, results are presented in two forms: percentage of annual runoff volume captured as a function of unit basin size and drawdown (drain) time, and as a percentage of total suspended solids captured, for the same two variables. Regional differences are apparent, driven mainly by variations in rainfall patterns around the country.  相似文献   
39.
Using a sample of Australian Multi-sector trusts we examine selectivity and market timing performance and extend the analysis to include the relatively new measure of volatility timing. This is of particular relevance to our data set, as high levels of volatility persistence are prevalent in Australia. In addition we consider the stability, asymmetry and seasonality of the various performance and risk measures. A survivorship adjustment procedure is also employed in order to assess the impact of survivorship on selectivity, market timing and volatility timing performance.  相似文献   
40.
The optical character of waste stabilization ponds (WSPs) is of concern for several reasons. Algal photosynthesis, which produces oxygen for waste oxidation in WSPs, is influenced by attenuation of sunlight in ponds. Disinfection in WSPs is influenced by optical characteristics because solar UV exposure usually dominates inactivation. The optical nature of WSPs effluent also affects assimilation by receiving waters. Despite the importance of light behaviour in WSPs, few studies have been made of their optical characteristics. We discuss simple optical measures suitable for routine monitoring of WSPs (including at sites remote from laboratories): optical density of filtrates - an index of dissolved coloured organic (humic) matter, visual clarity - to provide an estimate of the beam attenuation coefficient (a fundamental quantity needed for optical modelling) colour (hue) - as an indicator of general WSP 'condition' and irradiance attenuation quantifying depth of light penetration. The value of optical characterisation of WSPs is illustrated with reference to optical data for WSPs in NZ (including high-rate algal ponds) treating dairy cattle wastewater versus domestic sewage. We encourage increased research on optical characteristics of WSPs and the incorporation of optical measures in monitoring and modelling of WSP performance.  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