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   32902篇
  免费   347篇
  国内免费   245篇
安全科学   868篇
废物处理   1206篇
环保管理   4153篇
综合类   6273篇
基础理论   9104篇
环境理论   26篇
污染及防治   8375篇
评价与监测   1934篇
社会与环境   1349篇
灾害及防治   206篇
  2022年   227篇
  2021年   213篇
  2019年   243篇
  2018年   442篇
  2017年   426篇
  2016年   623篇
  2015年   537篇
  2014年   758篇
  2013年   2291篇
  2012年   943篇
  2011年   1317篇
  2010年   1122篇
  2009年   1131篇
  2008年   1355篇
  2007年   1453篇
  2006年   1276篇
  2005年   1085篇
  2004年   1074篇
  2003年   1034篇
  2002年   1000篇
  2001年   1293篇
  2000年   915篇
  1999年   609篇
  1998年   461篇
  1997年   476篇
  1996年   467篇
  1995年   518篇
  1994年   478篇
  1993年   421篇
  1992年   439篇
  1991年   404篇
  1990年   407篇
  1989年   439篇
  1988年   368篇
  1987年   316篇
  1986年   295篇
  1985年   330篇
  1984年   302篇
  1983年   332篇
  1982年   338篇
  1981年   288篇
  1980年   266篇
  1979年   298篇
  1978年   251篇
  1977年   219篇
  1976年   221篇
  1975年   209篇
  1974年   185篇
  1973年   197篇
  1972年   216篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
271.
272.
273.
This study uses an integrative approach to study the water-quality impacts of future global climate and land-use changes. In this study, changing land-use types was used as a mitigation strategy to reduce the adverse impacts of global climate change on water resources. The climate scenarios were based on projections made by the Intergovernmental Panel on Climate Change (IPCC) and the United Kingdom Hadley Centre's climate model (HadCM2). The Thornthwaite water-balance model was coupled with a land-use model (L-THIA) to investigate the hydrologic effects of future climate and land-use changes in the Ohio River Basin. The land-use model is based on the Soil Conservation Service's curve-number method. It uses the curve number, an index of land use and soil type, to calculate runoff volume and depth. The ArcView programming language, Avenue, was used to integrate the two models into a geographic information system (GIS). An output of the water-balance model, daily precipitation values adjusted for potential evapotranspiration, served as one of the inputs into the land-use model. Two watersheds were used in the present study: one containing the city of Cincinnati on the main stem of the Ohio River, and one containing the city of Columbus on a tributary of the Ohio River. These cities represent two major metropolitan areas in the Ohio River Basin with different land uses experiencing different rates of population growth. The projected hypothetical land-use changes were based on linear extrapolations of current population data. Results of the analyses indicate that conversion from agricultural land use to low-density residential land use may decrease the amount of surface runoff. The land-use practices which generate the least amount of runoff are forest, low-density residential, and agriculture; whereas high-density residential and commercial land-use types produce the highest runoff. The hydrologic soil type present was also an important factor in determining the amount of runoff and non-point-source pollution. A runoff-depth matrix and total nitrogen matrix were created for Cincinnati and Columbus to describe possible land-use mitigation measures in response to global climate change. The differences in Cincinnati and Columbus were due to differences in geographic location, air temperature, and total runoff. The results of this study may be useful to planners and policy makers for defining the possible impacts of future global climate and land-use changes on water resources.  相似文献   
274.
275.
276.
277.
278.
279.
280.
Social insects use complex chemical communication systems to govern many aspects of their life. We studied chemical changes in Dufours gland secretions associated with ovary development in several genotypes of honeybees. We found that C28–C38 esters were associated only with cavity nesting honeybee queens, while the alcohol eicosenol was associated only with their non-laying workers. In contrast, both egg-laying anarchistic workers and all parasitic Cape workers from queenright colonies showed the typical queen pattern (i.e. esters present and eicosenol absent), while egg-laying wild-type and anarchistic workers in queenless colonies showed an intermediate pattern, producing both esters and eicosenol but at intermediate levels. Furthermore, neither esters nor eicosenol were found in aerial nesting honeybee species. Both esters and eicosenol are biosynthetically similar compounds since both are recognizable products of fatty acid biosynthesis. Therefore, we propose that in honeybees the biosynthesis of esters and eicosenol in the Dufours gland is caste-regulated and this pathway has been conserved over evolutionary time.  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