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   160篇
  免费   3篇
测绘学   1篇
大气科学   11篇
地球物理   30篇
地质学   53篇
海洋学   14篇
天文学   19篇
自然地理   35篇
  2020年   3篇
  2016年   2篇
  2015年   2篇
  2014年   3篇
  2013年   9篇
  2012年   7篇
  2010年   6篇
  2009年   6篇
  2008年   4篇
  2007年   2篇
  2006年   3篇
  2005年   3篇
  2004年   4篇
  2003年   8篇
  2002年   3篇
  2001年   7篇
  2000年   7篇
  1999年   2篇
  1998年   5篇
  1997年   4篇
  1994年   3篇
  1993年   3篇
  1990年   1篇
  1989年   2篇
  1988年   2篇
  1987年   1篇
  1986年   2篇
  1985年   2篇
  1984年   2篇
  1983年   3篇
  1982年   4篇
  1981年   1篇
  1980年   2篇
  1979年   3篇
  1978年   2篇
  1977年   4篇
  1976年   1篇
  1975年   2篇
  1974年   3篇
  1973年   3篇
  1957年   1篇
  1954年   1篇
  1925年   2篇
  1910年   6篇
  1908年   4篇
  1905年   2篇
  1899年   1篇
  1898年   3篇
  1895年   1篇
  1875年   1篇
排序方式: 共有163条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
45.
A pilot cropland carbon sequestration program within north central Montana has allowed farmers to receive carbon credit for management adjustments associated with changing from tillage-based agricultural systems to no-till. Carbon credit can also be obtained by adopting conservation reserve, where cropland is planted into perennial vegetation. Summer fallowing is also considered within the crediting process as credit is not given in years that a field is left un-vegetated. The carbon sequestration program has been advocated as a means to mitigate climate change while providing an added source of income for Montana farmers. There is lack of data, however, pertaining to the percentage of lands within this region that have not converted to no-till management, lands under certain crop intensities (e.g. those that are cropped every growing season vs. those that use a fallow-crop-fallow system), or cropland that have converted to perennial vegetation outside of the popular Conservation Reserve Program. Data is also sparse concerning the amount of soil organic carbon that might be sequestered given a conversion to no-till or conservation reserve. This study established regional percentage estimates of cropland under no-till, various degrees of crop intensity, and conservation reserve within north central Montana. Literature-based carbon sequestration estimates were used to generate carbon gain data associated with the conversation to no-till and to conservation reserve. These estimates were then applied to the area-based cropland statistics to estimate potential regional carbon sequestration associated with these management changes.  相似文献   
46.
47.
This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of these recirculations gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program (June 2004–June 2006) and include current meters, pressure and current recording inverted echo sounders, and subsurface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114 Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio's transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current's transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.  相似文献   
48.
49.
Balloon observations of the X-ray source Sco X-1 carried out in November 1978 have revealed a thermal spectrum withkT?7 keV in the 20–60 keV energy band. In addition, there was evidence of a high energy component, possibly variable, above ~50 keV. The spectral form of this component could not be determined but was hard with a 60 keV flux of ~10?4 photons (keV cm2 s)?1.  相似文献   
50.
Understanding ecosystem processes from a functional point of view is essential to study relationships among climate variability, biogeochemical cycles, and surface-atmosphere interactions. Increasingly during the last decades, the eddy covariance (EC) method has been applied in terrestrial, marine and urban ecosystems to quantify fluxes of greenhouse gases (e.g., CO2, H2O) and energy (e.g., sensible and latent heat). Networks of EC systems have been established in different regions and have provided scientific information that has been used for designing environmental and adaptation policies. In this context, this article outlines the conceptual and technical framework for the establishment of an EC regional network (i.e., MexFlux) to measure the surface-atmosphere exchange of heat and greenhouse gases in Mexico. The goal of the network is to improve our understanding of how climate variability and environmental change influence the dynamics of Mexican ecosystems. First, we discuss the relevance of CO2 and water vapor exchange between terrestrial ecosystems and the atmosphere. Second, we briefly describe the EC basis and present examples of measurements in terrestrial and urban ecosystems of Mexico. Finally, we describe the conceptual and operational goals at short-, medium-, and long-term scales for continuity of the MexFlux network.  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