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   71篇
  免费   2篇
测绘学   1篇
大气科学   5篇
地球物理   23篇
地质学   26篇
海洋学   11篇
天文学   5篇
自然地理   2篇
  2019年   1篇
  2018年   1篇
  2017年   1篇
  2015年   2篇
  2014年   1篇
  2013年   3篇
  2012年   3篇
  2011年   2篇
  2010年   5篇
  2009年   5篇
  2008年   2篇
  2007年   3篇
  2006年   4篇
  2005年   3篇
  2004年   4篇
  2003年   3篇
  2001年   2篇
  2000年   2篇
  1999年   2篇
  1997年   2篇
  1996年   3篇
  1995年   1篇
  1994年   1篇
  1993年   2篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   3篇
  1983年   2篇
  1982年   2篇
  1974年   1篇
  1966年   1篇
  1965年   1篇
排序方式: 共有73条查询结果,搜索用时 343 毫秒
1.
2.
3.
The majority of emissions of nitrous oxide – a potent greenhouse gas (GHG) – are from agricultural sources, particularly nitrogen fertilizer applications. A growing focus on these emission sources has led to the development in the United States of GHG offset protocols that could enable payment to farmers for reducing fertilizer use or implementing other nitrogen management strategies. Despite the development of several protocols, the current regional scope is narrow, adoption by farmers is low, and policy implementation of protocols has a significant time lag. Here we utilize existing research and policy structures to propose an ‘umbrella’ approach for nitrogen management GHG emissions protocols that has the potential to streamline the policy implementation and acceptance of such protocols. We suggest that the umbrella protocol could set forth standard definitions common across multiple protocol options, and then modules could be further developed as scientific evidence advances. Modules could be developed for specific crops, regions, and practices. We identify a policy process that could facilitate this development in concert with emerging scientific research and conclude by acknowledging potential benefits and limitations of the approach.

Key policy insights

  • Agricultural greenhouse gas market options are growing, but are still underutilized

  • Streamlining protocol development through an umbrella process could enable quicker development of protocols across new crops, regions, and practices

  • Effective protocol development must not compromise best available science and should follow a rigorous pathway to ensure appropriate implementation

  相似文献   
4.
5.
6.
Ground water flow and travel time are dependent on stratigraphic architecture, which is governed by competing processes that control the spatial and temporal distribution of accommodation and sediment supply. Accommodation is the amount of space in which sediment may accumulate as defined by the difference between the energy gradient and the topographic surface. The temporal and spatial distribution of accommodation is affected by processes that change the distribution of energy (e.g., sea level or subsidence). Fluvial stratigraphic units, generated by FLUVSIM (a stratigraphic simulator based on accommodation and sediment supply), with varying magnitudes and causes of accommodation, were incorporated into a hydraulic regime using MODFLOW (a ground water flow simulator), and particles were tracked using MODPATH (a particle-tracking algorithm). These experiments illustrate that the dominant type of accommodation process influences the degree of continuity of stratigraphic units and thus affects ground water flow and transport. When the hydraulic gradient is parallel to the axis of the fluvial system in the depositional environment, shorter travel times occur in low-total accommodation environments and longer travel times in high-total accommodation environments. Given the same total accommodation, travel times are longer when sea-level change is the dominant process than those in systems dominated by subsidence.  相似文献   
7.
Ogallala gravels are an important lithic material source for stone tool manufacture across the Great Plains. The lithology of eight gravel sample locations along the eastern escarpment of the Southern High Plains has been examined to quantify the composition and variability in Ogallala deposits. In addition, three Ogallala gravel quarry sites have been analyzed to determine how the lithology of Ogallala gravels influenced procurement and tool production strategies. Results indicate deposits of Ogallala gravels differ in their composition related to their position on the landscape, and the types of rocks vary by size and shape. Investigation of the quarry sites has revealed that the lithology of Ogallala gravels had an impact on testing and the initial stages of lithic reduction. Different rock types, however, may have more of an influence on later stages of tool manufacture at nearby campsites and other locations where Ogallala gravels are not as dense. © 2009 Wiley Periodicals, Inc.  相似文献   
8.
Willapa Bay is a large, economically and ecologically important estuary on the Washington coast, USA for which the zooplankton community has not previously been studied. Thus, in 2006 and 2007, six stations within Willapa Bay were sampled biweekly for macrozooplankton, chlorophyll, and various abiotic variables to elucidate the processes underlying community composition and dynamics. Non-metric multidimensional scaling identified water temperature and upwelling values as major factors defining two distinct temporal communities. High densities and a community dominated by oceanic species (Calanus pacificus, Centropages abdominalis) marked the winter season, while summer (or the upwelling season) was dominated by estuarine species (Palaemonidae, Clevelandia ios). Smaller scale changes in the community were characterized by variation in chlorophyll a concentration and salinity and were marked by the presence of other taxa (Neotrypaea californiensis, Mysidae). These results point to the importance of physical processes, including the import of marine organisms and retention of estuarine organisms, in the structuring of the macrozooplankton community in Willapa Bay.  相似文献   
9.
The Las Campanas Observatory and Anglo-Australian Telescope Rich Cluster Survey (LARCS) is a panoramic imaging and spectroscopic survey of an X-ray luminosity-selected sample of 21 clusters of galaxies at     . Charge-coupled device (CCD) imaging was obtained in B and R of typically 2° wide regions centred on the 21 clusters, and the galaxy sample selected from the imaging is being used for an on-going spectroscopic survey of the clusters with the 2dF spectrograph on the Anglo-Australian Telescope. This paper presents the reduction of the imaging data and the photometric analysis used in the survey. Based on an overlapping area of 12.3 deg2 we compare the CCD-based LARCS catalogue with the photographic-based galaxy catalogue used for the input to the 2dF Galaxy Redshift Survey (2dFGRS) from the APM, to the completeness of the GRS/APM catalogue,     . This comparison confirms the reliability of the photometry across our mosaics and between the clusters in our survey. This comparison also provides useful information concerning the properties of the GRS/APM. The stellar contamination in the GRS/APM galaxy catalogue is confirmed as around     per cent, as originally estimated. However, using the superior sensitivity and spatial resolution in the LARCS survey evidence is found for four distinct populations of galaxies that are systematically omitted from the GRS/APM catalogue. The characteristics of the 'missing' galaxy populations are described, reasons for their absence examined and the impact they will have on the conclusions drawn from the 2dF Galaxy Redshift Survey are discussed.  相似文献   
10.
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