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   338篇
  免费   15篇
  国内免费   2篇
测绘学   7篇
大气科学   17篇
地球物理   72篇
地质学   152篇
海洋学   34篇
天文学   26篇
自然地理   47篇
  2021年   9篇
  2020年   11篇
  2019年   8篇
  2018年   12篇
  2017年   17篇
  2016年   9篇
  2015年   7篇
  2014年   11篇
  2013年   17篇
  2012年   17篇
  2011年   19篇
  2010年   15篇
  2009年   12篇
  2008年   12篇
  2007年   17篇
  2006年   12篇
  2005年   7篇
  2004年   14篇
  2003年   6篇
  2002年   7篇
  2001年   8篇
  2000年   13篇
  1999年   5篇
  1998年   7篇
  1997年   3篇
  1996年   5篇
  1995年   5篇
  1994年   4篇
  1993年   4篇
  1992年   3篇
  1991年   3篇
  1990年   5篇
  1988年   5篇
  1987年   4篇
  1986年   1篇
  1985年   4篇
  1984年   5篇
  1983年   5篇
  1982年   2篇
  1981年   4篇
  1979年   2篇
  1978年   4篇
  1977年   1篇
  1976年   1篇
  1975年   2篇
  1974年   2篇
  1973年   2篇
  1972年   1篇
  1968年   2篇
  1938年   1篇
排序方式: 共有355条查询结果,搜索用时 296 毫秒
81.
82.
Treatments of land surface processes in General Circulation Models are presently limited by the realism of the simulations of precipitation and surface radiation. We explore this thesis by examination of some of the climatological fields of a 6-year model simulation, using the Community Climate Model version 1 of the National Center for Atmospheric Research with addition of a diurnal cycle and coupled to a detailed treatment of land surface processes referred to as the Biosphere-Atmosphere Transfer Scheme. We examine July climatological surface fields over North America and note an excess of surface solar radiation over Eastern United States. Comparison with satellite derived cloud forcing suggests that the model underestimates the reduction of solar radiation by clouds over Eastern United States and in high latitudes, and so probably largely explaining the excess surface radiation. We consider the annual cycle of model hydrological fields (soil moisture, runoff, precipitation, evapotranspiration, net radiation) averaged over a box covering the central part of the United States (roughtly the Mississippi basin). The seasonal cycle of evapotranspiration over this box appears to be dominated by the variation of surface solar radiation and less related to that of precipitation.  相似文献   
83.
Crustal neon: a striking uniformity   总被引:1,自引:0,他引:1  
By combining data from a diverse suite of crustal fluid samples representing a broad geographical distribution, we have identified a well-defined nucleogenic (crustal) neon component. The neon is produced from (α, n) and (n, α) nuclear interactions involving nuclei of O, Mg, and F [1]. In the limiting case of 20Ne/22Ne = 0, the composition is: 21Ne/22Ne = 0.47 ± 0.01 and 21Ne/4He = (0.46 ± 0.08) × 10−7. A crustal O/F ratio of 110 (atomic) calculated from the 21Ne/22Ne ratio is 4–10 times less than the average crustal O/F ratio. The discrepancy can be accounted for by an enhanced O/F ratio within the 10–40 μm range of the U-Th-generated α-particles.  相似文献   
84.
Methane-hydrogen gas seeps with mantle-like C and noble gas isotopic characteristics issue from partially serpentinized ultramafic rocks in the Zambales ophiolite, Philippines. New measurements of noble gas and 14C isotope abundances, rock/mixed-volatile equilibrium calculations, and previous chemical and isotopic data suggest that these reduced gases are products of periodotite hydration. The gas seeps are produced in rock-dominated zones of serpentinization, and similar gases may be ubiquitous in ultramafic terranes undergoing serpentinization.  相似文献   
85.
Summary Data from two multiparameter radars are used to diagnose some microphysical characteristics of intense convective storms, in particular, the 24 June 1992 case near Fort Collins and Greeley, Colorado. Dual-polarization and dual-frequency radar measurements from the CSU-CHILL and NCAR/CP-2 radars provided the basis for microphysical interpretations. Supporting in-situ measurements were provided by several T-28 aircraft penetrations of updraft regions.Limited dual-Doppler synthesis as well as surface mesoscale features showed persistent regions of convergence and advection of moist air along the northeast side of the storm complex. The Fort Collins storm was analyzed in detail over its duration including an intercomparison of rainfall rates from raingage and as deduced from specific differential phase and attenuation measurements. Vertical sections of radar data taken parallel and perpendicular to the surface convergence axis showed interesting features such as positiveZ dr and attenuation columns with an LDR cap on the inflow side. Such columns provide evidence of the important role of warm cloud processes in this storm. NCAR/CP-2 radar data from a multi-cellular storm in central Florida are also analyzed as a contrast to the 24 June Colorado case.With 23 Figures  相似文献   
86.
Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8?‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to ?12.39?‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as ?12.4?‰) and some of the most highly spatially variable (values varied over 15.4?‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.  相似文献   
87.
The Caribbean subspecies of the Red‐tailed Hawk (Buteo jamaicensis solitudinus or B.j. jamaicensis) is the largest native carnivore on the island of La Gonave, Haiti, and the island's apex predator. On other Caribbean islands, raptors have been persecuted by local people typically because of their depredation of livestock. In addition to possible persecution from humans, Red‐tailed Hawks on La Gonave face challenges from island‐wide timber overharvest and rapid land‐cover changes. To better understand the human‐apex predator relationship on La Gonave, we surveyed islanders about their perceptions of, and interactions with, the Red‐tailed Hawk. According to 121 respondents from 10 villages, La Gonavans do not hunt, kill, or consume Red‐tailed Hawks. Our study revealed a notable absence of negative perceptions of the hawk by respondents despite intense hawk depredation of domestic chickens. While the chicken depredation may lead outside observers to consider the human‐hawk relationship on La Gonave as representative of a human‐wildlife conflict, our interview data do not support that conception. Our findings reflect an important conversation in related contemporary scientific literature about what constitutes human‐wildlife conflicts across various cultures. Despite tolerance by residents, Red‐tailed Hawks on La Gonave remain threatened by habitat degradation.  相似文献   
88.
89.
The Neogene and Quaternary sediments of the Faeroe-Shetland Channel and West Shetland shelf and slope rest upon a major regional unconformity, the Latest Oligocene Unconformity (LOU), and have been deposited through the interaction of downslope and parallel-to-slope depositional processes. The upper to middle continental slope is dominated by mass-transport deposits (debris flows), which progressively diminish downslope, and were largely generated and deposited during glacial cycles when ice sheets supplied large quantities of terrigeneous sediment to the upper slope and icebergs scoured sea-floor sediments on the outer shelf and uppermost slope. Large-scale sediment failures have also occurred on the upper slope and resulted in deposition of thick, regionally extensive mass-transport deposits on portions of the lower slope and channel floor. In contrast, large fields of migrating sediment waves and drift deposits dominate most of the middle to lower slope below 700 m water depth and represent deposition by strong contour currents of the various water masses moving northeastward and southwestward through the channel. These migrating sediment waves indicate strong northeastward current flow at water depths shallower than 700 m and strong southwestward current flow at water depths from 700 to >1,400 m. These flow directions are consistent with present-day water-mass flow through the Faeroe-Shetland Channel. The Faeroe-Shetland Channel floor is underlain by thin conformable sediments that appear to be predominantly glacial marine and hemipelagic with less common turbidites and debris flows. No evidence is observed in seismic or core data that indicates strong contour-current erosion or redistribution of sediments along the channel floor.  相似文献   
90.
ABSTRACT

Work Integration Social Enterprises (WISE) offer supported work environments for people experiencing disadvantage, including people with disability. This paper reflects on a research project that is mapping the ways in which social enterprises in regional Australian cities produce wellbeing for their employees. Through supported employment programs, these organisations are transforming individual lives, helping to build a sense of self-worth and purpose, and increase individuals’ social skills and capacities. Many of the perceived benefits are associated with a belief in the innate good of paid work. Based upon interviews with employees in the social enterprises, it is apparent that the hopes attached to providing ‘meaningful work’ are considerable. At the same time social enterprises are taking on social responsibilities that were once provided by the state, while also selling their services as employers of disabled and disadvantaged members of society and maintaining commercial viability of the enterprise. Using feminist political economy alongside discussion of the distributive economy, this paper explores how contemporary policy conditions, productivist biases and normative discourses about the value of wage work inflect the forms of wellbeing that can be experienced by WISE workers.  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