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   112篇
  免费   3篇
大气科学   12篇
地球物理   15篇
地质学   27篇
海洋学   12篇
天文学   33篇
自然地理   16篇
  2023年   1篇
  2022年   1篇
  2021年   2篇
  2018年   2篇
  2017年   1篇
  2016年   1篇
  2015年   2篇
  2014年   1篇
  2013年   4篇
  2012年   2篇
  2011年   4篇
  2010年   3篇
  2009年   7篇
  2008年   2篇
  2007年   7篇
  2006年   2篇
  2005年   5篇
  2004年   6篇
  2003年   4篇
  2002年   6篇
  2001年   4篇
  2000年   4篇
  1999年   4篇
  1998年   4篇
  1997年   1篇
  1996年   3篇
  1995年   2篇
  1994年   1篇
  1993年   2篇
  1991年   2篇
  1990年   1篇
  1989年   2篇
  1987年   2篇
  1986年   2篇
  1984年   1篇
  1983年   3篇
  1982年   4篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1977年   3篇
  1973年   1篇
  1972年   1篇
  1969年   1篇
  1914年   1篇
排序方式: 共有115条查询结果,搜索用时 46 毫秒
91.
Natural resource management approaches that deliver biodiversity conservation remain elusive, with evidence of a persistent implementation gap between biodiversity science and conservation projects. Scenarios have been identified as potentially useful in addressing the complex issues underlying this implementation gap, but have been infrequently applied to biodiversity conservation. Our paper reports on action co-research to develop, apply and assess the efficacy of scenarios within a community-based natural resource management (CBNRM) approach to biodiversity conservation at Mission Beach, a key site within the globally significant Wet Tropics bioregion. We focused on the capacity of scenarios to address the issues of contested interests and uncertainty, aiming specifically to engage the community to build a cohesive vision. The scenarios' headline messages included a projected substantial loss of habitat in coastal vegetation communities that are highly valued by all stakeholders. Our assessment identified that the use of scenarios fulfilled the intended aims, resulting in a vision for biodiversity conservation that has substantial community support. Three factors contributed to this efficacy of the scenarios: (1) the focus on threat; (2) biodiversity science integration; and (3) simplicity in presentation. Further investigation of the potential of scenarios as tools to overcome the implementation gap in biodiversity conservation is recommended.  相似文献   
92.
93.
A dated landscape history of the Allt nan Uamh valley in the Assynt area is constructed, spanning the last 300 ka, using geomorphological analysis, U-series speleothem dating, and existing cave surveys. The mean rate of valley deepening is estimated to lie between 47 and 68 m per glacial/interglacial cycle of 100 ka. This, combined with an estimated duration of glaciation, implies glacial erosion rates of about 2 mm a−1, in agreement with modern process measurements. © 1997 by John Wiley & Sons, Ltd.  相似文献   
94.
95.
Bindheimite has been identified in specimens from the Lvov Lease, N.W. Queensland. Chemical, microscopic and X‐ray data are given for the mineral which was found to possess a cubic lattice with a unit cell dimension of 10–427 A.  相似文献   
96.
We report the outcome of the deep optical/infrared photometric survey of the central region (33 × 33 arcmin2 or 0.9 pc2) of the η Chamaeleontis (η Cha) pre-main sequence star cluster. The completeness limits of the photometry are I = 19.1, J = 18.2 and H = 17.6, faint enough to reveal low-mass members down to the brown dwarf and planet boundary of ≈13 MJup. We found no such low-mass members in this region. Our result combined with a previous shallower ( I = 17) but larger area survey indicates that low-mass objects  (0.013 < M /M < 0.075)  were either not created in the η Cha cluster or lost due to the early dynamical history of the cluster and ejected to outside the surveyed areas.  相似文献   
97.
A bulk micromegas micropattern charge readout device has, for the first time, been operated at room temperature in low pressure carbon disulphide vapour. This is a key step opening prospects for use of micromegas readout for large volume negative ion time projection chambers (TPCs) without magnets, such as proposed for directional dark matter detectors and other rare event applications. The dependence of the gain on the amplification field, pressure and drift field has been evaluated. For the available gap size of 75 μm a maximum gain of 1300 ± 120 was achieved in 40 torr vapour with an energy resolution of 22% for 5.9 keV 55Fe X-rays. From a fit to the data, the Townsend coefficient gas parameters A and B have been derived. Operation has also been successfully achieved in xenon:carbon disulphide blends over a range of partial and total pressures. A gain of 890 ± 130 at an energy resolution of 35% has been recorded for a 1:1 blend at a total pressure of 80 torr. Possible improvements are discussed in the context of operation in directional dark matter TPCs as a replacement for multi-wire proportional counters.  相似文献   
98.
Cold, dense clouds of gas have been proposed to explain the dark matter in Galactic haloes, and have also been invoked in the Galactic disc as an explanation for the excess faint submillimetre sources detected by SCUBA. Even if their dust-to-gas ratio is only a small percentage of that in conventional gas clouds, these dense systems would be opaque to visible radiation. We examine the possibility that the data sets of microlensing experiments searching for massive compact halo objects can also be used to search for occultation signatures by such clouds. We compute the rate and time-scale distribution of stellar transits by clouds in the Galactic disc and halo. We find that, for cloud parameters typically advocated by theoretical models, thousands of transit events should already exist within microlensing survey data sets. We examine the seasonal modulation in the rate caused by the Earth's orbital motion and find it provides an excellent probe of whether detected clouds are of disc or halo origin.  相似文献   
99.
This paper presents side-by-side comparisons of blowcount values for the Texas cone penetration (TCP) test and the standard penetration test (SPT). The comparisons yielded statistically-significant regression models for both coarse-grained soils and fine-grained soils. Consistent with expected trends and published data, the TCP–SPT relationship is nonlinear, with weak to fair correlation strength (R2 = 23–44%). For TCP blowcounts (N60, TCP) varying from 25 to 200 blows/30 cm (1 ft), corresponding SPT blowcounts (N60, SPT) are typically 30–60% lower than N60, TCP in fine-grained soils. Likewise, corresponding N60, SPT blowcounts are 10–70% lower than N60, TCP in coarse-grained soils, all other things being equal. Comparative data were obtained from published sources and from project-specific field research sites used for full-scale deep foundation load tests. The final dataset consisted of 225 test pairs obtained in similar soils and geomaterials, at equivalent depths, with all blowcounts normalized to 30 cm (12 in.) penetration (i.e., blows/30 cm or blows/ft) within the bounds of typical test precision, and corrected to 60% hammer efficiency. The generally weak correlations do not support conversion of N60, TCP to N60, SPT (or vice versa) to compute foundation capacity for final design. But, engineers can certainly get an intuitive feel about site conditions and preliminary foundation capacity by using the correlation equations to translate their knowledge of one test to the other. This study extends previous work by formally comparing and contrasting the similar yet different SPT and TCP test methods in such a way as to make the results useful to users of both tests and to the broader geotechnical engineering community.  相似文献   
100.
The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity (K) has not previously been measured in neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon: Quistococha, San Jorge and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s?1, and at 90 cm, it varies between 0.00027 and 0.057 cm s?1. Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for ~20% of the within‐site variance and that depth is a good predictor of K. However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that ~18% of the variance in the K data can be explained by between‐site differences. Simulations using a simple hydrological model suggest that the relatively high K values could lead to lowering of the water table by >10 cm within ~48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K, peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration. Copyright © 2013 John Wiley & Sons, Ltd.  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