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   535篇
  免费   25篇
  国内免费   1篇
测绘学   8篇
大气科学   53篇
地球物理   110篇
地质学   203篇
海洋学   60篇
天文学   60篇
综合类   1篇
自然地理   66篇
  2023年   1篇
  2022年   2篇
  2021年   5篇
  2020年   5篇
  2019年   13篇
  2018年   15篇
  2017年   15篇
  2016年   19篇
  2015年   19篇
  2014年   30篇
  2013年   43篇
  2012年   31篇
  2011年   33篇
  2010年   38篇
  2009年   22篇
  2008年   31篇
  2007年   26篇
  2006年   25篇
  2005年   27篇
  2004年   17篇
  2003年   15篇
  2002年   17篇
  2001年   9篇
  2000年   10篇
  1999年   9篇
  1998年   4篇
  1997年   8篇
  1996年   4篇
  1995年   7篇
  1994年   4篇
  1993年   7篇
  1992年   4篇
  1991年   3篇
  1990年   4篇
  1989年   4篇
  1988年   2篇
  1986年   1篇
  1985年   9篇
  1984年   3篇
  1983年   1篇
  1982年   3篇
  1981年   4篇
  1979年   3篇
  1978年   1篇
  1977年   1篇
  1975年   1篇
  1974年   1篇
  1973年   3篇
  1971年   1篇
  1934年   1篇
排序方式: 共有561条查询结果,搜索用时 31 毫秒
221.
Responding to the threat of climate change, conserving freshwater ecosystems and securing adequate energy and water supplies are among the greatest challenges facing modern societies. Yet recognition of the interdependencies between climate, energy and water policy—with resulting synergies and trade-offs—remains limited, leaving societies and governments alike vulnerable to the dangers of conflicted or unintended policy outcomes from sectoral decisions. In this paper, we analyse current Australian climate, energy and water policies to identify the risks of perverse outcomes between the three policy sectors. In doing so we categorise the conflicts and synergies between particular energy generation, carbon sequestration and water supply policies to improve understandings of the challenges facing decision makers in Australia and internationally. Four types of interventions are identified that would enable integration and optimisation of policies, namely: better cross-sectoral knowledge to inform decisions; the identification of technologies with co-benefits; markets with broader cross-sectoral participation (including linking water and carbon markets); and better-integrated governance institutions.  相似文献   
222.
223.
Satellite-measured along-track and gridded sea surface height (SSH) anomaly products from AVISO are compared with in situ SSH anomaly measurements from an array of 43 pressure-recording inverted echo sounders (PIESs) in the Kuroshio Extension. PIESs measure bottom pressure (P bot) and round-trip acoustic travel time from the sea floor to the sea surface (τ). The P bot and τ measurements are used to estimate, respectively, the mass-loading and steric height variations in SSH anomaly. All comparisons are made after accurate removal of tidal components from all data. Overall good correlations are found between along-track and PIES-derived SSH anomalies with mean correlation coefficient of 0.97. Comparisons between the two measurements reveal that the mass-loading component estimated from P bot is relatively small in this geographical region. It improves regression coefficients about 5?% and decreases mean root-mean-squared (rms) differences from 7.8 to 6.4?cm. The AVISO up-to-date gridded product, which merges all available satellite measurements of Jason-1, Envisat, Geosat Follow-On, and TOPEX/Poseidon interlaced, shows better correlations and smaller rms differences than the AVISO reference gridded product, which merges only Jason-1 and Envisat. Especially, the up-to-date gridded product reveals 6.8?cm rms improvement on average at sites away from Jason-1 ground tracks. Gridded products exhibit low correlation (0.75–0.9) with PIES-derived SSH in a subregion where the SSH fluctuations have relatively high energy at periods shorter than 20?days.  相似文献   
224.
The 2010-2011 wet season was one of extreme weather for the State of Queensland, Australia. Major rivers adjacent to the Great Barrier Reef (GBR) were discharging at rates 1.5 to >3 times higher than their long term median. Exposure to photosystem II herbicides has been routinely monitored over a period of up to 5 years at 12 inshore GBR sites. The influence of this wet season on exposure to photosystem II herbicides was examined in the context of this long-term monitoring record and during flood plume events in specific regions. Median exposures expressed as diuron equivalent concentration were an average factor of 2.3 times higher but mostly not significantly different (p<0.05) to the median for the long-term monitoring record. The herbicides metolachlor and tebuthiuron were frequently detected in flood plume waters at concentrations that reached or exceeded relevant water quality guidelines (by up to 4.5 times).  相似文献   
225.
Declining sand inputs to a channel with bimodal bed sediment can lead to degradation, armoring, and reduced bedload transport rates. Where sand loading is episodic, channels may alternate between high‐sand and low‐sand conditions, with ensuing responses in bed texture and bedload transport rates. The effects of episodic sand loading are explored through flow, grain size, and bedload transport measurements on the Pasig‐Potrero River, a sediment‐rich channel draining Mount Pinatubo, Philippines. Sand loading on the Pasig‐Potrero River is highly seasonal, and channel adjustments between seasons are dramatic. In the rainy season, inputs from sand‐rich 1991 eruption deposits lead to active, sand‐bedded, braided channels. In the dry season, many precipitation‐driven sand sources are cut off, leading to incision, armoring, and significantly lower bedload transport rates. This seasonal transition offers an excellent opportunity to examine models of degradation, incision, and armoring as well as the effectiveness of sediment transport models that explicitly encapsulate the importance of sand on transport rates. During the fall 2009 seasonal transition, 7·6 km of channel incised and armored, carving a 2–3 m deep channel on the upper alluvial fan. Bedload transport rates measured in the August 2009 rainy season were over four orders of magnitude greater than gravel‐bedded dry‐season channels surveyed in January 2010, despite having similar shear stress and unit discharge conditions. Within dry‐season incised channels, bed armoring is rapid, leading to an abrupt gravel‐sand transition. Bedload transport rates adjust more slowly, creating a lag between armoring and commensurate reductions in transport. Seasonal channel incision occurred in steps, aided by lateral migration into sand‐rich banks. These lateral sand inputs may increase armor layer mobility, renewing incision, and forming terraces within the incised seasonal channel. The seasonal incised channel is currently being reset by precipitation‐driven sand loading during the next rainy season, and the cycle begins again. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
226.
As defined by the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), a surface washing agent (SWA) is a product that removes oil from solid surfaces, such as beaches, rocks, and concrete, through a detergency mechanism and that does not involve dispersing or solubilizing the oil into the water column. Commercial products require testing to qualify for listing on the NCP Product Schedule. Such testing is conducted both for toxicity and effectiveness. Protocols currently exist for bioremediation agents and dispersants, but not SWAs. The US Environmental Protection Agency (EPA) is developing a laboratory testing protocol to evaluate the effectiveness of SWAs in removing crude oil from a solid substrate. This paper summarizes some of the defining research supporting this new protocol. Multiple variables were tested to determine their effect on SWA performance. The protocol was most sensitive to SWA-to-oil ratio and rotational speed of mixing. Less sensitive variables were contact time, mixing time, and SWA concentration when total applied mass of active product was constant. EPA recommendations for the testing protocol will be made following round robin testing.  相似文献   
227.
Settled particles of fresh, labile organic matter may be a significant source of oxygen demand and nutrient regeneration in seasonally-hypoxic regions caused by nutrient inputs into stratified coastal zones. Studying the dynamics of this material requires sediment sampling methods that include flocculent organic materials and overlying water (OLW) at or above the sediment–water interface (SWI). A new coring device (“HYPOX” corer) was evaluated for examining nitrogen- (N) and oxygen-dynamics at the SWI and OLW in the northern Gulf of Mexico (NGOMEX). The HYPOX corer consists of a “Coring Head” with a check-valve, a weighted “Drive Unit,” and a “Lander,” constructed from inexpensive components. The corer collected undisturbed sediment cores and OLW from sediments at NGOMEX sampling sites with underlying substrates ranging from sand to dense clay. The HYPOX corer could be deployed in weather conditions similar to those needed for a multi-bottle rosette water-sampling system with 20 L bottles. As an example of corer applicability to NGOMEX issues, NH4+ cycling rates were examined at hypoxic and control sites by isotope dilution experiments. The objective was to determine if N-dynamics in OLW were different from those in the water column. “Ammonium demand,” as reflected by potential NH4+ uptake rates, was higher in OLW than in waters collected from a meter or more above the bottom at both sites, but the pattern was more pronounced at the hypoxia site. By contrast, NH4+ regeneration rates were low in all samples. These preliminary results suggest that heterotrophic activity and oxygen consumption in OLW in the hypoxic region may be regulated by the availability of NH4+, or other reduced N compounds, rather than by the lack of sufficient labile organic carbon.  相似文献   
228.
Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) brown dwarfs which are amongst the oldest objects in the Universe. Despite this diversity, solar system planets, extrasolar planets and brown dwarfs have broadly similar global temperatures between 300 and 2500 K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emissions. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emissions that potentially originate from accelerated electrons on brown dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation.  相似文献   
229.
Water security: Debating an emerging paradigm   总被引:2,自引:0,他引:2  
This paper presents a comprehensive review of the concept of water security, including both academic and policy literatures. The analysis indicates that the use of the term water security has increased significantly in the past decade, across multiple disciplines. The paper presents a comparison of definitions of, and analytical approaches to, water security across the natural and social sciences, which indicates that distinct, and at times incommensurable, methods and scales of analysis are being used. We consider the advantages and disadvantages of narrow versus broad and integrative framings of water security, and explore their utility with reference to integrated water resources management. In conclusion, we argue that an integrative approach to water security brings issues of good governance to the fore, and thus holds promise as a new approach to water management.  相似文献   
230.
There is now a substantial body of sociocultural research that has investigated the ways in which specific communities living in physical proximity with a variety of polluting or hazardous technological installations experience and respond to their exposure to the associated risk. Much of this research has sought to understand the apparent acceptance or acquiescence displayed by local populations towards established hazards of the kind that are typically resisted when the subject of siting proposals. However, recent theoretical contributions, produced largely outside the field of risk research, have problematised the objective distinction between proximity and distance. In this paper we explore the potential of some of these ideas for furthering our understanding of the relationship between place and the constitution of risk subjectivities. To do this we re-examine a number of existing sociocultural studies that are predicated on a localised approach and conceptualise the relationship of physically proximate sources of risk to everyday experience in terms of practices of ‘presencing’ and ‘absencing’. We conclude with some thoughts on the methodological and substantive implications of this reworking of proximity for future research into risk subjectivities.  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