全文获取类型
收费全文 | 4649篇 |
免费 | 180篇 |
国内免费 | 98篇 |
专业分类
测绘学 | 161篇 |
大气科学 | 365篇 |
地球物理 | 1198篇 |
地质学 | 1610篇 |
海洋学 | 407篇 |
天文学 | 768篇 |
综合类 | 23篇 |
自然地理 | 395篇 |
出版年
2022年 | 30篇 |
2021年 | 48篇 |
2020年 | 49篇 |
2019年 | 66篇 |
2018年 | 86篇 |
2017年 | 106篇 |
2016年 | 125篇 |
2015年 | 123篇 |
2014年 | 146篇 |
2013年 | 281篇 |
2012年 | 137篇 |
2011年 | 247篇 |
2010年 | 177篇 |
2009年 | 243篇 |
2008年 | 261篇 |
2007年 | 220篇 |
2006年 | 202篇 |
2005年 | 187篇 |
2004年 | 148篇 |
2003年 | 177篇 |
2002年 | 147篇 |
2001年 | 91篇 |
2000年 | 98篇 |
1999年 | 74篇 |
1998年 | 78篇 |
1997年 | 69篇 |
1996年 | 74篇 |
1995年 | 56篇 |
1994年 | 61篇 |
1993年 | 48篇 |
1992年 | 52篇 |
1991年 | 47篇 |
1990年 | 40篇 |
1989年 | 43篇 |
1988年 | 41篇 |
1987年 | 42篇 |
1986年 | 41篇 |
1985年 | 64篇 |
1984年 | 50篇 |
1983年 | 61篇 |
1982年 | 58篇 |
1981年 | 64篇 |
1980年 | 54篇 |
1979年 | 45篇 |
1978年 | 48篇 |
1977年 | 32篇 |
1976年 | 34篇 |
1975年 | 26篇 |
1974年 | 35篇 |
1973年 | 32篇 |
排序方式: 共有4927条查询结果,搜索用时 187 毫秒
91.
92.
A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data
collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine
typologies. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity
and composition, species abundance, nursery function, and trophic integrity. Metric reference conditions and scoring criteria
were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary
by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good).
A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively
differentiate between poor and good quality sites. Applying the EFCI to estuaries in which multiple samples were taken also
showed that the index is reproducible. The EFCI is both a robust and sensitive method for assessing the ecological condition
of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood
format for managers, policy makers, and the general public. 相似文献
93.
94.
The “region” and “regional change” have been elusive ideas within political and economic geography, and in essence require a greater understanding of their dynamic characteristics. Trailing in the backwaters of the devolution to the Celtic nations of Britain, the contemporary era of New Labour’s political-economic ideology, manifest through “third-way” governance in England places the region and its functional capacity into the heart of geographical inquiry. Drawing upon a new regionalist epistemology, this paper seeks to recover a sense of (regional) political economy through a critical investigation of the development and formulation of Blair’s “New Regional Policy” (NRP). I address how New Labour has attempted to marry economic regionalisation on the one hand, and democratic regionalism on the other. This paper specifically questions the wisdom of such a marriage of politically distinct ideologies through a critical investigation of the underlying contradictions of their strategy from both a theoretical and empirical standpoint. Demonstrated both in the North East “no” vote in 2004, and in the post-mortem undertaken by the ODPM Select Committee in 2005, the paper illustrates how a loss of political drive gradually undermined the capacity of devolution to deliver in England. Finally, I argue that through the lens of the NRP we can speculate on some of the wider issues and implications for the study of regional governance. 相似文献
95.
Janet M. Hergt L. Paul Bédard Etienne Deloule Klaus Peter Jochum Kathryn L. Linge Paul J. Sylvester Matthias Willbold Jon D. Woodhead 《Geostandards and Geoanalytical Research》2006,30(3):141-142
In 2005 Geostandards and Geoanalytical Research embarked upon a new initiative for its readers. Key researchers in various fields of geoanalytical technique development and their application were identified and invited to provide reviews pertinent to their expertise. As noted in the first of these publications "…instead of revisiting the historical context or decades of development in each analytical technique, the goal here has been to capture a snapshot of "hot topics" across a range of fields as represented in the… literature" (Hergt et al . 2005). Rather than prepare an annual review, a decision was taken earlier this year to provide a biennial summary of progress and accomplishments, in this case for the years 2004–2005. The principal techniques employed in Earth and environmental sciences are covered here, and include laser ablation and multicollector ICP-MS, ICP-AES, thermal ionisation and secondary ion mass spectrometry, as well as neutron activation analysis, X-ray fluorescence and atomic absorption spectrometry. A comprehensive review of the development of reference materials, often essential to these techniques, is also provided. The contributions assembled serve both to keep readers informed of advances they may be unfamiliar with, but also as a means of showcasing examples of the breadth and depth of work being conducted in these fields. 相似文献
96.
Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes 总被引:6,自引:0,他引:6
Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion. 相似文献
97.
98.
Complexity and emergence (another conversation) 总被引:1,自引:0,他引:1
99.
100.
Paul Augustinus Michael Reid Sofia Andersson Yanbin Deng Mark Horrocks 《Journal of Paleolimnology》2006,35(4):789-805
Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record
of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the
past c. 190 years. A reliable chronology is available from AMS 14C and 210Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of
introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope
geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke
catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent
nuisance algal blooms in 1933, 1934 and 1939 using CuSO4 addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the 210Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant
anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years.
The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually
allowed unambiguous identification of the signatures of change and their causes. 相似文献