全文获取类型
收费全文 | 2981篇 |
免费 | 79篇 |
国内免费 | 25篇 |
专业分类
测绘学 | 82篇 |
大气科学 | 244篇 |
地球物理 | 548篇 |
地质学 | 978篇 |
海洋学 | 300篇 |
天文学 | 695篇 |
综合类 | 5篇 |
自然地理 | 233篇 |
出版年
2021年 | 17篇 |
2020年 | 30篇 |
2019年 | 35篇 |
2018年 | 45篇 |
2017年 | 36篇 |
2016年 | 66篇 |
2015年 | 48篇 |
2014年 | 60篇 |
2013年 | 152篇 |
2012年 | 74篇 |
2011年 | 117篇 |
2010年 | 91篇 |
2009年 | 148篇 |
2008年 | 108篇 |
2007年 | 110篇 |
2006年 | 111篇 |
2005年 | 94篇 |
2004年 | 79篇 |
2003年 | 81篇 |
2002年 | 84篇 |
2001年 | 71篇 |
2000年 | 84篇 |
1999年 | 65篇 |
1998年 | 68篇 |
1997年 | 50篇 |
1996年 | 48篇 |
1995年 | 43篇 |
1994年 | 49篇 |
1993年 | 38篇 |
1992年 | 36篇 |
1991年 | 38篇 |
1990年 | 32篇 |
1989年 | 36篇 |
1988年 | 29篇 |
1987年 | 44篇 |
1986年 | 33篇 |
1985年 | 52篇 |
1984年 | 59篇 |
1983年 | 53篇 |
1982年 | 51篇 |
1981年 | 49篇 |
1980年 | 52篇 |
1979年 | 35篇 |
1978年 | 32篇 |
1977年 | 39篇 |
1976年 | 31篇 |
1975年 | 35篇 |
1973年 | 22篇 |
1972年 | 19篇 |
1971年 | 16篇 |
排序方式: 共有3085条查询结果,搜索用时 0 毫秒
51.
Nutrient enrichments and phytoplankton growth in the surface waters of the Louisiana Bight 总被引:1,自引:0,他引:1
Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities. 相似文献
52.
Elizabeth Holcombe Sarah Smith Edmund Wright Malcolm G. Anderson 《Natural Hazards》2012,61(2):351-385
Despite the recognition of the need for mitigation approaches to landslide risk in developing countries, the delivery of ‘on-the-ground’
measures is rarely undertaken. With respect to other ‘natural’ hazards, it is widely reported that mitigation can pay. However,
the lack of such an evidence base in relation to landslides in developing countries hinders advocacy amongst decision makers
for expenditure on ex-ante measures. This research addresses these limitations directly by developing and applying an integrated
risk assessment and cost–benefit analysis of physical landslide mitigation measures implemented in an unplanned community
in the Eastern Caribbean. In order to quantify the level of landslide risk reduction achieved, landslide hazard and vulnerability
were modelled (before and after the intervention), and project costs, direct and indirect benefits were monetised. It is shown
that the probability of landslide occurrence has been substantially reduced by implementing surface-water drainage measures
and that the benefits of the project outweigh the costs by a ratio of 2.7–1. This paper adds to the evidence base that ‘mitigation
pays’ with respect to landslide risk in the most vulnerable communities—thus strengthening the argument for ex-ante measures.
This integrated project evaluation methodology should be suitable for adoption as part of the community-based landslide mitigation
project cycle, and it is hoped that this resource, and the results of this study, will stimulate further such programmes. 相似文献
53.
Acta Geotechnica - The most common cause of slope instability is intense or sustained rainfall, which may induce reduction in soil suction, and thus, shear strength. Capillary barrier systems... 相似文献
54.
55.
We investigated spatial and temporal changes in spectral irradiance, phytoplankton community composition, and primary productivity in North Inlet Estuary, South Carolina, USA. High concentrations of colored dissolved organic matter (CDOM) were responsible for up to 84 % of the attenuation of photosynthetically available radiation (PAR). Green-yellow wavelengths were the predominant colors of light available at the two sampling sites: Clam Bank Creek and Oyster Landing. Vertical attenuation coefficients of PAR were 0.7–2.1 m?1 with corresponding euphotic zone depths of 1.5–6.7 m. Phytoplankton biomass (as chlorophyll a [chl a]) varied seasonally with a summer maximum of 16 μg chl a l?1 and a winter minimum of 1.4 μg chl a l?1. The phytoplankton community consisted mainly of diatoms, prasinophytes, cryptophytes and haptophytes, with diatoms and prasinophytes accounting for up to 67 % of total chl a. Changes in phytoplankton community composition showed strongest correlations with temperature. Light-saturated chl a-specific rates of photosynthesis and daily primary productivity varied with season and ranged from 1.6 to 14 mg C (mg chl a) ?1?h?1 (32–803 mg C m?3?day?1). Calculated daily rates added up to an annual carbon fixation rate of 84 g C m?3?year?1. Overall, changes in phytoplankton community composition and primary productivity in North Inlet showed a strong dependence on temperature, with PAR and spectral irradiance playing a relatively minor role due to short residence times, strong tidal forcing and vertical mixing. 相似文献
56.
The Berchtesgaden National Park (Bavaria, Germany): a platform for interdisciplinary catchment research 总被引:1,自引:1,他引:1
T. Marke U. Strasser G. Kraller M. Warscher H. Kunstmann H. Franz M. Vogel 《Environmental Earth Sciences》2013,69(2):679-694
The Berchtesgaden National Park (Bavaria, Germany), a study site of the UNESCO Man and the Biosphere program in the catchment of Berchtesgadener Ache, is introduced as a platform for interdisciplinary research. As the investigation of how human activities affect the natural resources in the park area, which has been defined a main aim of the program, naturally requires expertise from different scientific fields, interdisciplinary research has been fostered in the national park plan since the very beginning of the Man and the Biosphere program in 1981. To analyze the complex interactions and mutual dependencies between socio-economic and natural systems, a variety of monitoring programs have been initialized in different disciplines (e.g. climate sciences, zoology, botany) that are addressed in this paper. As a result of these research efforts, the park offers a profound data basis to be used in future studies (e.g. land cover classifications, maps of geological and soil conditions). Detailed information is provided on a climate monitoring network that has been installed in the park starting in the year 1993. The network has been continuously extended over the years and now provides extraordinary comprehensive information on meteorological conditions in the park, setting the basis for current as well as for potential future climate-related studies. A special characteristic of the station network is the fact that it covers a large range of elevations from 600 m a.s.l in the valleys to 2,600 m a.s.l in the summit regions and is therefore able to capture altitudinal gradients in meteorological variables as typical for Alpine regions. Due to the large number of stations in high elevations (15 stations are in elevations higher than 1,500 m a.s.l) the network provides information on the complex hydrometeorological conditions in summit regions which are often insufficiently represented in observation networks due to the increased costs for maintenance of climate stations in these locations. Beside the various monitoring programs, a variety of numerical models have been (further) developed for application in the park area that make extensive use of the different data collected and therefore largely benefit from the comprehensive data pool. The potential and necessity of the climate monitoring network for modelling studies is demonstrated by utilizing the meteorological recordings in the framework of a hydrometeorological simulation experiment. Further examples of environmental modelling efforts are shortly described together with preliminary model results. 相似文献
57.
58.
We report direct luminescence ages for the culture‐bearing sediments of the Kebaran site of Nahal Hadera V (NHV) in the coastal plain of Israel. Although the site contains, in addition to rich lithic deposits, plentiful mammalian bone, it has proved to be undatable using radiocarbon dating, in spite of the fact that the cultural context places the time of occupation well within the range of radiocarbon dating. In contrast, luminescence dating of the site sediments proved successful. Luminescence ages were determined using the single aliquot additive‐dose (SAA) method, applied to sand‐sized quartz extracts to determine past equivalent doses (De). Dose rates (R) were calculated using thick source alpha counting for the uranium (U) and thorium (Th) concentrations and x‐ray fluorescence analysis for the potassium (K20) concentration. Of the five samples collected at the site, four represent cultural and subcultural deposits and the fifth represents the geological substrate for the archaeological deposit, a quartz‐rich, carbonate‐cemented dune sand known as aeolianite or kurkar. The luminescence age of the kurkar is 42.7 ± 6.3 ka. Human occupation of the site occurred between 21.3 ka and 14.0 ka ago, during the Last Glacial Maximum. © 2003 Wiley Periodicals, Inc. 相似文献
59.
Geophysical exploration for epithermal gold deposits 总被引:1,自引:0,他引:1
Epithermal gold deposits are highly variable in form, ranging from thin quartz veins to large disseminated deposits, and are located in a variety of geological environments. Consequently, they exhibit a wide range of geophysical signatures. Hydrothermal alteration accompanying these deposits causes pronounced changes in the physical properties of the rocks. Magnetic susceptibility and remanence decrease; the potassium content commonly increases causing an increase in radioactivity; the electrical resistivity changes by up to two orders of magnitude; and the density increases or decreases depending on the nature of the host rock and alteration processes.Airborne magnetic surveys are effective in delineating major structures which may control the locations of epithermal gold deposits, and also in detecting magnetic lows or “flat zones” resulting from the destruction of magnetite by hydrothermal alteration. Radiometric surveys may detect potassium enrichment accompanying this alteration. Filtering and image-processing techniques are particularly useful in enhancing magnetic and radiometric data to reveal subtle structures and alteration systems.Ground geophysical techniques can play an important in defining drill targets. Well controlled gravity surveys may assist in the delineation of major structures, basement highs and alteration zones. Buried conductive alteration systems can be defined by resistivity, electromagnetic and magnetotelluric methods. Resistive gold-bearing silicified zones and shallow quartz vein systems are commonly detectable by conventional galvanic or inductive resistivity techniques.Geophysical surveys over a number of epithermal gold deposits are presented, including Bimurra-Conway (northeast Queensland, Australia), Gold Ridge (Solomon Islands), Ohui (Coromandel Peninsula, New Zealand), McLaughlin (California, USA), Rhyolite Creek (Victoria, Australia) and Mt Aubrey (New South Wales, Australia).Provided there is a clear appreciation of survey objectives and a good understanding of the changes in physical properties which rocks undergo during hydrothermal alteration, geophysics can make a substantial contribution to epithermal gold exploration. 相似文献
60.