全文获取类型
收费全文 | 727篇 |
免费 | 43篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 57篇 |
地球物理 | 259篇 |
地质学 | 245篇 |
海洋学 | 62篇 |
天文学 | 76篇 |
综合类 | 1篇 |
自然地理 | 58篇 |
出版年
2023年 | 6篇 |
2021年 | 18篇 |
2020年 | 10篇 |
2019年 | 14篇 |
2018年 | 31篇 |
2017年 | 24篇 |
2016年 | 23篇 |
2015年 | 24篇 |
2014年 | 29篇 |
2013年 | 49篇 |
2012年 | 32篇 |
2011年 | 38篇 |
2010年 | 49篇 |
2009年 | 39篇 |
2008年 | 40篇 |
2007年 | 41篇 |
2006年 | 28篇 |
2005年 | 30篇 |
2004年 | 30篇 |
2003年 | 32篇 |
2002年 | 25篇 |
2001年 | 14篇 |
2000年 | 12篇 |
1999年 | 14篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有779条查询结果,搜索用时 15 毫秒
81.
Bethany N. Deshpande Roxane Tremblay Reinhard Pienitz Warwick F. Vincent 《Journal of Paleolimnology》2014,52(3):171-184
Lac Saint-Augustin is an urban lake located on the outskirts of Quebec City, one of North America’s oldest cities. Anthropogenic inputs from land clearing, agriculture, highway development and urbanization in the surrounding catchment have resulted in strong impacts on the limnology of the lake throughout the past three centuries. In recent years, this lake has experienced severe eutrophication, including persistent cyanobacterial blooms. In winter 2011, a sediment core was extracted from the deepest area of the lake. A detailed paleopigment analysis was used to assess eutrophication processes in the lake and to determine the timing and appearance of cyanobacterial blooms and their subsequent variability. Extracted chlorophyll a, its degradation products and 11 carotenoid pigments were identified and quantified via reverse-phase high performance liquid chromatography to examine relative changes in the phytoplankton. The results revealed large variations in the phytoplankton community structure of Lac Saint-Augustin over the past 356 years. Chlorophyll a concentrations per unit organic matter (OM) increased significantly from the base of the core to present day, rising more than 15-fold from 18.4 µg (g OM)?1 at the base of the core to 287 µg (g OM)?1 in the most recent strata. Biostratigraphical analysis revealed three major periods of enrichment, with episodes of cyanobacterial abundance from the 1890s onwards. The greatest changes occurred in the most recent period (from the 1960s to the present) relative to earlier periods, with pigment increases for all phytoplankton groups. The cyanobacterial pigments canthaxanthin, echinenone and zeaxanthin (also a marker for green algae) showed concentrations in the surface sediments that were significantly above values at the bottom of the core, and these differences were large, even giving consideration to the lesser pigment degradation near the surface. Overall, the results indicate that cyanobacterial blooms are not a recent feature of Lac Saint-Augustin but began to occur soon after catchment modification 150 years ago. The pigment records also imply that cyanobacterial and associated algal populations have risen to unprecedented levels over the last few decades of ongoing development of the Lac Saint-Augustin catchment. This study highlights the utility of multiple pigment analysis of lake sediments for identifying the timing and magnitude of anthropogenic impacts. 相似文献
82.
Rock fall hazard zoning is a challenging yet necessary task to be accomplished for planning an appropriate land use in mountainous areas. Methodologies currently adopted for elaborating zoning maps do not provide satisfactory results though, due to uncertainties and related assumptions characterising hazard assessment. The new Cadanav methodology, presented in this paper, aims at improving quantitative hazard assessment and zoning at the local scale, by reducing uncertainties mainly related to the technique for combining rock fall intensity and frequency of occurrence. Starting from available information on rock fall failure frequency and trajectory simulation results, the procedure merges in a strict way temporal frequency, probability of reach and energy data and evaluates the hazard degree by means of “hazard curves”. These curves are described at each point of the slope by a series of energy–return period couples representing the hazardous conditions which may possibly affect that location. The new Cadanav methodology is here detailed and compared to its original version. Hazard zoning results are illustrated along two different 2D slope profiles, for linear homogeneous cliff configurations, and according to the Swiss intensity–frequency diagram for rock fall hazard zoning. However, the procedure can be easily used with any other intensity–frequency diagram prescribed in national guidelines and, additionally, extended to problems involving 3D topographies. 相似文献
83.
K. Deepthi Usha Natesan A. L. Muthulakshmi Vincent A. Ferrer V. P. Venugopalan S. V. Narasimhan 《Environmental Earth Sciences》2014,72(3):717-729
Kalpakkam, a tiny fishing hamlet dotting the east coast, halfway between Chennai and Pondicherry has become prominent due to the Madras Atomic Power Station. The present study aims at assessing the spatial and temporal distribution pattern of heavy metals (Cd, Pb, Zn, Cu, Ni, Cr, and Fe) from 12 stations along the inner shelf of Bay of Bengal, India, during pre-monsoon (PRM), monsoon, and post-monsoon (POM) seasons. The order of occurrence of the metals in sediments of Kalpakkam is Cr > Zn > Ni > Cu > Pb > Cd (excluding Fe since unit is in %) and exhibits a unique seasonal pattern with the highest values (average) during POM except for Cd which shows highest (average) concentration during PRM. In order to determine the sample association according to their geochemical composition and their granulometric characteristics, a correlation matrix was generated and sediment pollution indices viz., sediment enrichment factor and geoaccumulation index were computed. The results confirm anthropogenic input of Cd to nearshore sediments of Kalpakkam. Sources of Cd can be attributed to dredging activities at Edaiyur, direct dumping and sewage sludge from anthropogenic activities, which reach the study area through the Buckingham Canal opening at the backwaters—Sadras and Edaiyur, and extreme use of antifouling paints by fishing trawlers. 相似文献
84.
Roy James W. Bordeleau Genevive Rivard Christine Ryan M. Cathryn Malet Xavier Brown Susan J. Tremblay Vincent 《Hydrogeology Journal》2022,30(3):1005-1019
Hydrogeology Journal - Monitoring of dissolved methane concentrations in groundwater is required to identify impacts from oil and gas development and to understand temporal variability under... 相似文献
85.
86.
A second generation adjusted precipitation daily dataset has been prepared for trend analysis in Canada. Daily rainfall and snowfall amounts have been adjusted for 464 stations for known measurement issues such as wind undercatch, evaporation and wetting losses for each type of rain-gauge, snow water equivalent from ruler measurements, trace observations and accumulated amounts from several days. Observations from nearby stations were sometimes combined to create time series that are longer; hence, making them more useful for trend studies. In this new version, daily adjustments are an improvement over the previous version because they are derived from an extended dataset and enhanced metadata knowledge. Datasets were updated to cover recent years, including 2009. The impact of the adjustments on rainfall and snowfall total amounts and trends was examined in detail. As a result of adjustments, total rainfall amounts have increased by 5 to 10% in southern Canada and by more than 20% in the Canadian Arctic, compared to the original observations, while the effect of the adjustments on snowfall were larger and more variable throughout the country. The slope of the rain trend lines decreased as a result of the larger correction applied to the older rain-gauges while the slope of the snow trend lines increased, mainly along the west coast and in the Arctic. Finally, annual and seasonal rainfall and snowfall trends based on the adjusted series were computed for 1950–2009 and 1900–2009. Overall, rainfall has increased across the country while a mix of non-significant increasing and decreasing trends was found during the summer in the Canadian Prairies. Snowfall has increased mainly in the north while a significant decrease was observed in the southwestern part of the country for 1950–2009. 相似文献
87.
CMIP5 simulated climate conditions of the Greater Horn of Africa (GHA). Part 1: contemporary climate
The present study is a preliminary interrogation of the ability of ten Earth System Models (ESMs) from the fifth phase of coupled model intercomparison project to characterize seasonal and annual mean precipitation cycle over the Greater Horn of Africa region. Each ESM had at least 2 ensemble members. In spite of distributional anomalies of observations, ESM ensemble means were examined on the basis of gridded precipitation data. Majority of the ten ESMs analyzed correctly reproduce the mean seasonal and annual cycle of precipitation for the period 1979–2008 as compared to gridded satellite-derived observations. At the same time our analysis shows significant biases in individual models depending on region and season. Specifically, a modest number of models were able to capture correctly the peaks of bimodal (MAM and OND) and JJAS rainfall while a few either dragged the onset to subsequent months or displaced the locations of seasonal rainfall further north. Nearly all models were in agreement with their representation of the zonal orientation of spatial pattern of the leading EOF rainfall modes; more so, enhanced precipitation over the Indian Ocean and a dipole mode of precipitation pattern are captured in the first and second mode respectively. Further, the corresponding EOF time series of the ESMs rainfall modes were all in phase with observations. However, all models output were positively biased against observations, with large medians and varied range of anomalies. Therefore, caution needs to be taken when choosing models for applications over the region, especially when ensemble means have to be considered. 相似文献
88.
Andrew D. Gronewold Vincent Fortin Brent Lofgren Anne Clites Craig A. Stow Frank Quinn 《Climatic change》2013,120(4):697-711
The North American Laurentian Great Lakes hold nearly 20 % of the earth’s unfrozen fresh surface water and have a length of coastline, and a coastal population, comparable to frequently-studied marine coasts. The surface water elevations of the Great Lakes, in particular, are an ideal metric for understanding impacts of climate change on large hydrologic systems, and for assessing adaption measures for absorbing those impacts. In light of the importance of the Great Lakes to the North American and global economies, the Great Lakes and the surrounding region also serve as an important benchmark for hydroclimate research, and offer an example of successful adaptive management under changing climate conditions. Here, we communicate some of the important lessons to be learned from the Great Lakes by examining how the coastline, water level, and water budget dynamics of the Great Lakes relate to other large coastal systems, along with implications for water resource management strategies and climate scenario-derived projections of future conditions. This improved understanding fills a critical gap in freshwater and marine global coastal research. 相似文献
89.
Elena M. Blanter Jean-Louis Le Mouël Mikhail G. Shnirman Vincent Courtillot 《Solar physics》2014,289(11):4309-4333
We apply a Kuramoto model of nonlinear coupled oscillators to the simulation of slow variations of the phase difference between sunspot number [R I ] and geomagnetic indices [aa and ζ]. The Kuramoto model is described for the particular case of two oscillators connected by symmetric coupling with quasi-stationary behavior, and its properties are investigated. By solving an inverse problem, we reconstruct the evolution of the couplings between pairs of indices [R I and aa, R I and ζ, aa and ζ], and interpret these in terms of the physics of the solar dynamo. The de-correlation between R I and geomagnetic indices found in Solar Cycle 20 by Le Mouël et al. (J. Geophys. Res. 117, A09103, 2012) is successfully reproduced by the Kuramoto model and corresponds to the alternation of the leading oscillator. Application of the Kuramoto model to the cross-correlations [C(R I ,ζ) and C(aa,ζ)] for ζ-indices computed in eight geomagnetic stations shows the latitudinal dependence of the mean phase difference. We discuss these results in terms of the solar-wind contribution to local geomagnetic indices [ζ]. 相似文献
90.
Vincent Mannings 《Astrophysics and Space Science》1995,224(1-2):185-188
Studies of circumstellar dust and gas in pre-main-sequence systems are in part motivated by a desire to probe possible sites of future or on-going planet formation and, should accretion onto young stars be taking place, to determine whether the observed remnants of embryonic envelopes might yet contribute significantly to the final masses of stars as they evolve towards the main sequence. New 0.35- to 1.3-mm flux measurements have been made for 13 intermediate-mass pre-MS systems in order to estimate total dust+gas masses and to investigate the spatial distribution of circumstellar material. At least two sources may be dominated by compact circumstellar disks; emission from extended envelopes appears to be important for the majority of sources. HD 163296 is an unusual object and warrants further examination. 相似文献