全文获取类型
收费全文 | 4827篇 |
免费 | 217篇 |
国内免费 | 51篇 |
专业分类
测绘学 | 122篇 |
大气科学 | 415篇 |
地球物理 | 1101篇 |
地质学 | 1776篇 |
海洋学 | 354篇 |
天文学 | 866篇 |
综合类 | 17篇 |
自然地理 | 444篇 |
出版年
2022年 | 28篇 |
2021年 | 64篇 |
2020年 | 67篇 |
2019年 | 91篇 |
2018年 | 143篇 |
2017年 | 150篇 |
2016年 | 154篇 |
2015年 | 133篇 |
2014年 | 188篇 |
2013年 | 261篇 |
2012年 | 203篇 |
2011年 | 269篇 |
2010年 | 207篇 |
2009年 | 263篇 |
2008年 | 212篇 |
2007年 | 190篇 |
2006年 | 176篇 |
2005年 | 174篇 |
2004年 | 185篇 |
2003年 | 161篇 |
2002年 | 160篇 |
2001年 | 77篇 |
2000年 | 83篇 |
1999年 | 66篇 |
1998年 | 81篇 |
1997年 | 62篇 |
1996年 | 63篇 |
1995年 | 60篇 |
1994年 | 55篇 |
1993年 | 62篇 |
1992年 | 45篇 |
1991年 | 39篇 |
1990年 | 39篇 |
1989年 | 40篇 |
1988年 | 45篇 |
1987年 | 48篇 |
1986年 | 43篇 |
1985年 | 56篇 |
1984年 | 66篇 |
1983年 | 50篇 |
1982年 | 56篇 |
1981年 | 47篇 |
1980年 | 47篇 |
1979年 | 46篇 |
1978年 | 43篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1975年 | 32篇 |
1974年 | 33篇 |
1973年 | 31篇 |
排序方式: 共有5095条查询结果,搜索用时 15 毫秒
81.
Sandow Mark Yidana Samuel Ganyaglo Bruce Banoeng-Yakubo Thomas Akabzaa 《Journal of African Earth Sciences》2011,59(2-3):185-194
A conceptual groundwater flow model was developed for the crystalline aquifers in southeastern part of the Eastern region, Ghana. The objective was to determine approximate levels of groundwater recharge, estimate aquifer hydraulic parameters, and then test various scenarios of groundwater extraction under the current conditions of recharge. A steady state groundwater flow model has been calibrated against measured water levels of 19 wells in the area. The resulting recharge is estimated to range from 8.97 × 10?5 m/d to 7.14 × 10?4 m/d resulting in a basin wide average recharge of about 9.6% of total annual precipitation, which results in a basin wide quantitative recharge of about 2.4 million m3/d in the area. This compares to recharge estimated from the chloride mass balance of 7.6% of precipitation determined in this study. The general groundwater flow in the area has also been determined to conform to the general northeast–southwest structural grain of the country. The implication is that the general hydrogeology is controlled by post genetic structural entities imposed on the rocks to create ingresses for sufficient groundwater storage and transport. Calibrated aquifer hydraulic conductivities range between 0.99 m/d and over 19.4 m/d. There is a significant contribution of groundwater discharge to stream flow in the study area. Increasing groundwater extraction will have an effect on stream flow. This study finds that the current groundwater extraction levels represent only 0.17% of the annual recharge from precipitation, and that groundwater can sustain future increased groundwater demands from population growth and industrialization. 相似文献
82.
Triaxial shear testing of polished slickensided surfaces 总被引:1,自引:1,他引:0
Christopher L. Meehan Binod Tiwari Thomas L. Brandon J. Michael Duncan 《Landslides》2011,8(4):449-458
A series of consolidated-drained triaxial tests were performed on precut and polished clay specimens to measure their drained
residual strength. Two soils were tested during this research program: Rancho Solano Clay and San Francisco Bay Mud. Specimens
were tested using a specially modified triaxial compression device which was developed to minimize the effects of end-platen
restraint on the measured strengths. Special attention was paid to the influence of changing specimen area and membrane effects
during the test. Triaxial test results were compared with baseline measurements of drained residual shear strength that were
made for each of the clay soils using Bromhead ring shear tests and polished-specimen direct shear tests. Residual strength
values measured in the triaxial device were higher than the drained residual strengths measured in the Bromhead ring shear
apparatus and the direct shear device, indicating that this test approach is more challenging than the use of direct shear
tests conducted on polished slickensided surfaces. Comparison of single stage and multistage triaxial test data indicates
that multistage triaxial testing may work well for specimens that fail along a well-defined plane, provided that careful attention
is given to the effects of end platen restraint, membrane restraint, and changes in specimen area during shear. 相似文献
83.
Since the launch of the first satellite in 1972, ecologists have been equipped with new tools to address the degradation of
tropical forests, previously limited by field-based methods. This article is a review of the state of remote sensing technology
in characterizing the degradation of tropical forest. The factors responsible for the structural and functional degradation
of the tropical forest and its likely impacts are described in view of generating remote sensing based inputs. In order to
assess the degradation and utility of geo-informatics tools, 32 parameters are identified. The research developments at different
levels of information extraction from the historic to recent periods are elaborated, and future challenges are predicted.
The article concludes that an additional momentum of research is required to answer many unresolved questions of tropical
forest degradation. 相似文献
84.
Alexandre Forest Jean-Éric Tremblay Yves Gratton Johannie Martin Jonathan Gagnon Gérald Darnis Makoto Sampei Louis Fortier Mathieu Ardyna Michel Gosselin Hiroshi Hattori Dan Nguyen Roxane Maranger Dolors Vaqué Cèlia Marrasé Carlos Pedrós-Alió Amélie Sallon Christine Michel Colleen Kellogg Jody Deming Elizabeth Shadwick Helmuth Thomas Heike Link Philippe Archambault Dieter Piepenburg 《Progress in Oceanography》2011,91(4):410-436
Major pathways of biogenic carbon (C) flow are resolved for the planktonic food web of the flaw lead polynya system of the Amundsen Gulf (southeast Beaufort Sea, Arctic Ocean) in spring-summer 2008. This period was relevant to study the effect of climate change on Arctic marine ecosystems as it was characterized by unusually low ice cover and warm sea surface temperature. Our synthesis relied on a mass balance estimate of gross primary production (GPP) of 52.5 ± 12.5 g C m−2 calculated using the drawdown of nitrate and dissolved inorganic C, and a seasonal f-ratio of 0.64. Based on chlorophyll a biomass, we estimated that GPP was dominated by phytoplankton (93.6%) over ice algae (6.4%) and by large cells (>5 μm, 67.6%) over small cells (<5 μm, 32.4%). Ancillary in situ data on bacterial production, zooplankton biomass and respiration, herbivory, bacterivory, vertical particle fluxes, pools of particulate and dissolved organic carbon (POC, DOC), net community production (NCP), as well as selected variables from the literature were used to evaluate the fate of size-fractionated GPP in the ecosystem. The structure and functioning of the planktonic food web was elucidated through inverse analysis using the mean GPP and the 95% confidence limits of every other field measurement as lower and upper constraints. The model computed a net primary production of 49.2 g C m−2, which was directly channeled toward dominant calanoid copepods (i.e. Calanus hyperboreus 20%, Calanus glacialis 10%, and Metridia longa 10%), other mesozooplankton (12%), microzooplankton (14%), detrital POC (18%), and DOC (16%). Bacteria required 29.9 g C m−2, a demand met entirely by the DOC derived from local biological activities. The ultimate C outflow comprised respiration fluxes (82% of the initial GPP), a small sedimentation (3%), and a modest residual C flow (15%) resulting from NCP, dilution and accumulation. The sinking C flux at the model limit depth (395 m) supplied 60% of the estimated benthic C demand (2.8 g C m−2), suggesting that the benthos relied partly on other C sources within the bottom boundary layer to fuel its activity. In summary, our results illustrate that the ongoing decline in Arctic sea ice promotes the growth of pelagic communities in the Amundsen Gulf, which benefited from a ∼80% increase in GPP in spring-summer 2008 when compared to 2004 – a year of average ice conditions and relatively low GPP. However, 53% of the secondary production was generated within the microbial food web, the net ecological efficiency of zooplankton populations was not particularly high (13.4%), and the quantity of biogenic C available for trophic export remained low (6.6 g C m−2). Hence it is unlikely that the increase in lower food web productivity, such as the one observed in our study, could support new harvestable fishery resources in the offshore Beaufort Sea domain. 相似文献
85.
Charles A. Stock Michael A. Alexander Nicholas A. Bond Keith M. Brander William W.L. Cheung Enrique N. Curchitser Thomas L. Delworth John P. Dunne Stephen M. Griffies Melissa A. Haltuch Jonathan A. Hare Anne B. Hollowed Patrick Lehodey Simon A. Levin Jason S. Link Kenneth A. Rose Ryan R. Rykaczewski Jorge L. Sarmiento Ronald J. Stouffer Franklin B. Schwing Francisco E. Werner 《Progress in Oceanography》2011,88(1-4):1-27
The study of climate impacts on Living Marine Resources (LMRs) has increased rapidly in recent years with the availability of climate model simulations contributed to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Collaboration between climate and LMR scientists and shared understanding of critical challenges for such applications are essential for developing robust projections of climate impacts on LMRs. This paper assesses present approaches for generating projections of climate impacts on LMRs using IPCC-class climate models, recommends practices that should be followed for these applications, and identifies priority developments that could improve current projections. Understanding of the climate system and its representation within climate models has progressed to a point where many climate model outputs can now be used effectively to make LMR projections. However, uncertainty in climate model projections (particularly biases and inter-model spread at regional to local scales), coarse climate model resolution, and the uncertainty and potential complexity of the mechanisms underlying the response of LMRs to climate limit the robustness and precision of LMR projections. A variety of techniques including the analysis of multi-model ensembles, bias corrections, and statistical and dynamical downscaling can ameliorate some limitations, though the assumptions underlying these approaches and the sensitivity of results to their application must be assessed for each application. Developments in LMR science that could improve current projections of climate impacts on LMRs include improved understanding of the multi-scale mechanisms that link climate and LMRs and better representations of these mechanisms within more holistic LMR models. These developments require a strong baseline of field and laboratory observations including long time series and measurements over the broad range of spatial and temporal scales over which LMRs and climate interact. Priority developments for IPCC-class climate models include improved model accuracy (particularly at regional and local scales), inter-annual to decadal-scale predictions, and the continued development of earth system models capable of simulating the evolution of both the physical climate system and biosphere. Efforts to address these issues should occur in parallel and be informed by the continued application of existing climate and LMR models. 相似文献
86.
Robert S. Pickart Michael A. Spall G.W.K. Moore Thomas J. Weingartner Rebecca A. Woodgate Knut Aagaard Koji Shimada 《Progress in Oceanography》2011,88(1-4):78-100
The spin up and relaxation of an autumn upwelling event on the Beaufort slope is investigated using a combination of oceanic and atmospheric data and numerical models. The event occurred in November 2002 and was driven by an Aleutian low storm. The wind field was strongly influenced by the pack-ice distribution, resulting in enhanced winds over the open water of the Chukchi Sea. Flow distortion due to the Brooks mountain range was also evident. Mooring observations east of Barrow Canyon show that the Beaufort shelfbreak jet reversed to the west under strong easterly winds, followed by upwelling of Atlantic Water onto the shelf. After the winds subsided a deep eastward jet of Atlantic Water developed, centered at 250 m depth. An idealized numerical model reproduces these results and suggests that the oceanic response to the local winds is modulated by a propagating signal from the western edge of the storm. The disparity in wave speeds between the sea surface height signal—traveling at the fast barotropic shelf wave speed—versus the interior density signal—traveling at the slow baroclinic wave speed—leads to the deep eastward jet. The broad-scale response to the storm over the Chukchi Sea is investigated using a regional numerical model. The strong gradient in windspeed at the ice edge results in convergence of the offshore Ekman transport, leading to the establishment of an anti-cyclonic gyre in the northern Chukchi Sea. Accordingly, the Chukchi shelfbreak jet accelerates to the east into the wind during the storm, and no upwelling occurs west of Barrow Canyon. Hence the storm response is fundamentally different on the Beaufort slope (upwelling) versus the Chukchi slope (no upwelling). The regional numerical model results are supported by additional mooring data in the Chukchi Sea. 相似文献
87.
Chris McGonigle Jonathan H. Grabowski Craig J. Brown Thomas C. Weber Rory Quinn 《Estuarine, Coastal and Shelf Science》2011
Benthic macroalgae form an important part of temperate marine ecosystems, exhibiting a complex three-dimensional character which represents a vital foraging and spawning ground for many juvenile fish species. In this research, image-based techniques for classification of multibeam backscatter are explored for the detection of benthic macroalgae at Cashes Ledge in the Gulf of Maine, USA. Two classifications were performed using QTC-Multiview, differentiated by application of a threshold filter, and macroalgal signatures were independently extracted from the raw sonar datagrams in Matlab. All classifications were validated by comparison with video ground-truth data. The unfiltered classification shows a high degree of complexity in the shallowest areas within the study site; the filtered demonstrates markedly less variation by depth. The unfiltered classification shows a positive agreement with the video ground-truth data; 82.6% of observations recording Laminaria sp., 39.1% of Agarum cribrosum and 100.0% (n = 3) of mixed macroalgae occur within the same acoustically distinct group of classes. These are discrete from the 8.1% recorded agreement with absences and nulls (>40 m) of macrophytes (n = 32) from a total of 86 ground-truth locations. The results of the water column data extraction (WCDE) show similar success, accurately predicting 78.3% of Laminaria sp. and 30.4% of A. cribrosum observations. 相似文献
88.
Troy P. Sampere Thomas S. Bianchi Mead A. Allison Brent A. McKee 《Estuarine, Coastal and Shelf Science》2011
The primary focus of this paper is to better understand carbon burial on the Louisiana continental margin using spatial scales that covered various shelf depositional areas far-field and near-field (sediment and organic carbon inputs relative to river mouth proximity) and covering a variety of sedimentation rates. Box-cores samples were collected in July 2003; cores were collected along two depositional transects extending westward and southward from the Southwest Pass (SW Pass). A key difference between the two transects sampled in this study was the greater occurrence of mobile muds derived from spillover from shallower regions along the westward 50 m isobaths. The dominant mechanism for mixing in the surface active zone (SAZ) on the inner Louisiana shelf was due to physical, not biological, forces. Burial efficiencies for organic carbon (57.2–91.5%) and total nitrogen (44.2–86.9%) ranged widely across all shelf stations. Lower burial efficiencies for bulk organic carbon, total nitrogen, and pigment biomarkers were associated with mobile muds west of Southwest Pass. Chlorophyll a concentrations were significantly higher than pheopigments at depth at the Mississippi River and Southwest Pass stations, making up 40.4 and 77.4% of total pigment concentrations in the (SAZ) and 46.2 and 63.2% in the accumulation zone (AZ), respectively. These results are in agreement with earlier plant pigment studies which showed that a large fraction of the phytodetritus delivered to the inner shelf was derived from coastal and river diatoms. The amount of lignin preserved with depth decreased with increasing residence time in the SAZ and diagenetic zone (DZ) along the canyon transect but not along the western transect. Trends for lignin concentration followed previously identified surface sediment trends indicating overall lower burial of refractory terrestrial material at depth with greater distance offshore. 相似文献
89.
90.
Annemarie Christophersen Nicola Litchfield Kelvin Berryman Richard Thomas Roberto Basili Laura Wallace William Ries Gavin P. Hayes Kathleen M. Haller Toshikazu Yoshioka Richard D. Koehler Dan Clark Monica Wolfson-Schwehr Margaret S. Boettcher Pilar Villamor Nick Horspool Teraphan Ornthammarath Ramon Zuñiga Robert M. Langridge Mark W. Stirling Tatiana Goded Carlos Costa Robert Yeats 《Natural Hazards》2015,79(1):111-135