全文获取类型
收费全文 | 668篇 |
免费 | 61篇 |
国内免费 | 53篇 |
专业分类
测绘学 | 51篇 |
大气科学 | 97篇 |
地球物理 | 163篇 |
地质学 | 251篇 |
海洋学 | 48篇 |
天文学 | 95篇 |
综合类 | 14篇 |
自然地理 | 63篇 |
出版年
2024年 | 9篇 |
2022年 | 7篇 |
2021年 | 25篇 |
2020年 | 24篇 |
2019年 | 27篇 |
2018年 | 25篇 |
2017年 | 25篇 |
2016年 | 29篇 |
2015年 | 20篇 |
2014年 | 25篇 |
2013年 | 45篇 |
2012年 | 25篇 |
2011年 | 35篇 |
2010年 | 37篇 |
2009年 | 35篇 |
2008年 | 32篇 |
2007年 | 37篇 |
2006年 | 24篇 |
2005年 | 27篇 |
2004年 | 19篇 |
2003年 | 22篇 |
2002年 | 23篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 10篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 4篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1972年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有782条查询结果,搜索用时 0 毫秒
651.
Large barchan-shaped sand deposits have been observed in the north west of Torres Strait. These deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks. A study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns (10-15 m west per year), and that sediment is shed from their horns, features that are characteristic of barchan dunes. However the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually-evasive channels and circulation of sediment around the sand bank, a characteristic of subaqueous sand banks. The presence of mutually-evasive channels is the criteria used to categorise the deposits as sand banks.Barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes. In the Torres Strait both these criteria are met. Previous work has demonstrated the presence of a net westward current through the Torres Strait that is driven by the southeast trade winds. The relatively high displacement of the wind-driven currents during the trade wind season relative to the monsoon appears to provide the necessary ‘unidirectional’ regime to form barchans. The low, and typically eastwards, displacement of the residual monsoon season current appears to have a negligible affect on the barchan form. While seasonal wind-driven currents appear to maintain the barchan shape of the sand banks, tidal currents actively maintain mutually-evasive channels observed by variations in dune orientation on the sand banks. A sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form. 相似文献
652.
653.
Characterisation of mine waste rock with respect to acid generation potential is a necessary part of routine mine operations,
so that environmentally benign waste rock stacks can be constructed for permanent storage. Standard static characterisation
techniques, such as acid neutralisation capacity (ANC), maximum potential acidity, and associated acid–base accounting, require
laboratory tests that can be difficult to obtain rapidly at remote mine sites. We show that a combination of paste pH and
a simple portable carbonate dissolution test, both techniques that can be done in the field in a 15 min time-frame, is useful
for distinguishing rocks that are potentially acid-forming from those that are acid-neutralising. Use of these techniques
could allow characterisation of mine wastes at the metre scale during mine excavation operations. Our application of these
techniques to pyrite-bearing (total S = 1–4 wt%) but variably calcareous coal mine overburden shows that there is a strong
correlation between the portable carbonate dissolution technique and laboratory-determined ANC measurements (range of 0–10 wt%
calcite equivalent). Paste pH measurements on the same rocks are bimodal, with high-sulphur, low-calcite rocks yielding pH
near 3 after 10 min, whereas high-ANC rocks yield paste pH of 7–8. In our coal mine example, the field tests were most effective
when used in conjunction with stratigraphy. However, the same field tests have potential for routine use in any mine in which
distinction of acid-generating rocks from acid-neutralising rocks is required. Calibration of field-based acid–base accounting
characteristics of the rocks with laboratory-based static and/or kinetic tests is still necessary. 相似文献
654.
Changes in wintertime 10 m winds due to the El Niño-Southern Oscillation are examined using a 6 km resolution climate simulation of Southern California covering the period from 1959 through 2001. Wind speed statistics based on regional averages reveal a general signal of increased mean wind speeds and wind speed variability during El Niño across the region. An opposite and nearly as strong signal of decreased wind speed variability during La Niña is also found. These signals are generally more significant than the better-known signals in precipitation. In spite of these regional-scale generalizations, there are significant sub-regional mesoscale structures in the wind speed impacts. In some cases, impacts on mean winds and wind variability at the sub-regional scale are opposite to those of the region as a whole. All of these signals can be interpreted in terms of shifts in occurrences of the region’s main wind regimes due to the El Niño phenomenon. The results of this study can be used to understand how interannual wind speed variations in regions of Southern California are influenced by the El Niño phenomenon. 相似文献
655.
利用位于上海中心城区徐家汇站的涡动相关湍流通量资料,对该区域2012年12月至2013年11月二氧化碳(CO2)通量的时间变化、空间分布特征、年总排放量进行分析。结果表明:中心城区徐家汇为CO2通量的源区,CO2通量的日变化呈现与交通流量相对应的双峰现象,冬季的通量值普遍小于其他季节,尤其在早高峰时段更为明显。节假日CO2通量的早高峰效应并不明显,其峰值明显低于工作日,且有滞后的趋势。各方向CO2通量大小与其周边下垫面情况密切相关,夏季白天在商业建筑密集区和主要道路处,CO2通量明显高于其他季节。上海中心城区CO2通量的年总排放量为44.5 kg/(m2·a),高于国内外其他城市的市中心或高密度住宅区的数值,这主要和观测时段相对偏晚、植被覆盖率偏低、周边高层建筑和主干道偏多有关。 相似文献
656.
An efficient forward model of the climate controls on interannual variation in tree-ring width 总被引:1,自引:1,他引:0
Susan E. Tolwinski-Ward Michael N. Evans Malcolm K. Hughes Kevin J. Anchukaitis 《Climate Dynamics》2011,36(11-12):2419-2439
We present a simple, efficient, process-based forward model of tree-ring growth, called Vaganov–Shashkin-Lite (VS-Lite), that requires as inputs only latitude and monthly temperature and precipitation. Simulations of six bristlecone pine ring-width chronologies demonstrate the interpretability of model output as an accurate representation of the climatic controls on growth. Ensemble simulations by VS-Lite of two networks of North American ring-width chronologies correlate with observations at higher significance levels on average than simulations formed by regression of ring width on the principal components of the same monthly climate data. VS-Lite retains more skill outside of calibration intervals than does the principal components regression approach. It captures the dominant low- and high-frequency spatiotemporal ring-width signals in the network with an inhomogeneous, multivariate relationship to climate. Because continuous meteorological data are most widely available at monthly temporal resolution, our model extends the set of sites at which forward-modeling studies are possible. Other potential uses of VS-Lite include generation of synthetic ring-width series for pseudo-proxy studies, as a data level model in data assimilation-based climate reconstructions, and for bias estimation in actual ring-width index series. 相似文献
657.
Use of upscaled elevation and surface roughness data in two-dimensional surface water models 总被引:1,自引:0,他引:1
In this paper, we present an approach that uses a combination of cell-block- and cell-face-averaging of high-resolution cell elevation and roughness data to upscale hydraulic parameters and accurately simulate surface water flow in relatively low-resolution numerical models. The method developed allows channelized features that preferentially connect large-scale grid cells at cell interfaces to be represented in models where these features are significantly smaller than the selected grid size. The developed upscaling approach has been implemented in a two-dimensional finite difference model that solves a diffusive wave approximation of the depth-integrated shallow surface water equations using preconditioned Newton-Krylov methods. Computational results are presented to show the effectiveness of the mixed cell-block and cell-face averaging upscaling approach in maintaining model accuracy, reducing model run-times, and how decreased grid resolution affects errors. Application examples demonstrate that sub-grid roughness coefficient variations have a larger effect on simulated error than sub-grid elevation variations. 相似文献
658.
Amanda M. Vieillard Robinson W. Fulweiler Zoe J. Hughes Joanna C. Carey 《Estuarine, Coastal and Shelf Science》2011
Salt marshes are widely studied due to the broad range of ecosystem services they provide including serving as crucial wildlife habitat and as hotspots for biogeochemical cycling. Nutrients such as nitrogen (N), phosphorus (P), and carbon (C) are well studied in these systems. However, salt marshes may also be important environments for the cycling of another key nutrient, silica (Si). Found at the land–sea interface, these systems are silica replete with large stocks in plant biomass, sediments, and porewater, and therefore, have the potential to play a substantial role in the transformation and export of silica to coastal waters. In an effort to better understand this role, we measured the fluxes of dissolved (DSi) and biogenic (BSi) silica into and out of two tidal creeks in a temperate, North American (Rowley, Massachusetts, USA) salt marsh. One of the creeks has been fertilized from May to September for six years allowing us to examine the impacts of nutrient addition on silica dynamics within the marsh. High-resolution sampling in July 2010 showed no significant differences in Si concentrations between the fertilized and reference creeks with dissolved silica ranging from 0.5 to 108 μM and biogenic from 2.0 to 56 μM. Net fluxes indicated that the marsh is a point source of dissolved silica to the estuary in the summer with a net flux of approximately 169 mol h−1, demonstrating that this system exports DSi on the same magnitude as some nearby, mid-sized rivers. If these findings hold true for all salt marshes, then these already valuable regions are contributing yet another ecosystem service that has been previously overlooked; by exporting DSi to coastal receiving waters, salt marshes are actively providing this important nutrient for coastal primary productivity. 相似文献
659.
660.
Pavese et al. (1999) examined cation partitioning vs. temperature in a synthetic spinel of composition (Mg0.70 Fe0.23
3+) Al1.97 O4 using structure data obtained from in situ neutron powder diffraction. After imposing assumptions on the site assignment
of vacancies and Fe3+, they assigned the remaining cations by applying least-squares minimization to chemical constraints on site-occupancy sums,
site-scattering, chemical composition, and thermal expansion of the octahedral site. Their proposed site assignments exhibit
a sharp discontinuity in occupancy fractions versus temperature, a necessary consequence of their assumptions on vacancy assignments.
In this paper we reexamine the cation partitioning of the same spinel using the constrained least-squares formulation of OccQP (Wright et al. 2000), which optimizes site occupancies without ad hoc assumptions. We obtain strikingly different results,
supporting the general view that spinel undergoes a lambda transition at ∼1000 K. For all observed parameters, the residuals
obtained with the OccQP assignments are lower than those obtained with the Pavese et al. assignments, in some cases by more than 1 order of magnitude.
Received: 05 April 2000 / Accepted: 19 October 2000 相似文献