全文获取类型
收费全文 | 653篇 |
免费 | 57篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 100篇 |
地球物理 | 153篇 |
地质学 | 204篇 |
海洋学 | 54篇 |
天文学 | 117篇 |
综合类 | 1篇 |
自然地理 | 73篇 |
出版年
2024年 | 1篇 |
2022年 | 5篇 |
2021年 | 23篇 |
2020年 | 20篇 |
2019年 | 17篇 |
2018年 | 22篇 |
2017年 | 25篇 |
2016年 | 54篇 |
2015年 | 33篇 |
2014年 | 30篇 |
2013年 | 47篇 |
2012年 | 53篇 |
2011年 | 38篇 |
2010年 | 45篇 |
2009年 | 53篇 |
2008年 | 35篇 |
2007年 | 32篇 |
2006年 | 36篇 |
2005年 | 28篇 |
2004年 | 19篇 |
2003年 | 15篇 |
2002年 | 7篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有723条查询结果,搜索用时 15 毫秒
711.
A large, circular marking ∼1800 km across is seen in near-infrared images of Titan. The feature is centered at 10°S, 120°W on Titan, encompasses much of Titan’s western Xanadu region, and has an off-center, quasi-circular, inner margin about 700 km across, with lobate outer margins extending 200-500 km from the inner margin. On the feature’s southern flank is Tui Regio, an area that has very high reflectivity at 5 μm, and is hypothesized to exhibit geologically recent cryovolcanic flows (Barnes, J.W. et al. [2006]. Geophys. Res. Lett. 33), similar to flows seen in Hotei Regio, a cryovolcanic area whose morphology may be controlled by pre-existing, crustal fractures resulting from an ancient impact (Soderblom, L.A. et al. [2009]. Icarus, 204). The spectral reflectivity of the large, circular feature is quite different than that of its surroundings, making it compositionally distinct, and radar measurements of its topography, brightness temperature and volume scattering also suggest that the feature is quite distinct from its surroundings. These and several other lines of evidence, in addition to the feature’s morphology, suggest that it may occupy the site of an ancient impact. 相似文献
712.
Field study on influence of root characteristics on soil suction distribution in slopes vegetated with Cynodon dactylon and Schefflera heptaphylla 下载免费PDF全文
Vegetation evapotranspiration (ET) induced soil water suction reduces hydraulic conductivity and increases shear strength of slopes. Several field studies have been conducted to investigate suction distribution in vegetated slopes. However, these studies were conducted on natural slopes, which are prone to heterogeneity in vegetation and soil conditions. Moreover, studies quantifying the effect of different vegetation species, root characteristics (root depth and root area index) and transpiration reduction function (Trf) on suction in slopes under natural variation are rare. This study investigated the suction distribution and root characteristics in recompacted slopes vegetated with two different species, i.e. Cynodon dactylon (Bermuda grass) and Schefflera heptaphylla (ivy tree). Bare slope served as a control. Suction distributions during different seasons and rainfall events were monitored. It is found that during the dry season, slope vegetated with young Schefflera heptaphylla seedlings have substantially higher suction within the root zone compared with bare slope and slope vegetated with Cynodon dactylon. This is because Schefflera heptaphylla has a higher root biomass, Trf and ET than Cynodon dactylon. It was also found that suctions within root zones of vegetated slopes and bare slope were completely destroyed under rainfall events corresponding to 2 years and 20 years return period. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
713.
Maria Li. Calleja Cristina Barrón Jason A. Hale Thomas K. Frazer Carlos M. Duarte 《Estuaries and Coasts》2006,29(6):1255-1264
The effects of light reduction on community metabolism and sediment sulfate reduction rates (SRR) were assessed experimentally
in a shallow (<2.0 m) seagrass (Thalassia testudinum) meadow along Florida's north-central Gulf coast. Nine experimental plots (1.5 m×1.5 m) were shaded differentially to achieve
a 0–90% gradient in light reduction within the seagrass meadow. Gross primary production and net community production (NCP),
estimated with in situ benthic chamber incubations, decreased with increasing light reduction. The compensation irradiance
for community metabolism, i.e., the shading level at which NCP shifted from net autotrophic to net heterotrophic, was determined
to be 52.5% of the incoming irradiance at canopy height in the seagrass bed (308.7 μE m−2 s−1 PAR at noon). Sediment SRR, determined with the use of a35S−SO4
2− radiotracer technique, increased quickly (within 5 d) and markedly with increased shade, i.e., simulated light reduction.
SRR increased 50-fold when shading exceeded the light compensation point for the seagrass community, rendering the community
net heterotrophic. Five days after restoring ambient light conditions, SRR had decreased sharply for all shading treatments.
The observed decrease in NCP, coincident with the increase in the SRR with light reduction, suggests that light reduction
has an indirect influence on sediment SRR mediated through its effect on seagrass metabolism. 相似文献
714.
Jason D. Mann David W. Leverington John Rayburn James T. Teller 《Journal of Paleolimnology》1999,22(1):71-80
During the last retreat of the Laurentide Ice Sheet in North America, many proglacial lakes formed as continental drainage was impounded against the southern and western ice margin. Lake Agassiz was the largest of these lakes. The bathymetry of Lake Agassiz at the Herman and Upper Campbell beach levels – formed at about 11.5–11.0 ka and 9.9–9.5 ka, respectively – was computer modelled in this study by first collecting data for the isostatically-deformed paleowater planes of the two lake levels (derived from isobase lines constructed from beach elevations), and then subtracting these from the modern topography of the former lake floor. Pixels with dimensions of 1/30 × 1/30 of a degree were used in the model. Using these data, the area and volume of the lake were also calculated: at the Herman level these were 152 500 km2 and 13 100 km3 respectively; at the Upper Campbell level these were 350 400 km2 and 38 700 km3. Contour maps showing the paleobathymetry of both periods in the lake's history were also constructed. Determining the paleobathymetry and volume of Lake Agassiz is an important step in understanding the impact that the lake had on its surrounding environment and on the rivers, lakes, and oceans into which it flowed. 相似文献
715.
Mahmoud Aburihan † Jason D. Fiege Richard N. Henriksen Thibaut Lery 《Monthly notices of the Royal Astronomical Society》2001,326(4):1217-1227
The formation and collapse of a protostar involves the simultaneous infall and outflow of material in the presence of magnetic fields, self-gravity and rotation. We use self-similar techniques to self-consistently model the anisotropic collapse and outflow by using a set of angle-separated self-similar equations. The outflow is quite strong in our model, with the velocity increasing in proportion to radius, and material formally escaping to infinity in the finite time is required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating magnetohydrodynamics (MHD), which features co-existing regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late-time behaviour to be dominated by the singularity, rather than depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity. 相似文献
Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating magnetohydrodynamics (MHD), which features co-existing regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late-time behaviour to be dominated by the singularity, rather than depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity. 相似文献
716.
Jason Laura Sergio J. Rey Luc Anselin 《International journal of geographical information science》2013,27(4):536-555
In this paper, we report efforts to develop a parallel implementation of the p-compact regionalization problem suitable for multi-core desktop and high-performance computing environments. Regionalization for data aggregation is a key component of many spatial analytical workflows that are known to be NP-Hard. We utilize a low communication cost parallel implementation technique that provides a benchmark for more complex implementations of this algorithm. Both the initialization phase, utilizing a Memory-based Randomized Greedy and Edge Reassignment (MERGE) algorithm, and the local search phase, utilizing Simulated Annealing, are distributed over available compute cores. Our results suggest that the proposed parallelization strategy is capable of solving the compactness-driven regionalization problem both efficiently and effectively. We expect this work to advance CyberGIS research by extending its application areas into the regionalization world and to make a contribution to the spatial analysis community by proposing this parallelization strategy to solve large regionalization problems efficiently. 相似文献
717.
The equivalent widths of the oxygen lines at 7774 and 8446 and of H (and some H) have been measured for 22 early-type, emission-line stars. A strong correlation between H and 8446 intensities has been found, although there is no such correlation between H and 7774. This confirms the probability that Bowen's mechanism is operative (the neutral oxygen 33
D state is overpopulated because the excitation energy of Ly- nearly coincides with that of theOi 1025 line). The possibility of using 8446 and H equivalent widths for a comparison of oxygen to hydrogen abundances in these stars is discussed. 相似文献
718.
719.
Semiarid rivers are often characterized by chains of small pools connected by riffles and wet meadows. The pools can be maintained by wet season surface runoff, groundwater discharge, or some combination thereof. Using synoptic surveys for several environmental tracers (δD and δ18O of H2O, specific electrical conductance at 25°C [EC], chloride and 222Rn), we evaluated the groundwater—surface water connectivity of the Light River (South Australia) along an 8 km section in the vicinity of a proposed mining development. In all three surveys (representing spring, summer and winter conditions), the pools were maintained by regional groundwater discharge based on an elevated surface water EC (9–12 dS m−1) similar to regional groundwater, elevated radon-222 activities (0.09–3.0 Bq L−1) and low rainfall. Most pools were perennial, either because they directly received groundwater discharge or, indirectly, had an inflow originating from upstream groundwater-fed pools. The elevated salinity of regional groundwater is a key factor for the maintenance of perennial pools in the Light River because the potential for baseflow depletion by groundwater pumping is more limited. 相似文献
720.
Hydrologic studies require high-quality streamflow measurements. V-notch weirs provide accurate discharge measurements and when properly equipped produce continuous records of streamflow. However, sensor drift and accumulations of floating woody debris cause systematic errors in depth measurements behind weirs, particularly during periods of base flow between storms. Manual processing of high-frequency streamflow data is often subjective and difficult to reproduce. We developed a method to automatically correct erroneous depth time series to accelerate data processing and promote objectivity and reproducibility. The method inspects depth measurements, isolates and preserves periods of direct runoff during storms, and corrects erroneous inter-event baseflow. We applied experience obtained during manual data processing to implement an automatic correction method based on time series decomposition to produce a consistent time series of stage data that eliminates obvious errors due to sensor drift and clogging by woody debris. The automatic method offers an objective and reproducible procedure capable of efficiently processing 1 year of stage data in seconds. This method promotes reproducibility and may appeal to water resources experts working with large amounts of data especially at sites where distance, cost, or inclement weather prevents regular instrument maintenance. 相似文献