全文获取类型
收费全文 | 4328篇 |
免费 | 167篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 189篇 |
大气科学 | 651篇 |
地球物理 | 1005篇 |
地质学 | 1780篇 |
海洋学 | 178篇 |
天文学 | 534篇 |
综合类 | 18篇 |
自然地理 | 151篇 |
出版年
2021年 | 65篇 |
2020年 | 63篇 |
2019年 | 44篇 |
2018年 | 141篇 |
2017年 | 161篇 |
2016年 | 236篇 |
2015年 | 134篇 |
2014年 | 198篇 |
2013年 | 254篇 |
2012年 | 109篇 |
2011年 | 171篇 |
2010年 | 184篇 |
2009年 | 225篇 |
2008年 | 156篇 |
2007年 | 126篇 |
2006年 | 115篇 |
2005年 | 95篇 |
2004年 | 58篇 |
2003年 | 66篇 |
2002年 | 98篇 |
2001年 | 86篇 |
2000年 | 66篇 |
1999年 | 70篇 |
1998年 | 69篇 |
1997年 | 63篇 |
1996年 | 52篇 |
1995年 | 69篇 |
1994年 | 75篇 |
1993年 | 31篇 |
1992年 | 32篇 |
1991年 | 33篇 |
1990年 | 42篇 |
1989年 | 35篇 |
1988年 | 32篇 |
1987年 | 33篇 |
1985年 | 43篇 |
1984年 | 45篇 |
1983年 | 54篇 |
1982年 | 41篇 |
1981年 | 43篇 |
1980年 | 37篇 |
1979年 | 28篇 |
1978年 | 55篇 |
1977年 | 36篇 |
1976年 | 32篇 |
1975年 | 43篇 |
1974年 | 52篇 |
1973年 | 47篇 |
1969年 | 26篇 |
1968年 | 26篇 |
排序方式: 共有4506条查询结果,搜索用时 25 毫秒
1.
Ahmet Güllü Ercan Yüksel Cem Yalçın A. Anıl Dindar Hasan Özkaynak Oral Büyüköztürk 《地震工程与结构动力学》2019,48(1):27-45
Input energy is the principal component of the energy balance equation. It is beneficial to determine, through its components, how the recoverable and irrecoverable energies are distributed within the structural elements. Several equations and attenuation relations to define mass-normalized input energy spectra exist in the literature. They are mainly proposed for elastic systems subjected to far-fault EQs. There is a lack of experimental verification of these proposed spectra. In this paper, experimental assessment was performed to the existing spectra, and further improvements were accomplished. For this purpose, steel cantilever columns were tested on the shake table for two specific historical EQs coincidently having similar spectral acceleration values. Based on the experimental results, a three-part mass-normalized relative input energy spectrum was formulated including soil type, EQ (corner period, intensity, duration, spectral acceleration, and velocity), and structural behavioral characteristics (period and structural damping). The proposed input energy spectrum was experimentally calibrated and numerically validated for various EQs featuring near- and far-field types. Analytical and experimental comparisons were made between the previously developed spectrum and the newly proposed one. The validation studies and the statistical evaluations exposed that the proposed spectrum yielded better agreement with the experimental and numerical results. 相似文献
2.
Mesocosm experiments identifying hotspots of groundwater upwelling in a water column by fibre optic distributed temperature sensing 下载免费PDF全文
Amaya Irene Marruedo Arricibita Stefan Krause Jesus Gomez‐Velez David M. Hannah Jörg Lewandowski 《水文研究》2018,32(2):185-199
Lacustrine groundwater discharge (LGD) can substantially impact ecosystem characteristics and functions. Fibre optic distributed temperature sensing (FO‐DTS) has been successfully used to locate groundwater discharge into lakes and rivers at the sediment–water interface, but locating groundwater discharge would be easier if it could be detected from the more accessible water surface. So far, it is not clear if how and under which conditions the LGD signal propagates through the water column to the water surface–atmosphere interface, and what perturbations and signal losses occur along this pathway. In the present study, LGD was simulated in a mesocosm experiment. Under winter conditions, water with temperatures of 14 to 16 °C was discharged at the bottom of a 10 × 2.8‐m mesocosm. Water within this mesocosm ranged from 4.0 to 7.4 °C. Four layers (20, 40, 60, and 80 cm above the sediment) of the 82 cm deep mesocosm were equipped with FO‐DTS for tracing thermal patterns in the mesocosm. Aims are (a) to test whether the positive buoyancy of relatively warm groundwater imported by LGD into shallow water bodies allows detection of LGD at the lake's water surface–atmosphere interface by FO‐DTS, (b) to analyse the propagation of the temperature signal from the sediment‐water interface through the water column, and (c) to learn more about detectability of the signal under different discharge rates and weather conditions. The experiments supported the benchmarking of scale dependencies and robustness of FO‐DTS applications for measuring upwelling into aquatic environments and revealed that weather conditions can have important impacts on the detection of upwelling at water surface–atmosphere interfaces at larger scales. 相似文献
3.
4.
Evaluating Subsurface Parameterization to Simulate Hyporheic Exchange: The Steinlach River Test Site
Reynold Chow Jeremy Bennett Jürnjakob Dugge Thomas Wöhling Wolfgang Nowak 《Ground water》2020,58(1):93-109
Hyporheic exchange is the interaction of river water and groundwater, and is difficult to predict. One of the largest contributions to predictive uncertainty for hyporheic exchange has been attributed to the representation of heterogeneous subsurface properties. Our study evaluates the trade-offs between intrinsic (irreducible) and epistemic (reducible) model errors when choosing between homogeneous and highly complex subsurface parameter structures. We modeled the Steinlach River Test Site in Southwest Germany using a fully coupled surface water-groundwater model to simulate hyporheic exchange and to assess the predictive errors and uncertainties of transit time distributions. A highly parameterized model was built, treated as a “virtual reality” and used as a reference. We found that if the parameter structure is too simple, it will be limited by intrinsic model errors. By increasing subsurface complexity through the addition of zones or heterogeneity, we can begin to exchange intrinsic for epistemic errors. Thus, the appropriate level of detail to represent the subsurface depends on the acceptable range of intrinsic structural errors for the given modeling objectives and the available site data. We found that a zonated model is capable of reproducing the transit time distributions of a more detailed model, but only if the geological structures are known. An interpolated heterogeneous parameter field (cf. pilot points) showed the best trade-offs between the two errors, indicating fitness for practical applications. Parameter fields generated by multiple-point geostatistics (MPS) produce transit time distributions with the largest uncertainties, however, these are reducible by additional hydrogeological data, particularly flux measurements. 相似文献
5.
Petter Nyman Walter A.C. Box Justin C. Stout Gary J. Sheridan Saskia D. Keesstra Patrick N.J. Lane Christoph Langhans 《地球表面变化过程与地形》2020,45(5):1155-1167
Field studies that investigate sediment transport between debris-flow-producing headwaters and rivers are uncommon, particularly in forested settings, where debris flows are infrequent and opportunities for collecting data are limited. This study quantifies the volume and composition of sediment deposited in the arterial channel network of a 14-km2 catchment (Washington Creek) that connects small, burned and debris-flow-producing headwaters (<1 km2) with the Ovens River in SE Australia. We construct a sediment budget by combining new data on deposition with a sediment delivery model for post-fire debris flows. Data on deposits were plotted alongside the slope–area curve to examine links between processes, catchment morphometry and geomorphic process domains. The results show that large deposits are concentrated in the proximity of three major channel junctions, which correspond to breaks in channel slope. Hyperconcentrated flows are more prominent towards the catchment outlet, where the slope–area curve indicates a transition from debris flow to fluvial domains. This shift corresponds to a change in efficiency of the flow, determined from the ratio of median grain size to channel slope. Our sediment budget suggests a total sediment efflux from Washington Creek catchment of 61 × 103 m3. There are similar contributions from hillslopes (43 ± 14 × 103 m3), first to third stream order channel (35 ± 12 × 103 m3) and the arterial fourth to fifth stream order channel (31 ± 17 × 103 m3) to the total volume of erosion. Deposition (39 ± 17 × 103 m3) within the arterial channel was higher than erosion (31 ± 17 × 103 m3), which means a net sediment gain of about 8 × 103 m3 in the arterial channel. The ratio of total deposition to total erosion was 0.44. For fines <63 μm, this ratio was much smaller (0.11), which means that fines are preferentially exported. This has important implications for suspended sediment and water quality in downstream rivers. © 2019 John Wiley & Sons, Ltd. 相似文献
6.
J. Ignacio López-Moreno Leena Leppänen Bartłomiej Luks Ladislav Holko Ghislain Picard Alba Sanmiguel-Vallelado Esteban Alonso-González David C. Finger Ali N. Arslan Katalin Gillemot Aynur Sensoy Arda Sorman M. Cansaran Ertaş Steven R. Fassnacht Charles Fierz Christoph Marty 《水文研究》2020,34(14):3120-3133
Manually collected snow data are often considered as ground truth for many applications such as climatological or hydrological studies. However, there are many sources of uncertainty that are not quantified in detail. For the determination of water equivalent of snow cover (SWE), different snow core samplers and scales are used, but they are all based on the same measurement principle. We conducted two field campaigns with 9 samplers commonly used in observational measurements and research in Europe and northern America to better quantify uncertainties when measuring depth, density and SWE with core samplers. During the first campaign, as a first approach to distinguish snow variability measured at the plot and at the point scale, repeated measurements were taken along two 20 m long snow pits. The results revealed a much higher variability of SWE at the plot scale (resulting from both natural variability and instrumental bias) compared to repeated measurements at the same spot (resulting mostly from error induced by observers or very small scale variability of snow depth). The exceptionally homogeneous snowpack found in the second campaign permitted to almost neglect the natural variability of the snowpack properties and focus on the separation between instrumental bias and error induced by observers. Reported uncertainties refer to a shallow, homogeneous tundra-taiga snowpack less than 1 m deep (loose, mostly recrystallised snow and no wind impact). Under such measurement conditions, the uncertainty in bulk snow density estimation is about 5% for an individual instrument and is close to 10% among different instruments. Results confirmed that instrumental bias exceeded both the natural variability and the error induced by observers, even in the case when observers were not familiar with a given snow core sampler. 相似文献
7.
8.
Gualtiero Badin Jörn Behrens Christian Franzke Jens Rademacher 《地球物理与天体物理流体动力学》2019,113(5-6):425-427
9.
Ansgar Greshake Richard Wirth Jörg Fritz Tomasz Jakubowski Ute Böttger 《Meteoritics & planetary science》2018,53(3):467-481
Libyan Desert Glass (LDG) is a SiO2-rich natural glass whose origin, formation mechanism, and target material are highly debated. We here report on the finding of a lens-shaped whitish inclusion within LDG. The object is dominantly composed of siliceous glass and separated from the surrounding LDG by numerous cristobalite grains. Within cristobalite, several regions rich in mullite often associated with ilmenite were detected. Mineral assemblage, chemical composition, and grain morphologies suggest that mullite was formed by thermal decomposition of kaolinitic clay at atmospheric pressure and T ≥ 1600 °C and also attested to high cooling rates under nonequilibrium conditions. Cristobalite contains concentric and irregular internal cracks and is intensely twinned, indicating that first crystallized β-cristobalite inverted to α-cristobalite during cooling of the SiO2-rich melt. The accompanied volume reduction of 4% induced the high density of defects. The whitish inclusion also contains several partly molten rutile grains evidencing that at least locally the LDG melt was at T ≥ 1800 °C. Based on these observations, it is concluded that LDG was formed by high-temperature melting of kaolinitic clay-, rutile-, and ilmenite-bearing Cenozoic sandstone or sand very likely during an asteroid or comet impact onto Earth. While melting and ejection occurred at high pressures, the melt solidified quickly at atmospheric pressure. 相似文献
10.
Biotic drivers of river and floodplain geomorphology – New molecular methods for assessing present‐day and past biota 下载免费PDF全文
Geomorphology has increasingly considered the role of biotic factors as controls upon geomorphic processes across a wide range of spatial and temporal scales. Where timescales are long (centennial and longer), it has been possible to quantify relationships between geomorphic processes and vegetation using, for example, the pollen record. However, where the biotic agents are fauna, longer term reconstruction of the impacts of biological activity upon geomorphic processes is more challenging. Here, we review the prospect of using environmental DNA as a molecular proxy to decipher the presence and nature of faunal influences on geomorphic processes in both present and ancient deposits. When used appropriately, this method has the potential to improve our understanding of biotic drivers of geomorphic processes, notably fauna, over long timescales and so to reconstruct how such drivers might explain the landscape as we see it today. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献