全文获取类型
收费全文 | 1719篇 |
免费 | 66篇 |
国内免费 | 20篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 126篇 |
地球物理 | 418篇 |
地质学 | 734篇 |
海洋学 | 147篇 |
天文学 | 177篇 |
综合类 | 7篇 |
自然地理 | 156篇 |
出版年
2021年 | 18篇 |
2020年 | 20篇 |
2019年 | 14篇 |
2018年 | 37篇 |
2017年 | 48篇 |
2016年 | 65篇 |
2015年 | 42篇 |
2014年 | 50篇 |
2013年 | 107篇 |
2012年 | 73篇 |
2011年 | 93篇 |
2010年 | 92篇 |
2009年 | 112篇 |
2008年 | 84篇 |
2007年 | 90篇 |
2006年 | 80篇 |
2005年 | 96篇 |
2004年 | 83篇 |
2003年 | 58篇 |
2002年 | 57篇 |
2001年 | 43篇 |
2000年 | 25篇 |
1999年 | 33篇 |
1998年 | 27篇 |
1997年 | 15篇 |
1996年 | 22篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 21篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 4篇 |
1974年 | 14篇 |
1973年 | 5篇 |
1972年 | 3篇 |
排序方式: 共有1805条查询结果,搜索用时 31 毫秒
11.
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
12.
The role of baseflow in dissolved solids delivery to streams in the Upper Colorado River Basin 下载免费PDF全文
Christine A. Rumsey Matthew P. Miller Gregory E. Schwarz Robert M. Hirsch David D. Susong 《水文研究》2017,31(26):4705-4718
Salinity has a major effect on water users in the Colorado River Basin, estimated to cause almost $300 million per year in economic damages. The Colorado River Basin Salinity Control Program implements and manages projects to reduce salinity loads, investing millions of dollars per year in irrigation upgrades, canal projects, and other mitigation strategies. To inform and improve mitigation efforts, there is a need to better understand sources of salinity to streams and how salinity has changed over time. This study explores salinity in the baseflow fraction of streamflow, assessing whether groundwater is a significant contributor of dissolved solids to streams in the Upper Colorado River Basin (UCRB). Chemical hydrograph separation was used to estimate baseflow discharge and baseflow dissolved solids loads at stream gages (n = 69) across the UCRB. On average, it is estimated that 89% of dissolved solids loads originate from the baseflow fraction of streamflow, indicating that subsurface transport processes play a dominant role in delivering dissolved solids to streams in the UCRB. A statistical trend analysis using weighted regressions on time, discharge, and season was used to evaluate changes in baseflow dissolved solids loads in streams (n = 27) from 1986 to 2011. Decreasing trends in baseflow dissolved solids loads were observed at 63% of streams. At the three most downstream sites, Green River at Green River, UT, Colorado River at Cisco, UT, and the San Juan River near Bluff, UT, baseflow dissolved solids loads decreased by a combined 823,000 metric tons (mT), which is approximately 69% of projected basin‐scale decreases in total dissolved solids loads as a result of salinity control efforts. Decreasing trends in baseflow dissolved solids loads suggest that salinity mitigation projects, landscape changes, and/or climate are reducing dissolved solids transported to streams through the subsurface. Notably, the pace and extent of decreases in baseflow dissolved solids loads declined during the most recent decade; average decreasing loads during the 2000s (28,200 mT) were only 54% of average decreasing loads in the 1990s (51,700 mT). 相似文献
13.
Mathieu Duval Christophe Falguères Jean‐Jacques Bahain Rainer Grün Qingfeng Shao Maxime Aubert John Hellstrom Jean‐Michel Dolo Jordi Agusti Bienvenido Martínez‐Navarro Paul Palmqvist Isidro Toro‐Moyano 《第四纪科学杂志》2011,26(6):603-615
The palaeontological site of Venta Micena (Orce, Andalusia, Spain) lies in the eastern sector of the Guadix–Baza basin, one of the best documented areas in Europe for Plio‐Pleistocene biostratigraphy. The combination of biochronological and palaeomagnetic results, combined with the radiometric data obtained for Atapuerca Sima del Elefante, indicated that the Venta Micena stratum was formed between the Jaramillo and Olduvai palaeomagnetic events, most likely between 1.22 and 1.77 Ma. Five fossil teeth from two outcrops (sites A and B) were selected to assess the potential of combined uranium series–electron spin resonance (US‐ESR) dating of Early Pleistocene sites. Although the US‐ESR results of the first outcrop showed a large scatter between the three teeth, the mean age of 1.37 ± 0.24 Ma can be considered a reasonable age estimate for Venta Micena. The mean ESR age of 0.62 ± 0.03 Ma obtained for site B seems to be a severe underestimation when compared with the independent age control. This underestimation is attributed to a relative recent U‐mobilization event that led to some U‐leaching. The results show that any ESR age calculations of old samples are extremely sensitive to variations in the measured 230Th/234U ratios in dental tissues. Although the results demonstrate that ESR can in principle be applied to Early Pleistocene sites, they also reveal the complexity of dating such old teeth. It is necessary to continue research in several directions, such as study of the behaviour of ESR signals in old teeth and understanding recent U‐mobilization processes, to improve the reliability of the combined US‐ESR dating method applied to Early Pleistocene times, a period for which the number of available numerical dating techniques is very limited. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
14.
Diffusive mass exchange of non‐reactive substances in dual‐porosity porous systems – column experiments under saturated conditions 下载免费PDF全文
Diffusive mass exchange into immobile water regions within heterogeneous porous aquifers influences the fate of solutes. The percentage of immobile water is often unidentified in natural aquifers though. Hence, the mathematical prediction of solute transport in such heterogeneous aquifers remains challenging. The objective of this study was to find a simple analytical model approach that allows quantifying properties of mobile and immobile water regions and the portion of immobile water in a porous system. Therefore, the Single Fissure Dispersion Model (SFDM), which takes into account diffusive mass exchange between mobile and immobile water zones, was applied to model transport in well‐defined saturated dual‐porosity column experiments. Direct and indirect model validation was performed by running experiments at different flow velocities and using conservative tracer with different molecular diffusion coefficients. In another column setup, immobile water regions were randomly distributed to test the model applicability and to determine the portion of immobile water. In all setups, the tracer concentration curves showed differences in normalized maximum peak concentration, tailing and mass recovery according to their diffusion coefficients. These findings were more pronounced at lower flow rates (larger flow times) indicating the dependency of diffusive mass exchange into immobile water regions on tracers' molecular diffusion coefficients. The SFDM simulated all data with high model efficiency. Successful model validation supported the physical meaning of fitted model parameters. This study showed that the SFDM, developed for fissured aquifers, is applicable in porous media and can be used to determine porosity and volume of regions with immobile water. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
15.
16.
Marinka E.B. van Puijenbroek Juul Limpens Alma V. de Groot Michel J.P.M. Riksen Maurits Gleichman Pieter A. Slim Han F. van Dobben Frank Berendse 《地球表面变化过程与地形》2017,42(11):1733-1744
For development of embryo dunes on the highly dynamic land–sea boundary, summer growth and the absence of winter erosion are essential. Other than that, however, we know little about the specific conditions that favour embryo dune development. This study explores the boundary conditions for early dune development to enable better predictions of natural dune expansion. Using a 30 year time series of aerial photographs of 33 sites along the Dutch coast, we assessed the influence of beach morphology (beach width and tidal range), meteorological conditions (storm characteristics, wind speed, growing season precipitation, and temperature), and sand nourishment on early dune development. We examined the presence and area of embryo dunes in relation to beach width and tidal range, and compared changes in embryo dune area to meteorological conditions and whether sand nourishment had been applied. We found that the presence and area of embryo dunes increased with increasing beach width. Over time, embryo dune area was negatively correlated with storm intensity and frequency. Embryo dune area was positively correlated with precipitation in the growing season and sand nourishment. Embryo dune area increased in periods of low storm frequency and in wet summers, and decreased in periods of high storm frequency or intensity. We conclude that beach morphology is highly influential in determining the potential for new dune development, and wide beaches enable development of larger embryo dune fields. Sand nourishment stimulates dune development by increasing beach width. Finally, weather conditions and non‐interrupted sequences of years without high‐intensity storms determine whether progressive dune development will take place. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
17.
18.
19.
Derek G. Turner Brent C. Ward Duane G. Froese Michel Lamothe Jeffrey D. Bond Nancy H. Bigelow 《Boreas: An International Journal of Quaternary Research》2016,45(3):521-536
At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 14C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon. 相似文献
20.
Direct measurements of plasma motions in the photosphere are limited to the line-of-sight component of the velocity. Several algorithms have therefore been developed to reconstruct the transverse components from observed continuum images or magnetograms. We compare the space and time averages of horizontal velocity fields in the photosphere inferred from pairs of consecutive intensitygrams by the LCT, FLCT, and CST methods and the DeepVel neural network in order to identify the method that is best suited for generating synthetic observations to be used for data assimilation. The Stein and Nordlund (Astrophys. J. Lett.753, L13, 2012) magnetoconvection simulation is used to generate synthetic SDO/HMI intensitygrams and reference flows to train DeepVel. Inferred velocity fields show that DeepVel performs best at subgranular and granular scales and is second only to FLCT at mesogranular and supergranular scales. 相似文献