全文获取类型
收费全文 | 5975篇 |
免费 | 955篇 |
国内免费 | 943篇 |
专业分类
测绘学 | 116篇 |
大气科学 | 298篇 |
地球物理 | 511篇 |
地质学 | 3618篇 |
海洋学 | 306篇 |
天文学 | 2286篇 |
综合类 | 311篇 |
自然地理 | 427篇 |
出版年
2024年 | 13篇 |
2023年 | 52篇 |
2022年 | 145篇 |
2021年 | 148篇 |
2020年 | 149篇 |
2019年 | 171篇 |
2018年 | 147篇 |
2017年 | 164篇 |
2016年 | 178篇 |
2015年 | 189篇 |
2014年 | 259篇 |
2013年 | 260篇 |
2012年 | 288篇 |
2011年 | 294篇 |
2010年 | 270篇 |
2009年 | 497篇 |
2008年 | 389篇 |
2007年 | 498篇 |
2006年 | 469篇 |
2005年 | 433篇 |
2004年 | 412篇 |
2003年 | 393篇 |
2002年 | 352篇 |
2001年 | 301篇 |
2000年 | 256篇 |
1999年 | 242篇 |
1998年 | 224篇 |
1997年 | 114篇 |
1996年 | 76篇 |
1995年 | 105篇 |
1994年 | 78篇 |
1993年 | 58篇 |
1992年 | 69篇 |
1991年 | 35篇 |
1990年 | 33篇 |
1989年 | 40篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1986年 | 15篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有7873条查询结果,搜索用时 46 毫秒
1.
Hydrological trends and the evolution of catchment research in the Alptal valley,central Switzerland
Manfred Stähli Jan Seibert James W. Kirchner Jana von Freyberg Ilja van Meerveld 《水文研究》2021,35(4):e14113
When the observation of small headwater catchments in the pre-Alpine Alptal valley (central Switzerland) started in the late 1960s, the researchers were mainly interested in questions related to floods and forest management. Investigations of geomorphological processes in the steep torrent channels followed in the 1980s, along with detailed observations of biogeochemical and ecohydrological processes in individual forest stands. More recently, research in the Alptal has addressed the impacts of climate change on water supply and runoff generation. In this article, we describe, for the first time, the evolution of catchment research at Alptal, and present new analyses of long-term trends and short-term hydrologic behaviour. Hydrometeorological time series from the past 50 years show substantial interannual variability, but only minimal long-term trends, except for the ~2°C increase in mean annual air temperature over the 50-year period, and a corresponding shift towards earlier snowmelt. Similar to previous studies in larger Alpine catchments, the decadal variations in mean annual runoff in Alptal's small research catchments reflect the long-term variability in annual precipitation. In the Alptal valley, the most evident hydrological trends were observed in late spring and are related to the substantial change in the duration of the snow cover. Streamflow and water quality are highly variable within and between hydrological events, suggesting rapid shifts in flow pathways and mixing, as well as changing connectivity of runoff-generating areas. This overview illustrates how catchment research in the Alptal has evolved in response to changing societal concerns and emerging scientific questions. 相似文献
2.
Effects of the geometry of two‐dimensional fractures on their hydraulic aperture and on the validity of the local cubic law 下载免费PDF全文
Flow through rough fractures is investigated numerically in order to assess the validity of the local cubic law for different fracture geometries. Two‐dimensional channels with sinusoidal walls having different geometrical properties defined by the aperture, the amplitude, and the wavelength of the walls' corrugations, the corrugations asymmetry, and the phase shift between the two walls are considered to represent different fracture geometries. First, it is analytically shown that the hydraulic aperture clearly deviates from the mean aperture when the walls' roughness, the phase shift, and/or the asymmetry between the fracture walls are relatively high. The continuity and the Navier–Stokes equations are then solved by means of the finite element method and the numerical solutions compared to the theoretical predictions of the local cubic law. Reynolds numbers ranging from 0.066 to 66.66 are investigated so as to focus more particularly on the effect of flow inertial effects on the validity of the local cubic law. For low Reynolds number, typically less than 15, the local cubic law properly describes the fracture flow, especially when the fracture walls have small corrugation amplitudes. For Reynolds numbers higher than 15, the local cubic law is valid under the conditions that the fracture presents a low aspect ratio, small corrugation amplitudes, and a moderate phase lag between its walls. 相似文献
3.
A 43 cm long E271 sediment core collected near the East Pacific Rise(EPR) at 13°N were studied to investigate the origin of smectite for understanding better the geochemical behavior of hydrothermal material after deposition.E271 sediments are typical metalliferous sediments. After removal of organic matter, carbonate, biogenic opal,and Fe-Mn oxide by a series of chemical procedures, clay minerals(2 μm) were investigated by X-ray diffraction,chemical analysis and Si isotope analysis. Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. It is enriched in iron, and its contents are highest in clay minerals. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The Si that participates in this reaction may be derived from siliceous microfossils(diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And their δ30 Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Feoxyhydroxides. Sm/Fe mass ratios(a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and terrigenous clay minerals. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure, which decreases the value of metalliferous sediments as a potential resource for REE. 相似文献
4.
5.
6.
7.
8.
9.
10.