全文获取类型
收费全文 | 30656篇 |
免费 | 702篇 |
国内免费 | 335篇 |
专业分类
测绘学 | 902篇 |
大气科学 | 2171篇 |
地球物理 | 6946篇 |
地质学 | 10559篇 |
海洋学 | 2501篇 |
天文学 | 7048篇 |
综合类 | 83篇 |
自然地理 | 1483篇 |
出版年
2022年 | 146篇 |
2021年 | 302篇 |
2020年 | 340篇 |
2019年 | 371篇 |
2018年 | 851篇 |
2017年 | 815篇 |
2016年 | 1056篇 |
2015年 | 655篇 |
2014年 | 954篇 |
2013年 | 1643篇 |
2012年 | 1084篇 |
2011年 | 1284篇 |
2010年 | 1190篇 |
2009年 | 1495篇 |
2008年 | 1313篇 |
2007年 | 1288篇 |
2006年 | 1263篇 |
2005年 | 956篇 |
2004年 | 942篇 |
2003年 | 871篇 |
2002年 | 804篇 |
2001年 | 701篇 |
2000年 | 729篇 |
1999年 | 619篇 |
1998年 | 632篇 |
1997年 | 590篇 |
1996年 | 437篇 |
1995年 | 462篇 |
1994年 | 485篇 |
1993年 | 359篇 |
1992年 | 348篇 |
1991年 | 301篇 |
1990年 | 363篇 |
1989年 | 312篇 |
1988年 | 291篇 |
1987年 | 313篇 |
1986年 | 279篇 |
1985年 | 360篇 |
1984年 | 377篇 |
1983年 | 371篇 |
1982年 | 362篇 |
1981年 | 290篇 |
1980年 | 306篇 |
1979年 | 254篇 |
1978年 | 255篇 |
1977年 | 256篇 |
1976年 | 206篇 |
1975年 | 230篇 |
1974年 | 207篇 |
1973年 | 209篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
Cristián Frêne Juan J. Armesto Freddy Véliz Fernando D. Alfaro Kathleen C. Weathers 《水文研究》2021,35(5):e14175
The variability of rainfall-dependent streamflow at catchment scale modulates many ecosystem processes in wet temperate forests. Runoff in small mountain catchments is characterized by a quick response to rainfall pulses which affects biogeochemical fluxes to all downstream systems. In wet-temperate climates, water erosion is the most important natural factor driving downstream soil and nutrient losses from upland ecosystems. Most hydrochemical studies have focused on water flux measurements at hourly scales, along with weekly or monthly samples for water chemistry. Here, we assessed how water and element flows from broad-leaved, evergreen forested catchments in southwestern South America, are influenced by different successional stages, quantifying runoff, sediment transport and nutrient fluxes during hourly rainfall events of different intensities. Hydrograph comparisons among different successional stages indicated that forested catchments differed in their responses to high intensity rainfall, with greater runoff in areas covered by secondary forests (SF), compared to old-growth forest cover (OG) and dense scrub vegetation (CH). Further, throughfall water was greatly nutrient enriched for all forest types. Suspended sediment loads varied between successional stages. SF catchments exported 455 kg of sediments per ha, followed by OG with 91 kg/ha and CH with 14 kg/ha, corresponding to 11 rainfall events measured from December 2013 to April 2014. Total nitrogen (TN) and phosphorus (TP) concentrations in stream water also varied with rainfall intensity. In seven rainfall events sampled during the study period, CH catchments exported less nutrients (46 kg/ha TN and 7 kg/ha TP) than SF catchments (718 kg/ha TN and 107 kg/ha TP), while OG catchments exported intermediate sediment loads (201 kg/ha TN and 23 kg/ha TP). Further, we found significant effects of successional stage attributes (vegetation structure and soil physical properties) and catchment morphometry on runoff and sediment concentrations, and greater nutrients retention in OG and CH catchments. We conclude that in these southern hemisphere, broad-leaved evergreen temperate forests, hydrological processes are driven by multiple interacting phenomena, including climate, vegetation, soils, topography, and disturbance history. 相似文献
2.
Micromechanical approach to effective viscoelastic properties of micro‐fractured geomaterials
下载免费PDF全文
![点击此处可从《国际地质力学数值与分析法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The aim of this paper is to formulate a micromechanics‐based approach to non‐aging viscoelastic behavior of materials with randomly distributed micro‐fractures. Unlike cracks, fractures are discontinuities that are able to transfer stresses and can therefore be regarded from a mechanical viewpoint as interfaces endowed with a specific behavior under normal and shear loading. Making use of the elastic‐viscoelastic correspondence principle together with a Mori‐Tanka homogenization scheme, the effective viscoelastic behavior is assessed from properties of the material constituents and damage parameters related to density and size of fractures. It is notably shown that the homogenized behavior thus formulated can be described in most cases by means of a generalized Maxwell rheological model. For practical implementation in structural analyses, an approximate model for the isotropic homogenized fractured medium is formulated within the class of Burger models. Although the approximation is basically developed for short‐term and long‐term behaviors, numerical applications indicate that the approximate Burger model accurately reproduce the homogenized viscoelastic behavior also in the transient conditions. 相似文献
3.
4.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献
5.
6.
7.
Zsolt Sándor Bálint Érdi Carl D. Murray 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):355-368
The dynamics of co-orbital motion in the restricted three-body problem are investigated by symplectic mappings. Analytical and semi-numerical mappings have been developed and studied in detail. The mappings have been tested by numerical integration of the equations of motion. These mappings have been proved to be useful for a quick determination of the phase space structure reflecting the main characteristics of the dynamics of the co-orbital problem. 相似文献
8.
T.-C. Jim Yeh 《水文研究》1992,6(4):369-395
This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended. 相似文献
9.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献