全文获取类型
| 收费全文 | 2178篇 |
| 免费 | 943篇 |
| 国内免费 | 468篇 |
专业分类
| 测绘学 | 39篇 |
| 大气科学 | 33篇 |
| 地球物理 | 559篇 |
| 地质学 | 1937篇 |
| 海洋学 | 165篇 |
| 天文学 | 686篇 |
| 综合类 | 76篇 |
| 自然地理 | 94篇 |
出版年
| 2024年 | 8篇 |
| 2023年 | 24篇 |
| 2022年 | 54篇 |
| 2021年 | 81篇 |
| 2020年 | 104篇 |
| 2019年 | 103篇 |
| 2018年 | 114篇 |
| 2017年 | 117篇 |
| 2016年 | 151篇 |
| 2015年 | 129篇 |
| 2014年 | 164篇 |
| 2013年 | 175篇 |
| 2012年 | 154篇 |
| 2011年 | 147篇 |
| 2010年 | 129篇 |
| 2009年 | 199篇 |
| 2008年 | 143篇 |
| 2007年 | 171篇 |
| 2006年 | 211篇 |
| 2005年 | 172篇 |
| 2004年 | 157篇 |
| 2003年 | 130篇 |
| 2002年 | 111篇 |
| 2001年 | 101篇 |
| 2000年 | 108篇 |
| 1999年 | 106篇 |
| 1998年 | 72篇 |
| 1997年 | 52篇 |
| 1996年 | 35篇 |
| 1995年 | 27篇 |
| 1994年 | 37篇 |
| 1993年 | 21篇 |
| 1992年 | 16篇 |
| 1991年 | 21篇 |
| 1990年 | 19篇 |
| 1989年 | 4篇 |
| 1988年 | 7篇 |
| 1987年 | 2篇 |
| 1986年 | 4篇 |
| 1985年 | 6篇 |
| 1983年 | 2篇 |
| 1979年 | 1篇 |
排序方式： 共有3589条查询结果，搜索用时 793 毫秒
1.
Subsurface flow measurements using passive flux meters in variably-saturated cold-regions landscapes
To date, passive flux meters have predominantly been applied in temperate environments for tracking the movement of contaminants in groundwater. This study applies these instruments to reduce uncertainty in (typically instantaneous) flux measurements made in a low-gradient, wetland dominated, discontinuous permafrost environment. This method supports improved estimation of unsaturated and over-winter subsurface flows which are very difficult to quantify using hydraulic gradient-based approaches. Improved subsurface flow estimates can play a key role in understanding the water budget of this landscape. 相似文献
2.
黑洞X射线暂现源的迷你爆发是一类峰值光度较低、持续时间较短的爆发.由于观测数据较少,其物理机制仍不清楚.利用RXTE (Rossi X-ray Timing Explorer)卫星从2001年1月28日到3月14日的数据,研究了黑洞X射线暂现源XTE J1550–564 2001年迷你爆发的X射线能谱特性.发现在本次迷你爆发中, XTE J1550–564的X射线能谱可以用幂律谱很好地拟合.整个爆发的硬度强度图不是标准的q型,而是一直保持在最右侧.此外,还分析了此次爆发谱指数Γ与未吸收的2–10 keV能段的X射线流量F_(2–10 keV)的相关性,发现Γ-F_(2–10 keV)呈反相关关系,且谱指数Γ∈[1.35, 1.72].上述结果表明2001年这次爆发一直处于低/硬态,它的X射线辐射主要来自于辐射低效的吸积模式,如ADAF(Advection-Dominated Accretion Flow). 相似文献
3.
Solution for a plane strain rough‐walled hydraulic fracture driven by turbulent fluid through impermeable rock 
下载免费PDF全文
下载免费PDF全文 The impact of turbulent flow on plane strain fluid‐driven crack propagation is an important but still poorly understood consideration in hydraulic fracture modeling. The changes that hydraulic fracturing has experienced over the past decade, especially in the area of fracturing fluids, have played a major role in the transition of the typical fluid regime from laminar to turbulent flow. Motivated by the increasing preponderance of high‐rate, water‐driven hydraulic fractures with high Reynolds number, we present a semianalytical solution for the propagation of a plane strain hydraulic fracture driven by a turbulent fluid in an impermeable formation. The formulation uses a power law relationship between the Darcy‐Weisbach friction factor and the scale of the fracture roughness, where one specific manifestation of this generalized friction factor is the classical Gauckler‐Manning‐Strickler approximation for turbulent flow in a rough‐walled channel. Conservation of mass, elasticity, and crack propagation are also solved simultaneously. We obtain a semianalytical solution using an orthogonal polynomial series. An approximate closed‐form solution is enabled by a choice of orthogonal polynomials embedding the near‐tip asymptotic behavior and thus giving very rapid convergence; a precise solution is obtained with 2 terms of the series. By comparison with numerical simulations, we show that the transition region between the laminar and turbulent regimes can be relatively small so that full solutions can often be well approximated by either a fully laminar or fully turbulent solution. 相似文献
4.
This study evaluated the spatial variability of streambed vertical hydraulic conductivity (Kv) in different stream morphologies in the Frenchman Creek Watershed, Western Nebraska, using different variogram models. Streambed Kv values were determined in situ using permeameter tests at 10 sites in Frenchman, Stinking Water and Spring Creeks during the dry season at baseflow conditions. Measurements were taken both in straight and meandering stream channels during a 5 day period at similar flow conditions. Each test site comprised of at least three transects and each transect comprised of at least three Kv measurements. Linear, Gaussian, exponential and spherical variogram models were used with Kriging gridding method for the 10 sites. As a goodness-of-fit statistic for the variogram models, cross-validation results showed differences in the median absolute deviation and the standard deviation of the cross-validation residuals. Results show that using the geometric means of the 10 sites for gridding performs better than using either all the Kv values from the 93 permeameter tests or 10 Kv values from the middle transects and centre permeameters. Incorporating both the spatial variability and the uncertainty involved in the measurement at a reach segment can yield more accurate grid results that can be useful in calibrating Kv at watershed or sub-watershed scales in distributed hydrological models. 相似文献
5.
6.
7.
8.
9.
10.