全文获取类型
收费全文 | 81910篇 |
免费 | 1978篇 |
国内免费 | 700篇 |
专业分类
测绘学 | 2287篇 |
大气科学 | 6328篇 |
地球物理 | 17721篇 |
地质学 | 28552篇 |
海洋学 | 6668篇 |
天文学 | 17663篇 |
综合类 | 373篇 |
自然地理 | 4996篇 |
出版年
2021年 | 638篇 |
2020年 | 703篇 |
2019年 | 683篇 |
2018年 | 2049篇 |
2017年 | 1963篇 |
2016年 | 2284篇 |
2015年 | 1415篇 |
2014年 | 2155篇 |
2013年 | 4088篇 |
2012年 | 2632篇 |
2011年 | 3111篇 |
2010年 | 2632篇 |
2009年 | 3342篇 |
2008年 | 3002篇 |
2007年 | 2923篇 |
2006年 | 2782篇 |
2005年 | 3032篇 |
2004年 | 3010篇 |
2003年 | 2667篇 |
2002年 | 2161篇 |
2001年 | 1928篇 |
2000年 | 1792篇 |
1999年 | 1577篇 |
1998年 | 1527篇 |
1997年 | 1566篇 |
1996年 | 1256篇 |
1995年 | 1281篇 |
1994年 | 1231篇 |
1993年 | 1076篇 |
1992年 | 1047篇 |
1991年 | 993篇 |
1990年 | 1115篇 |
1989年 | 995篇 |
1988年 | 957篇 |
1987年 | 1055篇 |
1986年 | 911篇 |
1985年 | 1202篇 |
1984年 | 1300篇 |
1983年 | 1277篇 |
1982年 | 1235篇 |
1981年 | 1137篇 |
1980年 | 1078篇 |
1979年 | 988篇 |
1978年 | 978篇 |
1977年 | 896篇 |
1976年 | 836篇 |
1975年 | 771篇 |
1974年 | 826篇 |
1973年 | 829篇 |
1972年 | 522篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
11.
Colin P. R. McCarter Stephen D. Sebestyen Susan L. Eggert Randall K. Kolka Carl P. J. Mitchell 《水文研究》2020,34(26):5354-5369
Sustainable fuels legislation and volatility in energy prices have put additional pressures on the forestry sector to intensify the harvesting of biomass for “advanced biofuel” production. To better understand how residual biomass removal after harvest affects forest hydrology in relatively low slope terrain, a Before-After-Control-Impact (BACI) study was conducted in the USDA Forest Service's Marcell Experimental Forest, Minnesota, USA. Hydrological measurements were made from 2010–2013 on a forested hillslope that was divided into three treatment blocks, where one block was harvested and residual biomass removed (Biomass Removed), the second was harvested and residual biomass left (Biomass Left), and the last block was left as an Unharvested Control. The pre-harvest period (2 years) was 2010–11 and post-harvest (2 years) was 2012–13. Water table elevation at the upslope and downslope position, subsurface runoff, and soil moisture were measured between May–November. Mixed effect statistical models were used to compare both the before-after and “control” treatment ratios (ratios between harvested hillslopes and the Unharvested Control hillslope). Subsurface runoff significantly increased (p < .05) at both harvested hillslopes but to a greater degree on the Biomass Left hillslope. Greater subsurface runoff volumes at both harvested hillslopes were driven by substantial increases during fall, with additional significant increases during summer on the Biomass Left hillslope. The hydrological connectivity, inferred from event runoff ratios, increased due to harvesting at both hillslopes but only significantly on the Biomass Left hillslope. The winter harvest minimized soil disturbance, resulting in no change to the effective hydraulic conductivity distribution with depth. Thus, the observed hydrological changes were driven by increased effective precipitation and decreased evapotranspiration, increasing the duration that both harvested hillslopes were hydrologically active. The harvesting of residual biomass appears to lessen hydrological connectivity relative to leaving residual biomass on the hillslope, potentially decreasing downstream hydrological impacts of similar forestry operations. 相似文献
12.
13.
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
14.
Micromechanical approach to effective viscoelastic properties of micro‐fractured geomaterials 下载免费PDF全文
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
15.
The mass ratio of Charon to Pluto is a basic parameter describing the binary system and is necessary for determining the individual masses and densities of these two bodies. Previous measurements of the mass ratio have been made, but the solutions differ significantly (Null et al., 1993; Young et al., 1994; Null and Owen, 1996; Foust et al., 1997; Tholen and Buie, 1997). We present the first observations of Pluto and Charon with a well-calibrated astrometric instrument—the fine guidance sensors on the Hubble Space Telescope. We observed the motion of Pluto and Charon about the system barycenter over 4.4 days (69% of an orbital period) and determined the mass ratio to be 0.122±0.008 which implies a density of 1.8 to 2.1 g cm−3 for Pluto and 1.6 to 1.8 g cm−3 for Charon. The resulting rock-mass fractions for Pluto and Charon are higher than expected for bodies formed in the outer solar nebula, possibly indicating significant postaccretion loss of volatiles. 相似文献
16.
17.
18.
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
19.
20.