全文获取类型
收费全文 | 25886篇 |
免费 | 466篇 |
国内免费 | 213篇 |
专业分类
测绘学 | 520篇 |
大气科学 | 1939篇 |
地球物理 | 5200篇 |
地质学 | 9057篇 |
海洋学 | 2106篇 |
天文学 | 6011篇 |
综合类 | 47篇 |
自然地理 | 1685篇 |
出版年
2021年 | 160篇 |
2020年 | 197篇 |
2019年 | 209篇 |
2018年 | 498篇 |
2017年 | 462篇 |
2016年 | 566篇 |
2015年 | 436篇 |
2014年 | 606篇 |
2013年 | 1211篇 |
2012年 | 732篇 |
2011年 | 1015篇 |
2010年 | 875篇 |
2009年 | 1219篇 |
2008年 | 1052篇 |
2007年 | 1053篇 |
2006年 | 978篇 |
2005年 | 818篇 |
2004年 | 837篇 |
2003年 | 799篇 |
2002年 | 739篇 |
2001年 | 688篇 |
2000年 | 636篇 |
1999年 | 571篇 |
1998年 | 576篇 |
1997年 | 576篇 |
1996年 | 440篇 |
1995年 | 419篇 |
1994年 | 380篇 |
1993年 | 335篇 |
1992年 | 311篇 |
1991年 | 276篇 |
1990年 | 304篇 |
1989年 | 281篇 |
1988年 | 235篇 |
1987年 | 311篇 |
1986年 | 257篇 |
1985年 | 346篇 |
1984年 | 384篇 |
1983年 | 371篇 |
1982年 | 335篇 |
1981年 | 310篇 |
1980年 | 312篇 |
1979年 | 282篇 |
1978年 | 311篇 |
1977年 | 263篇 |
1976年 | 267篇 |
1975年 | 275篇 |
1974年 | 234篇 |
1973年 | 238篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Kaizad F. Patel Ivan J. Fernandez Sarah J. Nelson Stephen A. Norton Cheryl J. Spencer 《水文研究》2021,35(5):e14147
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture. 相似文献
2.
Edward P. F. Rose 《Geology Today》2019,35(3):107-114
During the Second World War, the Allied invasion of the French coast of Normandy on D‐Day, 6 June 1944, was the greatest amphibious assault in world history. An article in Geology Today (v.11, for 1995, pp.58–63) marked the 50th anniversary of the end of the war in Europe, on 8 May 1945, by describing how British military geologists had participated in planning for D‐Day and in the NW Europe campaign that followed it. The work of these geologists provides a classic case history, revealing that ‘military geology’ has many potential applications. Geological factors influenced site selection for temporary airfields, predictions of trafficability for the Normandy beaches, the development of potable water supplies, and quarrying for road metal—and more besides. This new article helps to mark the 75th anniversary of D‐Day by further details of how geologists and geology contributed to Allied victory. 相似文献
3.
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
4.
Revisiting the existence of an effective stress for wet granular soils with micromechanics
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)
A possible effective stress variable for wet granular materials is numerically investigated based on an adapted discrete element method (DEM) model for an ideal three‐phase system. The DEM simulations consider granular materials made of nearly monodisperse spherical particles, in the pendular regime with the pore fluid mixture consisting of distinct water menisci bridging particle pairs. The contact force‐related stress contribution to the total stresses is isolated and tested as the effective stress candidate for dense or loose systems. It is first recalled that this contact stress tensor is indeed an adequate effective stress that describes stress limit states of wet samples with the same Mohr‐Coulomb criterion associated with their dry counterparts. As for constitutive relationships, it is demonstrated that the contact stress tensor used in conjunction with dry constitutive relations does describe the strains of wet samples during an initial strain regime but not beyond. Outside this so‐called quasi‐static strain regime, whose extent is much greater for dense than loose materials, dramatic changes in the contact network prevent macroscale contact stress‐strain relationships to apply in the same manner to dry and unsaturated conditions. The presented numerical results also reveal unexpected constitutive bifurcations for the loose material, related to stick‐slip macrobehavior. 相似文献
5.
6.
7.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
8.
The UBV photometric observations of RT Per, from Sanwal and Chaubey (1981), were analyzed by the Wilson and Devinney code (1971). The light curves include reflection effects that for the first time has been suggested by Dugan (1911). RT Per has a semi-detached configuration where the lower-mass component is in contact with its respective Roche surface. The higher-mass component very nearly fills its Roche lobe. It has the characteristic of an Algol type system. The absolute dimensions for the primary and secondary of this system were calculated from its spectral types and by combining the photometric solution with inferred component radial velocities (Lu, 1990). 相似文献
9.
10.
An updated Lagrangian finite-element formulation has been developed for time-dependent problems of soil consolidation involving finite deformations. Large plastic strains as well as rotations occur in such problems and nominal stress measures are introduced in the formulation to redefine stresses. This leads to corrective terms for equilibrium and yield violations in addition to geometric stiffening terms in the governing integral equations. The soil is considered to be either a linear elastic or an elastoplastic, critical-state material. Some simple numerical examples are studied to validate the formulation, followed by a detailed analysis of the problem of penetration of a pile into soil. The results of this problem are viewed with emphasis on the physical interpretation and practical significance. 相似文献