全文获取类型
收费全文 | 5157篇 |
免费 | 419篇 |
国内免费 | 485篇 |
专业分类
测绘学 | 207篇 |
大气科学 | 146篇 |
地球物理 | 609篇 |
地质学 | 1290篇 |
海洋学 | 296篇 |
天文学 | 2812篇 |
综合类 | 81篇 |
自然地理 | 620篇 |
出版年
2024年 | 25篇 |
2023年 | 46篇 |
2022年 | 86篇 |
2021年 | 93篇 |
2020年 | 100篇 |
2019年 | 154篇 |
2018年 | 96篇 |
2017年 | 88篇 |
2016年 | 109篇 |
2015年 | 127篇 |
2014年 | 160篇 |
2013年 | 197篇 |
2012年 | 189篇 |
2011年 | 220篇 |
2010年 | 163篇 |
2009年 | 446篇 |
2008年 | 394篇 |
2007年 | 465篇 |
2006年 | 447篇 |
2005年 | 358篇 |
2004年 | 339篇 |
2003年 | 339篇 |
2002年 | 246篇 |
2001年 | 222篇 |
2000年 | 166篇 |
1999年 | 175篇 |
1998年 | 196篇 |
1997年 | 82篇 |
1996年 | 63篇 |
1995年 | 46篇 |
1994年 | 33篇 |
1993年 | 44篇 |
1992年 | 33篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 15篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1971年 | 2篇 |
排序方式: 共有6061条查询结果,搜索用时 31 毫秒
1.
The groundwater divide is a key feature of river basins and significantly influenced by subsurface hydrological processes. For an unconfined aquifer between two parallel rivers or ditches, it has long been defined as the top of the water table based on the Dupuit–Forchheimer approximation. However, the exact groundwater divide is subject to the interface between two local flow systems transporting groundwater to rivers from the infiltration recharge. This study contributes a new analytical model for two-dimensional groundwater flow between rivers of different water levels. The flownet is delineated in the model to identify groundwater flow systems and the exact groundwater divide. Formulas with two dimensionless parameters are derived to determine the distributed hydraulic head, the top of the water table and the groundwater divide. The locations of the groundwater divide and the top of the water table are not the same. The distance between them in horizontal can reach up to 8.9% of the distance between rivers. Numerical verifications indicate that simplifications in the analytical model do not significantly cause misestimates in the location of the groundwater divide. In contrast, the Dupuit–Forchheimer approximation yields an incorrect water table shape. The new analytical model is applied to investigate groundwater divides in the Loess Plateau, China, with a Monte Carlo simulation process taking into account the uncertainties in the parameters. 相似文献
2.
3.
4.
Janis Britals 《Celestial Mechanics and Dynamical Astronomy》1992,54(4):305-316
It is shown in this paper how to build a canonical transformation of variables, so that the eccentric anomaly becomes the new independent variable. In the case of eccentric elliptical orbits it changes the equations of motion so, that they can be integrated analytically to any order of approximation comparatively easy. 相似文献
5.
6.
7.
A highly automated moving object detection package 总被引:1,自引:0,他引:1
8.
9.
George Saklatvala Michael P. Hobson Stafford Withington 《Monthly notices of the Royal Astronomical Society》2006,367(3):1194-1200
We describe a procedure for the numerical modelling of astronomical interferometers, with particular relevance to far-infrared and submillimetre wavelengths. The scheme is based on identifying a set of modes that carry power from the sky to the detector. The procedure is extremely general, and can be used to model scalar or vector fields, in any state of coherence and polarization, the only limitation being that the propagation of a coherent field through the system be described by an integral transform, a constraint that is in practise always met.
We present simulations of ideal, multimode two-dimensional interferometers, and show that the modal theory reproduces the correct behaviour of both Michelson and Fizeau interferometers. We calculate simulated visibility data for a multimode bolometric Michelson interferometer, with a synthesized source, and produce a dirty map, recovering the original source with the usual artefacts associated with interferometers. 相似文献
We present simulations of ideal, multimode two-dimensional interferometers, and show that the modal theory reproduces the correct behaviour of both Michelson and Fizeau interferometers. We calculate simulated visibility data for a multimode bolometric Michelson interferometer, with a synthesized source, and produce a dirty map, recovering the original source with the usual artefacts associated with interferometers. 相似文献
10.