全文获取类型
收费全文 | 16544篇 |
免费 | 3932篇 |
国内免费 | 5065篇 |
专业分类
测绘学 | 1882篇 |
大气科学 | 1714篇 |
地球物理 | 4933篇 |
地质学 | 11381篇 |
海洋学 | 2676篇 |
天文学 | 255篇 |
综合类 | 1313篇 |
自然地理 | 1387篇 |
出版年
2024年 | 61篇 |
2023年 | 238篇 |
2022年 | 557篇 |
2021年 | 783篇 |
2020年 | 765篇 |
2019年 | 1019篇 |
2018年 | 734篇 |
2017年 | 831篇 |
2016年 | 829篇 |
2015年 | 962篇 |
2014年 | 1205篇 |
2013年 | 1136篇 |
2012年 | 1188篇 |
2011年 | 1303篇 |
2010年 | 1129篇 |
2009年 | 1240篇 |
2008年 | 1120篇 |
2007年 | 1247篇 |
2006年 | 1207篇 |
2005年 | 1025篇 |
2004年 | 979篇 |
2003年 | 832篇 |
2002年 | 658篇 |
2001年 | 550篇 |
2000年 | 557篇 |
1999年 | 518篇 |
1998年 | 465篇 |
1997年 | 422篇 |
1996年 | 359篇 |
1995年 | 304篇 |
1994年 | 285篇 |
1993年 | 219篇 |
1992年 | 202篇 |
1991年 | 130篇 |
1990年 | 110篇 |
1989年 | 134篇 |
1988年 | 74篇 |
1987年 | 56篇 |
1986年 | 27篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1954年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
A new and simple calculating technique for the Jacobian elliptic parameter is presented in this study. The technique is very useful in generating a train of cnoidal waves in both laboratory and numerical wave tanks. Upon specification of water depth, the wave height and either the wave period or the wavelength, the proposed technique uses the Newton–Raphson method to estimate the Jacobian elliptic parameter directly, without trial and error procedures or look-up in tables. It is shown that the technique provides equally accurate results as the ad hoc methods previously used. 相似文献
82.
1 .Introduction Owing to the decline of capture fisheries andsaturation of traditional inshorefarms ,sea cage cul-ture as a newtype of aquaculture is developingrapidlyin China .However ,cage culture is a highriskoperation.The practice is vulnerable to natural hazards ,such as strong tides ,storms and typhoons .So,it is necessaryto assess the behavior of a sea cage exposedto waves and current before the designandinstallation of the structure . Computer simulation,due to its many advantages ,ha… 相似文献
83.
1 .IntroductionThe lifting systemis composed of pipes andtheir connectors .It is not onlythe lifting channel formanganese nodule betweenthe miningship andintermediate bin,but alsothe supporting massfor deepsea miningequipment and cables .Sothe reliability… 相似文献
84.
Full-rangenonlinearanalysisoffatiguebehaviorsofreinforcedconcretestructuresbyfiniteelementmethod¥SongYupu;ZhaoShunbo;WangRuim... 相似文献
85.
W.R. Geyer 《Estuarine, Coastal and Shelf Science》1997,44(6):713-722
Observations of two small estuaries in Cape Cod, U.S.A. indicate large variations in salinity structure that are forced by variations in along-estuary wind stress. During onshore winds, the estuarine circulation is reduced, and the along-estuary salinity gradient increases as freshwater accumulates. During offshore winds, the surface outflow is enhanced, freshwater is flushed out of the estuary, and the along-estuary salinity gradient becomes weak. Constrictions block the wind-induced flushing, resulting in strong salinity fronts across the constrictions. The residence time of one of the estuaries varies by more than a factor of three in response to variations in wind-induced flushing. The other estuary has little variation of flushing associated with winds, due to a constriction at the mouth that inhibits the wind-induced exchange. The strong influence of winds on the flushing of these estuaries is due in part to their shallow depths, which accentuates the influence of wind stress relative to the effects of the horizontal density gradient. In addition, the residence times of the estuaries are comparable to the time scale of wind forcing, allowing large changes in water properties during wind events. 相似文献
86.
提出一种确定连续系统参数的时域方法。该方法利用方波输入和系统稳定性,离析出输出采样信号与结构参数的内在联系,推导出确定连续系统结构参数的算法.指出该算法与辨识方法相结合,可实现对含随机干扰的连续系统结构参数的估计. 相似文献
87.
88.
黄河三角洲沿岸及邻近海区细粒沉积物中的碳酸盐 总被引:4,自引:0,他引:4
本文用X-射线衍射法直接计算了来自黄河三角洲沿岸及邻近海区80个沉积样的细粒组分(<2μm)中的碳酸盐含量。计算结果与全样的容量法分析结果基本一致。研究结果表明,本区细粒沉积物中碳酸盐含量变化受沉积物来源、海区水动力作用、海底地貌特征和碳酸盐的地球化学行为等因素所影响。 相似文献
89.
In this study we test Talley's hypothesis that Oyashio winter mixed-layer water (26.5–26.6σ θ) increases its density to produce the North Pacific Intermediate Water (NPIW) salinity minimum (26.7– 26.8σθ) in the Mixed Water Region, assuming a combination of cabbeling and double diffusion. The possible density change of Oyashio
winter mixed-layer water is discussed using an instantaneous ratio of the change of temperature and salinity along any particular
intrusion (R
l
). We estimate the range of R
l
DD
required to convert Oyashio winter mixed-layer water to the NPIW salinity minimum due to double diffusion, and then assume
double-diffusive intrusions as this conversion mechanism. A double-diffusive intrusion model is used to estimate R
l
DD
in a situation where salt fingering dominates vertical mixing, as well as to determine whether Oyashio winter mixed-layer
water can become the NPIW salinity minimum. Possible density changes are estimated from the model R
l
DD
by assuming the amount of density change due to cabbeling. From these results, we conclude that Oyashio winter mixed-layer
water contributes to a freshening of the lighter layer of the NPIW salinity minimum (around 26.70σθ) in the MWR. 相似文献
90.
Nowadays there are some chronic serious environmental problems, such as eutrophication, blue tide and so on, in a complicated coastal zone or a semi-enclosed bay, because the water exchanges between an inner bay and an outer sea is weak compared with the supply of contaminant. Under this situation, a method to improve the water quality by 3-dimensional small unsymmetrical structures has been proposed by Komatsu et al. In this paper, several numerical simulations of the tidal current and concentration for various arrangements of bottom roughness in a semi-enclosed model bay are carfled out with a depth-averaged 2-D numerical model. The model is solved by the hybrid finite analytic method with nonstaggered grid. And the SIMPLES algorithm with Rhie and Chow' s momentum interpolation technique is used for the simulation. The effect of Komatsu' s method for water purification is examined by numerical simulation. The result of numerical experiment indicates that it is possible to generate a new tidal residual current and to activate a tidal exchange by bottom roughness arrangement only. 相似文献