全文获取类型
收费全文 | 16498篇 |
免费 | 3226篇 |
国内免费 | 3542篇 |
专业分类
测绘学 | 781篇 |
大气科学 | 1851篇 |
地球物理 | 4350篇 |
地质学 | 9322篇 |
海洋学 | 2950篇 |
天文学 | 140篇 |
综合类 | 1165篇 |
自然地理 | 2707篇 |
出版年
2024年 | 99篇 |
2023年 | 214篇 |
2022年 | 487篇 |
2021年 | 710篇 |
2020年 | 668篇 |
2019年 | 781篇 |
2018年 | 636篇 |
2017年 | 705篇 |
2016年 | 678篇 |
2015年 | 794篇 |
2014年 | 1018篇 |
2013年 | 1259篇 |
2012年 | 1013篇 |
2011年 | 1108篇 |
2010年 | 982篇 |
2009年 | 1040篇 |
2008年 | 1060篇 |
2007年 | 1107篇 |
2006年 | 1120篇 |
2005年 | 963篇 |
2004年 | 925篇 |
2003年 | 793篇 |
2002年 | 741篇 |
2001年 | 637篇 |
2000年 | 594篇 |
1999年 | 503篇 |
1998年 | 462篇 |
1997年 | 413篇 |
1996年 | 312篇 |
1995年 | 297篇 |
1994年 | 269篇 |
1993年 | 206篇 |
1992年 | 151篇 |
1991年 | 113篇 |
1990年 | 80篇 |
1989年 | 98篇 |
1988年 | 55篇 |
1987年 | 39篇 |
1986年 | 31篇 |
1985年 | 35篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 3篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 9篇 |
1973年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
83.
A nutrient dynamic model coupled with a 3D physical model has been developed to study the annual cycle of phytoplankton production in the Yellow Sea. The biological model involves interactions between inorganic nitrogen (nitrate and ammonium), phosphate and phytoplankton biomass. The model successfully reproduces the main features of phytoplankton-nutrient variation and dynamics of production. 1. The well-mixed coastal water is characterized by high primary production, as well as high new production. 2. In summer, the convergence of tidal front is an important hydrodynamic process, which contributes to high biomass at frontal areas. 3. The evolution of phytoplankton blooms and thermocline in the central region demonstrate that mixing is a dominant factor to the production in the Yellow Sea. In this simulation, nitrate- and ammonium-based productions are estimated regionally and temporally. The northern Yellow Sea is one of the highly ranked regions in the Yellow Sea for the capability of fixing carbon and nitrogen. The annual averaged f-ratio of 0.37 indicates that regenerated production prevails over the Yellow Sea. The result also shows that phosphate is the major nutrient, limiting phytoplankton growth throughout the year and it can be an indicator to predict the bloom magnitude. Finally, the relative roles of external nutrient sources have been evaluated, and benthic fluxes might play a significant role in compensating 54.6% of new nitrogen for new production consumption. 相似文献
84.
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
85.
Experiments carried out with models of floating production, storage and offloading platforms (FPSOs) showed that the flow of water over the deck edge, onto the deck resembled a suddenly released wall of water rather than a breaking wave. Therefore green water flow onto the deck was simulated using dam breaking theory, but the theory’s shallow-water assumptions may be limiting. In this paper a non-linear dam breaking problem is formulated. Equations of motion in the Lagrangian form are used and the solution is sought as an infinite series in time. Comparisons with the shallow water approximation are carried out. 相似文献
86.
Doaa Hassan Youssef 《Journal of Oceanography》2003,59(5):537-544
Concentrations of boron in seawater (from four regions along the Alexandria coastline, subjected to land disposal), brackish
water (Lake Edku) and drains water (e.g. El-Umum Drain) were determined during the period from February to August 2000. Boron
was determined spectrophotometrically by a modified curcumin method. For Lake Edku, boron concentration fluctuated between
0.023 and 0.105 mmol/l. There are several factors that affect the concentration of B in the Lake water: (a) effect of the
drainage water via El-Khairy and Barsiek Drains, (b) utilization of boron by hydrophytes, and (c) water exchange through the
sea-lake connection. It may be concluded that the level of boron in the Lake water cannot be considered a substantial hazard
to the Lake organisms. Boron concentration varied from 0.392 to 0.522 mmol/l in seawater samples and from 0.141 to 0.458 mmol/l
in the sites where the water from the drains (El-Umum Drain and El-Noubaria Canal) mixed with the seawater. The broader variation
of boron (mmol/l)/salinity ratios for seawater samples (0.0106–0.0138) may be due to that the samples were collected from
the upper seawater layers, where contributions from land run-off, atmospheric precipitation and differences in the biota affect
the concentration.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
87.
本文根据1988年10月20~24日“向阳红09号”调查船在奄美大岛以西海区进行调查时,在陆架斜坡上的表均温层的底部,即50m附近的深层上,出现一个黑潮锋面涡。不论在平面上形成的特点,还是在断面上水系配置的形式,它与春季黑潮锋面涡的特征极为相似。说明秋季东海同样存在陆架水与黑潮水在水平方向交换与混合。 相似文献
88.
Qiu Dahong 《中国海洋工程》1992,(2)
Based on the 1st order cnoidal wave theory, the nonlinear wave diffraction around a circular cylinder in shallow water is studied in this paper. The equation of the wave surface around the cylinder is formulated and by using this formula the wave surface elevation on the cylinder surface can be obtained. In this paper, the formula for calculating the cnoidal wave force on a circular cylinder is also derived. For the wave conditions which are often encountered in practical engineering designs, the ratios of the nonlinear wave forces to the linear wave forces are calculated, and the results are plotted in this paper for design purposes. In order to verify the theoretical results, model tests are conducted. After comparing the test results with the theoretical ones, it is concluded that, in shallow water, for the case of T g / d~(1/2) > 8-10 and H / d > 0.3, the cnoidal wave theory should be used to calculate the wave action on a cylindrical pier. 相似文献
89.
Data collected in three Californian estuaries indicate that hypersaline conditions exist during the dry summers typical of a Mediterranean climate. The generalised seasonal and longitudinal hydrographic structures are described and explained. It is argued that this seasonal hypersalinity is common and that it represents a major class of estuaries. The observed accumulation of salt indicates surprisingly long residence times in small basins which have free exchange with the ocean. This semi-isolation of the inner basin leads to a large build-up or severe depletion of nutrients, pollutants and plankton in these systems. Of concern are the trends to increase pollutant loading in the same systems that are experiencing an increase in residence times owing to freshwater extraction in the watershed. 相似文献
90.