全文获取类型
收费全文 | 26301篇 |
免费 | 4694篇 |
国内免费 | 7023篇 |
专业分类
测绘学 | 4822篇 |
大气科学 | 4765篇 |
地球物理 | 6061篇 |
地质学 | 12544篇 |
海洋学 | 3686篇 |
天文学 | 265篇 |
综合类 | 2104篇 |
自然地理 | 3771篇 |
出版年
2024年 | 169篇 |
2023年 | 457篇 |
2022年 | 944篇 |
2021年 | 1124篇 |
2020年 | 1305篇 |
2019年 | 1434篇 |
2018年 | 1185篇 |
2017年 | 1368篇 |
2016年 | 1527篇 |
2015年 | 1627篇 |
2014年 | 1747篇 |
2013年 | 1910篇 |
2012年 | 1778篇 |
2011年 | 1799篇 |
2010年 | 1492篇 |
2009年 | 1665篇 |
2008年 | 1653篇 |
2007年 | 1732篇 |
2006年 | 1644篇 |
2005年 | 1441篇 |
2004年 | 1305篇 |
2003年 | 1094篇 |
2002年 | 1065篇 |
2001年 | 886篇 |
2000年 | 823篇 |
1999年 | 766篇 |
1998年 | 683篇 |
1997年 | 608篇 |
1996年 | 545篇 |
1995年 | 477篇 |
1994年 | 426篇 |
1993年 | 368篇 |
1992年 | 222篇 |
1991年 | 198篇 |
1990年 | 131篇 |
1989年 | 99篇 |
1988年 | 104篇 |
1987年 | 68篇 |
1986年 | 29篇 |
1985年 | 31篇 |
1984年 | 22篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The probability of fractal determination of coastal types based on GIS is preliminarily discussed with China as an example. Finally, some significant conclusions are drawn: (1) The fractal dimension of coastline of the bedrock coast is larger than that of the plain coast on the same scale map; (2) As far as the bedrock coast is concerned, the larger fractal dimension of coastline of the bedrock coast on the same scale map indicates that the bedrock coast is probably not typical; (3) As far as the plain coast is concerned, the smaller fractal dimension of coastline of the plain coast on the same scale map indicates that it is probably the silt plain coast; (4) The different substantial compositions affect the fractal dimensions of coastlines of different coastal types. In general, the coast which lies in the north of the Hangzhou Bay consists of sand mainly, its surface is flat, and it is connected with the coastal plain, its landform is broad shoal, its total change is comparatively homogenous in the tidal dynamic process, and thus, the relatively smaller fractal dimension of coastline results from this. For the bedrock coast, there is more bedrock, the coastline is comparatively smooth and straight, being affected by the faults and ocean dynamic process, which result in the larger fractal dimension. 相似文献
102.
103.
Modelling nitrogen and phosphorus cycles and dissolved oxygen in the Zhujiang Estuary Ⅰ. Model development 总被引:1,自引:0,他引:1
wrmcrloxHuman activities related to the population growth and developrnent of industry and rnwhci-pality have led to the incrouing hadings of various POllutants into estudries during the past fewdecades. These increasing edlutant lOadings have caused declined estuallne hedth which can bemereured by a vdriety of indices. In order to obtain solutions to environrnent problerns, re-sources manageTnnt apencies are supporting a holistic approach to envirorirnental management.An effcient strategy t… 相似文献
104.
The European Regional Seas Ecosystem Model (ERSEM) has been coupled with a two-dimensional depth-averaged transport model of the Humber plume region and run to simulate 1988–1989. Simulations of the spatial and temporal variations in chlorophyll-a, nitrate, phosphate and suspended particulate matter distributions in winter, spring and summer show how the development of the spring bloom and subsequent maintenance of primary production is controlled by the physicochemical environment of the plume zone. Results are also shown for two stations, one characterised by the high nutrient and suspended matter concentrations of the plume and the other by the relatively low nutrient and sediment concentrations of the offshore waters. The modelled net primary production at the plume site was 105 g C m−2 a−1 and 127 g C m−2 a−1 offshore. Primary production was controlled by light limitation between October and March and by the availability of nutrients during the rest of the year. The phytoplankton nutrient demand is met by in-situ recycling processes during the summer. The likely effect of increasing and decreasing anthropogenic riverine inputs of nitrate and phosphate upon ecosystem function was also investigated. Modelling experiments indicate that increasing the nitrogen to silicate ratio in freshwater inputs increased the production of non-siliceous phytoplankton in the plume. The results of this model have been used to calculate the annual and quarterly mass balances describing the usage of inorganic nitrogen, phosphate and silicate within the plume zone for the period of the NERC North Sea survey (September 1988 to October 1989). The modelled Humber plume retains 3.9% of the freshwater dissolved inorganic nitrogen, 2.2% of the freshwater phosphate and 1.3% of the freshwater silicate input over the simulated seasonal cycle. The remainder is transported into the southern North Sea in either dissolved or particulate form. The reliability of these results is discussed. 相似文献
105.
根据湍流封闭理论,建立一种适用于正压浅海湍流运动的雷诺应力封闭模型(RSM),以代替目前三维浅海动力学模型中普遍采用的湍粘性系数的传统假设。通过直接建立并模化f—平面上正压海洋的雷诺应力传输方程,分别得到的微分形式和代数形式的RSM方程组。并讨论了进行数值计算所需要的边界条件。利用该模型可以进一步研究浅海潮流、风暴潮流及风海流等浅海流动的三维结构和湍流特性。 相似文献
106.
根据大量田间试验和调查资料分析,作者提出烟单14号玉米生长过程中,小喇叭口期土壤中全氮、大喇叭口期土壤中速效磷的含量与产量相关密切。并据此建立了产量及构成三因素与土壤养分间的最优相关模型。 相似文献
107.
宋丽娜 《中国海洋大学学报(自然科学版)》1987,(2)
本文将流速分解模型应用于作为超浅海风暴潮的渤海风潮,并讨论了变湍粘性系数的确定。作为一个初步的,但较为成功的数值试验例子,描述了实际风场作用下的渤海风潮,比较了变湍粘性系数模型与常湍粘性系数模型的计算结果间的差异。 相似文献
108.
对含多资料的分潮波模型的超定问题进行探讨,建立一种有限差分—最小二乘方法,该方法使得岸边资料直接进入了计算方程,并保证岸界法向流速为零条件。通过对渤海的四大分潮的试算,证实本方法可行。 相似文献
109.
A vertical two-dimensional turbulence numerical model for the interaction of waves and currents is developed in the paper based on the nonlinear two-equation k-ε model with the VOF method.The one-dimensional equivalent advection velocity and equivalent mixing coefficient are defined and the solving process is introduced:The pollutant concentration field,generated by an instant source in waves and currents,is calculated with the model,and then the equivalent advection velocity and equivalent mixing coefficient are obtained by calculating the time derivative of the mean and variance of pollutant concentration probability distribution.The effects of wave period and wave height on the equivalent mixing coefficient for waves and wave-currents are also investigated. 相似文献
110.
The mixing characteristics of particles such as dredged sediment of variable size discharged into cross flow are studied by a 3D numerical model, which is developed to model the particle-fluid two-phase flow. The Eulerian method with the modified k-ε parameterization of turbulence for the fluid phase is used to solve fluid phase, while a Lagrangian method for the solid phase (particles), both the processes are coupled through the momentum sources. In the model the wake turbulence induced by particles has been included as additional source term in the k-ε model; and the variable drift velocities of the particles are treated efficiently by the Lagrangian method in which the particles are tracked explicitly and the diffusion process is approximated by a random walk model. The hydrodynamic behavior of dumping a cloud of particles is governed by the total buoyancy of the cloud, the drag force on each particle and the velocity of cross-flow. The computed results show a roughly linear relationship between the displacement of the frontal position and the longitudinal width of the particle cloud. The particle size in the cloud and the velocity of cross flow dominate the flow behavior. The computed results are compared with the results of laboratory experiments and satisfactory agreement is obtained. 相似文献