全文获取类型
收费全文 | 49047篇 |
免费 | 3719篇 |
国内免费 | 5936篇 |
专业分类
测绘学 | 2721篇 |
大气科学 | 6171篇 |
地球物理 | 9995篇 |
地质学 | 24149篇 |
海洋学 | 4169篇 |
天文学 | 3944篇 |
综合类 | 3759篇 |
自然地理 | 3794篇 |
出版年
2024年 | 164篇 |
2023年 | 475篇 |
2022年 | 1122篇 |
2021年 | 1388篇 |
2020年 | 1240篇 |
2019年 | 1367篇 |
2018年 | 6096篇 |
2017年 | 5246篇 |
2016年 | 3918篇 |
2015年 | 1417篇 |
2014年 | 1617篇 |
2013年 | 1545篇 |
2012年 | 2423篇 |
2011年 | 4253篇 |
2010年 | 3447篇 |
2009年 | 3648篇 |
2008年 | 3135篇 |
2007年 | 3624篇 |
2006年 | 1195篇 |
2005年 | 900篇 |
2004年 | 1016篇 |
2003年 | 1074篇 |
2002年 | 1019篇 |
2001年 | 744篇 |
2000年 | 685篇 |
1999年 | 752篇 |
1998年 | 624篇 |
1997年 | 634篇 |
1996年 | 525篇 |
1995年 | 493篇 |
1994年 | 516篇 |
1993年 | 394篇 |
1992年 | 325篇 |
1991年 | 238篇 |
1990年 | 198篇 |
1989年 | 187篇 |
1988年 | 176篇 |
1987年 | 113篇 |
1986年 | 101篇 |
1985年 | 86篇 |
1984年 | 78篇 |
1983年 | 73篇 |
1982年 | 75篇 |
1981年 | 74篇 |
1980年 | 49篇 |
1979年 | 39篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1976年 | 22篇 |
1975年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Zhao Yiyang 《中国海洋湖沼学报》1985,3(2):200-211
Geochemical patterns of shelf sediments of China Seas are discussed in the following order: 1) the grain-size control of elements,
2) zonal distribution of elements along the coast, 3) estuarine enrichment of elements, 4) correlation of elements, 5) philo-detrital
property of elements, 6) philo-continental property of elements, 7) rapid accumulation rate of elements. Statistics show that
the abundance of indicator elements in shelf sediments is relatively close to that in terrestrial rocks and continental crust,
and the abundance distribution pattern of elements in shelf sediments is similar to that in continental crust. 相似文献
22.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
23.
Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH4). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
24.
We discuss the determination of membership of 42 open clusters. Our analysis shows that Vasilevskis' mathematical model can be reasonably applied to this case. Our improved version of Sanders' method and our definition of cluster member based on the principles of discriminatory analysis effectively exclude stars of low probabilities. It is important in the study of open cluster to use only those with high probabilities. The effectiveness of the statistical method is closely related to the velocity distributions of the member and field stars. For fields where the error rate is high, it is better to combine other data than proper motion in determining membership. 相似文献
25.
Technique for Controlling Spread of Limnotic Oncomelania 总被引:1,自引:0,他引:1
Schistosomiasis is a parasitic disease mostly found in areas along the Changjiang River of China. The disease is spread solely through an intermediary named oncomelania, so its spread of schistosomiasis can be controlled by properly designing water intakes which prevent oncomelania from entering farming land or residential areas. This paper reports a successful design process and a new oncomelania-free intake device. The design of the new intake is based on a sound research program in which extensive experimental studies were carried out to gain knowledge of oncomelania eco-hydraulic behavior and detailed flow field information through CFD simulation. 相似文献
26.
27.
28.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
29.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
30.
DharmalingamVENUGOPAL 《山地科学学报》2004,1(1):74-80
The Nilgiri Mountains of south India is considered unique by anthropologists, geologists, climatologists, botanists as well as tourists. It has remained a subject of constant study and research over the last two centuries. Man-nature balance had continued undisturbed in the Nilgiris for thousands of years until the early 19th century when it became a British colony attracting, in due course, various developmental activities. Subsequently, the Nilgiris and its popular hill stations emerged as favourite places for the British population in India for rest and recuperation, game and for raising commercial plantations. In the process, the traditional indigenous crops were replaced by “English“ vegetables and the natural forests gave way to commercial plantations of coffee, tea and other exotic species of trees. 相似文献