全文获取类型
收费全文 | 77224篇 |
免费 | 720篇 |
国内免费 | 1360篇 |
专业分类
测绘学 | 2750篇 |
大气科学 | 5701篇 |
地球物理 | 14364篇 |
地质学 | 30061篇 |
海洋学 | 5905篇 |
天文学 | 14449篇 |
综合类 | 2342篇 |
自然地理 | 3732篇 |
出版年
2022年 | 376篇 |
2021年 | 664篇 |
2020年 | 708篇 |
2019年 | 756篇 |
2018年 | 6101篇 |
2017年 | 5296篇 |
2016年 | 4361篇 |
2015年 | 1233篇 |
2014年 | 1852篇 |
2013年 | 2918篇 |
2012年 | 2774篇 |
2011年 | 4906篇 |
2010年 | 3965篇 |
2009年 | 4829篇 |
2008年 | 4076篇 |
2007年 | 4573篇 |
2006年 | 2258篇 |
2005年 | 1816篇 |
2004年 | 2025篇 |
2003年 | 1930篇 |
2002年 | 1713篇 |
2001年 | 1344篇 |
2000年 | 1271篇 |
1999年 | 1008篇 |
1998年 | 1066篇 |
1997年 | 948篇 |
1996年 | 821篇 |
1995年 | 786篇 |
1994年 | 686篇 |
1993年 | 606篇 |
1992年 | 592篇 |
1991年 | 595篇 |
1990年 | 622篇 |
1989年 | 494篇 |
1988年 | 504篇 |
1987年 | 529篇 |
1986年 | 489篇 |
1985年 | 612篇 |
1984年 | 674篇 |
1983年 | 591篇 |
1982年 | 562篇 |
1981年 | 519篇 |
1980年 | 487篇 |
1979年 | 479篇 |
1978年 | 458篇 |
1977年 | 368篇 |
1976年 | 352篇 |
1975年 | 357篇 |
1974年 | 309篇 |
1973年 | 344篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
162.
Dongqi Sun Liang Zhou Yu Li Haimeng Liu Xiaoyan Shen Zedong Wang Xixi Wang 《地理学报(英文版)》2017,27(8):943-966
The future development of new-type urbanization has drawn great attention from both the government and public alike. In this context, the present study had three related research aims. Firstly, it sought to predict the urbanization and population dynamics in China at both national and provincial levels for the period of 2015 to 2030. Secondly, on this basis, it sought to examine the spatial variation of urbanization given the predicted national urbanization rate of 70.12%. Thirdly, it sought to estimate and evaluate the national and provincial demands of investment in the development of new-type urbanization. The main conclusions from this study were as follows: (1) The population size and urbanization rate will reach 1.445 billion and 70.12%, respectively, from 2015 to 2030. (2) The demographic dividend will vanish when the population pressure reaches its maximum. During this period, there will be 70.16 million urban population born. The suburban population that becomes urbanized will be 316.7 million, and thus the net increase in urban population will reach 386 million. (3) Although the urbanization rate of every Chinese province will increase during 2015–2030, it will do so unequally, while differences in urbanization quality among provinces will also be substantial. In some provinces, moreover, the urbanization quality is not compatible with their eco-social development. (4) A total of 4,105,380 billion yuan is required to fund new-type urbanization and the investment demand for each province varies greatly; for example, Guangdong province requires the most funding, amounting to approximately 148 times that required by Tibet, the province in least need of funding. In the final part of this study, policy suggestions concerning the investment of the new-type urbanization are put forward and discussed. 相似文献
163.
H.S. Yoon O.A. El-Samni A.T. Huynh H.H. Chun H.J. Kim A.H. Pham I.R. Park 《Ocean Engineering》2009,36(9-10):697-707
The present study numerically investigates the characteristics of three-dimensional turbulent flow in a wavy channel. For the purpose of a careful observation of the effect of the wave amplitude on the turbulent flow, numerical simulations are performed at a various range of the wave amplitude to wavelength ratio (0.01?α/λ?0.05), where the wavelength is fixed with the same value of the mean channel height (H). The immersed boundary method is used to handle the wavy surface in a rectangular grid system, using the finite volume method. The Reynolds number (Re=UbH/ν) based on the bulk velocity (Ub) is fixed at 6760. The present computational results for a wavy surface are well compared with those of references. When α/λ=0.02, the small recirculating flow occurs near the trough at the instant, but the mean reverse flow is not observed. In the mean flow field, the reverse flow appears from α/λ=0.03 among the wave amplitude considered in this study. The domain of the mean reverse flow defined by the locations of separation and reattachment depends strongly on the wave amplitude. The pressure drag coefficient augments with increasing the wave amplitude. The friction drag coefficient shows the increase and decrease behavior according to the wave amplitude. The quantitative information about the flow variables such as the distribution of pressure and shear stress on the wavy surface is highlighted. 相似文献
164.
165.
J. I. S. Zonneveld Dr. 《GeoJournal》1983,7(2):121-129
Modern geography is based on ecological thinking. In this paper some geoecological notions are discussed: the scales and levels regarding spatial magnitude and complexity, the “hierarchy” of landscape units and espcially the scale of time and the concept of the supper-imposing “stamps”. This last mentioned concept is very important in relation to the problem regarding regionalization and its possibilities. Three “stamps” are recognized: 1. the substrate, 2. the climatic conditions, 3. the human (noötic) influence. Holistic regionalization in which all elements (including the human influences) are telling to full advantage, is unsatisfactory. 相似文献
166.
M.R. Kaplan A. Coronato N.R.J. Hulton J.O. Rabassa P.W. Kubik S.P.H.T. Freeman 《Geomorphology》2007,87(4):284-301
We measured in situ 10Be, 26Al and36Cl on glacial deposits as old as 1.1 Myr in the southernmost part of Patagonia and on northern Tierra del Fuego to understand boulder and moraine and, by inference, landscape changes. Nuclide concentrations indicate that surface boulders have been exposed for far less time than the ages of moraines they sit upon. The moraine ages are themselves constrained by previously obtained 40Ar/39Ar ages on interbedded lava flows or U-series and amino acid measurements on related (non-glacial) marine deposits. We suggest that a combination of boulder erosion and their exhumation from the moraine matrix could cause the erratics to have a large age variance and often short exposure histories, despite the fact that some moraine landforms are demonstrably 1 Myr old. We hypothesize that fast or episodic rates of landscape change occurred during glacial times or near the sea during interglacials. Comparison with boulder erosion rates and exhumation histories derived for the middle latitudes of semi-arid Patagonia imply different geomorphic processes operating in southernmost South America. We infer a faster rate of landscape degradation towards the higher latitudes where conditions have been colder and wetter. 相似文献
167.
Aggeliki Adamaki Eleftheria E. Papadimitriou George M. Tsaklidis Vassilios Karakostas 《Acta Geophysica》2011,59(4):748-769
Aftershock rates seem to follow a power law decay, but the assessment of the aftershock frequency immediately after an earthquake,
as well as during the evolution of a seismic excitation remains a demand for the imminent seismic hazard. The purpose of this
work is to study the temporal distribution of triggered earthquakes in short time scales following a strong event, and thus
a multiple seismic sequence was chosen for this purpose. Statistical models are applied to the 1981 Corinth Gulf sequence,
comprising three strong (M = 6.7, M = 6.5, and M = 6.3) events between 24 February and 4 March. The non-homogeneous Poisson process outperforms the simple Poisson process
in order to model the aftershock sequence, whereas the Weibull process is more appropriate to capture the features of the
short-term behavior, but not the most proper for describing the seismicity in long term. The aftershock data defines a smooth
curve of the declining rate and a long-tail theoretical model is more appropriate to fit the data than a rapidly declining
exponential function, as supported by the quantitative results derived from the survival function. An autoregressive model
is also applied to the seismic sequence, shedding more light on the stationarity of the time series. 相似文献
168.
LIU ZhiQiang LAI YuanMing ZHANG MingYi & ZHANG XueFu State Key Laboratory of Frozen Soil Engineering Cold Arid Regions Environmental Engineering Research Institute Chi- nese Academy of Sciences Lanzhou China School of Civil Engineering Lanzhou Jiao Tong University Lanzhou China Chongqing Jiao Tong University Chongqing China 《中国科学D辑(英文版)》2007,50(3):404-410
The stochastic finite element equations for random temperature are obtained using the first-order per-turbation technique taking into account the random thermal properties and boundary condition, based on heat transfer variational principle. The local average method for 2-D is used to discretize random fields. Then, the random temperature fields of embankment in cold regions are investigated on condi-tion that the thermal properties and boundary condition are taken as random fields, respectively, by using the program, which is written by the methods. The expected value of temperature field and the standard deviation of the temperature field of embankment in cold regions are obtained and analyzed. 相似文献
169.
Mass spectra in the range 2 ≦ M/Q ≦ 3 provided by a high resolution mode of the ISEE-3 Plasma Composition Experiment were evaluated for three selected periods during early 1980. The observed Ne/O ratios are compatible with estimated solar abundance ratios. In two of the three periods, the He/Ne-ratios agree with the Apollo foil results. Freezing-in temperatures for oxygen are similar to those obtained by other groups. Possible reasons for an unexpectedly high flux at M/Q = 2.4 are discussed. 相似文献
170.