首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   29401篇
  免费   1017篇
  国内免费   2163篇
测绘学   1708篇
大气科学   2879篇
地球物理   5753篇
地质学   14366篇
海洋学   1720篇
天文学   1840篇
综合类   2612篇
自然地理   1703篇
  2024年   22篇
  2023年   81篇
  2022年   209篇
  2021年   249篇
  2020年   201篇
  2019年   239篇
  2018年   4987篇
  2017年   4207篇
  2016年   2811篇
  2015年   481篇
  2014年   379篇
  2013年   278篇
  2012年   1240篇
  2011年   2992篇
  2010年   2293篇
  2009年   2591篇
  2008年   2127篇
  2007年   2599篇
  2006年   230篇
  2005年   401篇
  2004年   538篇
  2003年   595篇
  2002年   442篇
  2001年   215篇
  2000年   230篇
  1999年   273篇
  1998年   202篇
  1997年   211篇
  1996年   191篇
  1995年   163篇
  1994年   133篇
  1993年   155篇
  1992年   124篇
  1991年   82篇
  1990年   78篇
  1989年   66篇
  1988年   47篇
  1987年   33篇
  1986年   31篇
  1985年   23篇
  1984年   17篇
  1983年   10篇
  1982年   25篇
  1981年   29篇
  1980年   30篇
  1979年   6篇
  1978年   3篇
  1976年   6篇
  1964年   2篇
  1958年   3篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
91.
The ability of artificial neural network to differentiate water samples from the two aquifers of Kuwait on the basis of their major ion chemistry has been demonstrated. The major ion concentration distribution in the groundwater of the Kuwait Group and the Dammam Formation aquifers of Kuwait appears very similar. Cross-plots, supported by the discriminant function analysis of the data, however, suggest that there are some subtle differences in the overall composition of the water from the two aquifers that make it possible to differentiate the water from the two aquifers in almost 80% of the cases. An artificial neural network improved the differentiation capability to 90% of the cases. It is also possible to estimate the fraction of Kuwait Group water in the flow stream of dually completed wells with the help of an artificial neural network developed for this purpose. Electronic Publication  相似文献   
92.
The skill and efficiency of a numerical model mostly varies with the quality of initial values, accuracy on parameterization of physical processes and horizontal and vertical resolution of the model. Commonly used low-resolution reanalyses are hardly able to capture the prominent features associated with organized convective processes in a monsoon depression. The objective is to prepare improved high-resolution analysis by the use of MM5 modelling system developed by the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR). It requires the objective comparison of high and low-resolution analysis datasets in assessing the specific convective features of a monsoon depression. For this purpose, reanalysis datasets of NCAR/NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) at a horizontal resolution of 2.5‡ (latitude/longitude) have been used as first guess in the objective analysis scheme. The additional asynoptic datasets obtained during BOBMEX-99 are utilized within the assimilation process. Cloud Motion Wind (CMW) data of METEOSAT satellite and SSM/I surface wind data are included for the improvement of derived analysis. The multiquadric (MQD) interpolation technique is selected and applied for meteorological objective analysis at a horizontal resolution of 30 km. After a successful inclusion of additional data, the resulting reanalysis is able to produce the structure of convective organization as well as prominent synoptic features associated with monsoon depression. Comparison and error verifications have been done with the help of available upper-air station data. The objective verification reveals the efficiency of the analysis scheme.  相似文献   
93.
We consider radiative transfer in C18O, HCO+, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure.  相似文献   
94.
95.
96.
Using analytic signal method, interpretation of pole-pole secondary electric potentials due to 2D conductive/resistive prisms is presented. The estimated parameters are the location, lateral extent or width and depth to top surface of the prism. Forward modelling is attempted by 2D-Finite Difference method. The proposed stabilised analytic signal algorithm (RES2AS) uses Tikhonov’s regularization scheme and FFT routines. The algorithm is tested on three theoretical examples and field data from the campus of Roorkee University. The stability of RES2AS is also tested on synthetic error prone secondary pole-pole potential data.  相似文献   
97.
98.
INTRODUCTIONThenotionof“learningorganization”hasbecomeanewbuzzwordinthemanagementandbusinesslitera turefromthelate 1980s (WangandAhmed ,2 0 0 3;Stewart,2 0 0 1;Garavan ,1997) .Thereappearstobemoreconsensusaboutthatonesourceofcompetitivead vantageistheabilityandrateatwhichanorganizationcanlearnandreactmorequicklythanitscompetitors(Stewart ,2 0 0 1;Garavan ,1997;Senge ,1990 ) ,soanumberofcompaniesandorganizationsaspiretocreateanddevelopintolearningorganizationsinordertofacili tatecontin…  相似文献   
99.
100.
Blue crabsCallinectes sapidus in lower Chesapeake Bay are subject to high rates of predation during the late summer of their first year of growth as they migrate out of vegetated nursery habitats. Predators, potentially contributing to this pattern, were identified in video-recorded field observations of tethered juvenile crabs (20–25 mm carapace width). Predators were also tested in large laboratory tanks containing similarly-sized untethered crabs as prey. Seven different predators attacked tethered crabs in the field. Only two predators, larger blue crabs and northern puffers,Sphoeroides maculatus, consistently succeeded in preying on crabs in both field and laboratory settings. These results confirm the importance of cannibalism on juvenile blue crabs and identify puffers as a potentially overlooked source of predation pressure.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号