全文获取类型
收费全文 | 20848篇 |
免费 | 4488篇 |
国内免费 | 5592篇 |
专业分类
测绘学 | 1959篇 |
大气科学 | 2801篇 |
地球物理 | 7124篇 |
地质学 | 11463篇 |
海洋学 | 3329篇 |
天文学 | 478篇 |
综合类 | 1468篇 |
自然地理 | 2306篇 |
出版年
2024年 | 67篇 |
2023年 | 252篇 |
2022年 | 583篇 |
2021年 | 820篇 |
2020年 | 894篇 |
2019年 | 1168篇 |
2018年 | 922篇 |
2017年 | 1013篇 |
2016年 | 1039篇 |
2015年 | 1199篇 |
2014年 | 1450篇 |
2013年 | 1499篇 |
2012年 | 1415篇 |
2011年 | 1576篇 |
2010年 | 1367篇 |
2009年 | 1436篇 |
2008年 | 1388篇 |
2007年 | 1567篇 |
2006年 | 1527篇 |
2005年 | 1238篇 |
2004年 | 1135篇 |
2003年 | 953篇 |
2002年 | 805篇 |
2001年 | 680篇 |
2000年 | 659篇 |
1999年 | 645篇 |
1998年 | 582篇 |
1997年 | 516篇 |
1996年 | 464篇 |
1995年 | 381篇 |
1994年 | 373篇 |
1993年 | 263篇 |
1992年 | 251篇 |
1991年 | 183篇 |
1990年 | 127篇 |
1989年 | 151篇 |
1988年 | 100篇 |
1987年 | 63篇 |
1986年 | 41篇 |
1985年 | 32篇 |
1984年 | 16篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 17篇 |
1980年 | 6篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1954年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Based on the Intensive Field Campaign(IFC-1)data of Boreal Ecosystem-Atmosphere Study(BOREAS).a three-dimensional meso-β scale model is used to simulate the effect of boreal forests onthe lower atmosphere.A fine horizontal resolution of 2 km×2 km is used in order to distinguish thevegetative heterogeneity in the boreal region.A total of 20×25 grid points cover the entire sub-modeling area in BOREAS' South Study Area(SSA).The ecosystem types and their coverage ineach grid square are extracted from the North American Land Cover Characteristics Data Base(NALCCD)generated by the U.S.Geographical Survey(USGS)and the University of Nebraska-Lincoln(UNL).The topography of the study area is taken from the Digital Elevation Map(DEM)of USGS.The model outputs include the components of the energy balance budget within the canopyand at the ground.the turbulence parameters in the atmospheric boundary layer and the wind.temperature and humidity profiles extending up to a height of 1500 m.In addition to the fine timeand spatial step,the unique feature of the present model is the incorporation of both dynamic andbiological effects of the Boreal forest into the model parameterization scheme.The model resultscompare favorably with BOREAS' IFC-1 data in 1994 when the forest was in the luxuriant growingperiod. 相似文献
32.
Numerical methods are usually used for the computation of ephemerides with perturbations for the precise orbital determination of an artificial satellite. But their numerical stability will be encountered in a long arc. In this case the use the improved Encke special perturbation methods has been suggested. The results of this paper show that Encke's method does indeed have a certain effectiveness, but cannot yet completely resolve the numerical stability, and the more efficient method is to use the energy integral or its variational relation to control the growth of the along-track error in general numerical calculations so that the aim of stabilization can be achieved. 相似文献
33.
34.
在太阳活动区的物理研究中,特别是在二维动力学光谱分析中,迫切需要相应活动区的磁图资料。本文介绍了在太阳光谱仪的入射狭缝后安装一种新型偏振器进行活动区二维磁场观测的新方法。这种方法不仅能获得日面上任一点的磁场强度,且可快速获得活动区的纵向场磁图。除此之外,还可利用多条谱线的同时观测,获得有关磁力线管结构等方面的资料。 相似文献
35.
The energy flow ofBranchiura sowerbyi was studied for the first time in China in a shallow macrophytic lake, Biandantang Lake, Hubei Province. The energy flow
was calculated from the measurement of flesh production (12.5241kJ/m2a), egestion (517.7302kJ/m2a), metabolism (38.3273 kJ/m2a), and excretion (4.3798kJ/m2a). The net growth efficiency of the species is about 22.7%, which accords well with the generally reported value for oligochaetes.
In addition, the relationship between starvation respiration (R, mgO2/ind·d), wet weight (Ww, mg) and temperature (T, °C) were also measured, with the regression function beingR=0.008Ww0.736 e0.050T.
Project supported by NSFC (30270278, 3960019), the foundation of the government of Hubei Province (No. 2000J109), and the
foundation of Ecological Station, CAS in the Institute of Hydrobiology. 相似文献
36.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
37.
The energy budget ofBellamya earuginosa in a shallow algal lake, Houhu Lake (Wuhan, China) was investigated by the measurement of flesh production (32.8 kJ/(m2·a)), egestion (337.7 kJ/(m2·a)), metabolism (246.7 kJ/(m2·a)), and estimation of excretion (21.4kJ/(m2·a)). The net growth efficiency of the species is about 10.9%, which accords with the generally reported value for gastropods.
In addition, the relationships between starvation respiration (R, mgO2/(Ind·d)), body weight (Wd, mg in dry wt) and temperature (T, °C) were also determined. The regression equationR=0.044Wd
0.537
e
0.061T
was obtained by the least square method, The measured SDA of the species is 26.51% of its gross metabolism.
Project No. 3960019 and 39430101 supported by NSFC and also a granted for the East Lake Ecological Experimental Station, CAS,
Wuhan. 相似文献
38.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
39.
CLIMATICTRENDINDICATEDBYVARIATIONSOFGLACIERSANDLAKESINTHETIANSHANMOUNTAINS¥HuRuji;YangChuande;MaHong;JiangFengqing(XinjiangIn... 相似文献
40.
本文简介海南省1:5万坝王岭、猴狝岭幅区域地质调查的地球化学简单分段法在地层划分中的应用。根据对下古生界变质岩与白垩系红层两条地层实测剖面的地球化学资料整理,用简单分段法进行划分、其效果较好。它的地球化学组分基本能反映原始沉积物的特点和其沉积环境,且与岩石地层的划分相互印证。 相似文献