全文获取类型
收费全文 | 25445篇 |
免费 | 5240篇 |
国内免费 | 7155篇 |
专业分类
测绘学 | 2683篇 |
大气科学 | 4662篇 |
地球物理 | 6074篇 |
地质学 | 14432篇 |
海洋学 | 3547篇 |
天文学 | 842篇 |
综合类 | 2609篇 |
自然地理 | 2991篇 |
出版年
2024年 | 228篇 |
2023年 | 675篇 |
2022年 | 1408篇 |
2021年 | 1645篇 |
2020年 | 1366篇 |
2019年 | 1442篇 |
2018年 | 1648篇 |
2017年 | 1403篇 |
2016年 | 1608篇 |
2015年 | 1536篇 |
2014年 | 1756篇 |
2013年 | 1775篇 |
2012年 | 1784篇 |
2011年 | 1842篇 |
2010年 | 1812篇 |
2009年 | 1682篇 |
2008年 | 1482篇 |
2007年 | 1362篇 |
2006年 | 1151篇 |
2005年 | 1023篇 |
2004年 | 736篇 |
2003年 | 681篇 |
2002年 | 722篇 |
2001年 | 705篇 |
2000年 | 673篇 |
1999年 | 817篇 |
1998年 | 658篇 |
1997年 | 632篇 |
1996年 | 594篇 |
1995年 | 456篇 |
1994年 | 479篇 |
1993年 | 394篇 |
1992年 | 342篇 |
1991年 | 267篇 |
1990年 | 214篇 |
1989年 | 177篇 |
1988年 | 161篇 |
1987年 | 89篇 |
1986年 | 85篇 |
1985年 | 68篇 |
1984年 | 41篇 |
1983年 | 29篇 |
1982年 | 47篇 |
1981年 | 20篇 |
1980年 | 30篇 |
1979年 | 28篇 |
1978年 | 8篇 |
1976年 | 8篇 |
1958年 | 21篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
概略地叙述了西宁地区的泥石流形成条件和形成机理,并将西宁地区泥石流划分为稀性泥石流类及泥石质泥石流种类。在此基础上提出防治泥石流灾害的意见。 相似文献
13.
The ordinary kriging method, a geostatistical interpolation technique, was applied for developing contour maps of design storm depth in northern Taiwan using intensity–duration–frequency (IDF) data. Results of variogram modelling on design storm depths indicate that the design storms can be categorized into two distinct storm types: (i) storms of short duration and high spatial variation and (ii) storms of long duration and less spatial variation. For storms of the first category, the influence range of rainfall depth decreases when the recurrence interval increases, owing to the increasing degree of their spatial independence. However, for storms of the second category, the influence range of rainfall depth does not change significantly and has an average of approximately 72 km. For very extreme events, such as events of short duration and long recurrence interval, we do not recommend usage of the established design storm contours, because most of the interstation distances exceed the influence ranges. Our study concludes that the influence range of the design storm depth is dependent on the design duration and recurrence interval and is a key factor in developing design storm contours. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
14.
本文讨论了我国南水北调东线水质污染的产生及其对调水水质的影响,同时对调水区域内工业排水与治理经济模式和湖泊水生经济系统的最优化分析进行了探论. 相似文献
15.
1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t… 相似文献
16.
In order to understand the process of surface erosion and acquire basic data of conditions on hillslope without vege tation, a sprinkling experiment is conducted on a bare slope in Mt. Tanakami in the central part of Japan. Based on the mea surements of runoff, mean soil erosion depth, and sediment yield, etc. , the results suggest the following characteristics in the process of surface erosion in the experimental area. (1) The occurrence of sediment discharge is interrupted; (2) Surface runoff is a saturated overland flow; (3) The mean soil erosion depth is thick compared with other areas in Mt. Tanakami;(4) Sediment discharge process is detachment- limited. 相似文献
17.
对20个氯代苯酚类化合物进行了CNDO/2量子化学计算,讨论了化合物电子结构与其对水生物翻车鱼(Bluegill)、花鳉鱼(Guppy)、和虾(Shrimp)半致死量之间的关系,分别获得了表示其构效关系的三个线性方程,它们的显著性均远高于α=0.01水平。结果表明:氯代苯酚化合物苯环上碳原子的兀电荷之和(∑Qπ_R)越大苯环上碳原子的净电荷之和(∑Q_R)越大,或LUMO轨道能(E_(LUMO))越低,均使化合物对Bluegill和Guppy水生物的毒性增大;而化合物苯环上羟基邻位的碳原子的兀电荷布居(P_3)越大,同时HOMO轨道能E_(HOMO)越高,则化合物对Shrimp毒性也越大。据此,可预测氯代苯酚类化合物对上述三种水生物的毒性。 相似文献
18.
An improved approach is given for deriving the physical parameters of a molecular cloud by its NH3 rotation-inversion hyperfine spectra. The optical depth τ0(1, 1) of NH3 (1,1) is obtained by considering the blending effect of the magnetic hyperfine spectral lines, the (1-1) excitation temperature Tex (1, 1) is calculated by a two level model, the effect of different collision rate and the thermalization of the (1, 1) inversion lines are discussed, the rotation temperature TR ( 2, 1) between the NH3 levels (2, 2) and (1, 1) and the column density N( 1, 1) of the NH3 (1, 1) inversion level are derived, the results of the total column density from different assumptions for the abundance ratio of ortho-NH3 and para-NH3 are also discussed.
This approach is used for the molecular cloud NGC 2023 by using the relevant observed data and its optical depth and other physical parameters are obtained. 相似文献
19.
The accuracy of the AE index as a measure of the joule heat production rate is examined for a typical substorm event on 18 March 1978 by estimating the global joule heat production rate, as a function of time, using data obtained from the IMS six meridian chains. In spite of the fact that the AE index had had an initial slow growth which was followed by a rapid growth, the joule heat production rate attained a high level during the slow growth and thus their variations were considerably different from those of the AE index. Therefore, although the AE index is statistically linearly related to the global joule heat production rate, one should be cautious in assuming that details of time variations of the AE index during individual events represent those of the joule heat production rate. 相似文献
20.
本矿床是我国金红石矿床重要的成因及工业类型,为具有特定化学成分的岩石在特定变质条件下形成的。本文通过对大量数据的处理,查明了化学成分与成矿的关系:高Al2O3、MgO、Fe2O3+FeO、K2O、P2O5、H2O+、MnO,低SiO2、CaO、Na2O、CO2、S等,利于钛富集;高TiO2、MgO、P2O5、S,低SiO2、CaO、K2O、Al2O3、Fe2O3/FeO,利于金红石形成;高Sc、Zr、Ni、Zn及La/Yb,低Y、Nb、Th、Cd、Ba、Be、ΣREE,利于金红石纯度提高。对研究矿床成因及寻找新的同类矿床等有重要意义 相似文献