全文获取类型
收费全文 | 19929篇 |
免费 | 3419篇 |
国内免费 | 6614篇 |
专业分类
测绘学 | 2622篇 |
大气科学 | 5903篇 |
地球物理 | 4073篇 |
地质学 | 7574篇 |
海洋学 | 3900篇 |
天文学 | 1471篇 |
综合类 | 1659篇 |
自然地理 | 2760篇 |
出版年
2024年 | 166篇 |
2023年 | 391篇 |
2022年 | 724篇 |
2021年 | 836篇 |
2020年 | 1024篇 |
2019年 | 1155篇 |
2018年 | 865篇 |
2017年 | 1030篇 |
2016年 | 1046篇 |
2015年 | 1175篇 |
2014年 | 1186篇 |
2013年 | 1485篇 |
2012年 | 1341篇 |
2011年 | 1270篇 |
2010年 | 1037篇 |
2009年 | 1255篇 |
2008年 | 1337篇 |
2007年 | 1555篇 |
2006年 | 1502篇 |
2005年 | 1334篇 |
2004年 | 1136篇 |
2003年 | 930篇 |
2002年 | 893篇 |
2001年 | 675篇 |
2000年 | 786篇 |
1999年 | 727篇 |
1998年 | 597篇 |
1997年 | 470篇 |
1996年 | 378篇 |
1995年 | 322篇 |
1994年 | 296篇 |
1993年 | 249篇 |
1992年 | 201篇 |
1991年 | 143篇 |
1990年 | 74篇 |
1989年 | 99篇 |
1988年 | 59篇 |
1987年 | 45篇 |
1986年 | 47篇 |
1985年 | 30篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 8篇 |
1954年 | 4篇 |
1877年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Using a zonally averaged, one-hemispheric numerical model of the thermohaline circulation, the dependence of the overturning strength on the surface equator-to-pole density difference is investigated. It is found that the qualitative behavior of the thermohaline circulation depends crucially on the nature of the small-scale vertical mixing in the interior of the ocean. Two different representations of this process are considered: constant vertical diffusivity and the case where the rate of mixing energy supply is taken to be a fixed quantity, implying that the vertical diffusivity decreases with increasing stability of the water column. When the stability-dependent diffusivity parameterization is applied, a weaker density difference is associated with a stronger circulation, contrary to the results for a fixed diffusivity. A counterintuitive consequence of the stability-dependent mixing is that the poleward atmospheric freshwater flux, which acts to reduce the thermally imposed density contrast, strengthens the thermally dominated circulation and its attendant poleward heat transport. However, for a critical value of the freshwater forcing, the thermally dominated branch of steady states becomes unstable, and is succeeded by strongly time-dependent states that oscillate between phases of forward and partly reversed circulation. When a constant vertical diffusivity is employed, on the other hand, the thermally dominated circulation is replaced by a steady salinity-dominated state with reversed flow. Thus in this model, the features of the vertical mixing are essential for the steady-state response to freshwater forcing as well as for the character of flow that is attained when the thermally dominated circulation becomes unstable.Responsible Editor: Jin-Song von Storch 相似文献
102.
LIU Xiangwen YANG Guangming ZOU Huamin QU Jing & ZHAO Wenxia . Graduate School China University of Geosciences Wuhan China . Test Center China University of Geosciences Wuhan China . Physics Technology College Wuhan University Wuhan China . Test Center Zhongshan University Guangzhou China 《中国科学D辑(英文版)》2004,47(3)
Sakhaite was first discovered by Ostrovskaya, Pertsev and Nikitina at Siberia in the former Soviet Union. By using the X-ray diffraction technique, they proved that the crystal system of sakhaite was cubic (a = 1.464 nm), its possible space groups were Fm3m, F432, F43m, Fm3, F23, and its crystal chemical for-mula was Ca48Mg16(CO3)16(BO3)28Cl4(OH)8 4H2O[1]. Chichagov, Simonov and Belov studied the crystal structure of synthetic sakhaite and determined that its space group was F4132, … 相似文献
103.
104.
105.
The Zhangjiakou–Penglai seismotectonic zone (ZPSZ) lies in the northern part of North China and extends along the Zhangjiakou–Beijing–Tianjin–Bohai Bay–Penglai–Yellow Sea. It is about 900 km long and some 250 km wide in a northwest direction. The great Sanhe-Pinggu (MS=8.0) earthquake occurred on September 1679 and the Tangshan (MS=7.8) earthquake on July 1976 caused serious economic and life losses. According to some differences in crust structure and regional tectonic stress field, the ZPSZ is divided into western and eastern segment by the 117°E line for study on long-term seismic hazard analysis. An analysis of Gutenberg–Richter's empirical relation of earthquake-frequency and time process of historic and recent earthquakes along the eastern and western segments shows that the earthquake activity obeys a Poisson process, and these calculations indicate that the earthquake occurrence probability of MS=6.0–6.9 is 0.77–0.83 in the eastern segment and the earthquake occurrence probability of MS=7.0–7.9 is 0.78–0.80 in the western segment of the ZPSZ during a period from 2005 to 2015. 相似文献
106.
A rainfall-induced shallow landslide is a major hazard in mountainous terrain, but a time-space based approach is still an unsettled issue for mapping rainfall-induced shallow landslide hazards. Rain induces a rise of the groundwater level and an increase in pore water pressure that results in slope failures. In this study, an integrated infinite slope analysis model has been developed to evaluate the influence of infiltration on surficial stability of slopes by the limit equilibrium method. Based on this new integrated infinite slope analysis model, a time-space based approach has been implemented to map the distributed landslide hazard in a GIS (Geographic Information Systems) and to evaluate the shallow slope failure induced by a particular rainfall event that accounts for the rainfall intensity and duration. The case study results in a comprehensive time-space landslide hazard map that illustrates the change of the safety factor and the depth of the wetting front over time. 相似文献
107.
于3个假说和林冠上方2m处的气象变量,采用Penman-Monteith组合模型估算了一个生长季节内川西亚高山林区分别以云杉(SF)、冷杉(FF)和白桦(BF)为优势树种的3个林分的湿林冠蒸发速率(Er)。研究结果表明,SF、FF和BF的湿林冠蒸发量(E)分别为44.51mm、88.51mm和57.8mm,分别占总降雨量的9.2%、16.6%和10.2%。与SF和BF相比,FF具有最高的月平均Er和蒸发比例。SF、FF和BF的平均Er分别为0.097mm/h(变化范围:0.028-0.487mm/h)、0.242mm/h(变化范围:0.068~0.711mm/h)和0.149mm/h(0.060~0.576mm/h)。最高和最低的月平均Er分别在6月(SF、FF和BF分别为0.120mm/h、0.317mm/h和0.169mm/h)和10月(SF、FF和BF分别为0.083mm/h、0.187mm/h和0.101mm/h)。8:00至16:00期间的平均点Er显著高于0:00至8:00以及16:00至0:00期间的平均Er。Er显著的日变化和月变化主要归因于林冠上方的太阳辐射、空气温度和相对湿度的变化。 相似文献
108.
GPS广播星历参数拟合算法 总被引:25,自引:2,他引:25
介绍了GPS广播星历参数的最小二乘估计方法。推导了相应的计算公式。计算结果表明。文中给出的公式是正确的,其拟合精度以用户距离误差(URE)示时,对预报轨道的损失小于1cm。 相似文献
109.
110.