全文获取类型
收费全文 | 11908篇 |
免费 | 2108篇 |
国内免费 | 2477篇 |
专业分类
测绘学 | 1150篇 |
大气科学 | 624篇 |
地球物理 | 4084篇 |
地质学 | 7035篇 |
海洋学 | 994篇 |
天文学 | 66篇 |
综合类 | 1081篇 |
自然地理 | 1459篇 |
出版年
2024年 | 33篇 |
2023年 | 133篇 |
2022年 | 307篇 |
2021年 | 406篇 |
2020年 | 464篇 |
2019年 | 527篇 |
2018年 | 448篇 |
2017年 | 526篇 |
2016年 | 518篇 |
2015年 | 547篇 |
2014年 | 704篇 |
2013年 | 845篇 |
2012年 | 656篇 |
2011年 | 761篇 |
2010年 | 633篇 |
2009年 | 820篇 |
2008年 | 850篇 |
2007年 | 782篇 |
2006年 | 804篇 |
2005年 | 618篇 |
2004年 | 617篇 |
2003年 | 571篇 |
2002年 | 493篇 |
2001年 | 426篇 |
2000年 | 400篇 |
1999年 | 391篇 |
1998年 | 331篇 |
1997年 | 320篇 |
1996年 | 281篇 |
1995年 | 227篇 |
1994年 | 205篇 |
1993年 | 194篇 |
1992年 | 154篇 |
1991年 | 110篇 |
1990年 | 103篇 |
1989年 | 69篇 |
1988年 | 69篇 |
1987年 | 36篇 |
1986年 | 26篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 16篇 |
1977年 | 5篇 |
1972年 | 1篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
992.
993.
反相流动注射催化光度法测定废水中的亚硝酸根 总被引:3,自引:0,他引:3
采用反相流动注射技术与催化光度法相结合,用微机化流动注射分析仪准确控制反应时间,优化了实验条件,克服了催化光度法反应时间难以控制引起的方法精密度、准确度不高的弱点,拟定了测定痕量亚硝酸根的流动注射催化光度方法。分析速度第小时20次,NO2^- 质量浓度在0.00-2.00mg/L,回归方程斜率为0.155。方法直接测定电厂测水中NO2^-的结果与Griess法相符合,RSD<5.0%(n=5)。 相似文献
994.
岩石变形所反映的地震作用过程--第13届国际变形机理、流变学和构造学学术会议概述 总被引:2,自引:0,他引:2
简要介绍了第13届国际变形机理、流变学和构造学学术会议的概况和特点,综述了地震变形作用的研究现状。 相似文献
995.
Strain and tilt changes measured during a water injection experiment at the Nojima Fault zone, Japan 总被引:2,自引:0,他引:2
Kunio Fujimori Hiroshi Ishii Atsushi Mukai Shigeru Nakao Shigeo Matsumoto Yasuhiro Hirata 《Island Arc》2001,10(3-4):228-234
Abstract In order to make geophysical and geological investigations of the Nojima Fault on Awaji Island, Japan, three boreholes measuring 1800 m, 800 m and 500 m deep were drilled into the fault zone. The fault is one of the seismic source faults of the 1995 Hyogo-ken Nanbu earthquake of M 7.2. A new multicomponent borehole instrument was installed at the bottom of the 800 m borehole and continuous observations of crustal strain and tilt have been made using this instrument since May 1996. A high-pressure water injection experiment within the 1800 m borehole was done in February and March 1997 to study the geophysical response, behavior, permeability, and other aspects of the fault zone. The injection site was located approximately 140 m horizontally and 800 m vertically from the instrument. Associated with the water injection, contraction of approximately 0.7 × 10−7 str (almost parallel to the fault) and tilt of approximately 1 × 10-7 rad in the sense of upheaval toward the injection site were observed. In addition to these controlled experiments, the strainmeter and tiltmeter also recorded daily variations. We interpret strain and tilt changes to be related to groundwater discharge and increased ultra-micro seismicity induced by the injected water. 相似文献
996.
The flow pattern around a cylinder, installed in a scoured channel bed, was experimentally investigated. Detailed measurements of the instantaneous 3D velocities were performed by using an Acoustic Doppler Velocity Profiler (ADVP), from which the profiles of the time-averaged velocities and turbulence stresses were obtained. It is shown that the influence of the cylinder and of the scour hole alters the approach flow; this is essentially confined to the vicinity of the cylinder and to the inside of the scour hole. The horseshoe vortex is measured as a flow reversal inside the scour hole, formed by the downward flow along the cylinder face and the reversed flow at the scour bed. 相似文献
997.
Statistical self-similarity in the spatial and temporal variability of rainfall, river networks, and runoff processes has been observed in many empirical studies. To theoretically investigate the relationships between the various time and space scales of variability in rainfall and runoff process we propose a simplified, yet physically based model of a catchment–rainfall interaction. The channel network is presented as a random binary tree, having topological and hydraulic geometry properties typically observed in real river networks. The continuous rainfall model consists of individual storms separated by dry periods. Each given storm is disaggregated in space and time using the random cascade model. The flow routing is modelled by the network of topologically connected nonlinear reservoirs, each representing a link in the channel network. Running the model for many years of synthetic rainfall time series and a continuous water balance model we generate an output, in the form of continuous time series of water discharge in all links in the channel network. The main subject of study is the annual peak flow as a function of catchment area and various characteristics of rainfall. The model enables us to identify different physical processes responsible for the empirically observed scaling properties of peak flows. 相似文献
998.
IntroductionWith the most feasible and powerful technical support provided by the high-precision GPS measurement for the study on horizontal crustal deformation, the obtainment of various-scale horizontal movement data and its study over the whole world is now in the ascendant, and its applicable fields is now expanding successively. In China, besides the GPS monitoring networks for different applications arranged by many agencies and departments, the key scientific project Crustal Movement… 相似文献
999.
Pebble clusters are common small‐scale morphological features in gravel‐bed rivers, occupying as much as 10 per cent of the bed surface. Important links exist between the presence of pebble clusters and the development of flow structures. These links are poorly understood at the three‐dimensional level. Particularly neglected has been the effect of clusters on the lateral flow characteristics. A laboratory study was conducted using a hydraulic flume, within which simulated pebble clusters were superimposed onto a plane bed of gravel material. High‐resolution three‐dimensional flow data were collected above the bed at two different flow depths using an acoustic Doppler velocimeter. The results present evidence of the importance of lateral flow in the development of turbulent flow structure. Narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three‐dimensional separation zone in their lee. This may indicate the presence of horseshoe‐type vortical structures analogous to those identified in less hydraulically rough environments. However, it is likely that these structures are more complicated given the mutual interference of the surrounding medium. The lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters. The results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters: flow acceleration up the stoss‐side of the cluster; recirculation behind the cluster in the wake region; vortex shedding from the pebble crest and shear layer; flow reattachment downstream of the cluster; upwelling of flow downstream of the point of reattachment; and recovery of flow. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
1000.
Flume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献