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   16634篇
  免费   1943篇
  国内免费   2925篇
测绘学   893篇
大气科学   2465篇
地球物理   3335篇
地质学   8117篇
海洋学   2270篇
天文学   1859篇
综合类   1002篇
自然地理   1561篇
  2024年   69篇
  2023年   221篇
  2022年   686篇
  2021年   894篇
  2020年   808篇
  2019年   866篇
  2018年   1083篇
  2017年   1001篇
  2016年   992篇
  2015年   751篇
  2014年   950篇
  2013年   1033篇
  2012年   999篇
  2011年   1097篇
  2010年   987篇
  2009年   934篇
  2008年   893篇
  2007年   927篇
  2006年   827篇
  2005年   438篇
  2004年   374篇
  2003年   426篇
  2002年   490篇
  2001年   455篇
  2000年   341篇
  1999年   364篇
  1998年   284篇
  1997年   308篇
  1996年   256篇
  1995年   257篇
  1994年   229篇
  1993年   181篇
  1992年   163篇
  1991年   107篇
  1990年   92篇
  1989年   109篇
  1988年   79篇
  1987年   63篇
  1986年   57篇
  1985年   43篇
  1984年   42篇
  1983年   41篇
  1982年   40篇
  1981年   24篇
  1980年   23篇
  1979年   28篇
  1978年   17篇
  1977年   17篇
  1975年   22篇
  1973年   17篇
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
881.
秦晓楠  程钰 《地理科学》2019,39(1):156-163
采用 DPSIR概念模型作为旅游生态安全系统的基本形态,引入网络 DEA模型测度系统运行效率,从“投入-产出”的研究视角探究旅游生态安全系统内部作用机理。采用结构方程模型对网络 DEA模型的结构及权重加以限制,构建具有节点权重约束的网络 DEA评价模型,对主要旅游城市生态安全系统实例进行实证研究。研究结果认为:将生态安全系统分为自然运行阶段、管理反馈阶段2阶段进行效率评价,加权综合得出各旅游城市生态安全系统的综合评价值,以此为依据将样本城市划分为按系统效率的评价结果将其分划为绿色发展型、稳步发展型、高效发展型、双向提升型和管理能力缺失型,针对各类型旅游城生态安全系统特点提出改善其生态安全系统状况的建议。  相似文献   
882.
狄乾斌  於哲  徐礼祥 《地理科学》2019,39(10):1621-1630
海洋经济协调发展是科学评价区域海洋经济发展质量的重要依据。遵循城市与海洋经济协同的发展理念,构建环渤海城市海洋经济发展时空协调度评价体系,运用复杂系统时空协调度评价模型,计算并分析2008~2016年环渤海城市海洋经济发展时空协调度,通过SaTScan软件分析其时空聚类区。研究结果显示:①海洋经济各系统发展水平中,社会系统呈大幅度分散上升趋势,经济系统、生态系统与综合系统呈小幅度集中与分散交替上升趋势;协调等级时序变化稳定型城市主要是大连、潍坊、青岛、烟台、威海。②协调度时空聚类区既有空间分异又有重叠交集,社会系统、综合系统、经济系统聚类区依次相互覆盖,生态系统与其它三系统既有重叠区也有独立区。③聚类区内部,社会系统最为稳定,综合系统、经济系统次之,生态系统最为薄弱。  相似文献   
883.
The population distribution grid at fine scales better reflects the distribution of residents and plays an important role in investigating urban systems. The recent years have witnessed a growing trend of applying the nighttime light data to the estimation of population at micro levels. However, using the nighttime light data alone to estimate population may cause the overestimation problem due to excessively high light radiance in specific types of areas such as commercial zones and transportation hubs. In dealing with this issue, this study used taxi trajectory data that delineate people’s movements, and explored the utility of integrating the nighttime light and taxi trajectory data in the estimation of population in Shanghai at the spatial resolution of 500 m. First, the initial population distribution grid was generated based on the NPP-VIIRS nighttime light data. Then, a calibration grid was created with taxi trajectory data, whereby the initial population grid was optimized. The accuracy of the resultant population grid was assessed by comparing it with the refined survey data. The result indicates that the final population distribution grid performed better than the initial population grid, which reflects the effectiveness of the proposed calibration process.  相似文献   
884.
Groundwater is a key factor controlling the growth of vegetation in desert riparian systems. It is important to recognise how groundwater changes affect the riparian forest ecosystem. This information will not only help us to understand the ecological and hydrological process of the riparian forest but also provide support for ecological recovery of riparian forests and water-resources management of arid inland river basins. This study aims to estimate the suitability of the Water Vegetation Energy and Solute Modelling(WAVES) model to simulate the Ejina Desert riparian forest ecosystem changes,China, to assess effects of groundwater-depth change on the canopy leaf area index(LAI) and water budgets, and to ascertain the suitable groundwater depth for preserving the stability and structure of desert riparian forest. Results demonstrated that the WAVES model can simulate changes to ecological and hydrological processes. The annual mean water consumption of a Tamarix chinensis riparian forest was less than that of a Populus euphratica riparian forest, and the canopy LAI of the desert riparian forest should increase as groundwater depth decreases. Groundwater changes could significantly influence water budgets for T. chinensis and P. euphratica riparian forests and show the positive and negative effects on vegetation growth and water budgets of riparian forests. Maintaining the annual mean groundwater depth at around 1.7-2.7 m is critical for healthy riparian forest growth. This study highlights the importance of considering groundwater-change impacts on desert riparian vegetation and water-balance applications in ecological restoration and efficient water-resource management in the Heihe River Basin.  相似文献   
885.
Izvestiya, Atmospheric and Oceanic Physics - Bioparticles constitute a significant fraction of atmospheric aerosol. Their size range varies from nanometers (macromolecules) to hundreds of...  相似文献   
886.
Izvestiya, Atmospheric and Oceanic Physics - The aim of this work is to obtain estimates and parameterization of the dissipation rate of the turbulence kinetic energy (TKE dissipation), ε,...  相似文献   
887.
Izvestiya, Atmospheric and Oceanic Physics - Tsunami forecast possibilities for areas with a small base of historical tsunamis have been discussed using the Probabilistic Tsunami Hazard Assessment...  相似文献   
888.
Artemov  Yu. G.  Egorov  V. N.  Gulin  S. B. 《Oceanology》2019,59(6):860-870
Oceanology - Based on long-term data on the spatial distribution and fluxes of streaming (bubbling) methane in the Black Sea, the study considers various types of streaming gas emissions...  相似文献   
889.
Potalova  E. Yu.  Kleshcheva  T. I.  Permyakov  M. S. 《Oceanology》2019,59(6):814-822
Oceanology - The article considers a particular event in the influence of a mesoscale cyclone (MC) on the northern Sea of Japan on February 10–11, 2013. The passage of the MC was accompanied...  相似文献   
890.
The spatial and temporal variability of tidal mixing in Bohai Sea is studied using a numerical approach. In calculating tidal mixing, accurate barotropic tidal current is obtained via a harmonic analysis package utilizing the simulated current output from a high-resolution regional ocean model. And a “small-scale” roughness map is adopted to describe the detailed topographic features of Bohai Sea. It is shown that the tidal mixing estimated in Bohai Sea is much higher than the level of global background, and fluctuates considerably at some regions within a single day. In Liaodong Bay, Bohai Bay and Bohai Strait, the mixing varies greatly, with the peak value of O (10?2) m2 s?1. The order of magnitude of mixing in Laizhou Bay is about O (10?5~10?3) m2 s?1. Mixing with background level of O (10?5) m2 s?1 only appears in central area. Result also shows that rough topography plays relatively a more important role than tidal current in enhancing diapycnal mixing in Bohai Sea. The distributions of tidal mixing in selected sections reveal that the vertical stratification in Bohai Sea is not obvious, generally renders a barotropic structure.  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