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   23417篇
  免费   4133篇
  国内免费   5381篇
测绘学   1314篇
大气科学   4756篇
地球物理   6531篇
地质学   11209篇
海洋学   3176篇
天文学   1040篇
综合类   2326篇
自然地理   2579篇
  2024年   124篇
  2023年   431篇
  2022年   1017篇
  2021年   1185篇
  2020年   955篇
  2019年   1023篇
  2018年   1163篇
  2017年   1097篇
  2016年   1302篇
  2015年   1088篇
  2014年   1305篇
  2013年   1360篇
  2012年   1207篇
  2011年   1279篇
  2010年   1335篇
  2009年   1303篇
  2008年   1127篇
  2007年   1154篇
  2006年   849篇
  2005年   855篇
  2004年   626篇
  2003年   670篇
  2002年   675篇
  2001年   655篇
  2000年   771篇
  1999年   1132篇
  1998年   960篇
  1997年   924篇
  1996年   907篇
  1995年   780篇
  1994年   699篇
  1993年   630篇
  1992年   514篇
  1991年   348篇
  1990年   280篇
  1989年   232篇
  1988年   219篇
  1987年   123篇
  1986年   135篇
  1985年   77篇
  1984年   68篇
  1983年   66篇
  1982年   63篇
  1981年   48篇
  1980年   51篇
  1979年   37篇
  1978年   16篇
  1977年   10篇
  1974年   9篇
  1958年   20篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Pavlova viridis sp. nov. is described on the basis of light and electron microscope observations. The material was collected from the coast of haiyang county of Shandong, China. It is characterized by 1) yellowish green to green chloroplast, 2) rudimentary short flagellum 0.3 μm long and hook-shaped, 3) long flagellum with small spherical knob-scales, 4) absence of pyrenoid and stigma, and 5) marine habitat. This new species andP. salina seem to be most closely related to each other but their colour, the insertion of their three appendages, the shape of their knob scales, and their cell periplasts are quite different from one another. Contribution No. 1879 from the Institute of Oceanology, Academia Sinica  相似文献   
72.
信阳上天梯矿区钙基膨润土可分为四个自然类型。根据矿石理化性能和蒙脱石晶体结构特征,选用第Ⅲ类矿石,运用对辊挤压钠化法和最佳工艺进行实验研究,获得钻井泥浆用A.P.I级膨润土。  相似文献   
73.
The HAMSOM(Hamburg Shelf Ocean Model),a high-resolution regional ice-ocean coupled model,was applied to investigate the seasonal evolution of Bohai Sea ice for winter 2015/2016.HAM SOM was initialized with monthly climatological temperature and salinity data from WOA13 and driven by hourly meteorological data obtained from the NCEP above the sea surface and tides at the open boundary.The ice model used here is a modified Hibler-type dynamic-thermodynamic sea ice model based upon viscous-plastic rheology.The ice extent,concentration,area,thickness,length of ice season as well as the distance between the top of Liaodong Bay(North China) and the outer ice edge line were simulated and compared with the observed data.Three types of modeling experiments were carried out to investigate the effects of wind,tide,and both wind and tide on Bohai Sea ice.The re sults show that wind,as both a dynamic and a thermodynamic factor,has a significant impact on the ice thickness,ice area,and ice-freezing and ice breakup dates as well as the ice velocity,while tides are a dynamic factor that influences only the ice velocity.During the severe ice period,the wind speed intensity increased by 25%,the average ice thickness thickened by approximately 4.0 cm in Liaodong B ay,approximately 2.1 cm in Bohai B ay and approximately2.5 cm in Laizhou B ay,and the total ice coverage area and total ice actual area increased by about 2×10~4 km~2 and 1.4×10~4 km2,respectively.While the tidal amplitude intensity increased by 25%,the average ice velocity increased by approximately 0.1 m/s.  相似文献   
74.
???????????????????????????????????????????б??????????????б?????????????????????????????????????????????????????????б????????????????????????????????????б??????????????????????λ???????????????????????????????????????????Ч???????????????????????  相似文献   
75.
Marsupenaeusjaponicus Bate is one of the most valuable cultured shrimp species in China and outdoor earthen pond farming is the most common method of culturing this organism. The need to increase soil usage efficiency in aquaculture has been recognized and a great deal of research effort has been directed toward development of super-intensive farming systems. However, current research and development in this field is largely devoted to Litopenaeus vannamei Boone, while to M.japonicus Bate it has been neglected. In this study, a layered fanning system was designed and a 66-day study was conducted in M.japonicus Bate culture. The system comprised bracket and sand layers that divided a shrimp tank filled to a depth of 1.2 m into four water layers. Conventional tank culture (unlayered) was used as a control. The results show that survival rate, feed conversion efficiency and production of M.japonicus Bate in the layered farming system were 68%, 18%, and 0.59 kg/m^2, respectively, all of which are significantly higher than in the unlayered fanning system (P〈0.01). These findings confirmed the possibility of using a layered system to culture M. japonicus Bate.  相似文献   
76.
The detention pond is one of the crucial items in detention facilities. It may effectively alleviate the occurrence of peak discharge, control the center of flood flow, and reduce the amount of soil loss. The objective of this study is analyzing the detention volume change of a detention pond with long-duration rainfall under the known isosceles trapezoidal inflow hydrograph model. The volume change of detention, which is under the influences of a givenisosceles trapezoidal inflow hydrograph and the extent of peak attenuation, is investigated by using the non-dimensional detention theory and the related mathematical analyses. The minimum detention volume of a detention pond can therefore be calculated based on the estimated of volume change of detention. The proposed detention volume estimation model can be used for the design of detention of facilities during the hillside development.  相似文献   
77.
??GNSS??????????У??轫GNSS?????????????????????????????????????????GNSS???????????????????????????????????????????????????????????????????????????????????????????????????????????????е???????????????????????е????λ???ó????????????????????????????????????????????????????????????????о???GNSS????????????????????????????????????????????????????????????????????????????????????????????????????????????  相似文献   
78.
The Geo-anchored method, based on a moment-type estimator, has been developed for estimating parent population properties from a successive sample of discoveries. By substituting the expectation of the waiting time z (n+1) of the (n + 1) th discovery to occurrence for an unknown parameter in the anchored method, the Geo-anchored method allows estimation of inclusion probabilities directly from observed data, thus eliminating the need for a priori selection of a value of N, R, or some other feature of the parent population. Because direct estimation of N and R requires an ordered sample, the Geo-anchored method is more sensitive to the data-generating process than the anchored method. This paper presents a sensitivity study on the Geo-anchored method. The test is based on simulated discovery sequences with different assumptions regarding discovery efficiency, exploration maturity, and the shape of the parent field size distribution. As a reference for comparison, estimates from the Horvitz–Thompson estimator also are presented.  相似文献   
79.
首先从人口、经济、用地3个维度综合考察武汉市增长与收缩的全貌,并采用县区及街道2个尺度的数据定量描述了武汉市增长与收缩的特征与空间格局,发现武汉市下辖青山区、硚口区、汉阳区和蔡甸区存在局部较严重的收缩现象,空间上形成集聚,形态上呈“穿孔式”。进一步以青山区为案例,着重从资本视角探讨发生局部收缩的内在机制,发现其存在老龄化、少子化趋势,但局部收缩的主因是资本从产业部门的“逃逸”。  相似文献   
80.
Climate condition over a region is mostly determined by the changes in precipitation, temperature and evaporation as the key climate variables. The countries belong to the Belt and Road region are subjected to face strong changes in future climate. In this paper, we used five global climate models from the latest Sixth Phase of Coupled Model Intercomparison Project (CMIP6) to evaluate future climate changes under seven combined scenarios of the Shared Socioeconomic Pathways and the Representative Concentration Pathways (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0 and SSP5-8.5) across the Belt and Road region. This study focuses on undertaking a climate change assessment in terms of future changes in precipitation, air temperature and actual evaporation for the three distinct periods as near-term period (2021-2040), mid-term period (2041-2060) and long-term period (2081-2100). To discern spatial structure, K?ppen-Geiger Climate Classification method has been used in this study. In relative terms, the results indicate an evidence of increasing tendency in all the studied variables, where significant changes are anticipated mostly in the long-term period. In addition to, though it is projected to increase under all the SSP-RCP scenarios, greater increases will be happened under higher emission scenarios (SSP5-8.5 and SSP3-7.0). For temperature, robust increases in annual mean temperature is found to be 5.2 °C under SSP3-7.0, and highest 7.0 °C under SSP5-8.5 scenario relative to present day. The northern part especially Cold and Polar region will be even more warmer (+6.1 °C) in the long-term (2081-2100) period under SSP5-8.5. Similarly, at the end of the twenty-first century, annual mean precipitation is inclined to increase largely with a rate of 2.1% and 2.8% per decade under SSP3-7.0 and SSP5-8.5 respectively. Spatial distribution demonstrates that the largest precipitation increases are to be pronounced in the Polar and Arid regions. Precipitation is projected to increase with response to increasing warming most of the regions. Finally, the actual evaporation is projected to increase significantly with rate of 20.3% under SSP3-7.0 and greatest 27.0% for SSP5-8.5 by the end of the century. It is important to note that the changes in evaporation respond to global mean temperature rise consistently in terms of similar spatial pattern for all the scenarios where stronger increase found in the Cold and Polar regions. The increase in precipitation is overruled by enhanced evaporation over the region. However, this study reveals that the CMIP6 models can simulate temperature better than precipitation over the Belt and Road region. Findings of this study could be the reliable basis for initiating policies against further climate induced impacts in the regional scale.  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