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   32241篇
  免费   429篇
  国内免费   374篇
测绘学   1221篇
大气科学   2451篇
地球物理   6270篇
地质学   11382篇
海洋学   2493篇
天文学   7676篇
综合类   143篇
自然地理   1408篇
  2021年   282篇
  2020年   278篇
  2019年   325篇
  2018年   819篇
  2017年   784篇
  2016年   1025篇
  2015年   591篇
  2014年   960篇
  2013年   1696篇
  2012年   1047篇
  2011年   1284篇
  2010年   1078篇
  2009年   1405篇
  2008年   1213篇
  2007年   1164篇
  2006年   1179篇
  2005年   990篇
  2004年   874篇
  2003年   872篇
  2002年   861篇
  2001年   776篇
  2000年   749篇
  1999年   678篇
  1998年   619篇
  1997年   639篇
  1996年   571篇
  1995年   537篇
  1994年   511篇
  1993年   432篇
  1992年   385篇
  1991年   413篇
  1990年   416篇
  1989年   388篇
  1988年   363篇
  1987年   432篇
  1986年   364篇
  1985年   445篇
  1984年   480篇
  1983年   465篇
  1982年   445篇
  1981年   357篇
  1980年   362篇
  1979年   312篇
  1978年   307篇
  1977年   294篇
  1976年   259篇
  1975年   256篇
  1974年   285篇
  1973年   308篇
  1972年   197篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
The objective of this study was to clarify whether the changes in percent cover of plant functional types (i.e., life forms and growth forms) along a grazing gradient reflect the livestock number, which would reinforce the reliability of using a grazing gradient design and improve the management of rangeland. We selected two livestock camps that for many years have had different numbers of livestock, with approximately six times more sheep-equivalents at site 1 than at site 2. Vegetation was sampled in 10 quadrats on five transects along the grazing gradient at each site. In each quadrat, we recorded percent cover of each plant species. Our findings suggested that vegetation changes along the grazing gradient under different livestock numbers were characterized by changes in the cover of life forms: perennial species were replaced by annual species near the camps (10–50 m). However, we did not find growth form change that reflected the difference in the number of livestock.  相似文献   
122.
123.
124.
The genetic and synoptic classifications of the Novorossiysk Bora are created using the data of daily observations at the Novorossiysk meteorological station and other available synoptic information. Obtained are the quantitative criteria of these classifications, and on this base worked out are the basic scenarios of the generation and evolution of this dangerous phenomenon on the Black Sea coast of Russia. According to the genetic classification, the Bora was divided into four types: frontal, air-mass, monsoon, and gravity. Quantitative criteria worked out for each type can be used for the more accurate forecast of this destructive phenomenon near Novorossiysk. According to the synoptic classification, four classes were distinguished: Azores, North Atlantic, Siberian, and Arctic.  相似文献   
125.
In order to check the premature siltation of the reservoirs and guard against the drop in the irrigation potential, the Government of India has launched the schemes of soil conservation and integrated watershed management in the catchments of RVPs and Flood Prone rivers. Owing to the large financial and manpower commitments needed to implement and execute soil conservation measures over vast catchment areas, a priority approach for treatment was identified. The methodology developed for prioritization of watersheds of a catchment area conceptualizes sedimentation of the reservoirs as a multiplicative function of erosivity value and the delivery ratio. This paper deals with the development of a computerized data base software module ‘WEIGHT’ for determination of erosivity values for the mapping units comprising assemblages of the varying combinations of climate, physiography and slope, land use and cover conditions, soil characteristics (texture, solumn thickness, permeability and pH) and the existing erosion and soil conservation measures. The WEIGHT software package is coded in FORTRON-4 for PDP 11/83 operating system. the data base comprises storage of the attributes of the different erosivity determinants of the mapping units with predetermined erosivity values sequentially on a disk and comparing the attributes of a new mapping unit to get the most probabilities erosivity value. The objective has been to eliminate the personal bias and bring about the objectivity in the process of assigning erosivity values to the different mapping units. The data base design, design logic and operational sequence of the data base are discussed in the paper.  相似文献   
126.
In the present paper, a new foundation model has been proposed by introducing a stretched rough elastic membrane in the Pasternak shear layer sandwiched between two spring layers which is an extension of Kerr model. Considering the equilibrium of different elements, the equations governing the elastic settlement response of the model are derived. Finite difference scheme has been employed to solve the governing equations. The parametric studies carried out show the effect of several parameters on the elastic settlement response of the model. The proposed model is well suited for idealizing the behavior of geosynthetic-reinforced granular fill—soft soil system besides other applications.  相似文献   
127.
128.
Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.  相似文献   
129.
130.
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