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   33514篇
  免费   1002篇
  国内免费   982篇
测绘学   975篇
大气科学   3334篇
地球物理   6970篇
地质学   12206篇
海洋学   2677篇
天文学   6898篇
综合类   353篇
自然地理   2085篇
  2021年   263篇
  2020年   272篇
  2019年   293篇
  2018年   640篇
  2017年   595篇
  2016年   822篇
  2015年   581篇
  2014年   825篇
  2013年   1524篇
  2012年   879篇
  2011年   1165篇
  2010年   1018篇
  2009年   1363篇
  2008年   1194篇
  2007年   1077篇
  2006年   1133篇
  2005年   985篇
  2004年   932篇
  2003年   976篇
  2002年   979篇
  2001年   851篇
  2000年   896篇
  1999年   812篇
  1998年   715篇
  1997年   774篇
  1996年   682篇
  1995年   625篇
  1994年   548篇
  1993年   497篇
  1992年   479篇
  1991年   451篇
  1990年   462篇
  1989年   429篇
  1988年   401篇
  1987年   483篇
  1986年   451篇
  1985年   473篇
  1984年   567篇
  1983年   566篇
  1982年   515篇
  1981年   493篇
  1980年   453篇
  1979年   437篇
  1978年   449篇
  1977年   394篇
  1976年   355篇
  1975年   355篇
  1974年   405篇
  1973年   389篇
  1972年   245篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Surveys of breeding birds on small islands in Penny Strait and Queens Channel, Nunavut Territory, Canada, were conducted in July 2002 and 2003. Approximately 3600 marine birds were observed, with the most common species being Arctic terns ( Sterna paradisaea , N=2400) and common eiders ( Somateria mollissima borealis , N=620). We observed no Ross's gulls ( Rhodostethia rosed ) in either year, and we found ivory gulls ( Pagophila eburned ) only in 2003, even though these species commonly bred here in the 1970s. This previously unsurveyed region supports numerous breeding marine birds, but reproductive success on these small islands may be dependent on annual ice conditions and consequent movements of Arctic foxes ( Alopex lagopus ).  相似文献   
932.
Wind velocity and sand transport on a barchan dune   总被引:2,自引:0,他引:2  
We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.  相似文献   
933.
Hugh H. Mills   《Geomorphology》2003,55(1-4):263
Measures of local relief, regional relief, and slope were calculated from digital elevation models (DEMs) for 50 bedrock units in the Ridge and Valley and Blue Ridge provinces of Tennessee. Each of these measures was normalized and the three were then averaged to produce the erosional resistance index (ERI). Bedrock units with higher ERI values include coarse clastics, intermediate clastics, and metaplutonics. Units with lower values include shales, limestones, limestones plus dolostones, and carbonates plus fine clastics. Dolostones tend to have intermediate values. The calculated ERI values were compared with subjective ratings by a geologist with decades of field experience in east Tennessee. Generally, the agreement between the two ratings was good, the most glaring exception being several shales with improbably high ERI values. These turned out to be thin units cropping out beneath very hard sandstones, allowing them to stand higher and steeper than would otherwise be possible. A systematic method for detecting such erroneously high ERI values is suggested. Inspection of a drainage map superimposed on the geology map shows that in a given area, streams tend to flow on rock units with the lowest ERI values. In addition, statistical analysis shows that bedrock units with the lowest ERI values are, on average, almost three times closer to the nearest stream and six times as likely to have streams flowing on them than are units with highest values. Further, the effect of ERI on stream location is strongest for streams with drainage areas between 1 and 30 km2. Thus, small streams appear to be subject to greater lithologic control than are larger streams.  相似文献   
934.
The Colville basin drains the North Slope of Alaska and is one of several large Arctic river systems located within permafrost. The timing and style of fluvial processes in the earth's permafrost regions differ from those occurring in midlatitude settings. Moreover, in comparison to temperate-zone systems, rivers located entirely within permafrost perform most of their work during relatively short periods of time. This paper examines river ice hydrology and the resulting geomorphic processes that occur within the Colville delta, Alaska. Fluvial processes and landform development within the Colville delta occur after the flood-pulse is initiated by the breakup of river ice. During this 4-month period, the geomorphic processes are largely influenced by the movement of ice. The flood-pulse and accompanying river ice influences erosional and depositional processes and results in unique styles of sediment transport, deposition, and riverbank erosion.  相似文献   
935.
Soil water repellency can enhance overland flow and erosion and may be altered by fire. The Christmas 2001 bushfires near Sydney allowed investigation of the relationship between fire severity, water repellency and hydrogeomorphological changes. For two sub-catchments with differences in fire severities in Nattai National Park, south-west of Sydney, this paper considers: (1) the links between fire severity based on SPOT image analysis and ground observation of fire severity and repellency; (2) the textural and organic/minerogenic characteristics of eroded sediment; and (3) erodibility, erosion and deposition of soils in both catchments. Ground surveys show that image analysis reflects well the degree of vegetation consumption by fire, but cannot adequately predict the degree of ground litter consumption, associated soil heating and repellency effects. Fire had varying effects on repellency, leaving it unchanged, destroying it or enhancing it, depending on the soil temperature reached. The main post-fire hydrogeomorphological changes have been widespread erosion and colluvial and alluvial deposition of topsoil in foot-slope locations and river systems, but only localised redistribution of the highly erodible, repellent sandy subsurface layer. The fire did not trigger major geomorphological change in the study area, but fires probably cause important topsoil and nutrient depletion and may also affect water quality.  相似文献   
936.
This paper considers the likely future population in Australia to 2050 and is cast within the context of environmental limitations, to which Griffith Taylor alerted the nation in the 1920s and 1930s, and for which he was vilified in several quarters. While acknowledging the relative accuracy of his long-range forecasts, the arguments here depart from environmental determinism, although varying sets of environmental ethics and values are considered in relation to Australia's global responsibilities and international commitments. It is argued that an increase in population to 26 million by 2050 will not place severe stresses on the physical environment, provided environmental and resource management strategies are put firmly in place, and if consumption and resource use practices are significantly modified. An ideological linkage exists between a resurgent Australian nationalism and a number of environmental perspectives, but one which rejects growth and biological diversity among humans while embracing environmentalism. This inward-looking nationalism–environmentalism is seen as harmful both to Australia's moral integrity as a nation, and in local and world citizenship.  相似文献   
937.
试述福建顺昌南舟萤石矿地质特征   总被引:1,自引:0,他引:1  
傅树超 《福建地质》2003,22(2):53-61
顺昌南舟萤石矿位于顺昌县埔上镇南舟村境内,1996年福建省区域地质调查队开展1:5万埔上幅区调时发现矿体,后开展普查工作表明该矿床规模大、交通方便,易于开采,现已投入开发利用。南舟萤石矿成矿地质特征在闽北丰富的萤石资源中具有一定代表性。  相似文献   
938.
根据矿物差热曲线特征温度,拟定使用热重差分析岩矿中全碳、结晶水、硫化物硫和二氧化碳等项目的灼烧温度范围,并进行热重差分析。所得的结果与其他方法结果一致。  相似文献   
939.
新疆伊宁盆地活动断裂新活动特征研究   总被引:3,自引:0,他引:3  
新疆伊宁盆地主要分布有巩留南、喀什河、雅马特等6条活动断裂,断裂走向近东西向与北酉一北北西向。其中巩留南断裂、喀什河断裂、雅马特断裂等在晚更新世一全新世时期有过多次显著的新活动,切错了晚更新世一全新世堆积物。在喀什河断裂上1812年发生了8级大展,形成长约100km的地震形变带。在巩留南断裂、雅马特断裂、伊宁断裂上也有受控于断裂近代新活动的中等地震发生。  相似文献   
940.
苏鲁褶皱带形成于元古宙 (2 2 33~ 185 5Ma)典型优地槽构造环境 ,主要由石榴橄榄岩、石榴辉石岩、榴辉岩等侵入岩 (柯石英深度相地幔岩浆房中形成 )和它们的火山沉积建造围岩一起经褶皱、变质而形成。变质作用经历了先蓝片岩相 (前花岗岩 )后片麻岩混合岩相过程。由于变质作用的不规律性 ,苏鲁褶皱带可分为 2个构造带 :(1)东部构造带 (蓝片岩 )和 (2 )西部构造带 (片麻岩混合岩 )。根据A·都城秋穗所识别的变质带系统 ,可将其作为一个双变质带。东部构造带以出现许多块状、条带状榴辉岩辉石岩橄榄岩组合的残余岩块为特征 ,其中还残留着高压的矿物 (石榴石、绿辉石、柯石英 ) ,而且有被混合岩和各种交代岩替代的显著标志。在中生代 ,苏鲁元古褶皱带受造山作用的影响活化 ,导致许多花岗岩体的侵入 ,使交代岩广泛发育。  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