全文获取类型
收费全文 | 33514篇 |
免费 | 1002篇 |
国内免费 | 982篇 |
专业分类
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 |
出版年
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 31 毫秒
961.
R. A. SHAKESBY C. J. CHAFER S. H. DOERR W. H. BLAKE P. WALLBRINK G. S. HUMPHREYS 《The Australian geographer》2003,34(2):147-175
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
962.
I. H. Burnley 《The Australian geographer》2003,34(3):267-280
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
963.
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
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
964.
965.
966.
景观指数的粒度变化效应 总被引:66,自引:0,他引:66
本文以延河流域的1:250000和1:500000土地利用图为对象,以景观格局分析程序Fragstats3.3为分析工具,探讨了不同比例尺条件下景观指数随粒度增加的变化特征.研究结果表明,随着粒度值由25m到400m的逐渐增加,除斑块丰富度外的景观指数均具有明显的尺度效应,其中聚集度和集合度没有尺度转折点,其他指数具有明显或不明显的尺度转折点.对比分析1:250000和1:500000土地利用格局指数的计算结果可以发现:1)尺度转折点不是一个值,而是一个相对较小的区间;2)尺度转折点与研究图件的比例尺有关,比例尺越大,所发生的第一次尺度转折点的粒度就越小;3)第一尺度域是选择适宜粒度的较好取值范围.对延河流域1:250000土地利用图进行景观指数计算的适宜粒度范围是70~90m,1:500000土地利用图的适宜粒度范围是90~120m. 相似文献
967.
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
968.
1 INTRODUCTION Agricultural use of pesticides has been recognized as one of the important non-point pollution sources in the world, and the pesticide application to agricultural lands has led to large amounts of residues discharged into surface water through runoff (Leonard, 1990). There have been growing concerns about the fate and transport of pesticides in agricultural lands, due to the associated adverse impacts on water environment and human health (Agassi et al., 1995; Ng and Clegg… 相似文献
969.
The response of marine microorganism community to variations in the concentrations of heavy metals in the environment is investigated for the coastal waters of Peter the Great Bay and the northern part of Primor'e. The fact of the existence and sensitivity of the individual response of plankton community microorganisms to variations in the concentrations of metals in the water under the conditions of water contamination with a complex of metals are confirmed on the basis of microbiological data and the results of their comparison to the data of chemical analysis. Microbial indication is shown to serve as an operative method of monitoring and short-term forecasting of changes in the environmental conditions of sea coastal waters. This method allows obtaining preliminary information at the stage of selecting environmental quality observation stations. 相似文献
970.
— Analytical expressions to predict the enhancement of permeability due to stress-induced microcracking in initially low porosity rock are presented. A fracture mechanical microcrack model is employed to derive integrated effective hydraulic variables as a function of stress, which are then used to calculate the evolution of permeability using the statistically-based Dienes model. The model enables determination of permeability enhancement as a function of two loading parameters and three material parameters. Results are in reasonable agreement with experimental measurements and indicate that appreciable increases in permeability can be anticipated during brittle failure. The analytical nature of the model makes it easily incorporatable into numerical models that require quantification of the permeability evolution as a function of stress, for which there is currently no law. 相似文献