全文获取类型
收费全文 | 25985篇 |
免费 | 3700篇 |
国内免费 | 5073篇 |
专业分类
测绘学 | 3262篇 |
大气科学 | 3916篇 |
地球物理 | 6326篇 |
地质学 | 10220篇 |
海洋学 | 3284篇 |
天文学 | 1961篇 |
综合类 | 1800篇 |
自然地理 | 3989篇 |
出版年
2024年 | 91篇 |
2023年 | 287篇 |
2022年 | 709篇 |
2021年 | 919篇 |
2020年 | 1056篇 |
2019年 | 1322篇 |
2018年 | 901篇 |
2017年 | 1199篇 |
2016年 | 1179篇 |
2015年 | 1267篇 |
2014年 | 1595篇 |
2013年 | 1885篇 |
2012年 | 1586篇 |
2011年 | 1699篇 |
2010年 | 1337篇 |
2009年 | 1747篇 |
2008年 | 1714篇 |
2007年 | 1814篇 |
2006年 | 1750篇 |
2005年 | 1407篇 |
2004年 | 1319篇 |
2003年 | 1119篇 |
2002年 | 876篇 |
2001年 | 764篇 |
2000年 | 700篇 |
1999年 | 658篇 |
1998年 | 690篇 |
1997年 | 520篇 |
1996年 | 429篇 |
1995年 | 395篇 |
1994年 | 327篇 |
1993年 | 312篇 |
1992年 | 221篇 |
1991年 | 177篇 |
1990年 | 139篇 |
1989年 | 133篇 |
1988年 | 107篇 |
1987年 | 66篇 |
1986年 | 53篇 |
1985年 | 53篇 |
1984年 | 30篇 |
1983年 | 14篇 |
1982年 | 25篇 |
1981年 | 19篇 |
1980年 | 22篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 26篇 |
1973年 | 17篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
901.
This paper identifies contrasting environmental discourses on bushfire from the public debate that followed the Victorian fires of 2002–03. Submissions to the inquiry into the 2002–03 Victorian bushfires provide a particularly rich source of documentation. It is argued that environmental events such as bushfire only become political issues, or problems, when they are constituted as such through environmental discourse. Through the analysis, three contrasting discourses on bushfire are identified—here labelled the ‘conservationist’, ‘ruralist’ and ‘wise use’ discourses. In examining how different ‘constructions’ of bushfire have led to conflict, each discourse is shown to consist of a range of actors who draw on shared storylines. Having established a better understanding of the different attitudes, beliefs, interests and values that underpin debates about bushfire, the paper concludes with a brief discussion of the prospects for compromise among the discourses and the potential for improved land and fire management outcomes. 相似文献
902.
This article presents an econometric analysis of land‐cover change in western Honduras. Ground‐truthed satellite image analysis indicates that between 1987 and 1996 net reforestation occurred in the 1,015‐km2 study region. While some reforestation can be attributed to a 1987 ban on logging, the area of reforestation greatly exceeds that of previously clear‐cut areas. Further, new area was also deforested between 1987 and 1996. Thus, the observed land‐cover changes represent a complex mosaic of changing land‐use patterns across time and space. The analysis contributes to the literature on land‐cover change modeling in that: (1) it compares two econometric approaches to capture complex and often bidirectional changes in land cover from 1987 to 1996 as a function of agricultural suitability and transportation costs, and (2) it addresses techniques to identify and correct for spatial autocorrelation in a categorical regression framework. 相似文献
903.
904.
905.
906.
907.
908.
909.
Abstract The focus in the present study is on characterizing spatial patterns of textural and petrological variabilities, and on evaluating mechanisms influencing the textural and petrological components of modern river, beach and shelf sands in a volcanically active back‐arc tectonic setting. Abashiri Bay and the surrounding area in eastern Hokkaido, Japan, has volcanic source land within a back‐arc region associated with subduction of the Pacific Plate beneath the Okhotsk (North American) Plate. A total of 41 river, beach and shelf sands were obtained for grain‐size and modal composition analyses. Multivariate analytical techniques of hierarchical cluster and principal component analyses were performed on the textural and petrological data for investigating relations among quantitative variables. On the basis of grain‐size data, four sedimentary zones were identified: zone I, palimpsest zone; zone II, relict zone; zone III, modern (proteric) zone; zone IV, coastal sedimentary zone. All sands are feldspatholithic and quartz‐deficient. The framework (quartz, feldspar and rock fragment) modal compositions were also classified into four clusters, A–D. The characteristic components of each cluster are as follows: cluster A, felsic volcanic rock fragments; cluster B, andesitic–basaltic volcanic rock fragments; cluster C, mixed or plagioclase; cluster D, sedimentary rock fragments. Almost all sands in western and central Abashiri Bay belong to cluster A, where the original compositions are influenced by Kutcharo pyroclastic flow deposits. Andesitic–basaltic lava and Neogene volcaniclastic and sedimentary rocks have a major influence on the compositions of shelf sands in eastern Abashiri Bay. The modal compositions are basically controlled by the source lithology. Compositional maturity (percentage of quartz to feldspar and rock fragments; Q/FR%) slightly increased, in order, from river (1.2), zone IV (coastal, 1.7), zone II (relict, 2.2), zone I (palimpsest, 3.6), to zone III (modern proteric, 7.0). Greater maturity in the recycled sediments is indicative of weathering under the sea or abrasion by transportation induced by sea‐level fluctuations, waves, or sea currents. Several controlling factors – (i) source lithological; (ii) mineralogical; (iii) climatic; and (iv) geomorphological controls – might still cause low maturity through all sedimentary zones other than the continental margin sands previously reported. 相似文献
910.
P.N. Psarropoulos G. Klonaris G. Gazetas 《Soil Dynamics and Earthquake Engineering》2005,25(7-10):795-809
While limiting-equilibrium Mononobe–Okabe type solutions are still widely used in designing rigid gravity and flexible cantilever retaining walls against earthquakes, elasticity-based solutions have been given a new impetus following the analytical work of Veletsos and Younan [23]. The present paper develops a more general finite-element method of solution, the results of which are shown to be in agreement with the available analytical results for the distribution of dynamic earth pressures on rigid and flexible walls. The method is then employed to further investigate parametrically the effects of flexural wall rigidity and the rocking base compliance. Both homogeneous and inhomogeneous retained soil is considered, while a second soil layer is introduced as the foundation of the retaining system. The results confirm the approximate convergence between Mononobe–Okabe and elasticity-based solutions for structurally or rotationally flexible walls. At the same time they show the beneficial effect of soil inhomogeneity and that wave propagation in the underlying foundation layer may have an effect that cannot be simply accounted for with an appropriate rocking spring at the base. 相似文献