全文获取类型
收费全文 | 16811篇 |
免费 | 331篇 |
国内免费 | 114篇 |
专业分类
测绘学 | 345篇 |
大气科学 | 1160篇 |
地球物理 | 3486篇 |
地质学 | 6318篇 |
海洋学 | 1659篇 |
天文学 | 3262篇 |
综合类 | 66篇 |
自然地理 | 960篇 |
出版年
2022年 | 168篇 |
2021年 | 264篇 |
2020年 | 255篇 |
2019年 | 298篇 |
2018年 | 625篇 |
2017年 | 529篇 |
2016年 | 611篇 |
2015年 | 324篇 |
2014年 | 552篇 |
2013年 | 847篇 |
2012年 | 617篇 |
2011年 | 817篇 |
2010年 | 739篇 |
2009年 | 901篇 |
2008年 | 797篇 |
2007年 | 779篇 |
2006年 | 705篇 |
2005年 | 508篇 |
2004年 | 485篇 |
2003年 | 432篇 |
2002年 | 428篇 |
2001年 | 323篇 |
2000年 | 351篇 |
1999年 | 255篇 |
1998年 | 268篇 |
1997年 | 236篇 |
1996年 | 181篇 |
1995年 | 217篇 |
1994年 | 197篇 |
1993年 | 156篇 |
1992年 | 138篇 |
1991年 | 153篇 |
1990年 | 165篇 |
1989年 | 134篇 |
1988年 | 137篇 |
1987年 | 163篇 |
1986年 | 140篇 |
1985年 | 183篇 |
1984年 | 191篇 |
1983年 | 174篇 |
1982年 | 156篇 |
1981年 | 141篇 |
1980年 | 167篇 |
1979年 | 143篇 |
1978年 | 143篇 |
1977年 | 117篇 |
1976年 | 108篇 |
1975年 | 115篇 |
1974年 | 111篇 |
1973年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 366 毫秒
991.
992.
As the economic base of most American metropolitan areas relies increasingly on services, office industries have shifted to a more pivotal role in the new metropolitan service economy. Using primary office-space usage data of the time period 1985–1990, the changing spatial patterns of this new metropolitan service economy are investigated. It is found that the primary office activities are predominantly concentrated in a relatively small number of the largest metropolitan areas. However, an emerging spatial dispersion in primary office activities is also exhibited. Such dispersion is extremely strong at the upper end of the urban hierarchy. The Sunbelt-Snowbelt dichotomy fails to reveal any differences in the growth pattern of the new metropolitan service economy. A more diversified growth pattern of primary office activities is observed. Concentrated dispersion is the major spatial characteristic of this new metropolitan service economy. 相似文献
993.
Mapping the Geographical Diffusion of a Finnish Smallpox Epidemic from Historical Population Records
James L. Wilson 《The Professional geographer》1993,45(3):276-286
Smallpox mortality from an early 19th century epidemic in Finland is mapped at the parish village level. First, geographically referenced historical materials are used to construct a map showing the spread of smallpox mortality among villages. Next, the diffusion of smallpox morbidity is simulated by computer for the same set of villages. These two maps are then compared. Results indicate that at this scale maps of mortality diffusion can show general trends but have no direct spatial correspondence to the underlying pattern of morbidity diffusion. Mortality maps cannot be used as a surrogate measure of infectious contact behavior at micro scales of analysis. 相似文献
994.
James Pooler 《The Professional geographer》1993,45(3):297-305
This paper identifies a concept of structural spatial interaction. Although it is defined differently from its counterpart of structural unemployment in economics, it attempts to define a similar idea. In economics, the focus is on equilibrium in the supply and demand of labor. In this paper structural interaction is that which minimizes aggregate distance travelled, subject to constraints on origins and destinations. The structural interaction model produces an equilibrium, therefore, in the supply and demand of movers, while identifying a spatially efficient trip distribution. It is portrayed as a type of relaxed interaction model and is illustrated using an example of structural migration with Canadian interprovincial migration data. 相似文献
995.
James H. Cowan 《Estuaries and Coasts》1993,16(4):923-924
996.
In laboratory experiments, two fungi that commonly colonize leaves of Spartina aZterniJEoru(L oisel.) were studied to determine if they contribute to the formation of humic substances, which have been isolated earlier from salt marsh estuaries. In incubations of up to one year in duration, Phaeosphaeria spartinicola and Phaeosphaeria halima did not form dark pigments when grown in an artificial, full-nutrient medium. However, if the nutrient solution was enriched with a water extract of S. alterniJEoro, appreciable amounts of dark brown substances were produced. Elemental (C, H, N, 0) and spectral (ultraviolet, visible, and Fourier Transform Infrared) analyses of these dark brown substances show them to be similar to salt marsh humic substances. 相似文献
997.
GeoJournal - Inspite of step-up efforts at controlling flood hazards in Third World cities, the frequency and magnitude of the flood problem have more than doubled in recent years. A survey of the... 相似文献
998.
999.
Summary Evidence for changes in the annual and growing season rainfall series for the period 1919 to 1985 in Nigeria are examined on a regional basis, using power-spectral and lowpass filter techniques, and the Mann-Kendall rank statistic. Four regions, the Coastal Zone, the Guinea-Savanna Zone, the Midland area and the Sahel, are used in the investigation of rainfall variation from south to north across the country.Quasi-periodic oscillations in the annual and growing season rainfall series are found to be concentrated in four spectral bands: 2.0–2.4, 2.7–2.9, 3.2–3.6 and 5.6–6.3 years. The spatial coherence of the fluctuations in annual and growing season rainfall is found to be limited to Nigeria south of 11 degrees north latitude. Evidence also emerges of a progressive decline in annual and growing season rainfall for northern Nigeria, north of nine degrees north latitude, for the period 1939–1985.With 5 Figures 相似文献
1000.
Estimates from semiempirical models that characterize surface heat flux, mixing depth, and profiles of temperature, wind, and turbulence are compared with observations from atmospheric field studies conducted in Colorado, Illinois, Indiana, and Minnesota. Sodar observations are compared with tower measurements at the Colorado site, for wind and turbulence profiles. The median surface heat flux, as calculated using surface-layer flux-profile relationships and an energy budget model, was consistently overestimated by 20 to 80%. Several mixing-depth models were evaluated: (1) integration of the hourly surface heat flux and friction velocity, (2) solving for the time rate of change of profiles of virtual potential temperature, and (3) an interpolation scheme used by the U.S. Environmental Protection Agency in regulatory dispersion models. For the late afternoon, 80 to 90% of the estimates from the first and third models were within 40% of the observed values. For the morning hours after sunrise, all were less accurate. Temperature estimates from surface-layer flux-profile relationships compared well with observations within the mixed layer, but were too low for the inversion layer aloft. Wind profiles were derived using surface-layer flux-profile relationships, a windprofile power-law based on Pasquill stability category, and sodar measurements. The sodar measurements were superior to both types of model estimates. Turbulence profiles were derived from sodar measurements and from semiempirical similarity relationships based on mixing depth and Obukhov length. The scatter in the comparisons with the sodar observations is twice that seen in the comparisons with empirical profile relationships. Overall, it appears that uncertainty of as low as 20 to 30% in the characterization of the diffusion meteorology is the exception rather than the rule.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce.Disclaimer: Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. 相似文献