全文获取类型
收费全文 | 32209篇 |
免费 | 308篇 |
国内免费 | 289篇 |
专业分类
测绘学 | 811篇 |
大气科学 | 2883篇 |
地球物理 | 6645篇 |
地质学 | 11102篇 |
海洋学 | 2487篇 |
天文学 | 6890篇 |
综合类 | 82篇 |
自然地理 | 1906篇 |
出版年
2020年 | 229篇 |
2019年 | 242篇 |
2018年 | 518篇 |
2017年 | 516篇 |
2016年 | 739篇 |
2015年 | 481篇 |
2014年 | 723篇 |
2013年 | 1453篇 |
2012年 | 780篇 |
2011年 | 1080篇 |
2010年 | 923篇 |
2009年 | 1296篇 |
2008年 | 1114篇 |
2007年 | 975篇 |
2006年 | 1077篇 |
2005年 | 912篇 |
2004年 | 878篇 |
2003年 | 887篇 |
2002年 | 892篇 |
2001年 | 760篇 |
2000年 | 797篇 |
1999年 | 676篇 |
1998年 | 648篇 |
1997年 | 676篇 |
1996年 | 583篇 |
1995年 | 544篇 |
1994年 | 488篇 |
1993年 | 436篇 |
1992年 | 420篇 |
1991年 | 419篇 |
1990年 | 426篇 |
1989年 | 399篇 |
1988年 | 382篇 |
1987年 | 474篇 |
1986年 | 436篇 |
1985年 | 469篇 |
1984年 | 565篇 |
1983年 | 562篇 |
1982年 | 505篇 |
1981年 | 491篇 |
1980年 | 449篇 |
1979年 | 433篇 |
1978年 | 448篇 |
1977年 | 394篇 |
1976年 | 355篇 |
1975年 | 355篇 |
1974年 | 407篇 |
1973年 | 390篇 |
1972年 | 245篇 |
1971年 | 225篇 |
排序方式: 共有10000条查询结果,搜索用时 27 毫秒
941.
Adam C. Landon Maarten H. Jacobs Craig A. Miller Jerry J. Vaske Brent D. Williams 《社会与自然资源》2020,33(5):574-593
AbstractIncreasing wolf populations are a concern for wildlife managers in the Midwestern U.S. Understanding the psychological mechanisms that contribute to public perceptions of risk will enable development of strategies that seek to mitigate these risks, and suggest where outreach efforts may facilitate acceptance of wolves. We examined the psychological factors that influence Illinois residents’ perceived risks from wolves. We hypothesized that individuals’ perceived risks from wolves were a function of their attitudes toward wolves, negative affect toward wolves, and basic beliefs about wildlife. Data were obtained from a survey of the Illinois public (n?=?784). Negative affect and attitudes toward wolves were direct predictors of perceived risks. Basic beliefs predicted attitudes and negative affect toward wolves. Negative affect predicted attitudes. Basic beliefs had direct and indirect effects on perceived risks. 相似文献
942.
943.
944.
945.
Rowland RH 《Post-Soviet geography》1995,36(3):133-156
"Using urban places of 15,000 or more inhabitants as its point of reference, this paper identifies and investigates the most rapidly growing towns and cities of the USSR during the recent intercensal periods of 1970-79 and 1979-89. Rapidly growing towns are defined as towns that grew by at least 50 percent overall for the intercensal period and at a rate of equal to or exceeding 4.1 percent annually. In addition, a category of 'doubling towns' is investigated, defined as towns that increased in population by at least 100 percent (or 6.3 percent or more annually). Special attention is devoted to the geographical aspects, economic functions, and size characteristics of the towns involved. Comparisons with 1959-70, an overall examination from 1959-89, and an update for contemporary Russia during 1989-93 also are undertaken." 相似文献
946.
F. García-Rodríguez N. Mazzeo P. Sprechmann D. Metzeltin F. Sosa H.C. Treutler M. Renom B. Scharf C. Gaucher 《Journal of Paleolimnology》2002,28(4):457-468
Paleolimnological techniques were used to assess human impacts onLake Blanca, a small (0.6 km2), coastal fresh waterbodyin southern Uruguay, which is the drinking water source for 100,000 localresidents. We retrieved a core that extends to about 1100 14Cyr BP. 210Pb ages, organic mater, CO3, totalcarbon, nutrients, fossil pigments and diatoms allowed us to establishlimnological conditions before and after cultural impacts. Soil removal(1880–1960) and intensive cattle and sheep grazing (1943–1966) ledto gully formation in the catchment. This watershed erosion resulted inincreased sedimentation rates. The aquatic system appeared to be mesotrophicwith dominance of epibenthic diatoms until 1966, at which timeeutrophication intensified with forestry activities. Increases in nutrients, aswell as blooms of planktonic diatoms, were observed. During the last decade,tourist/urban development as well as high drinking water demand caused areduction in lake area. Subsequent marked increases in rainfall led to furtherphytoplankton blooms and macrophyte proliferation. 相似文献
947.
Frozen ground phenomena in the Northern Foothills, Northern Victoria Land, Antarctica, include large–scale polygons, 15–20 m in diameter, and small frost mounds, 1–5 m high. The polygons are most widespread on terrain formed upon Younger Drift and are usually surrounded by interpolygon furrows or troughs, 10–30 cm deep and 10–100 cm wide. The troughs contain shallow wedges of sandy gravel (sand wedges) near the surface but excavations into underlying permafrost indicate that small ice wedges or ice veins are locally present. Field and anecdotal evidence suggest that thermal contraction cracking is active under today's climate. Frost mounds occur in association with a number of perennially frozen lakes in the region. In most cases they appear related to frost and icing blister activity caused by the episodic injection of free water from below. The debris–covered nature of the centre of Enigma Lake is best explained in terms of basal ice accretion beneath the lake–ice cover. 相似文献
948.
949.
950.
J. N. Maina S. M. Kisia C. M. Wood A. B. Narahara H. L. Bergman P. Laurent P. J. Walsh 《International Journal of Salt Lake Research》1996,5(2):131-156
A morphometric analysis of the gills ofOreochromis alcalicus grahami has been carried out on specimens from ecologically distinct lagoons and a water-holding tank of Lake Magadi, a highly alkaline salt lake situated in a volcanically active region of the southern part of the Great Rift Valley in Kenya. The data were compared with those fromOreochromis niloticus, a close relative that lives in fresh water and with data from other fresh water and marine fish. Our primary goal was to identify the possible adaptive features which enable the fish to survive in an environment characterized by severely fluctuating levels of oxygen, a condition exacerbated by factors such as high temperature, alkalinity and osmolarity. The specimens ofO. a. grahami from the south-western lagoons of the lake had gills better adapted for gas exchange with a body mass specific diffusing capacity for oxygen which was about 2 times greater than that of the gills of the specimens from the fish spring lagoons and 2.5 times that of those from the water-holding tanks. Some parameters of the gills ofO. a. grahami, e.g. the gill filament length and number of gill filaments are significantly greater than those ofO. niloticus but the number of secondary lamellae, area of secondary lamellae and the diffusing capacity of the gills are similar in the two species. Compared with most other fish, the gills ofO. a. grahami appear to be particularly well adapted for gas exchange especially by having a thin water-blood barrier. Perhaps in no other extant fish have the gills had to be so exquisitely designed to meet environmental extremes and regulate complex and at times conflicting functions such as gas exchange, iono-regulation, acid-base balance and nitrogenous waste excretion as inO. a. grahami 相似文献