全文获取类型
收费全文 | 49505篇 |
免费 | 685篇 |
国内免费 | 300篇 |
专业分类
测绘学 | 1334篇 |
大气科学 | 3836篇 |
地球物理 | 9297篇 |
地质学 | 16316篇 |
海洋学 | 4177篇 |
天文学 | 12207篇 |
综合类 | 117篇 |
自然地理 | 3206篇 |
出版年
2020年 | 365篇 |
2019年 | 338篇 |
2018年 | 829篇 |
2017年 | 811篇 |
2016年 | 1053篇 |
2015年 | 699篇 |
2014年 | 1170篇 |
2013年 | 2443篇 |
2012年 | 1112篇 |
2011年 | 1546篇 |
2010年 | 1438篇 |
2009年 | 1936篇 |
2008年 | 1774篇 |
2007年 | 1779篇 |
2006年 | 1681篇 |
2005年 | 1544篇 |
2004年 | 1509篇 |
2003年 | 1413篇 |
2002年 | 1357篇 |
2001年 | 1230篇 |
2000年 | 1155篇 |
1999年 | 1121篇 |
1998年 | 1056篇 |
1997年 | 1062篇 |
1996年 | 844篇 |
1995年 | 836篇 |
1994年 | 794篇 |
1993年 | 738篇 |
1992年 | 710篇 |
1991年 | 681篇 |
1990年 | 776篇 |
1989年 | 674篇 |
1988年 | 648篇 |
1987年 | 741篇 |
1986年 | 620篇 |
1985年 | 825篇 |
1984年 | 947篇 |
1983年 | 912篇 |
1982年 | 861篇 |
1981年 | 819篇 |
1980年 | 722篇 |
1979年 | 698篇 |
1978年 | 692篇 |
1977年 | 627篇 |
1976年 | 596篇 |
1975年 | 517篇 |
1974年 | 597篇 |
1973年 | 586篇 |
1972年 | 362篇 |
1971年 | 334篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
A wide-ranging set of physical, urban, demographic, socioeconomic, and policy characteristics determines the spatial distribution of urban forests. Information on the characteristics surrounding tree removals on both public and private properties has received less attention in the literature. The purpose of this research was to analyze the spatiotemporal trends and geographic patterns of tree removals in Austin, Texas, between 2002 and 2011 in an effort to understand how site-specific characteristics influence urban tree removal and affect the overall distribution of Austin's urban forest. We examined permitted tree removals using a geographic information system (GIS) as well as spatial and statistical analyses. Specifically, we evaluated the degree to which variables related to various physical, urban, and socioeconomic conditions predicted tree removals. The results indicate that permitted tree removals and their associated characteristics in Austin have varied over the ten-year study period. Permitted tree removals increased over the study period and took place in the urban core and along the urban periphery. Permitted tree removals were more likely to be undertaken by college graduates and owner-occupants and to occur in more densely populated areas, closer to major streets, and on properties with older structures. The results of this research provide urban forest professionals with information on the location and intensity of permitted tree removals and the significant characteristics driving urban tree loss. 相似文献
112.
In the present study analysis of Landsat MSS, TM and SPOT imagery and digital analysis of IRS LISS-1 data of Doon Valley was carried out. Various geomorphic features were identified and classified, morphostratigraphy of the area has been established. Main geomorphic units of the area are Mussoorie Hill Range (Denudational), Siwalik Hills (Structural), Remnant Hills (Residual). Siwalik Piedmont. Doon Piedmont, River Terraces and Flood Plain. Three large fan lobes are identified on Doon Piedmont deposits, viz., Western fan lobe, Central fan lobe and Eastern fan lobe. Average slope of these three fan lobes are 2°21′, 2°3′, and 1°24′ for the western, central and eastern fan lobes respectively. Western and central fan lobes have been affected by neotectonic activity which is reflected in transverse profiles. 相似文献
113.
Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation. 相似文献
114.
115.
K. R. Grange 《新西兰海洋与淡水研究杂志》2013,47(1):203-216
Adults of three species of common intertidal gastropods, Melagraphia aethiops, Zediloma atrovirens, and Lunella smaragda, were collected from Waitemata Harbour, Auckland (36° 51 S, 174° 47 E) and brought to the laboratory, where several methods of artificial spawning were attempted throughout the year. The one successful method, involving vigorous water movement around ripe adults, induced them to release gametes during their respective spawning seasons, and was equally effective in the natural habitat. Larval stages were found in the plankton only during periods of on‐shore wind speeds greater than 10 knots, with which their presence was correlated, rather than with tides, lunar cycles, or sea temperatures. 相似文献
116.
— Using P and S arrival times, which occurred within the Huabei seismic network, we carried out a tomographic inversion and compared results with the earthquake catalogue of the last 1000?years in the area. The results are as follows:¶1) The hypocenters of most of the strong shocks are distributed in the transitional zones between high- and low-velocity areas in the crust, especially at edges of high-velocity blocks.¶2) Strong shocks predominantly lie above low-velocity blocks, or in transitional zones between low- and high-velocity areas, in the lower crust.¶3) The tectonic settings for the Tangshan and the Sanhe-Pingu earthquakes are similar. Both are not known near large fault belts, and in zones with a sharp lateral velocity gradient.¶4) The Ninghe, Tangshan and Luanxia earthquakes are located in high-velocity blocks that differ in size and depth. This difference can explain the focal depth distribution of the Tangshan earthquake sequence, i.e., earthquakes are shallower in the northeastern Luanxian area but deeper in the southwestern Ninghe area. 相似文献
117.
118.
M. M. Alconada-Magliano J. R. Fagundo-Castillo J. J. Carrillo-Rivera P. G. Hernández 《Environmental Earth Sciences》2011,64(1):57-71
Hydrogeochemical behaviour of samples of surface and groundwater collected on a cross-section from Mendoza to the Buenos Aires
provinces was studied based on chemical trends, mass balance and water mixing. Hydrogeochemical modelling included major,
minor elements as well as stable isotopes (deuterium and 18-O). The area investigated is located in the “Médanos Longitudinales” (longitudinal dunes) of the northwest of Buenos Aires province, Argentina. The study area is subject to alternating flooding
and drought. Rainfall and surface water subsequently transferred by rivers, canals and lagoons have been usually considered
responsible for local flooding. For this study, origins of excess water were investigated using physical and chemical characteristics
of the water involved. The prevalence of groundwater inflow to rainfall events was proposed based on data interpretation.
Groundwater influence of flows of local and intermediate nature were defined and the importance of recharge, transit and discharge
zones was highlighted. Lagoon floodwater, as well as groundwater from observation wells and production boreholes, show components
of intermediate origin. Regional recharge water was identified in Mendoza and San Luis provinces. Their discharge zone was
inferred to be located beyond the Buenos Aires province. 相似文献
119.
The IGS VTEC maps: a reliable source of ionospheric information since 1998 总被引:10,自引:15,他引:10
M. Hernández-Pajares J. M. Juan J. Sanz R. Orus A. Garcia-Rigo J. Feltens A. Komjathy S. C. Schaer A. Krankowski 《Journal of Geodesy》2009,83(3-4):263-275
The International GNSS Service (IGS) Working Group on Ionosphere was created in 1998. Since then, the Scientific community behind IGS, in particular CODE, ESA, JPL and UPC, have been continuosly contributing to reliable IGS combined vertical total electron content (VTEC) maps in both rapid and final schedules. The details on how these products are being generated, performance numbers, proposed improvement as far as VTEC evolution trends during near one Solar Cycle, are summarized in this paper. The confirmation of (1) the good performance of the IGS combined VTEC maps, and (2) the characteristic VTEC variability periods, are two main results of this work. 相似文献
120.
Fedorov V. M. Frolov D. M. Velasco Herrera V. M. N. Soon W. W.-H. Cionco R. G. 《Izvestiya Atmospheric and Oceanic Physics》2021,57(10):1239-1253
Izvestiya, Atmospheric and Oceanic Physics - On the basis of calculations of insolation and insolation characteristics, taking into account changes in solar activity, the causes of global climatic... 相似文献