全文获取类型
收费全文 | 103388篇 |
免费 | 1312篇 |
国内免费 | 700篇 |
专业分类
测绘学 | 2259篇 |
大气科学 | 6553篇 |
地球物理 | 20019篇 |
地质学 | 37001篇 |
海洋学 | 9337篇 |
天文学 | 24492篇 |
综合类 | 294篇 |
自然地理 | 5445篇 |
出版年
2022年 | 757篇 |
2021年 | 1282篇 |
2020年 | 1361篇 |
2019年 | 1505篇 |
2018年 | 3129篇 |
2017年 | 2894篇 |
2016年 | 3360篇 |
2015年 | 1666篇 |
2014年 | 3238篇 |
2013年 | 5393篇 |
2012年 | 3547篇 |
2011年 | 4523篇 |
2010年 | 4078篇 |
2009年 | 5179篇 |
2008年 | 4537篇 |
2007年 | 4700篇 |
2006年 | 4284篇 |
2005年 | 3027篇 |
2004年 | 2914篇 |
2003年 | 2725篇 |
2002年 | 2683篇 |
2001年 | 2401篇 |
2000年 | 2289篇 |
1999年 | 1798篇 |
1998年 | 1851篇 |
1997年 | 1747篇 |
1996年 | 1469篇 |
1995年 | 1466篇 |
1994年 | 1253篇 |
1993年 | 1187篇 |
1992年 | 1125篇 |
1991年 | 1129篇 |
1990年 | 1136篇 |
1989年 | 995篇 |
1988年 | 908篇 |
1987年 | 1071篇 |
1986年 | 910篇 |
1985年 | 1148篇 |
1984年 | 1290篇 |
1983年 | 1237篇 |
1982年 | 1140篇 |
1981年 | 1062篇 |
1980年 | 994篇 |
1979年 | 880篇 |
1978年 | 898篇 |
1977年 | 763篇 |
1976年 | 741篇 |
1975年 | 741篇 |
1974年 | 718篇 |
1973年 | 791篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
In 1988, four states in the northeastern USA commissioned a study to address land use changes in the Northern Forest, 26 million acres of temperate and boreal forest extending from Maine to eastern New York State. Against a backdrop of economic destabilization and concerns regarding social and ecological implications of a real estate boom, the sustained deliberative dialogue catalyzed by this study has come to rely heavily on the ambiguous concept of “working forest.” To clarify political and environmental dynamics in the region, we analyzed how people respond to and seek to capitalize on the interpretive flexibility of the term working forest. We combine an analysis of socio-political discourses of working forest based on a structured literature review with an assessment of local peoples’ definitions of working forest based on a survey conducted in a pair of contrasting New York State communities. The first study site represents an amenity-oriented community (i.e. a place where the forest supports a service economy including recreation and tourism) and the other study site represents a timber-dependent community. By linking data from community-level analysis to data derived from a general analysis of forest politics, we seek to develop a more robust perspective. By comparing discourses across differently structured communities, we investigate how local forest politics are mediated by local economic development processes. Our study empirically illustrates contested and geographically uneven processes of social construction of environment and rural development in a region confronting pressures of globalization. Results indicate that timber harvesting is a heavily privileged management objective, as a logic of ‘the forest that pays is the forest that stays’ dominates. Environmental politics in the region, and perhaps more generally, increasingly conforms to a form of pragmatism in which economic opportunities structure conservation planning and investment. 相似文献
995.
996.
J.T. Caulfield S.P. Turner A. Dosseto N.J. Pearson C. Beier 《Earth and Planetary Science Letters》2008,273(3-4):279-288
The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism.We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (< 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back-arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~ 75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab. 相似文献
997.
River restoration and bank stabilization programs often use vegetation for improving stream corridor habitat, aesthetic and function. Yet no study has examined the use of managed vegetation plantings to transform a straight, degraded stream corridor into an ecologically functional meandering channel. Experimental data collected using a distorted Froude‐scaled flume analysis show that channel expansion and widening, thalweg meandering and riffle and pool development are possible using discrete plantings of rigid, emergent vegetation, and the magnitudes of these adjustments depend on the shape of the vegetation zone and the density of the vegetation. These experimental results were verified and validated using a recently developed numerical model, and model output was then used to discuss mechanistically how rivers respond to the introduction of in‐stream woody vegetation. Finally, a hybrid method of meander design is proposed herein where managed vegetation plantings are used to trigger or force the desired morphologic response, transforming a straight, degraded reach into a more functional meandering corridor. It is envisioned that such numerical models could become the primary tool for designing future stream restoration programs involving vegetation and assessing the long‐term stability of such activities. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
998.
I. M. Vasilyev Yu. P. Kozhevnikov S. A. Matsiyevskiy V. G. Mezhberg A. V. Rykov I. V. Ulomov 《Seismic Instruments》2009,45(1):60-68
This paper considers the possibilities for modernizing the KSESh broadband seismic station developed earlier at the IPE AS USSR. The goal of the modernization was to extend the frequency and dynamic ranges and to decrease nonlinear distortions. The ways and concrete methods for improving the mechanical construction and electronic components of the station are shown and the intermediate results are described. As preliminary estimates show, there is an opportunity to build devices with the velocimeter response up to 520–540 s periods on the basis of this pendulum. 相似文献
999.
V. A. Vavilin 《洁净——土壤、空气、水》1985,13(1):75-83
In the process of the biological treatment of sewage, the multicomponent system of the substrate is converted by an association of microorganisms. The composition of species of microorganisms in the system depends on the substrate available and on the conversion of substrate. For this, a mathematical model is presented, which describes the change of the composition of species according to the rate of growth of the individual species on the basis of load (substrate available/biomass). The diversity of the biological system increases with decreasing load. The model is tested by practical examples. 相似文献
1000.
Summary The morphology of the Wadati-Benioff zone in the region of Southern Kuriles and Hokkaido, based on the distribution of 4015 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A paleosubduction zone activated by an intermediate depth collision with the active subduction zone was found and described.
u Wadati-Benioff amu uu - u a¶rt;, aa a an¶rt;uu 4015 a mu, nm¶rt;ua mau n¶rt; au u amu a¶rt;um au. a a¶rt;a u nuaa a na¶rt;uu, amuuuaa nmu mu amu ¶rt;uu.相似文献