全文获取类型
收费全文 | 90799篇 |
免费 | 1813篇 |
国内免费 | 823篇 |
专业分类
测绘学 | 2692篇 |
大气科学 | 7104篇 |
地球物理 | 18576篇 |
地质学 | 30663篇 |
海洋学 | 7616篇 |
天文学 | 20750篇 |
综合类 | 273篇 |
自然地理 | 5761篇 |
出版年
2021年 | 588篇 |
2020年 | 716篇 |
2019年 | 783篇 |
2018年 | 1667篇 |
2017年 | 1593篇 |
2016年 | 2244篇 |
2015年 | 1489篇 |
2014年 | 2213篇 |
2013年 | 4606篇 |
2012年 | 2327篇 |
2011年 | 3285篇 |
2010年 | 2800篇 |
2009年 | 3966篇 |
2008年 | 3520篇 |
2007年 | 3240篇 |
2006年 | 3312篇 |
2005年 | 2821篇 |
2004年 | 2849篇 |
2003年 | 2662篇 |
2002年 | 2557篇 |
2001年 | 2305篇 |
2000年 | 2281篇 |
1999年 | 2006篇 |
1998年 | 1921篇 |
1997年 | 1929篇 |
1996年 | 1687篇 |
1995年 | 1617篇 |
1994年 | 1469篇 |
1993年 | 1326篇 |
1992年 | 1225篇 |
1991年 | 1125篇 |
1990年 | 1332篇 |
1989年 | 1152篇 |
1988年 | 1049篇 |
1987年 | 1267篇 |
1986年 | 1121篇 |
1985年 | 1369篇 |
1984年 | 1562篇 |
1983年 | 1479篇 |
1982年 | 1384篇 |
1981年 | 1267篇 |
1980年 | 1124篇 |
1979年 | 1062篇 |
1978年 | 1123篇 |
1977年 | 1023篇 |
1976年 | 959篇 |
1975年 | 890篇 |
1974年 | 927篇 |
1973年 | 966篇 |
1972年 | 601篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
11.
Colin P. R. McCarter Stephen D. Sebestyen Susan L. Eggert Randall K. Kolka Carl P. J. Mitchell 《水文研究》2020,34(26):5354-5369
Sustainable fuels legislation and volatility in energy prices have put additional pressures on the forestry sector to intensify the harvesting of biomass for “advanced biofuel” production. To better understand how residual biomass removal after harvest affects forest hydrology in relatively low slope terrain, a Before-After-Control-Impact (BACI) study was conducted in the USDA Forest Service's Marcell Experimental Forest, Minnesota, USA. Hydrological measurements were made from 2010–2013 on a forested hillslope that was divided into three treatment blocks, where one block was harvested and residual biomass removed (Biomass Removed), the second was harvested and residual biomass left (Biomass Left), and the last block was left as an Unharvested Control. The pre-harvest period (2 years) was 2010–11 and post-harvest (2 years) was 2012–13. Water table elevation at the upslope and downslope position, subsurface runoff, and soil moisture were measured between May–November. Mixed effect statistical models were used to compare both the before-after and “control” treatment ratios (ratios between harvested hillslopes and the Unharvested Control hillslope). Subsurface runoff significantly increased (p < .05) at both harvested hillslopes but to a greater degree on the Biomass Left hillslope. Greater subsurface runoff volumes at both harvested hillslopes were driven by substantial increases during fall, with additional significant increases during summer on the Biomass Left hillslope. The hydrological connectivity, inferred from event runoff ratios, increased due to harvesting at both hillslopes but only significantly on the Biomass Left hillslope. The winter harvest minimized soil disturbance, resulting in no change to the effective hydraulic conductivity distribution with depth. Thus, the observed hydrological changes were driven by increased effective precipitation and decreased evapotranspiration, increasing the duration that both harvested hillslopes were hydrologically active. The harvesting of residual biomass appears to lessen hydrological connectivity relative to leaving residual biomass on the hillslope, potentially decreasing downstream hydrological impacts of similar forestry operations. 相似文献
12.
Zhi-gen Yang Wen-yao Zhu C. K. Shum Feng-chun Shu 《Chinese Astronomy and Astrophysics》2002,26(4):303-502
The vertical deformation rates (VDRs) and horizontal deformation rates (HDRs) of Shanghai VLBI station in China and Kashima and Kashima34 VLBI stations in Japan were re-analysed using the baseline length change rates from Shanghai to 13 global VLBI stations, and from Kashima to 27 stations and from Kashima34 to 12 stations, based on the NASA VLBI global solution glb1123 (Ma, 1999). The velocity vectors of the global VLBI stations were referred to the ITRF97 reference frame, and the Eulerian vectors of different models of plate motion were used for comparative solutions. The VDR of Shanghai station is estimated to be −1.91±0.56 mm/yr, and those of Kashima and Kashima34 stations, −3.72±0.74mm/yr and −8.81±0.84mm/yr, respectively. The difference between the last two was verified by further analysis. Similar estimates were also made for the Kokee, Kauai and MK_VLBA VLBI stations in mid-Pacific. 相似文献
13.
Abstract— The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high‐resolution, low Sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging) will take images with viewing geometries and resolutions appropriate for the identification of such features. 相似文献
14.
15.
The economic development in China and the rising of the living standard need to speed up the urbanisa-tion.The development of small towns is an important way to Chinese urbanization.Land use plays a very important role in the development of small towns.However there are many problems in the development of small towns,esp.in land use.The paper first discusses the land problems in the development of small towns.Such as much cultivated land lies idle,under-utilization and waste of land,increasing illegal use of land,unstable contractural relationship for land use.The relationship between the development of small towns and land use is also studied.Then the guidelines for the land system innovations of small towns are put forward.Namely the sustainability of social and economic development,the bal-ance between land reservation and land utilization,the provision of service to village ,agriculture and farmer,the manage-ment of land resource and land assets,the parsimonious andlegal use of land.The basic framework of land system innova-tions of small towns is put forward finally.It include the land replacement policy for small towns and the permanent tenan-cy of farmland,the overall plan for land uses and other plans in harmony,the establishment of a flexible system of land supplies,using land with payments ,the transfer of agricultural land and refining the land law related to the construction of small towns. 相似文献
16.
17.
The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause. 相似文献
18.
19.
20.