全文获取类型
收费全文 | 831篇 |
免费 | 27篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 81篇 |
地球物理 | 213篇 |
地质学 | 304篇 |
海洋学 | 51篇 |
天文学 | 122篇 |
综合类 | 1篇 |
自然地理 | 75篇 |
出版年
2021年 | 16篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 21篇 |
2016年 | 19篇 |
2015年 | 27篇 |
2014年 | 30篇 |
2013年 | 41篇 |
2012年 | 33篇 |
2011年 | 42篇 |
2010年 | 39篇 |
2009年 | 36篇 |
2008年 | 51篇 |
2007年 | 29篇 |
2006年 | 24篇 |
2005年 | 29篇 |
2004年 | 33篇 |
2003年 | 17篇 |
2002年 | 27篇 |
2001年 | 17篇 |
2000年 | 24篇 |
1999年 | 14篇 |
1998年 | 7篇 |
1997年 | 13篇 |
1996年 | 16篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1985年 | 4篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 15篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1972年 | 7篇 |
1944年 | 4篇 |
排序方式: 共有865条查询结果,搜索用时 16 毫秒
21.
22.
L. Vočadlo K. S. Knight G. D. Price I. G. Wood 《Physics and Chemistry of Minerals》2002,29(2):132-139
The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173?K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5)?×?10?5?K?1. The cell volume was fitted over the complete temperature range using Grüneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Grüneisen second-order approximation gave the following parameters: θD=445(11)?K, V 0=89.596(8)?Å3, K 0′=4.4(4) and γ′=2.33(3), where θD is the Debye temperature, V 0 is V at T=0?K, K 0′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Grüneisen parameter. The thermodynamic Grüneisen parameter, γth, has been calculated from experimental data in the range 4–400?K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases. 相似文献
23.
Robert H Wood 《Geochimica et cosmochimica acta》2002,66(24):4365
24.
25.
26.
Small-scale vegetation dynamics were followed for ecotones and in uniform stands inArtemisia-dominated steppe vegetation under grazing and when recovering from heavy grazing. Species composition was followed annually for 5 years in 1 m2and 0·25 m2plots for (1) presence–absence; (2) density; and (3) biomass.More rapid vegetation dynamics, in terms of change of type of vegetation and distance moved in DCA species space between sampling occasions, were observed in smaller plots and in early seral stages, where species were few and alpha diversity low. Only the plots recently protected from grazing showed a directional vegetation change; those protected for more than 3 years and those under continued grazing showed, at this scale, non-directional dynamics. 相似文献
27.
28.
Mitigating the Effects of Climate Change on the Water Resources of the Columbia River Basin 总被引:5,自引:3,他引:5
Jeffrey T. Payne Andrew W. Wood Alan F. Hamlet Richard N. Palmer Dennis P. Lettenmaier 《Climatic change》2004,62(1-3):233-256
The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM). This study focuses on three climate projections for the 21st century based on a `business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes were statistically downscaled and temporally disaggregated to produce daily forcings that drove a macro-scale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution. For comparison with the direct statistical downscaling approach, a dynamical downscaling approach using a regional climate model (RCM) was also used to derive hydrologic model forcings for 20-year subsets from the PCM control climate (1995–2015) scenario and from the three BAU climate(2040–2060) projections. The statistically downscaled PCM scenario results were assessed for three analysis periods (denoted Periods 1–3: 2010–2039,2040–2069, 2070–2098) in which changes in annual average temperature were +0.5,+1.3 and +2.1 °C, respectively, while critical winter season precipitation changes were –3, +5 and +1 percent. For RCM, the predicted temperature change for the 2040–2060 period was +1.2 °C and the average winter precipitation change was –3 percent, relative to the RCM controlclimate. Due to the modest changes in winter precipitation, temperature changes dominated the simulated hydrologic effects by reducing winter snow accumulation, thus shifting summer streamflow to the winter. The hydrologic changes caused increased competition for reservoir storage between firm hydropower and instream flow targets developed pursuant to the Endangered Species Act listing of Columbia River salmonids. We examined several alternative reservoir operating policies designed to mitigate reservoir system performance losses. In general, the combination of earlier reservoir refill with greater storage allocations for instream flow targets mitigated some of the negative impacts to flow, but only with significant losses in firm hydropower production (ranging from –9 percent in Period1 to –35 percent for RCM). Simulated hydropower revenue changes were lessthan 5 percent for all scenarios, however, primarily due to small changes inannual runoff. 相似文献
29.
John A. Wood 《Geochimica et cosmochimica acta》2004,68(19):4007-4021
This study attempts to identify the astrophysical setting in which properties of the Ca,Al-rich inclusions (CAIs) found in chondritic meteorites are best understood. Importance is attached to the short time period in which most or all of the CAIs were formed (<∼0.5 Myr, corresponding to the observed dispersion of values of initial 26Al/27Al about the canonical value of ∼5 × 10−5), a constraint that has been overlooked. This period is dissimilar to the time scale of evolution of T Tauri stars, ∼10 Myr; it corresponds instead to the time scale of Class 0 and Class I young stellar objects, protostars as they exist during the massive infall of interstellar material that creates stars. The innermost portion of the sun’s rapidly accreting nebular disk, kept hot during that period by viscous dissipation, is the most plausible site for CAI formation. Once condensed, CAIs must be taken out of that hot zone rather promptly in order to preserve their specialized mineralogical compositions, and they must be transported to the radial distance of the asteroid belt to be available for accretion into the chondrites that contain them today. Though this paper is critical of some aspects of the x-wind model of CAI formation, something akin to the x-wind may be the best way of understanding this extraction and transport of CAIs. 相似文献
30.
This article provides an analysis of a wetland site in southern Illinois from presettlement to the present. The study area is part of the Cache River‐Cypress Creek Wetland, which has international importance, as recognized by the Ramsar Convention on Wetlands. Land‐cover data for 1807, 1938, and 1993 were created and analyzed with a geographic information system (GIS). Land‐use change by topographic setting (uplands, transitional, and bottomlands) and soil productivity was quantified and studied. Interviews with local experts informed this analysis. Results illustrate the complexity of environmental change and its driving forces. First, notable forest and swamp acreage was converted to cropland between 1807 and 1938 and, to a lesser degree, from 1938 to 1993. Second, there were land‐use variations by topographic region. Between 1807 and 1938, the largest transformation occurred in the uplands, with substantial acreage converted from forest to cropland. Between 1938 and 1993, however, agriculture decreased in the upland areas as hilly areas reverted to forest cover. At the same time, agriculture expanded in the bottomlands as this land was drained for farming. Third, there are interesting patterns within these categories of land‐use change, as soil productivity is an indicator of what lands were taken out of cropland and converted back to grassland and forest. 相似文献