全文获取类型
收费全文 | 4138篇 |
免费 | 64篇 |
国内免费 | 56篇 |
专业分类
测绘学 | 104篇 |
大气科学 | 289篇 |
地球物理 | 820篇 |
地质学 | 1650篇 |
海洋学 | 252篇 |
天文学 | 938篇 |
综合类 | 7篇 |
自然地理 | 198篇 |
出版年
2023年 | 19篇 |
2022年 | 30篇 |
2021年 | 34篇 |
2020年 | 30篇 |
2019年 | 43篇 |
2018年 | 132篇 |
2017年 | 130篇 |
2016年 | 180篇 |
2015年 | 84篇 |
2014年 | 174篇 |
2013年 | 236篇 |
2012年 | 163篇 |
2011年 | 189篇 |
2010年 | 197篇 |
2009年 | 225篇 |
2008年 | 177篇 |
2007年 | 173篇 |
2006年 | 204篇 |
2005年 | 133篇 |
2004年 | 132篇 |
2003年 | 112篇 |
2002年 | 80篇 |
2001年 | 74篇 |
2000年 | 75篇 |
1999年 | 81篇 |
1998年 | 77篇 |
1997年 | 87篇 |
1996年 | 77篇 |
1995年 | 47篇 |
1994年 | 32篇 |
1993年 | 57篇 |
1992年 | 34篇 |
1991年 | 43篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 36篇 |
1987年 | 42篇 |
1986年 | 52篇 |
1985年 | 41篇 |
1984年 | 45篇 |
1983年 | 42篇 |
1982年 | 45篇 |
1981年 | 34篇 |
1980年 | 31篇 |
1979年 | 35篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 24篇 |
1974年 | 20篇 |
1973年 | 23篇 |
排序方式: 共有4258条查询结果,搜索用时 140 毫秒
1.
Cristián Frêne Juan J. Armesto Freddy Véliz Fernando D. Alfaro Kathleen C. Weathers 《水文研究》2021,35(5):e14175
The variability of rainfall-dependent streamflow at catchment scale modulates many ecosystem processes in wet temperate forests. Runoff in small mountain catchments is characterized by a quick response to rainfall pulses which affects biogeochemical fluxes to all downstream systems. In wet-temperate climates, water erosion is the most important natural factor driving downstream soil and nutrient losses from upland ecosystems. Most hydrochemical studies have focused on water flux measurements at hourly scales, along with weekly or monthly samples for water chemistry. Here, we assessed how water and element flows from broad-leaved, evergreen forested catchments in southwestern South America, are influenced by different successional stages, quantifying runoff, sediment transport and nutrient fluxes during hourly rainfall events of different intensities. Hydrograph comparisons among different successional stages indicated that forested catchments differed in their responses to high intensity rainfall, with greater runoff in areas covered by secondary forests (SF), compared to old-growth forest cover (OG) and dense scrub vegetation (CH). Further, throughfall water was greatly nutrient enriched for all forest types. Suspended sediment loads varied between successional stages. SF catchments exported 455 kg of sediments per ha, followed by OG with 91 kg/ha and CH with 14 kg/ha, corresponding to 11 rainfall events measured from December 2013 to April 2014. Total nitrogen (TN) and phosphorus (TP) concentrations in stream water also varied with rainfall intensity. In seven rainfall events sampled during the study period, CH catchments exported less nutrients (46 kg/ha TN and 7 kg/ha TP) than SF catchments (718 kg/ha TN and 107 kg/ha TP), while OG catchments exported intermediate sediment loads (201 kg/ha TN and 23 kg/ha TP). Further, we found significant effects of successional stage attributes (vegetation structure and soil physical properties) and catchment morphometry on runoff and sediment concentrations, and greater nutrients retention in OG and CH catchments. We conclude that in these southern hemisphere, broad-leaved evergreen temperate forests, hydrological processes are driven by multiple interacting phenomena, including climate, vegetation, soils, topography, and disturbance history. 相似文献
2.
3.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A. 相似文献
4.
5.
6.
A. Habib J. Vernin Z. Benkhaldoun H. Lanteri 《Monthly notices of the Royal Astronomical Society》2006,368(3):1456-1462
In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed v ( h i ) , the standard deviation of the horizontal wind speed fluctuations σ v ( hi ) and the integrated value of C 2 n over the thickness Δ hi of the turbulent layer C 2 n ( hi )Δ hi , where hi is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using C 2 n ( hi ) and v ( hi ) values. The results of other techniques support this new method. 相似文献
7.
An empirical kinematic method developed by Hakamada and Akasofu (1982) is calibrated on the basis of a one-dimensional MHD solution. The calibrated results are used to simulate the stream-stream interaction and the background corotating structure in a simple situation and also during 22 November–6 December 1977. The solar wind disturbances caused by solar activities during this period are then introduced into the above background stream in simulating the heliospheric disturbance event which was observed by an aligned set of spacecraft at distances between 0.6 and 1.6 a.u. The observations and the simulated results are satisfactory, and a little more refinement in the simulation could reconstruct reasonably well the data by filling the data gaps in the solar wind speed, the density and the IMF magnitude. 相似文献
8.
We have developed a method for analytically solving the porous medium flow equation in many different geometries for horizontal (two‐dimensional), homogeneous and isotropic aquifers containing impermeable boundaries and any number of pumping or injection wells located at arbitrary positions within the system. Solutions and results are presented for rectangular and circular aquifers but the method presented here is easily extendible to many geometries. Results are also presented for systems where constant head boundary conditions can be emulated internal to the aquifer boundary. Recommendations for extensions of the present work are briefly discussed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
9.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
10.
Samuel Lévêque Bertrand Koehler Oskar von der Lühe 《Astrophysics and Space Science》1996,239(2):305-314
The Very Large Telescope Interferometer [1,13] will be operated in air which will introduce small optical path fluctuations due to internal turbulence [4,6] and dispersion effects. Both may contribute to fringe contrast decrease. Longitudinal dispersion effects can be corrected by inserting an appropriate glass of variable thickness in one arm of the interferometer [3,5,11,12]. This paper presents a new method applied to VLTI in order to select the optimum glass, according to both the observing wavelength and the spectral bandpath, and to calculate its thickness. Results are presented in terms of improvement on the fringe contrast. 相似文献