全文获取类型
收费全文 | 1853篇 |
免费 | 114篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 65篇 |
大气科学 | 189篇 |
地球物理 | 585篇 |
地质学 | 689篇 |
海洋学 | 117篇 |
天文学 | 223篇 |
综合类 | 21篇 |
自然地理 | 88篇 |
出版年
2023年 | 6篇 |
2022年 | 16篇 |
2021年 | 44篇 |
2020年 | 43篇 |
2019年 | 34篇 |
2018年 | 79篇 |
2017年 | 84篇 |
2016年 | 121篇 |
2015年 | 101篇 |
2014年 | 114篇 |
2013年 | 186篇 |
2012年 | 145篇 |
2011年 | 121篇 |
2010年 | 101篇 |
2009年 | 86篇 |
2008年 | 68篇 |
2007年 | 50篇 |
2006年 | 62篇 |
2005年 | 39篇 |
2004年 | 37篇 |
2003年 | 39篇 |
2002年 | 45篇 |
2001年 | 28篇 |
2000年 | 15篇 |
1999年 | 14篇 |
1998年 | 15篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 19篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 20篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 14篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1969年 | 7篇 |
排序方式: 共有1977条查询结果,搜索用时 0 毫秒
31.
Abstract This paper investigates for a 25-year period the sediment distribution in a semi-arid Brazilian basin (2 × 104 km2) with a network containing more than 4000 surface reservoirs. The methodology is based on rating curves and fitted parameters derived from field data. The results showed that suspended load corresponded to 70% of the total sediment yield (148 t km-2 year-1). The relatively low contribution of the suspended load (compared with other semi-arid regions) was attributed to the impact of the numerous upstream reservoirs, which retained 235 t km-2 year-1. The micro (<1 hm3), small (1–10 hm3), medium-sized (10–50 hm3), and large or strategic (>50 hm3) reservoirs responded to, respectively, 5, 17, 30 and 48% of the total sediment retention by the reservoir network. This indicates that retention in the non-strategic reservoirs has a positive impact on water availability, since siltation of the strategic reservoirs would be expected to more than double if only such reservoirs existed. Citation Lima Neto, I. E., Wiegand, M. C. &; de Araújo, J. C. (2011) Sediment redistribution due to a dense reservoir network in a large semi-arid Brazilian basin. Hydrol. Sci. J. 56(2), 319–333. 相似文献
32.
In this study, we investigate the accuracy of approximating constant‐Q wave propagation by series of Zener or standard linear solid (SLS) mechanisms. Modelling in viscoacoustic and viscoelastic media is implemented in the time domain using the finite‐difference (FD) method. The accuracy of numerical solutions is evaluated by comparison with the analytical solution in homogeneous media. We found that the FD solutions using three SLS relaxation mechanisms as well as a single SLS mechanism, with properly chosen relaxation times, are quite accurate for both weak and strong attenuation. Although the RMS errors of FD simulations using a single relaxation mechanism increase with increasing offset, especially for strong attenuation (Q = 20), the results are still acceptable for practical applications. The synthetic data of the Marmousi‐II model further illustrate that the single SLS mechanism, to model constant Q, is efficient and sufficiently accurate. Moreover, it benefits from less computational costs in computer time and memory. 相似文献
33.
Renaud F Oberhänsli F Teyssié JL Miramand P Temara A Warnau M 《Marine pollution bulletin》2011,62(5):942-947
Linear Alkylbenzene Sulfonates (LAS) are ubiquitous surfactants. Traces can be found in coastal environments. Sorption and toxicity of C12-LAS congeners were studied in controlled conditions (2-3500 μg C12LAS/L) in five marine phytoplanktonic species, using standardized methods. IC50 values ranged from 0.5 to 2 mg LAS/L. Sorption of 14C12-6 LAS isomer was measured at environmentally relevant trace levels (4 μg/L) using liquid scintillation counting. Steady-state sorption on algae was reached within 5 h in the order dinoflagellate > diatoms > green algae. The sorption data, fitted a L-type Freundlich isotherm, indicating saturation. Desorption was rapid but a low LAS fraction was still sorbed after 24 h. Toxic cell concentration was 0.38 ± 0.09 mg/g for the studied species. LAS toxicity results from sorption on biological membranes leading to non-specific disturbance of algal growth. Results indicate that LAS concentrations in coastal environments do not represent a risk for these organisms. 相似文献
34.
Emilio Rodríguez-Caballero José Raúl Román Sonia Chamizo Beatriz Roncero Ramos Yolanda Cantón 《地球表面变化过程与地形》2019,44(14):2771-2779
Biological soil crust, or biocrust communities, are the dominating life form in many extreme habitats, such as arid and semiarid badlands, where water scarcity and highly erodible substrates limit vegetation cover. While climate, soil and biotic factors have been described as environmental filters influencing biocrust distribution in such biomes, little is known about the effect of terrain attributes on creating specific microhabitats that promote or restrict biocrust colonization. This study aimed to identify the main terrain attributes controlling biocrust distribution in the driest badland system in Europe, the Tabernas Badlands (SE Spain). To do this, we analysed the influence of different terrain attributes related to landscape stability and microclimate formation on the spatial distribution of lichen and cyanobacteria, using field measurements and topographical information from a LiDAR survey. Our results showed that the spatial distribution of cyanobacteria-dominated biocrusts, which are physiologically and morphologically adapted to extreme drought and high UVA radiation, was mostly associated with areas of high potential incoming solar radiation. The exception was bare south-aspect hillslopes with very high sediment transport potential, where bare physically crusted soils were the dominant ground cover. Lichen-dominated biocrusts, in contrast, colonized near the top of north-aspect hillslopes, characterized by low potential incoming solar radiation and potential evapotranspiration, and their cover decreased downstream, as conditions became good enough for vascular plants. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd. 相似文献
35.
36.
José Antonio Belinchón 《Astrophysics and Space Science》2009,323(3):307-315
We study a massive cosmic strings with BII symmetries cosmological models in two contexts. The first of them is the standard
one with a barotropic equation of state. In the second one we explore the possibility of taking into account variable “constants”
(G and Λ). Both models are studied under the self-similar hypothesis. We put special emphasis in calculating the numerical values
for the equations of state. We find that for ω∈(0,1], G, is a growing time function while Λ, behaves as positive decreasing time function. If ω=0, both “constants”, G and Λ, behave as true constants. 相似文献
37.
André Duijveman 《Solar physics》1983,84(1-2):189-203
Shortly after the occurrence of the impulsive spikes of the two-ribbon flare of May 21, 1980, a temperature analysis of the X-ray emitting flare plasma showed the presence of a low-temperature component [n = 15 × 1010 cm#X2212;3; T = 20 × 106 K] and a high-temperature component [n = 2 × 1010 cm#X2212;3; T = 40 × 106 K]. The mean free path of an electron in the hot component is comparable to the size of the source (≈ 104 km). Heat losses from the hot source can therefore not be described with classical formulae. Theoretical arguments show that most likely the electron to ion temperature ratio T e/Ti in the hot plasma is close to unity. This implies the presence of a hot ion component (T i ≈ 40 × 106 K) as well. Under these conditions (T e ≈ T i) heat flux limitation by electrostatic turbulence is ineffective. However, reduction of the heat flux is still possible due to the breakdown of classical theory. It is demonstrated that only non-classical current dissipation processes can sustain a hot source against cooling by a saturated heat flux. Investigation of the collisionality as a function of position along a magnetic loop shows that the breakdown of classical theory should be expected to occur first near the base of the loop. We conclude that the newly discovered hot source is important for the energy budget of the flare, even if the heat losses are considerably reduced. It is estimated that for the May 21, 1980 flare a total of about 1031 ergs were necessary to maintain the hot source against heat losses over the time period that it was observed (≈ 10 min). 相似文献
38.
Jordi LLORCA Ignasi CASANOVA Josep M. TRIGO‐RODRÍGUEZ José M. MADIEDO Julia ROSZJAR Addi BISCHOFF Ulrich OTT Ian A. FRANCHI Richard C. GREENWOOD Matthias LAUBENSTEIN 《Meteoritics & planetary science》2009,44(2):159-174
Abstract— Puerto Lápice is a new eucrite fall (Castilla‐La Mancha, Spain, 10 May 2007). In this paper, we report its detailed petrography, magnetic characterization, mineral and bulk chemistry, oxygen and noble gas isotope systematics, and radionuclide data. Study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature, and it contains basaltic and granulitic clasts, as well as recrystallized impact melt and breccia fragments. Shock veins are ubiquitous and show evidence of at least three different shock events. Bulk chemical analyses suggest that Puerto Lápice belongs to the main group of basaltic eucrites, although it has a significantly higher Cr content. Oxygen isotopes also confirm that the meteorite is a normal member of the HED suite. Noble gas abundances show typical patterns, with dominant cosmogenic and radiogenic contributions, and indicate an average exposure age of 19 ± 2 Ma, and a Pu‐fission Xe age well within typical eucrite values. Cosmogenic radionuclides suggest a preatmospheric size of about 20–30 cm in diameter. 相似文献
39.
Garrelt Mellema Léon V. E. Koopmans Filipe A. Abdalla Gianni Bernardi Benedetta Ciardi Soobash Daiboo A. G. de Bruyn Kanan K. Datta Heino Falcke Andrea Ferrara Ilian T. Iliev Fabio Iocco Vibor Jelić Hannes Jensen Ronniy Joseph Panos Labroupoulos Avery Meiksin Andrei Mesinger André R. Offringa V. N. Pandey Jonathan R. Pritchard Mario G. Santos Dominik J. Schwarz Benoit Semelin Harish Vedantham Sarod Yatawatta Saleem Zaroubi 《Experimental Astronomy》2013,36(1-2):235-318
The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope. 相似文献
40.
Abstract This document presents fundamental terms describing the transfer of radiative energy and relevant optical properties of natural waters. These are primarily based upon the terminology of the “Système International d'Unités (SI)”; and the “International Commission on Illumination (CIE).’’ Quantities and Symbols as proposed in the present terminology follow and extend the terminology that was recommended by the Committee on Radiant Energy in the Sea (of the International Association of Physical Oceanography, IAPO), as published by Jerlov (1968, Optical Oceanography Elsevier Oceanography Series, Vol. 5; 1976, Marine Optics Elsevier Oceanography Series, Vol. 14). 相似文献