全文获取类型
收费全文 | 65046篇 |
免费 | 619篇 |
国内免费 | 1231篇 |
专业分类
测绘学 | 2339篇 |
大气科学 | 4771篇 |
地球物理 | 11990篇 |
地质学 | 27193篇 |
海洋学 | 4650篇 |
天文学 | 10913篇 |
综合类 | 2216篇 |
自然地理 | 2824篇 |
出版年
2021年 | 206篇 |
2020年 | 254篇 |
2019年 | 308篇 |
2018年 | 7077篇 |
2017年 | 6358篇 |
2016年 | 4261篇 |
2015年 | 700篇 |
2014年 | 801篇 |
2013年 | 1378篇 |
2012年 | 2493篇 |
2011年 | 5437篇 |
2010年 | 4768篇 |
2009年 | 5300篇 |
2008年 | 4320篇 |
2007年 | 5296篇 |
2006年 | 1252篇 |
2005年 | 1491篇 |
2004年 | 1540篇 |
2003年 | 1519篇 |
2002年 | 1161篇 |
2001年 | 698篇 |
2000年 | 740篇 |
1999年 | 589篇 |
1998年 | 591篇 |
1997年 | 542篇 |
1996年 | 405篇 |
1995年 | 405篇 |
1994年 | 415篇 |
1993年 | 323篇 |
1992年 | 316篇 |
1991年 | 267篇 |
1990年 | 320篇 |
1989年 | 279篇 |
1988年 | 264篇 |
1987年 | 291篇 |
1986年 | 244篇 |
1985年 | 335篇 |
1984年 | 354篇 |
1983年 | 349篇 |
1982年 | 324篇 |
1981年 | 285篇 |
1980年 | 294篇 |
1979年 | 227篇 |
1978年 | 216篇 |
1977年 | 221篇 |
1976年 | 192篇 |
1975年 | 196篇 |
1974年 | 182篇 |
1973年 | 175篇 |
1972年 | 115篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Kaizad F. Patel Ivan J. Fernandez Sarah J. Nelson Stephen A. Norton Cheryl J. Spencer 《水文研究》2021,35(5):e14147
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture. 相似文献
2.
3.
4.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
5.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献
6.
Neill Reid 《Monthly notices of the Royal Astronomical Society》2003,342(3):837-850
We present high-resolution echelle spectroscopy of 20 stars in 16 systems catalogued as members of the TW Hydrae association, and 16 stars identified as possible new members. We have calibrated the range of coronal and chromospheric activity expected for such young stars as a function of spectral type by combining our observations with literature data for field and open cluster stars. We also compute space motions for TWA members and candidate members with proper motion measurements, using two techniques to estimate distances to stars lacking direct trigonometric parallax measurements. The mean space motion of the four TWA members with known parallaxes is ( U , V , W : −10.0, −17.8, −4.6) km s−1 . 14 of the candidates have properties inconsistent with cluster membership; the remaining two are potential new members, although further observations are required to confirm this possibility. 相似文献
7.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin. 相似文献
8.
9.
10.
T.-C. Jim Yeh 《水文研究》1992,6(4):369-395
This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended. 相似文献