全文获取类型
收费全文 | 83272篇 |
免费 | 1374篇 |
国内免费 | 711篇 |
专业分类
测绘学 | 2112篇 |
大气科学 | 6540篇 |
地球物理 | 17545篇 |
地质学 | 27724篇 |
海洋学 | 6996篇 |
天文学 | 18376篇 |
综合类 | 162篇 |
自然地理 | 5902篇 |
出版年
2020年 | 567篇 |
2019年 | 575篇 |
2018年 | 1133篇 |
2017年 | 1099篇 |
2016年 | 1622篇 |
2015年 | 1219篇 |
2014年 | 1680篇 |
2013年 | 4020篇 |
2012年 | 1842篇 |
2011年 | 2777篇 |
2010年 | 2369篇 |
2009年 | 3479篇 |
2008年 | 3170篇 |
2007年 | 2900篇 |
2006年 | 2974篇 |
2005年 | 2576篇 |
2004年 | 2691篇 |
2003年 | 2511篇 |
2002年 | 2420篇 |
2001年 | 2132篇 |
2000年 | 2116篇 |
1999年 | 1858篇 |
1998年 | 1849篇 |
1997年 | 1822篇 |
1996年 | 1578篇 |
1995年 | 1506篇 |
1994年 | 1390篇 |
1993年 | 1261篇 |
1992年 | 1201篇 |
1991年 | 1028篇 |
1990年 | 1255篇 |
1989年 | 1064篇 |
1988年 | 991篇 |
1987年 | 1201篇 |
1986年 | 1050篇 |
1985年 | 1364篇 |
1984年 | 1548篇 |
1983年 | 1486篇 |
1982年 | 1317篇 |
1981年 | 1286篇 |
1980年 | 1153篇 |
1979年 | 1094篇 |
1978年 | 1151篇 |
1977年 | 1037篇 |
1976年 | 1032篇 |
1975年 | 942篇 |
1974年 | 965篇 |
1973年 | 982篇 |
1972年 | 611篇 |
1971年 | 532篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
In 1903 the Swiss Federal Research Institute WSL started its first forest hydrology measurements with the aim to deliver a sound scientific basis for the implementation of new forest legislation introduced in Switzerland in 1876. This legislation was triggered by several large floods that occurred in Switzerland, for which a major cause was widely seen as the poor condition of forests at that time. Consequently, hydrologic research at WSL first focused on the influence of forests on floods. In the second half of the 20th century, other hydrological issues such as water quality, snow hydrology and sediment transport complemented the hydrologic research at WSL. Some recent results of this work are presented in three papers joining this introductory paper to mark the 100th anniversary of hydrologic research at WSL. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
112.
Christopher J. Duffin 《Geology Today》2005,21(2):58-63
Nowadays, when someone refers to lapidary they usually mean the process of polishing, engraving or working stones. The term is much broader than that, however, encompassing an entire genre of literature which formed the basis of much geological observation even before the word 'geology' was ever coined. 相似文献
113.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley
– Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as
emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite
and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and
a lower crustal source.
Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper
mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts
of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper
crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material. 相似文献
114.
OSCR is an HF radar system that has been developed for high spatial resolution coastal surface current measurement. This paper describes preliminary results that demonstrate that wave measurement can be successfully obtained from suitably processed OSCR data. Comparisons with data from a WAVEC directional buoy are presented and show encouraging agreement. Some of the limitations to the measurement process are discussed and indicate a maximum range of about 20 km. Surface current variability on short time scales presents the most serious obstacle to wave measurement. This appears to be more of a problem when the mean currents are large, in that in these circumstances the data fail initial quality control criteria. However, in lower mean currents, the effect is often still present and leads to errors in long wave measurement 相似文献
115.
The self-starter is improved using the operator of the split-step Pade solution. In addition to providing greater stability and being applicable closer to the source, the improved self-starter is an efficient forward model for geoacoustic inversion. It is necessary to solve only O(10) tridiagonal systems of equations to obtain the acoustic field on a vertical array located O(10) wavelengths from a source. This experimental configuration is effective for geoacoustic inverse problems involving unknown parameters deep in the ocean bottom. For problems involving depth-dependent acoustic parameters, the improved self-starter can be used to solve nonlinear inverse problems involving O(10) unknown sediment parameters in less than a minute on the current generation of workstations 相似文献
116.
117.
The paper provides a new stereo‐analytical method, which is a combination of the stereographic method and analytical methods, to separate finite removable blocks from the infinite and tapered blocks in discontinuous rock masses. The methodology has applicability to both convex and concave blocks. Application of the methodology is illustrated through examples. Addition of this method to the existing block theory procedures available in the literature improves the capability of block theory in solving practical problems in rock engineering. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
118.
Infiltration experiments have been performed at three sites along a well-known catena under virgin tropical rain forest using a portable sprinkling infiltrometer. Experimentally determined infiltration curves are presented. Infiltration curves are also simulated on the basis of the Mein-Larson equation. The parameters for this model have been obtained from the infiltration curves (saturated conductivity) and simple soil moisture determinations (fillable porosity). The agreement between experimentally determined and modelled infiltration is reasonable, provided (a) saturated conductivity as derived from the experimental data is corrected, (b) a storage parameter, also derived from the experimental data, is added to the Mein-Larson model, and (c) the decline in soil porosity with depth is either small or occurs abruptly at shallow depth. Comparison of observed infiltration rates with rainfall intensity shows that Horton Overland Flow has to occur naturally at least on the middle and lower section of the catena. Despite the fact that most parameters can be estimated in principle from basic soil data, it remains advisable to obtain sprinkling infiltrometer field measurements, because of soil variability due to dynamic surface conditions, macroporosity, air entrapment, and irregularity of the wetting front. 相似文献
119.
120.
High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ18O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献