全文获取类型
收费全文 | 37061篇 |
免费 | 5956篇 |
国内免费 | 8236篇 |
专业分类
测绘学 | 3494篇 |
大气科学 | 4918篇 |
地球物理 | 6511篇 |
地质学 | 20161篇 |
海洋学 | 4641篇 |
天文学 | 3389篇 |
综合类 | 2431篇 |
自然地理 | 5708篇 |
出版年
2024年 | 254篇 |
2023年 | 683篇 |
2022年 | 1345篇 |
2021年 | 1558篇 |
2020年 | 1429篇 |
2019年 | 1775篇 |
2018年 | 1242篇 |
2017年 | 1474篇 |
2016年 | 1496篇 |
2015年 | 1679篇 |
2014年 | 2103篇 |
2013年 | 2083篇 |
2012年 | 2186篇 |
2011年 | 2312篇 |
2010年 | 2019篇 |
2009年 | 2498篇 |
2008年 | 2421篇 |
2007年 | 2480篇 |
2006年 | 2450篇 |
2005年 | 2323篇 |
2004年 | 2010篇 |
2003年 | 1929篇 |
2002年 | 1689篇 |
2001年 | 1481篇 |
2000年 | 1498篇 |
1999年 | 1269篇 |
1998年 | 1128篇 |
1997年 | 801篇 |
1996年 | 662篇 |
1995年 | 577篇 |
1994年 | 544篇 |
1993年 | 454篇 |
1992年 | 338篇 |
1991年 | 286篇 |
1990年 | 195篇 |
1989年 | 151篇 |
1988年 | 127篇 |
1987年 | 76篇 |
1986年 | 41篇 |
1985年 | 38篇 |
1984年 | 22篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 14篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
周全浩 《云南地理环境研究》1990,2(1):1-9
本文分析了中国石油生产近20年变化的原因,对一些学者认为80年代中国会成为超级产油大国,后又悲观断定80年代中国可能变成石油净进口国作了评述。同时对90年代中国石油生产前景作了初步展望。 相似文献
12.
The summer monsoon season of the year 2006 was highlighted by an unprecedented number of monsoon lows over the central and
the western parts of India, particularly giving widespread rainfall over Gujarat and Rajasthan. Ahmedabad had received 540.2mm
of rainfall in the month of August 2006 against the climatological mean of 219.8mm. The two spells of very heavy rainfall
of 108.4mm and 97.7mm were recorded on 8 and 12 August 2006 respectively. Due to meteorological complexities involved in replicating
the rainfall occurrences over a region, the Weather Research and Forecast (WRF-ARW version) modeling system with two different
cumulus schemes in a nested configuration is chosen for simulating these events. The spatial distributions of large-scale
circulation and moisture fields have been simulated reasonably well in this model, though there are some spatial biases in
the simulated rainfall pattern. The rainfall amount over Ahmedabad has been underestimated by both the cumulus parameterization
schemes. The quantitative validation of the simulated rainfall is done by calculating the categorical skill scores like frequency
bias, threat scores (TS) and equitable threat scores (ETS). In this case the KF scheme has outperformed the GD scheme for
the low precipitation threshold. 相似文献
13.
14.
Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark.Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals.The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats. 相似文献
15.
Data collected in three Californian estuaries indicate that hypersaline conditions exist during the dry summers typical of a Mediterranean climate. The generalised seasonal and longitudinal hydrographic structures are described and explained. It is argued that this seasonal hypersalinity is common and that it represents a major class of estuaries. The observed accumulation of salt indicates surprisingly long residence times in small basins which have free exchange with the ocean. This semi-isolation of the inner basin leads to a large build-up or severe depletion of nutrients, pollutants and plankton in these systems. Of concern are the trends to increase pollutant loading in the same systems that are experiencing an increase in residence times owing to freshwater extraction in the watershed. 相似文献
16.
Management of summer-spawning herring off Iceland 总被引:1,自引:0,他引:1
17.
18.
The photochemical instability of several related pteridines in seawater was investigated by aseptic incubation of solutions at 20–22°C under illumination from cool-white light of intensity 6 kerg cm−2 sec−1, and the chemical changes were spectrophotometrically monitored. All the pteridines showed markedly accelerated degradation from this illumination relative to their behaviour in total darkness.Pterin and lumazine were degraded very slowly with zero-order reaction kinetics, while the other pteridines photolysed rapidly (according to first-order kinetics) with decomposition rates increasing in the order dioxylumazine (2,4,6,7-tetrahydroxypteridine) < leucopterin < isoxanthopterin < xanthopterin < oxylumazine (2,4,6-trihydroxypteridine). Excepting leucopterin and dioxylumazine, the photolysis rates were attributable to the pH of seawater and not its salt content; this was also the case with oxylumazine which required the salt content of seawater for decomposition in darkness. Leucopterin and dioxylumazine (both 6,7-dihydroxylated pteridines) gave evidence of complexation with the major divalent cations (Ca2+, Mg2+) of seawater, by virtue of which their photolytic degradation rates were enhanced to magnitudes obtained in pH-10 buffer without seawater. It is proposed that such complexation produces structural forms of these pteridines analogous to their normal ionic forms at pH 10–12.The photolysis of the 6-hydroxylated pteridines (xanthopterin, oxylumazine) proceeded via intermediate formation of their corresponding 7-hydroxylated derivatives (leucopterin, dioxylumazine). 相似文献
19.
A preliminary optical classification of lakes in Estonia and south Finland which can also be used for small bays of the Baltic Sea is elaborated. The classification is based on the optical properties of water (diffuse attenuation coefficient, diffuse reflectance) and parameters that are routinely monitored in water bodies (Secchi depth, concentration of chlorophyll-a, total suspended matter and yellow substance). The data complex used for our classification covers different types of water ecosystems (ranging from oligotrophic to hypertrophic) and the variability of water constituent concentrations in the ice-free period in Estonia and south Finland. Using cluster analysis, we found 5 optical classes of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). There is satisfactory correspondence between class of water, shape of diffuse attenuation coefficient and diffuse reflectance spectra and trophic state of the lakes. 相似文献
20.