全文获取类型
收费全文 | 865篇 |
免费 | 77篇 |
国内免费 | 140篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 30篇 |
地球物理 | 190篇 |
地质学 | 332篇 |
海洋学 | 137篇 |
天文学 | 6篇 |
综合类 | 41篇 |
自然地理 | 311篇 |
出版年
2024年 | 4篇 |
2023年 | 16篇 |
2022年 | 22篇 |
2021年 | 26篇 |
2020年 | 41篇 |
2019年 | 29篇 |
2018年 | 26篇 |
2017年 | 24篇 |
2016年 | 31篇 |
2015年 | 52篇 |
2014年 | 47篇 |
2013年 | 46篇 |
2012年 | 42篇 |
2011年 | 64篇 |
2010年 | 48篇 |
2009年 | 57篇 |
2008年 | 50篇 |
2007年 | 57篇 |
2006年 | 54篇 |
2005年 | 50篇 |
2004年 | 49篇 |
2003年 | 46篇 |
2002年 | 32篇 |
2001年 | 33篇 |
2000年 | 24篇 |
1999年 | 21篇 |
1998年 | 25篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有1082条查询结果,搜索用时 0 毫秒
971.
Wenjun Han Jingyan Gu Qiujie Yan Jungang Li Zhihong Wu Qianqun Gu Yuezhong Li 《中国海洋大学学报(英文版)》2012,11(3):375-382
Bacteria of the genus Flammeovirga can digest complex polysaccharides (CPs), but no details have been reported regarding the CP depolymerases of these bacteria. MY04, an agarolytic marine bacterium isolated from coastal sediments, has been identified as a new member of the genus Flammeovirga. The MY04 strain is able to utilize multiple CPs as a sole carbon source and grows well on agarose, mannan, or xylan. This strain produces high concentrations of extracellular proteins (490 mg L-1 ± 18.2 mg L-1 liquid culture) that exhibit efficient and extensive degradation activities on various polysaccharides, especially agarose. These proteins have an activity of 310 U mg-1 ± 9.6 U mg-1 proteins. The extracellular agarase system (EAS) in the crude extracellular enzymes contains at least four agarose depolymerases, which are with molecular masses of approximately 30-70 kDa. The EAS is stable at a wide range of pH values (6.0-11.0), temperatures (0-50℃), and sodium chloride (NaCl) concentrations (0- 0.9 mol L-1). Two major degradation products generated from agarose by the EAS are identified to be neoagarotetraose and neoagarohexaose, suggesting that β-agarases are the major constituents of the MY04 EAS. These results suggest that the Flammeovirga strain MY04 and its polysac-charide-degradation system hold great promise in industrial applications. 相似文献
972.
D. Poppe T. Brauers H. -P. Dorn M. Karl T. Mentel E. Schlosser R. Tillmann R. Wegener A. Wahner 《Journal of Atmospheric Chemistry》2007,57(3):203-214
Degradation of isoprene, m-xylene, n-octane, propene, and methacrolein by hydroxyl radicals has been studied in the simulation chamber SAPHIR under burden of
trace gases as they are typical for the moderately polluted planetary boundary layer. Measured time series of the hydrocarbon
mixing ratios and the OH concentrations were used to determine the rate constants. The hydrocarbons were measured with gas
chromatography and proton transfer reaction mass spectrometry. OH was measured with the Jülich DOAS (differential optical
absorption spectroscopy) instrument. In all cases except methacrolein good agreement was found with the reference rate constants
taken from the Master Chemical Mechanism (MCM3.1). The data for methacrolein are consistent with the results of Karl et al.
(J. Atmos. Chem 55, 2006, doi:) who reported a 12% smaller value. The degradation of hydrocarbons provides an independent method to analyse precision and
accuracy of the OH measurements. A precision of better than 4% over a period of nearly 4 months was found. The accuracy is
within the limitations given by the light absorption cross section of OH. Both results are consistent with earlier results
by Hausmann et al. (J. Geophys. Res. 102:16011–16022, 1997). 相似文献
973.
黄河源区水环境变化及其生态环境地质效应 总被引:9,自引:0,他引:9
为揭示黄河源区生态环境恶化的地质原因,本文依据野外调查资料,通过综合分析,认为水环境变异是源区生态环境恶化的主要原因,人类工程经济活动为次要原因.提出的"控制水文网下蚀,提高侵蚀基准面,遏制冻结层上水水位下降,恢复湿地、植被"的生态地质环境治理及保护措施具一定的应用价值. 相似文献
974.
以二氯乙酸(DCAA)和三氯乙酸(TCAA)为目标污染物,研究了铁(Fe0)和超声辅助铁(US-Fe0)还原降解水溶液中氯乙酸,以及溶液初始pH值、Fe0投加量、反应温度、反应时间、氯乙酸初始浓度对降解率的影响,并对降解的动力学进行了初步研究。结果表明,Fe0还原氯乙酸的最佳条件是:pH值为4.0、铁投加量为4g·L-1,室温条件下反应16h。超声辅助对Fe0还原水中氯乙酸的反应具有显著促进作用。在初始浓度为50μg·L-1时,US-Fe0还原降解DCAA和TCAA的降解率分别为87.3%和82.0%。Fe0和US-Fe0还原降解氯乙酸均符合准一级反应动力学(对氯乙酸),降解的表观速率常数分别为1.03×10-3 s-1(Fe0还原DCAA)、5.70×10-3 s-1(US-Fe0还原DCAA)和5.63×10-4 s-1(Fe0还原TCAA)、2.58×10-3 s-1(US-Fe0还原TCAA)。TCAA脱氯生成DCAA是降解的速率控制步骤。 相似文献
975.
黄河源生态环境变化与成因分析 总被引:29,自引:25,他引:29
以70年代、80年代和90年代3个时期的卫星影像资料为基础,结合野外调查,对70年代以来黄河源区生态环境演变过程及趋势进行了对比分析,并依据时期的气候变化、人为活动强度分析,对该区域生态环境变化的产生原因进行探讨,研究结果表明,与70年代相比,80年代和90年代以高寒沼泽草甸、高寒草罗和高山草原化草甸为代表的主要生态体系均呈明显退化,尤其90年代中期以来,高寒草原与高寒草甸植被退化剧烈,荒漠化发展 相似文献
976.
In February 2005, a landslide of significant dimensions occurred at Bol on the Island of Brač, in Croatia. The location of
the landslide was in a geological structure of weakly bounded conglomerates lying above a flysch, in an area where numerous
slides have been previously reported. In the paper the results of a geological engineering investigation of the Bol landslide
are presented. The investigation results show that the landslide was triggered by a combination of improperly started earthworks,
an increase in air temperature in days preceding landslide that caused snow melt, and abundant rainfall, but the primary cause
of the landslide was weathering of the flysch. In other words, despite the fact that contact between the flysch and the conglomerates
slopes is in approximate correspondence with the slope of the hillside, the landslide surface was not in contact between the
conglomerates and marl, but deeper in the flysch, provoked by the penetration of the weathering front into the flysch. Numerical
modelling results were able to reproduce this mechanism by incorporating strength degradation into the weathered zone, determined
by field investigation. The results of the research can be used to predict potential slides not only in the area under study. 相似文献
977.
Forest cover disturbances due to processes such as logging and forest fires are a widespread issue especially in the tropics, and have heavily affected forest biomass and functioning in the Brazilian Amazon in the past decades. Satellite remote sensing has played a key role for assessing logging activities in this region; however, there are still remaining challenges regarding the quantification and monitoring of these processes affecting forested lands. In this study, we propose a new method for monitoring areas affected by selective logging in one of the hotspots of Mato Grosso state in the Brazilian Amazon, based on a combination of object-based and pixel-based classification approaches applied on remote sensing data. Logging intensity and changes over time are assessed within grid cells of 300 m × 300 m spatial resolution. Our method encompassed three main steps: (1) mapping forest/non-forest areas through an object-based classification approach applied to a temporal series of Landsat images during the period 2000–2015, (2) mapping yearly logging activities from soil fraction images on the same Landsat data series, and (3) integrating information from previous steps within a regular grid-cell of 300 m × 300 m in order to monitor disturbance intensities over this 15-years period. The overall accuracy of the baseline forest/non-forest mask (year 2000) and of the undisturbed vs disturbed forest (for selected years) were 93% and 84% respectively. Our results indicate that annual forest disturbance rates, mainly due to logging activities, were higher than annual deforestation rates during the whole period of study. The deforested areas correspond to circa 25% of the areas affected by forest disturbances. Deforestation rates were highest from 2001 to 2005 and then decreased considerably after 2006. In contrast, the annual forest disturbance rates show high temporal variability with a slow decrease over the 15-year period, resulting in a significant increase of the ratio between disturbed and deforested areas. Although the majority of the areas, which have been affected by selective logging during the period 2000–2014, were not deforested by 2015, more than 70% of the deforested areas in 2015 had been at least once identified as disturbed forest during that period. 相似文献
978.
979.
Xiqiang Wang Rensheng Chen Guohua Liu Chuntan Han Yong Yang Yaoxuan Song Junfeng Liu Zhangwen Liu Xiaojiao Liu Shuhai Guo Lei Wang Qin Zheng 《水文研究》2019,33(1):66-75
The increase in low flows (winter discharge and minimum monthly discharge), caused primarily by permafrost degradation, is common in high‐latitude permafrost regions, whereas the dynamics of low flows in high‐altitude permafrost regions remain largely unknown. Long‐term discharge data from 28 unregulated catchments in western China were analysed, and the findings showed that winter discharge/minimum monthly discharge significantly increased (p ≤ 0.1) in 82/82%, 55/64%, and 0/0% of the catchments in the higher‐latitude mountain permafrost regions (Tienshan Mountains), mid‐latitude mountain permafrost regions (Qilian Mountains), and mid‐ to low‐latitude plateau permafrost regions (the source regions of the Yangtze and Yellow rivers), respectively. The differences in permafrost type and the distribution of permafrost and alpine cold desert (which is similar to tundra) were found to be the main causes for the different responses in the low flows. The rate of change of low flows (winter discharge and minimum monthly discharge) was negatively and linearly correlated with permafrost coverage when coverage was less than 40% of the catchment area, whereas the low flows changed only slightly when the permafrost coverage exceeded 40%. A significant thickening of the active layer increased the low flows in the lower permafrost‐covered catchments, which are dominated by warm permafrost. However, in the higher permafrost‐covered catchments with cold permafrost and a cold climate, only an increase in permafrost temperature (without a notable thickening of the active layer) occurred, resulting in non‐significant changes in low flows. 相似文献
980.
Abstract Changes in development and use of water resources over the last 30 years have led to significant environmental and hydrological degradation of the River Heihe basin in the People's Republic of China. Water discharge of the lower reaches has been seriously compromised and more than 30 tributaries and terminal lakes have dried up. The water output of springs has dropped significantly. Also water quality problems have been observed, in both surface waters and groundwater. Hydrological changes have resulted in a marked degradation of aquatic habitats, and have caused substantial, and expanding, land salinization and desertification. Solving these problems, which have been largely caused by human activities, requires massive and long-lasting concerted efforts. 相似文献