全文获取类型
收费全文 | 951篇 |
免费 | 38篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 102篇 |
地球物理 | 186篇 |
地质学 | 370篇 |
海洋学 | 69篇 |
天文学 | 138篇 |
综合类 | 2篇 |
自然地理 | 95篇 |
出版年
2023年 | 4篇 |
2021年 | 14篇 |
2020年 | 7篇 |
2019年 | 22篇 |
2018年 | 23篇 |
2017年 | 21篇 |
2016年 | 29篇 |
2015年 | 30篇 |
2014年 | 42篇 |
2013年 | 61篇 |
2012年 | 46篇 |
2011年 | 51篇 |
2010年 | 59篇 |
2009年 | 41篇 |
2008年 | 50篇 |
2007年 | 41篇 |
2006年 | 35篇 |
2005年 | 44篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 30篇 |
2001年 | 22篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 13篇 |
1994年 | 8篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1985年 | 14篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1977年 | 3篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1973年 | 13篇 |
1971年 | 6篇 |
1970年 | 4篇 |
1969年 | 3篇 |
排序方式: 共有992条查询结果,搜索用时 15 毫秒
91.
An analysis of observations of airflow over Lake Ontario in a stably stratified atmosphere is presented. The relationship with theory is mentioned only in terms of what may be important to future work. Detailed air motion, potential temperature and mixing-ratio measurements are presented which show pervasive variability in the horizontal. There is no evidence of gravity waves. It is suggested that vertical mixing of the atmosphere may occur in stable conditions by a mechanism quite apart from the usual concept of the small-scale random eddy motion usually associated with surface convection. Vertical soundings are analyzed as well as two long constant-height observational runs. 相似文献
92.
Olivia H. Devereux Karen L. Prestegaard Brian A. Needelman Allen C. Gellis 《水文研究》2010,24(11):1391-1403
Fine sediment sources were characterized by chemical composition in an urban watershed, the Northeast Branch Anacostia River, which drains to the Chesapeake Bay. Concentrations of 63 elements and two radionuclides were measured in possible land‐based sediment sources and suspended sediment collected from the water column at the watershed outlet during storm events. These tracer concentrations were used to determine the relative quantity of suspended sediment contributed by each source. Although this is an urbanized watershed, there was not a distinct urban signature that can be evaluated except for the contributions from road surfaces. We identified the sources of fine sediment by both physiographic province (Piedmont and Coastal Plain) and source locale (streambanks, upland and street residue) by using different sets of elemental tracers. The Piedmont contributed the majority of the fine sediment for seven of the eight measured storms. The streambanks contributed the greatest quantity of fine sediment when evaluated by source locale. Street residue contributed 13% of the total suspended sediment on average and was the source most concentrated in anthropogenically enriched elements. Combining results from the source locale and physiographic province analyses, most fine sediment in the Northeast Branch watershed is derived from streambanks that contain sediment eroded from the Piedmont physiographic province of the watershed. Sediment fingerprinting analyses are most useful when longer term evaluations of sediment erosion and storage are also available from streambank‐erosion measurements, sediment budget and other methods. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
93.
94.
The Loreto Bay National Park was established to protect the area's ecosystems from habitat destruction and overexploitation. However, the park has not met two of its primary goals: recovery of commercially valuable fish populations and their sustainable use by stakeholders. Based on evidence from the literature, dialogue with stakeholders, a literature review on using recreational fees for marine protected area (MPA) management, and an economic valuation survey, we provide practical guidelines for addressing factors hindering the park's success. These include: 1) the implementation of management based upon ecosystem science, and rigorous monitoring of park ecosystems, 2) increased communication among stake-holder groups, outreach and education, and 3) a re-organization of the park's administrative structure that would allow for more efficient use of funds. Our results also suggest that the park entrance fee could be raised to support these proposed improvements. 相似文献
95.
Magnetic fields control the inconstant Sun. The key to understanding solar variability and its direct impact on the Earth rests with understanding all aspects of these magnetic fields. The Advanced Technology Solar Telescope (ATST) has been design specifically for magnetic remote sensing. Its collecting area, spatial resolution, scattered light, polarization properties, and wavelength performance all insure ATST will be able to observe magnetic fields at all heights in the solar atmosphere from photosphere to corona. After several years of design efforts, ATST has been approved by the U.S. National Science Foundation to begin construction with a not to exceed cost cap of approximately $298M. Work packages for major telescope components will be released for bid over the next several months. An application for a building permit has been submitted (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
96.
In this article, disasters are understood as processes that have different impacts on social routines in terms of scale, scope
and duration. The extent of adaptive processes in society can provide the ground for a rough classification of disaster types.
Such classification has, on the one hand, practical and analytical advantages. On the other hand, they harbour the danger
of overlooking transitions of scale and discourage comprehensive scale-related learning forms. Based on the disaster scale
by Fischer (Int J Mass Emerg Disasters 1:91–107, 2003), flash floods in mountain rivers and torrents are described as extreme emergencies or small-town disasters. Three given
examples will clearly show that learning rarely takes place within an institutional setting that is subjected to small disasters, because the stakeholder’s focus remains on only one level. Therefore, we propose to implement a system of self-organised
and scale-independent learning, so called deutero learning, within the political subsystem. Following a damaging event, participative
processes that involve all levels should be initialised. Their task would be to assess the combination of causes and draw
conclusions for mitigation measures. An aggregation of these assessments would help the responsible political subsystems to
adapt the current natural disasters policy to the changing environmental conditions. 相似文献
97.
We investigated if the success of the invasive common reed Phragmites australis could be attributed to a competitive ability to use dissolved organic nitrogen (DON) when compared to the dominant macrophyte
Spartina alterniflora in tidal wetlands. Short-term nutrient uptake experiments were performed in the laboratory on two genetic lineages of Phragmites (native and introduced to North America) and S. alterniflora. Our results provide the first evidence for direct assimilation of DON by temperate marsh plants and indicate that amino
acids are assimilated intact by all plant types at similar rates. Both Phragmites lineages had significantly greater urea–N assimilation rates than S. alterniflora, and the affinity for dissolved inorganic nitrogen (DIN) species was the greatest in native Phragmites > introduced Phragmites > S. alterniflora. Field studies demonstrated uptake of both DON and DIN in similar proportion as those determined in the laboratory experiments.
Based on these uptake rates, we estimate that DON has the potential to account for up to 47% of N demand for Phragmites plants, and up to 24% for S. alterniflora plants. Additionally, we suggest that differences in N uptake between native and introduced Phragmites lineages explain one mechanism for the success of the introduced type under increasingly eutrophic conditions. 相似文献
98.
Philip J. Wallis Michael B. Ward Jamie Pittock Karen Hussey Howard Bamsey Amandine Denis Steven J. Kenway Carey W. King Shahbaz Mushtaq Monique L. Retamal Brian R. Spies 《Climatic change》2014,125(2):209-220
A variety of proposed activities to mitigate greenhouse gas emissions will impact on scarce water resources, which are coming under increasing pressure in many countries due to population growth and shifting weather patterns. However, the integrated analysis of water and carbon impacts has been given limited attention in greenhouse mitigation planning. In this Australian case study, we analyse a suite of 74 mitigation measures ranked as highest priority by one influential analysis, and we find that they have highly variable consequences for water quantity. We find: (1) The largest impacts result from land-based sequestration, which has the potential to intercept large quantities of water and reduce catchment yields, estimated to exceed 100 Mm3/MtCO2-e of carbon mitigated (100,000 l per tonne CO2-e). (2) Moderate impacts result from some renewable power options, including solar thermal power with a water cost estimated at nearly 4 Mm3/MtCO2-e. However, the water impacts of solar thermal power facilities could be reduced by designing them to use existing power-related water supplies or to use air or salt-water cooling. (3) Wind power, biogas, solar photovoltaics, energy efficiency and operational improvements to existing power sources can reduce water demand through offsetting the water used to cool thermal power generation, with minor savings estimated at 2 Mm3/MtCO2-e and amounting to nearly 100 Mm3 of water saved in Australia per annum in 2020. This integrated analysis significantly changes the attractiveness of some mitigation options, compared to the case where water impacts are not considered. 相似文献
99.
Mark W. Shephard Eva Mekis Robert J. Morris Yang Feng Xuebin Zhang Karen Kilcup 《大气与海洋》2014,52(5):398-417
Short-duration (5 minutes to 24 hours) rainfall extremes are important for a number of purposes, including engineering infrastructure design, because they represent the different meteorological scales of extreme rainfall events. Both single location and regional analyses of the changes in short-duration extreme rainfall amounts across Canada, as observed by tipping bucket rain gauges from 1965 to 2005, are presented. The single station analysis shows a general lack of a detectable trend signal, at the 5% significance level, because of the large variability and the relatively short period of record of the extreme short-duration rainfall amounts. The single station 30-minute to 24-hour durations show that, on average, 4% of the total number of stations have statistically significant increasing amounts of rainfall, whereas 1.6% of the cases have significantly decreasing amounts. However, regional spatial patterns are apparent in the single station trend results. Thus, for the same durations regional trends are presented by grouping the single station trend statistics across Canada. This regional trend analysis shows that at least two-thirds of the regions across Canada have increasing trends in extreme rainfall amounts, with up to 33% being significant (depending on location and duration). Both the southwest and the east (Newfoundland) coastal regions generally show significant increasing regional trends for 1- and 2-hour extreme rainfall durations. For the shortest durations of 5–15 minutes, the general overall regional trends in the extreme amounts are more variable, with increasing and decreasing trends occurring with similar frequency; however, there is no evidence of statistically significant decreasing regional trends in extreme rainfall amounts. The decreasing regional trends for the 5- to 15-minute duration amounts tend to be located in the St. Lawrence region of southern Quebec and in the Atlantic provinces. Additional analysis using criteria specified for traditional water management practice (e.g., Intensity-Duration-Frequency (IDF)) shows that fewer than 5.6% and 3.4% of the stations have significant increasing and decreasing trends, respectively, in extreme annual maximum single location observation amounts. This indicates that at most locations across Canada the traditional single station IDF assumption that historical extreme rainfall observations are stationary (in terms of the mean) over the period of record for an individual station is not violated. However, the trend information is still useful complementary information that can be considered for water management purposes, especially in terms of regional analysis. 相似文献
100.
Threshold behavior in hydrological systems generally involves a qualitative change of a single process, the system response or the functioning of the system. Different types of thresholds and their underlying controls are examined using the example of the Lurbach karst system (Austria). This karst system receives allogenic recharge from the sinking stream Lurbach, which under low-flow conditions only resurges at the Hammerbach spring. Yet, under medium- to high-flow conditions an overflow toward another spring, the Schmelzbach outlet occurs. Thresholds in physicochemical spring responses and their underlying controls are identified from the analysis of heat and solute transport processes in karst conduits. Applying this concept to the Hammerbach spring suggests that the threshold controlling the response of the spring water temperature was changed in the time period from 2006 to 2009 relative to the years before. At the same time, changes are observed in the behavior of the spring hydrograph and the discharge threshold at which the overflow to the Schmelzbach system is activated. All of these observations can be consistently explained by a decreased diameter of the conduit pathways within the indicated time period, presumably caused by the redistribution of sediments due to a flood event in 2005. Thus, thresholds in the physicochemical spring response were successfully employed to support the identification of a change in the functioning of the Lurbach karst system, which occurred possibly because a threshold related to the sediment transport within the karst conduits was crossed. 相似文献