全文获取类型
收费全文 | 1379篇 |
免费 | 52篇 |
国内免费 | 25篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 114篇 |
地球物理 | 284篇 |
地质学 | 507篇 |
海洋学 | 120篇 |
天文学 | 220篇 |
综合类 | 15篇 |
自然地理 | 169篇 |
出版年
2023年 | 7篇 |
2021年 | 15篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 35篇 |
2017年 | 24篇 |
2016年 | 23篇 |
2015年 | 39篇 |
2014年 | 40篇 |
2013年 | 85篇 |
2012年 | 49篇 |
2011年 | 86篇 |
2010年 | 59篇 |
2009年 | 69篇 |
2008年 | 60篇 |
2007年 | 76篇 |
2006年 | 58篇 |
2005年 | 46篇 |
2004年 | 37篇 |
2003年 | 33篇 |
2002年 | 40篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 24篇 |
1995年 | 14篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 13篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 23篇 |
1988年 | 16篇 |
1987年 | 23篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 27篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 16篇 |
1980年 | 21篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1975年 | 13篇 |
1974年 | 7篇 |
1973年 | 12篇 |
排序方式: 共有1456条查询结果,搜索用时 15 毫秒
141.
TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz 总被引:5,自引:2,他引:3
Jay B. Thomas E. Bruce Watson Frank S. Spear Philip T. Shemella Saroj K. Nayak Antonio Lanzirotti 《Contributions to Mineralogy and Petrology》2010,160(5):743-759
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder
apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility
occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements
and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for
the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments
to the simple expression
RTlnX\textTiO 2 \textquartz = - 60952 + 1.520 ·T(K) - 1741 ·P(kbar) + RTlna\textTiO 2 RT\ln X_{{{\text{TiO}}_{ 2} }}^{\text{quartz}} = - 60952 + 1.520 \cdot T(K) - 1741 \cdot P(kbar) + RT\ln a_{{{\text{TiO}}_{ 2} }} 相似文献
142.
An unusual jaw found in a calcite nodule from Collishaw Point, Hornby Island, British Columbia (off the east coast of Vancouver Island) represents the first definitive pterosaur found in British Columbia, and the first istiodactylid from Canada. 相似文献
143.
Variations in long term wind speed during different decades in Arabian Sea and Bay of Bengal 总被引:1,自引:0,他引:1
A study has been carried out by comparing the extreme wind speeds estimated based on NCEP/NCAR reanalysis data for 100 years
return period using Fischer Tippet-1 (commonly known as Gumbel) and Weibull distributions for three locations (off Goa, Visakhapatnam
and Machilipatnam) in the north Indian Ocean. The wind dataset for Goa is compared with that from ERA-40 data. For higher
wind speeds (12–20m s−1), NCEP wind speed has higher percentage of occurrence than that of ERA-40. Analysis has shown slight upward trend in the
annual maximum wind for location off Machilipatnam with an increase of 1.2 cm s−1 per year and a decreasing trend of −1.3 cm s−1 per year in the case of Goa. The Weibull distribution with shape parameter 2 fits the annual maximum wind data better than
FT-1 distribution. 相似文献
144.
Government policies intended to reduce flood losses can increase the potential for catastrophe by stimulating development
inside the floodplain, a phenomenon referred to as the “safe development paradox.” New Urbanist design has the potential to
both exacerbate and alleviate flood risks. Because they are built at relatively high densities, New Urbanist developments
can exacerbate risk by placing more people and property in harm’s way. Conversely, New Urbanist design features theoretically
better enable designers of New Urbanist developments to avoid floodplain portions of project sites than designers of conventional
subdivisions. Using a sample of New Urbanist developments in the US that contain floodplain portions within their boundaries,
this paper focuses on whether and why these developments locate built structures inside the floodplain. The authors find that
roughly 30% of the developments locate structures inside the floodplain, and that the odds of locating structures inside the
floodplain increase with the proportion of the project site located inside the floodplain and decrease with the presence of
government policies that prohibit residential development in the floodplain. The authors also identify confusion among government
planners regarding the distinction between pre and postconstruction floodplain boundaries. A subset of New Urbanist developments
is found to have built structures located outside the postconstruction floodplain boundary, but inside the preconstruction
floodplain boundary. This finding is cited as an example of the “safe development paradox” in action. The authors recommend
changes in New Urbanist design codes and local government floodplain management to increasingly direct new development away
from the floodplain. 相似文献
145.
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
146.
Measurements have been made in both a neutral and a stable boundary layer as part of an investigation of the wakes of wind turbines in an offshore environment, in the EnFlo stratified flow wind tunnel. The working section is long enough for the flow to have become very nearly invariant with streamwise distance. In order to be systematic, the flow profile generators of Irwin-type spires and surface roughness were the same for both neutral and stable conditions. Achieving the required profiles by adjusting the flow generators, even for neutral flow, is a highly iterative art, and the present results indicate that it will be no less iterative for a stable flow (as well as there being more conditions to meet), so this was not attempted in the present investigation. The stable-case flow conformed in most respects to Monin–Obukhov similarity in the surface layer. A linear temperature profile was applied at the working section inlet, resulting in a near-linear profile in the developed flow above the boundary layer and ‘strong’ imposed stability, while the condition at the surface was ‘weak’. Aerodynamic roughness length (mean velocity) was not affected by stability even though the roughness Reynolds number ${<}1$ , while the thermal roughness length was much smaller, as is to be expected. The neutral case was Reynolds-number independent, and by inference, the stable case was also Reynolds-number independent. 相似文献
147.
Arnold J. H. Reesink Philip J. Ashworth Gregory H. Sambrook Smith James L. Best Daniel R. Parsons Mario L. Amsler Richard J. Hardy Stuart N. Lane Andrew P. Nicholas Oscar Orfeo Steven D. Sandbach Christopher J. Simpson Ricardo N. Szupiany 《Sedimentology》2014,61(4):1055-1085
To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre‐scale bars vary within a multi‐kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre‐scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná – Río Paraguay confluence, where a significant volume of fine‐grained suspended sediment is introduced into the network. Bar‐scale cross‐stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar‐scale sets are found on top of finer‐grained ripple‐laminated bar‐trough deposits. Bar‐scale sets make up as much as 58% of the volume of the deposits in small, incipient mid‐channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of the sedimentary structures found in the Río Paraná is similar in scale to those found in much smaller rivers. In other words, large river deposits are not always characterized by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small‐scale structures, such as ripple sets, to be grouped into thicker cosets, which indicate river scale even when no obvious large‐scale sets are present. The results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the Río Paraguay. Relative to other controls on downstream fining, the tributary input of fine‐grained suspended material from the Río Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ca 5 m of mid‐channel bars shows: (i) an increase in the abundance and thickness (up to metre‐scale) of laterally extensive (hundreds of metres) fine‐grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar‐trough deposits and a corresponding decrease in bar‐scale cross‐strata (<10%). The thalweg deposits of the Río Paraná are composed of dune sets, even directly downstream from the Río Paraguay where the upper channel deposits are dominantly fine‐grained. Thus, the change in sedimentary facies due to a tributary point‐source of fine‐grained sediment is primarily expressed in the composition of the upper bar deposits. 相似文献
148.
In Jakarta, climate change has been detected through rising air temperatures, increased intensity of rainfall in the wet season, and sea level rise. The coupling of such changes with local anthropogenic driven modifications in the environmental setting could contribute to an increased probability of flooding, due to increase in both extreme river discharge and sedimentation (as a result of erosion in the watersheds above Jakarta and as indicated by sediment yield in the downstream area). In order to respond to the observed and projected changes in river discharge and sediment yield, and their secondary impacts, adaptation strategies are required. A possible adaptation strategy is through policy making in the field of spatial planning. For example, in Indonesia, presidential regulation number 54 year 2008 (Peraturan Presiden Nomor 54 Tahun 2008—Perpres 54/2008) was issued as a reference for the implementation of water and soil conservation. This paper assesses the impact of climate and land cover change on river discharge and sediment yield, as well as the effects of Perpres 54/2008 on that river discharge and sediment yield. The spatial water balance model Spatial Tools for River Basins and Environmental and Analysis of Management Option was used for the runoff computations, whilst the Spatial Decision Assistance of Watershed Sedimentation model was used to simulate erosion, Sediment Delivery Ratio, and sediment yield. The computation period is from January 1901 to December 2005, at the scale of the following watersheds: Ciujung, Cisadane, Ciliwung, and Citarum. During the twentieth century, computed average discharge in the downstream area (near Jakarta) increased between 2.5 and 35 m3/s/month, and sediment yield increased between 1 × 103 and 42 × 103 tons/year. These changes were caused by changes in both land cover and climate, with the former playing a stronger role. Based on a computation under a theoretical full implementation of the spatial plan proposed by Perpres 54/2008, river discharge would decrease by up to 5 % in the Ciliwung watershed and 26 % in the Cisadane watershed. The implementation of Perpres 54/2008 could also decrease the sediment yield, by up to 61 and 22 % in the Ciliwung and Cisadane watersheds, respectively. These findings show that the implementation of the spatial plan of Perpres 54/2008 could significantly improve watershed response to runoff and erosion. This study may serve as a tool for assessing the reduction in climate change impacts and evaluating the role of spatial planning for adaptation strategies. 相似文献
149.
150.
Changes to sediment sources following wildfire in a forested upland catchment,southeastern Australia
Hugh G. Smith Gary J. Sheridan Patrick N. J. Lane Philip J. Noske Henk Heijnis 《水文研究》2011,25(18):2878-2889
Few investigations link post‐fire changes to sediment sources and erosion processes with sediment yield response at the catchment scale. This linkage is essential if downstream impacts on sediment transport after fire are to be understood in the context of fire effects across different forest environments. In this study, we quantify changing source contributions to fine sediment (<63 µm) exported from a eucalypt forest catchment (136 ha) burnt by wildfire. The study catchment is one of a pair of research catchments located in the East Kiewa River valley in southeastern Australia that have been the subject of a research program investigating wildfire effects on runoff, erosion, and catchment sediment/nutrient exports. This previous research provided the opportunity to couple insights gained from a range of measurement techniques with the application of fallout radionuclides 137Cs and 210Pbex to trace sediment sources. It was found that hillslope surface erosion dominated exports throughout the 3·5‐year post‐fire measurement period. During this time there was a pronounced decline in the proportional surface contribution from close to 100% in the first six months to 58% in the fourth year after fire. Over the study period, hillslope surface sources accounted for 93% of the fine sediment yield from the burnt catchment. The largest decline in the hillslope contribution occurred between the first and second years after fire, which corresponded with the previously reported large decline in sediment yield, breakdown of water repellency in burnt soils, substantial reduction in hillslope erodibility, and rapid surface vegetation recovery. Coupling the information on sediment sources with hillslope process measurements indicated that only a small proportion of slopes contributed sediment to the catchment outlet, with material derived from near‐channel areas dominating the post‐fire catchment sediment yield response. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
|