全文获取类型
收费全文 | 425篇 |
免费 | 80篇 |
国内免费 | 77篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 160篇 |
地球物理 | 189篇 |
地质学 | 100篇 |
海洋学 | 34篇 |
天文学 | 19篇 |
综合类 | 14篇 |
自然地理 | 31篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 19篇 |
2017年 | 29篇 |
2016年 | 16篇 |
2015年 | 23篇 |
2014年 | 17篇 |
2013年 | 29篇 |
2012年 | 20篇 |
2011年 | 20篇 |
2010年 | 22篇 |
2009年 | 20篇 |
2008年 | 30篇 |
2007年 | 26篇 |
2006年 | 27篇 |
2005年 | 23篇 |
2004年 | 15篇 |
2003年 | 19篇 |
2002年 | 8篇 |
2001年 | 19篇 |
2000年 | 13篇 |
1999年 | 17篇 |
1998年 | 17篇 |
1997年 | 21篇 |
1996年 | 18篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有582条查询结果,搜索用时 500 毫秒
501.
The California Cooperative Oceanic Fisheries Investigations (CalCOFI) program has been systematically sampling zooplankton off the west coast of North America since 1949. In 1978, the 1-m diameter ring net used by the program was replaced with a bongo net, which consists of two 0.71-m diameter nets on a single frame. This study compares paired zooplankton samples taken with a ring net and a 0.71-m or 0.6-m bongo net to determine the relative performances of the two net types for catching calanoid copepods. Thirty-one species and stages were enumerated, along with the category ‘total female calanoids’. Twenty-one categories of calanoid copepods were abundant enough to test for effects of changes in net type. No significant differences between the nets were found after correcting for multiple testing. Statistical power was then estimated for a range of potential net effects equivalent to ratios of copepod densities between the nets of 1.1–3.0. The probability of detecting differences greater than a factor of 1.5–3.0 was high (≥80%) for total female calanoids, Metridia pacifica, Pleuromamma abdominalis edentata, P. borealis, Calanus pacificus, Eucalanus californicus and Rhincalanus nasutus. For these categories of copepods, any population changes greater than a factor of 1.5–3.0 that might be found from the CalCOFI data set can be assumed to be the result of factors other than the change in net type. 相似文献
502.
503.
504.
Zafer Ekinci Enes ŞayanAyşe Vildan Beşe Osman Nuri Ata 《International Journal of Mineral Processing》2007
In this study, the boric acid extraction from colemanite ore in aqueous media saturated by CO2 and SO2 gases was studied and the effects of relevant parameters, namely; reaction temperature, solid-to-liquid ratio, mean particle size, stirring speed and reaction time have been investigated on the boric acid extraction from colemanite ore by using the fractional factorial design and central composite design methods. The chosen experimental parameter levels were as follows: reaction temperature, 11.4–58.6 °C; solid-to-liquid ratio, 0.0685–0.1315 g/mL; mean particle size, 0.2835–3 mm; stirring speed, 107–893 rpm; reaction time, 7.125–22.875 min. A model has been developed between the boric acid extraction efficiency from colemanite ore and relevant parameters by means of variance analysis by using the matlab computer software and the obtained model was used to determine optimum conditions. The optimum conditions were found to be as follows: reaction temperature, 41 °C; solid-to-liquid ratio, 0.0685 g/mL; mean particle size, 0.2835 mm; stirring speed, 266 rpm; reaction time, 7 min. The calculated boric acid extraction efficiency from colemanite ore was approximately 99.9% under the optimum conditions. 相似文献
505.
Well-developed karst aquifers tend to be heterogeneous and consist of variable porosities. Groundwater monitoring and the
associated data interpretations in such aquifers are often more complicated than porous medium aquifers. Collection of representative
data in karst aquifers often requires monitoring at appropriately located wells and/or springs that are proven to connect
to the groundwater system. Water samples are to be collected under different flow conditions, including base flow, high-flow,
and low-flow. The sampling frequencies may vary from several months for base flows to minutes in response to recharge events.
The groundwater monitoring program presented in this paper is for a cement kiln dust mono-fill site in a karst area of southern
Indiana. Following dye tracing and extensive geophysical investigations, one spring was selected as a monitoring location.
A second spring should be used as a monitoring location when the last cell of the mono-fill begins receiving the wastes. The
paper discusses results from the first spring, at which nine background sampling events were completed to evaluate the natural
variations of the water quality. Based on the background data, a statistical evaluation plan was developed for 12 water-quality
parameters to determine the integrity of the landfill. The statistical power of the statistical analyses was evaluated by
Monte Carlo simulations. 相似文献
506.
The electricity generation capacity in the Limay River basin is approximately 26% of the total electrical power generation
in Argentina. Assessing the potential effects of climate change on the hydrological regime of this basin is an important issue
for water resources management. This study explores the presence of trends in streamflow series, evaluates climate sensitivity
and studies the effects on the flow regime of predicted changes in precipitation in the basin. In order to identify and quantify
changes in observed streamflow series, the Mann–Kendall test, with a modification for autocorrelated data, and an estimator
of the magnitude of the trend are applied. In order to evaluate the sensitivity of streamflow to changes in climate, the concept
of elasticity is used. Precipitation elasticity of streamflow is used to quantify the sensitivity of streamflow to changes
in precipitation and is estimated using a power law model and a linear statistical model in two sub-basins, Aluminé and Nahuel
Huapi. The effects on flow regime of the predicted changes in precipitation under different scenarios are studied. Climatic
results for different scenarios of growth in greenhouse gases from some General Circulation Models are used as inputs into
the proposed models. The analysis identifies decreasing trends in mean and minimum annual flows and in the low flow season.
The estimates of the precipitation elasticity imply that changes in precipitation produce similar changes in streamflow and
the climatic results for different scenarios show that the variations are moderate. 相似文献
507.
Several downscaling techniques, comprising fully dynamical and statistical–dynamical methods applied to near-shore local wave climate, are tested and assessed in terms of wave statistics with respect to the added value that can be achieved compared to larger scale data. The techniques are applied for the example of Helgoland, a small island in the German Bight. It was found that an improved representation could generally be obtained from all downscaling techniques by comparing the near-shore wave climate. Based on a balance between the required computer resources and the improvements achieved, it is suggested, to this end, that a dynamical–statistical approach based on high-resolution coastal wave modeling and linear regression provides the optimal choice. 相似文献
508.
This paper reviews the developments of the singular evolutive extended Kalman (SEEK) filter method used for data assimilation
in oceanography, since the original paper by Pham et al. (J Mar Syst 16:3–4, 323–340 1998a). First, a short review is presented of the context of data assimilation in oceanography and of the variety of numerical
ocean codes and configurations in which the SEEK filter has been implemented using different data sets for assimilation. Then,
the paper provides an exposition of the different versions of the SEEK filter developed during the past 10 years and discusses
their relative merits for scientific or operational implementations. A classification of the algorithmic variants is proposed,
and several possible improvements of the generic methodology are mentioned in the perspective of new assimilation challenges. 相似文献
509.
Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model. 相似文献
510.
The purpose of this study is to investigate the statistical relationship between hardness value and physicomechanical properties of constructional and cover rocks. The definition, measurement of hardness and classification of the rocks used are very important in construction sector. From this point of view, rock hardness is one of the most important parameters for the determination of rock properties. In this study, the determination of hardness and the physicomechanical properties of constructional and cover rocks in Çukurova region was accomplished using various methods in the laboratory. Statistical relations between physicomechanical properties and hardness of rocks were also determined. High correlations were found between the hardness methods (Shore Scleroscope, Schmidt hammer hardness), which are cheap and easy to use, and other physical and mechanical properties. It was found that physicomechanical properties can be estimated using hardness methods and compared with the calculated value from different empirical equations. 相似文献