全文获取类型
收费全文 | 4116篇 |
免费 | 826篇 |
国内免费 | 1007篇 |
专业分类
测绘学 | 288篇 |
大气科学 | 831篇 |
地球物理 | 1191篇 |
地质学 | 2081篇 |
海洋学 | 500篇 |
天文学 | 137篇 |
综合类 | 423篇 |
自然地理 | 498篇 |
出版年
2024年 | 10篇 |
2023年 | 60篇 |
2022年 | 172篇 |
2021年 | 217篇 |
2020年 | 146篇 |
2019年 | 177篇 |
2018年 | 200篇 |
2017年 | 215篇 |
2016年 | 236篇 |
2015年 | 206篇 |
2014年 | 246篇 |
2013年 | 240篇 |
2012年 | 225篇 |
2011年 | 212篇 |
2010年 | 199篇 |
2009年 | 233篇 |
2008年 | 230篇 |
2007年 | 200篇 |
2006年 | 140篇 |
2005年 | 123篇 |
2004年 | 142篇 |
2003年 | 183篇 |
2002年 | 203篇 |
2001年 | 178篇 |
2000年 | 187篇 |
1999年 | 209篇 |
1998年 | 134篇 |
1997年 | 160篇 |
1996年 | 161篇 |
1995年 | 132篇 |
1994年 | 102篇 |
1993年 | 97篇 |
1992年 | 76篇 |
1991年 | 55篇 |
1990年 | 59篇 |
1989年 | 39篇 |
1988年 | 34篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1977年 | 2篇 |
1962年 | 4篇 |
1958年 | 4篇 |
1957年 | 2篇 |
1941年 | 1篇 |
排序方式: 共有5949条查询结果,搜索用时 31 毫秒
991.
We examined the structure and spatial organization of ciliature base-associated microtubules (BAM) in three hypotrichous ciliates
(Stylonychia mytilus, Pseudourostyla cristata, Euplotes woodruffi) in fluorescence microscopy. The results revealed that BAM, including the anterior (ALM), posterior longitudinal microtubule
(PLM) and the transverse microtubule (TM) bands, are composed of tubulin. The respective microtubular bands have cytoplasmic
polarization patterns that are significantly asymmetric. The BAM of the midventral files in P. cristata appear cord-shaped compared with the ALM bands of transverse cirri in both S. mytilus and E. woodruffi, which extend to the left anterior side of the cell before converging. The TM bands of the left marginal cirri (MC) in S. mytilus extend to the right side of the cell, while those of the right MC bands extend to the left. Our observations suggest that
BAM traits are common in hypotrichous ciliates even though different species possess different microtubule arrangements related
to the conserved cirral morphogenetic patterns in the respective species. The differing development of BAM in the three ciliate
suggests that the microtubules may be conserved in different hypotrichs. We have also demonstrated that the BAM, which appear
polar and asymmetric, are localized in specific cytoskeletal positions and extend in different orientations within the cortex
to connect with other ciliature-associated structures and, thus, strengthen the cortex. These BAM features indicate that they
are directly associated with cell motion. 相似文献
992.
We deployed two ADCP mooring systems west of the Luzon Strait in August 2008, and measured the upper ocean currents at high
frequency. Two typhoons passed over the moorings during approximately one-month observation period. Using ADCP observations,
satellite wind and heat flux measurements, and high-resolution model assimilation products, we studied the response of the
upper ocean to typhoons. The first typhoon, Nuri, passed over one of the moorings, resulting in strong Ekman divergence and
significant surface cooling. The cooling of surface water lagged the typhoon wind forcing about one day and lasted about five
days. The second typhoon, Sinlaku, moved northward east of the Luzon Strait, and did not directly impact currents near the
observation regions. Sinlaku increased anomalous surface water transport exchange across the Luzon Strait, which modulated
the surface layer current of the Kuroshio. 相似文献
993.
A comparative study of numerical approaches to risk assessment of contaminant transport 总被引:3,自引:2,他引:1
Dongxiao Zhang Liangsheng Shi Haibin Chang Jinzhong Yang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(7):971-984
In risk analysis, a complete characterization of the concentration distribution is necessary to determine the probability
of exceeding a threshold value. The most popular method for predicting concentration distribution is Monte Carlo simulation,
which samples the cumulative distribution function with a large number of repeated operations. In this paper, we first review
three most commonly used Monte Carlo (MC) techniques: the standard Monte Carlo, Latin Hypercube sampling, and Quasi Monte
Carlo. The performance of these three MC approaches is investigated. We then apply stochastic collocation method (SCM) to
risk assessment. Unlike the MC simulations, the SCM does not require a large number of simulations of flow and solute equations.
In particular, the sparse grid collocation method and probabilistic collocation method are employed to represent the concentration
in terms of polynomials and unknown coefficients. The sparse grid collocation method takes advantage of Lagrange interpolation
polynomials while the probabilistic collocation method relies on polynomials chaos expansions. In both methods, the stochastic
equations are reduced to a system of decoupled equations, which can be solved with existing solvers and whose results are
used to obtain the expansion coefficients. Then the cumulative distribution function is obtained by sampling the approximate
polynomials. Our synthetic examples show that among the MC methods, the Quasi Monte Carlo gives the smallest variance for
the predicted threshold probability due to its superior convergence property and that the stochastic collocation method is
an accurate and efficient alternative to MC simulations. 相似文献
994.
Xiao Fu Jia-Fu Zhang Duo-Wen Mo Chen-Xi Shi Hui Liu Yi-Yin Li Li-Ping Zhou 《Quaternary Geochronology》2010,5(2-3):353-359
The Qujialing site is a representative Neolithic archaeological site in the middle reaches of the Yangtze River, China. Absence of suitable material for radiocarbon dating in this region makes the timing of the similar sites difficult. Here we applied optically stimulated luminescence (OSL–SAR) and thermoluminescence (TL–SAR) techniques to date the archaeological and natural deposits from the Qujialing site with known age, testing the techniques on samples at archaeological sites in this region. The results showed that the luminescence properties of quartz from sediment and baked earth samples are very similar. The quartz OSL ages obtained for a sediment sample and a baked earth sample from the cultural layer are 5.4 ± 0.3 and 5.1 ± 0.3 ka, respectively. The quartz TL age of the baked earth sample is 5.6 ± 0.5 ka. These dates are consistent with the calibrated radiocarbon ages (4.9 ± 0.1 and 5.1 ± 0.1 ka cal BP (±1σ)) of the two charcoal samples from the cultural layer at a nearby locality, and are also in agreement with the age of Qujialing culture period. The results indicate that the OSL dating techniques can be applied to date similar archaeological sites in the middle reaches of the Yangtze River, China. 相似文献
995.
Method for calibrating a theoretical model in karst springs: an example for a hydropower station in South China 总被引:1,自引:0,他引:1 下载免费PDF全文
Mingming Luo Zhihua Chen Robert E. Criss Hong Zhou Hamza Jakada Tingting Shi 《水文研究》2016,30(25):4815-4825
A theoretical, dimensionless rainfall–runoff model was used to simulate the discharge of Wulongdong spring in western Hubei province, South China. The single parameter (time constant τ) in the model is easy to obtain by fitting the recession rate of the observed hydrographs. The model was scaled by simply matching the total annual flow volume of the model to the observed value. Annual distribution of actual evapotranspiration was embedded in the model input to calculate the accumulated deficit of soil moisture before each rain event. Hourly precipitation input data performed better than daily data, defining τ of 0.85 days and returning a Nash–Sutcliffe efficiency of 0.89 and the root mean square error of 0.07. This model offers an effective way to simulate the discharge of karst springs that respond sensitively to rainfall events. The model parameters of a successful simulation can be used to estimate the recharge area and indicate the intrinsic response time of the basin. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
996.
997.
Contribution of climatic and technological factors to crop yield: empirical evidence from late paddy rice in Hunan Province,China 总被引:2,自引:1,他引:1
Zhu Wang Tao Ye Jun Wang Zhuo Cheng Peijun Shi 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(7):2019-2030
Climatic and technological factors are two remarkable aspects that are thought to contribute to crop yield change. However, the most significant factors and their contribution rate remain debatable. Selecting Hunan Province, which is one of the largest paddy rice producing regions in China as the research area, the marginal contributions of climatic and technological factors to late paddy yield change are estimated using a county-level panel data regression model with explicit consideration of technological variables during 2000–2011. The results indicate that the mean daily temperature and adoption rate of hybrid rice have significant positive impacts on late paddy rice yield. During the research period, 1 °C temperature increase will cause 7.220 t/km2 increase in late paddy rice yield, and 1 % increase in the adoption rate of hybrid rice will cause 0.212 t/km2 increase. Climatic factors, especially their annual variability, exhibit a stronger overall linkage to changes in late paddy rice yield in the study area compared with the technological factors. Climatic factors accounted for 67.645 and 74.585 % of the trend and variability in late paddy rice yield, respectively. The results of this study can provide valuable information for stakeholders to adjust the input on late paddy rice production and better adapt to the effect of climate change. 相似文献
998.
Tao Gao Xiaohui Shi 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(5):1421-1440
Analyses of the spatio-temporal variability of precipitation extremes defined by eleven extreme precipitation indices in Shandong were conducted by utilizing the methods of linear regression, ensemble empirical mode decomposition (EEMD) and Mann–Kendall test. The results revealed that statistically significant decreasing trends existed for almost all extreme precipitation indices except for the consecutive dry days (CDD) and simple daily intensity index. A periodicity of 10–15 years for precipitation extremes is detected by EEMD analysis. Greatest 5-day total rainfall (RX5day), very wet days (R95p) and annual total wet-day precipitation (PRCPTOT) experienced decreasing trends in the region stretching from the southeast coast to the west, while the spatial distribution of the decreasing trends for other indices was more complicated. Moreover, the frequency of occurrence in precipitation extremes at Changdao station, surrounded by the sea in the northeast region, increased in contrast to surrounding stations. This may suggest a possible effect from the local marine environment on extreme precipitation. In addition, the stations with statistically significant positive trends for CDD were mainly located in mid-west Shandong and along the southeast coast, where the extreme precipitation and total rainfall were, on the contrary, characterized by decreasing trends. These results indicate that drought or severe drought events have become more frequent in those regions. Analysis of large-scale atmospheric circulation changes indicates that a strengthening anticyclonic circulation and increasing geopotential height as well as decreasing strength of monsoonal flow in recent decades may have contributed to the variations in extreme precipitation in Shandong. 相似文献
999.
Hu Yang Jiping Liu Gerrit Lohmann Xiaoxu Shi Yongyun Hu Xueen Chen 《Ocean Dynamics》2016,66(3):353-365
Three prominent features of ocean surface turbulent heat fluxes (THF) trends during 1958–2013 are identified based on the Objectively Analyzed air-sea Fluxes (OAFlux) data set. The associated ocean-atmosphere dynamics changes are further investigated based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. First, the THF are enhanced over the mid-latitude expansions of the subtropical western boundary currents (WBCs). An intensified oceanic heat transport, forced by stronger near-surface zonal wind, is likely to be the cause of such THF tendency. Second, the THF are reduced over the tropical eastern Pacific Ocean, which is primarily caused by the decreasing near-surface wind speed and sea surface temperature (SST), associated with a local coupled ocean-atmosphere cooling mode. Finally, the THF are reduced over the northern tropical Atlantic Ocean, which is attributed to the decreasing air-sea humidity and temperature differences as a result of the convergence of near-surface air and the divergence of ocean currents (upwelling). 相似文献
1000.
At the early stages of deep-water oil exploration and development, fewer and further apart wells are drilled than in onshore oilfields. Supervised least squares support vector machine algorithms are used to predict the reservoir parameters but the prediction accuracy is low. We combined the least squares support vector machine (LSSVM) algorithm with semi-supervised learning and established a semi-supervised regression model, which we call the semi-supervised least squares support vector machine (SLSSVM) model. The iterative matrix inversion is also introduced to improve the training ability and training time of the model. We use the UCI data to test the generalization of a semi-supervised and a supervised LSSVM models. The test results suggest that the generalization performance of the LSSVM model greatly improves and with decreasing training samples the generalization performance is better. Moreover, for small-sample models, the SLSSVM method has higher precision than the semi-supervised K-nearest neighbor (SKNN) method. The new semisupervised LSSVM algorithm was used to predict the distribution of porosity and sandstone in the Jingzhou study area. 相似文献