全文获取类型
收费全文 | 2945篇 |
免费 | 542篇 |
国内免费 | 779篇 |
专业分类
测绘学 | 223篇 |
大气科学 | 525篇 |
地球物理 | 701篇 |
地质学 | 1673篇 |
海洋学 | 393篇 |
天文学 | 161篇 |
综合类 | 221篇 |
自然地理 | 369篇 |
出版年
2024年 | 19篇 |
2023年 | 55篇 |
2022年 | 167篇 |
2021年 | 161篇 |
2020年 | 146篇 |
2019年 | 206篇 |
2018年 | 175篇 |
2017年 | 159篇 |
2016年 | 164篇 |
2015年 | 166篇 |
2014年 | 173篇 |
2013年 | 243篇 |
2012年 | 200篇 |
2011年 | 181篇 |
2010年 | 172篇 |
2009年 | 169篇 |
2008年 | 138篇 |
2007年 | 143篇 |
2006年 | 130篇 |
2005年 | 89篇 |
2004年 | 67篇 |
2003年 | 71篇 |
2002年 | 84篇 |
2001年 | 86篇 |
2000年 | 91篇 |
1999年 | 103篇 |
1998年 | 102篇 |
1997年 | 83篇 |
1996年 | 85篇 |
1995年 | 68篇 |
1994年 | 55篇 |
1993年 | 56篇 |
1992年 | 56篇 |
1991年 | 36篇 |
1990年 | 36篇 |
1989年 | 26篇 |
1988年 | 19篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 13篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 8篇 |
1976年 | 2篇 |
1969年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有4266条查询结果,搜索用时 31 毫秒
991.
本文系根据1980~1989年福建省在台湾海峡生产的48组具有代表性的标本船之技术经济资料,以柯布·道格拉斯生产函数和经济指标评析其生产状况及技术经济效果。结果表明,近年来,福建省灯光围网渔船的宏观技术水平已明显提高;然而因其捕捞努力量大幅度增加,单位捕捞努力量渔获量曲线呈抛物线状,其长期趋势为CPUE=-0.311+0.9032f-0.06397f~2;预测投资在功率范围为147~294kW的灯光圈网渔船在台湾海峡生产可望受益。 相似文献
992.
Some observational results of sea storm current 总被引:3,自引:0,他引:3
INTRODUCTIONDr. Hollister, a marine geologist of the Woods Hole Oceanographic Institution, firstpointed out that there was ocean storm current in the ocean. He found out the wavy texture inthe seabed core samples, and suggested that this wavy texture was caused by the high speedsea current in remote antiquity. He then suggested a bold hypothesis that there existed a benthic storm current near the ocean bottom, and presented this hypothesis at the IUGG confer-ence held at San Francisco… 相似文献
993.
Nanxiaohong and Nangang main south channel are chosen as the typical flood and ebb channels. Hydrodynamics analysis based on field hydrological and sediment data is conducted with Gao-Collins model to analyse sediment transport trends. Also, the grain size distribution analysis of the bottom sediment sampled in Sep. 2001 is used as the base of the analysis. The result shows that the sediment in Nanxiaohong is from the rive mouth area. The sediment transports upwards with the flood flow which is stronger than the ebb flow, i.e., in the direction of SE-WN. The sediment in main south channel comes from upward. They transport downwards with the ebb flow. which is stronger than the flood flow, i.e., in the direction of WN-SE. The directions, sources and mechanism of sediment transport are identified according to comprehensive analyses of the observed data on hydrodynamics and sediment. 相似文献
994.
The ages of coral reef samples from several shallow drill holes in the South China Sea are determined by ESR and U-series (230Th/234U) methods. The experimental results show ideal agreement between ESR and U-series ages and that the coral reefs were formed in the early Holocene. In the determination of natural total dose of coral reef by use of the additive dose method good results can be obtained by exponential fitting, no matter how the effect of dose saturation is. It was found that the ratio of the natural ESR signal intensity ( I0) to the ESR signal intensity at dose saturation ( Imax) of sample can reflect the significance of a irradiation efficiency-k value. Using the k value of sample determined by the formula given in this paper, the precision of ESR dating of marine carbonates can be improved. 相似文献
995.
996.
Multi‐model ensemble prediction of terrestrial evapotranspiration across north China using Bayesian model averaging 下载免费PDF全文
Gaofeng Zhu Xin Li Kun Zhang Zhenyu Ding Tuo Han Jinzhu Ma Chunlin Huang Jianhua He Ting Ma 《水文研究》2016,30(16):2861-2879
Using high‐quality dataset from 12 flux towers in north China, the performance of four evapotranspiration (ET) models and the multi‐model ensemble approaches including the simple averaging (SA) and Bayesian model average (BMA) were systematically evaluated in this study. The four models were the single‐layer Penman–Monteith (P–M) model, the two‐layer Shuttleworthe–Wallace (S–W) model, the advection–aridity (A–A) model, and a modified Priestley–Taylor (PT‐JPL). Based on the mean value of Taylor skill (S) and the regression slope between measured and simulated ET values across all sites, the order of overall performance of the individual models from the best to the worst were: S–W (0.88, 0.87), PT‐JPL (0.80, 1.17), P–M (0.63, 1.73) and A–A (0.60, 1.68) [statistics stated as (Taylor skill, regression slope)]. Here, all models used the same values of parameters, LAI and fractional vegetation cover as well as the forcing meteorological data. Thus, the differences in model performance were mainly attributed to errors in model structure. To the ensemble approach, the BMA method has the advantage of generating more skillful and reliable predictions than the SA scheme. However, successful implementation of BMA requires accurate estimates of its parameters, and some degradation in performance were observed when the BMA parameters generated from the training period were used for the validation period. Thus, it is necessary to explore the seasonal variations of the BMA parameters according the different growth stages. Finally, the optimal conditional density function of half‐hourly ET approximated well by the double‐exponential distribution. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
997.
Graphical methods have been widely used for visualization, classification, and interpretation of aqueous geochemical data to obtain a better understanding of surface and subsurface hydrologic systems. This method note presents WQChartPy, an open-source Python package developed to plot a total of 12 diagrams for analysis of aqueous geochemical data. WQChartPy can handle various data formats including Microsoft Excel, comma-separated values (CSV), and general delimited text. The 12 diagrams include eight traditional diagrams (trilinear Piper, Durov, Stiff, Chernoff face, Schoeller, Gibbs, Chadha, and Gaillardet) and four recently proposed diagrams (rectangle Piper, color-coded Piper, contour-filled Piper, and HFE-D) that have not been implemented in existing graphing software. The diagrams generated by WQChartPy can be saved as portable network graphics (PNG), scalable vector graphics (SVG), or portable document format (PDF) files for scientific publications. Jupyter and Google Colab notebooks are available online to illustrate how to use WQChartPy with example datasets. The geochemical diagrams can be generated with several lines of Python codes. Source codes of WQChartPy are publicly available at GitHub ( https://github.com/jyangfsu/WQChartPy ) and PyPI ( https://pypi.org/project/wqchartpy/ ). 相似文献
998.
999.
The seasonal variability of leaf litter removal by crabs was observed from May 2006 to April 2007 in a Kandelia candel mangrove forest in Jiulongjiang Estuary, China. Daily average quantities of leaf fall ranged 0.85–3.86 gDW m−2 d−1, with high values in May, August, October and November. The whole-year's leaf fall was 6.48 t ha−1 yr−1 (1.81 gDW m−2 d−1). The standing stock of leaf litter on the forest floor was 7.78 gDW m−2 averaged from the whole year's data, with the lowest value in December (1.23 gDW m−2) and the highest in April (16.18 gDW m−2). Annually averaged removal (consumption on mangrove floor + burial in burrows) rate of leaf litter by crabs was 0.59 gDW m−2 d−1. High seasonal variability was observed in the removal rates of leaf litter by crabs. Removal rates in the winter months (December, January and February) were 0.07–0.09 gDW m−2 d−1, much lower than those in other months with values of 0.59–1.18 gDW m−2 d−1. Annually averaged percentage of leaf fall removed by crabs was 33%, with the highest values in September (reached 76%) and the lowest values in winter months. Of leaf litter removed by crabs, a large proportion was buried by crabs, and only 12% was consumed by crabs on the forest floor. Leaf litter removal rate, consumption rate on the forest floor, percentages of leaf fall and standing stock removed on the forest floor were significantly positively correlated with air temperature, indicating that leaf removal ability by crabs was higher in warm months than in cold months. 相似文献
1000.
基于空域的帧内预测是H.264采用的一项重要技术,对编码性能的提高具有重要作用。但是由于支持的帧内预测模式较多,使用率失真优化模型计算的编码复杂度很高。利用像素边缘方向信息,相邻块预测模式间的相关性提出了一种帧内模式选择快速算法,可以预先排除约60%的预测模式,从而减小对率失真优化模型的调用。实验结果表明,算法在基本保持H.264原算法性能的同时,编码时间减少70%以上。 相似文献