全文获取类型
收费全文 | 5809篇 |
免费 | 918篇 |
国内免费 | 908篇 |
专业分类
测绘学 | 1403篇 |
大气科学 | 654篇 |
地球物理 | 1193篇 |
地质学 | 2545篇 |
海洋学 | 755篇 |
天文学 | 254篇 |
综合类 | 346篇 |
自然地理 | 485篇 |
出版年
2024年 | 9篇 |
2023年 | 54篇 |
2022年 | 148篇 |
2021年 | 182篇 |
2020年 | 191篇 |
2019年 | 254篇 |
2018年 | 168篇 |
2017年 | 224篇 |
2016年 | 213篇 |
2015年 | 225篇 |
2014年 | 368篇 |
2013年 | 345篇 |
2012年 | 421篇 |
2011年 | 387篇 |
2010年 | 367篇 |
2009年 | 337篇 |
2008年 | 339篇 |
2007年 | 450篇 |
2006年 | 390篇 |
2005年 | 340篇 |
2004年 | 320篇 |
2003年 | 315篇 |
2002年 | 219篇 |
2001年 | 221篇 |
2000年 | 206篇 |
1999年 | 170篇 |
1998年 | 152篇 |
1997年 | 107篇 |
1996年 | 107篇 |
1995年 | 90篇 |
1994年 | 79篇 |
1993年 | 48篇 |
1992年 | 44篇 |
1991年 | 25篇 |
1990年 | 22篇 |
1989年 | 27篇 |
1988年 | 22篇 |
1987年 | 16篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1977年 | 2篇 |
排序方式: 共有7635条查询结果,搜索用时 31 毫秒
1.
Reservoirs of lowland floodplain rivers with eutrophic backgrounds cause variations in the hydrological and hydraulic conditions of estuaries and low-dam reservoir areas, which can promote planktonic algae to proliferate and algal bloom outbreaks. Understanding the ecological effects of variations in hydrological and hydraulic processes in lowland rivers is important for algal bloom control. In this study, the middle and lower reaches of the Han River, China, a typical regulated lowland river with a eutrophic background, are selected. Based on the effect of hydrological and hydraulic variability on algal blooms, a hydrological management strategy for river algal bloom control is proposed. The results showed that (a) differences in river morphology and background nutrient levels cause significant differences in the critical threshold flow velocities for algal bloom outbreaks between natural river and low-dam reservoir sections; there is no uniform threshold flow velocity for algal bloom control. (b) There are significant differences in the river hydrological/hydraulic conditions between years with and without algal blooms. The average river flow, water level and velocity in years with algal blooms are significantly lower than those in years without algal blooms. (c) For different river sections where algal blooms occur and to meet the threshold flow velocities, the joint operation of cascade reservoirs and diversion projects is an effective method to prevent and control algal blooms in regulated lowland rivers. This study is expected to deepen our understanding of the ecological significance of special hydrological processes and guide algal bloom management in regulated lowland rivers. 相似文献
2.
Study of the flood control scheduling scheme for the Three Gorges Reservoir in a catastrophic flood 下载免费PDF全文
The Three Gorges Project is the world's largest water conservancy project. According to the design standards for the 1,000‐year flood, flood diversion areas in the Jingjiang reach of the Yangtze River must be utilized to ensure the safety of the Jingjiang area and the city of Wuhan. However, once these areas are used, the economic and life loss in these areas may be very great. Therefore, it is vital to reduce this loss by developing a scheme that reduces the use of the flood diversion areas through flood regulation by the Three Gorges Reservoir (TGR), under the premise of ensuring the safety of the Three Gorges Dam. For a 1,000‐year flood on the basis of a highly destructive flood in 1954, this paper evaluates scheduling schemes in which flood diversion areas are or are not used. The schemes are simulated based on 2.5‐m resolution reservoir topography and an optimized model of dynamic capacity flood regulation. The simulation results show the following. (a) In accord with the normal flood‐control regulation discharge, the maximum water level above the dam should be not more than 175 m, which ensures the safety of the dam and reservoir area. However, it is necessary to utilize the flood diversion areas within the Jingjiang area, and flood discharge can reach 2.81 billion m3. (b) In the case of relying on the TGR to impound floodwaters independently rather than using the flood diversion areas, the maximum water level above the dam reaches 177.35 m, which is less than the flood check level of 180.4 m to ensure the safety of the Three Gorges Dam. The average increase of the TGR water level in the Chongqing area is not more than 0.11 m, which indicates no significant effect on the upstream reservoir area. Comparing the various scheduling schemes, when the flood diversion areas are not used, it is believed that the TGR can execute safe flood control for a 1,000‐year flood, thereby greatly reducing flood damage. 相似文献
3.
通过将车流量的增大或减小转化为路长权重的变化。将交通流量的动态问题转化为静态问题,用解决最短路问题的Dijkstra方法,给出交通流量实时最优控制的可行性模型及其有效算法。 相似文献
4.
月日潮汐摩擦和地球惯量矩变化是日长长期变化的主要原因.在本文中,利用最新的地球物理和古生物钟数据,对过去15亿年以来的月日潮汐摩擦、地球惯量矩变化和日长长期变化等作了数值对比研究.由此得到二个重要结论:一是仅利用地球的自转形变不能解释J2的变化,这说明地球的重力分异现象至今仍存在着;其二是在几亿年前的潮汐摩擦比现在大得多,若取潮汐耗散与距离的立方成反比时,理论结果与由古生物钟得到的回归年日数和朔望月日数数据较为符合。 相似文献
5.
本文提出了激变变星0623十71的4个时间分辨光谱,覆盖轨道周期的73%.其中的3个谱清楚地显示了各种光谱特征随轨道运动的变化,在流量、能谱以及谱线轮廓方面的这种变化正是对一个倾角不太大的激变变星所期望的.然而另一个曝光对分钟的谱,流量和视向速度与预期的相比都太大,很难加以解释,可能它是一次象AEAqr中观测到的跃发事件. 相似文献
6.
国家重大科学工程项目LAMOST总控系统的程序设计是一个庞大的软件工程.其总控系统下共包括8个既相互独立又互有关联的子系统.如何通过良好的软件设计方法来处理好这8个子系统之间的关系使其协调运转,以及如何确保软件开发的可维护性、可持续性是研究重点. 相似文献
7.
宣森 《广东海洋大学学报》1993,(1)
将专家系统技术与常规控制、模糊控制、超前控制、仿人智能操作控制以及计算机控制技术相结合,设计了一种新的基于专家系统决策的多模式控制(MCDBES)。经试验和实际运行结果表明,MCDBES系统对未知数学模型的被控对象,具有良好的控制性能。本文提出的MCDBES已成功地应用于燃油陶瓷窑炉计算机控制系统。 相似文献
8.
我们于1989年11月30日晚对PG0027+260进行了时间分辨率为108秒的高速CCD测光,得到了一条完整的光变曲线,从而确认其为激变食变星系统,轨道周期0.146~d 相似文献
9.
Technique for Controlling Spread of Limnotic Oncomelania 总被引:1,自引:0,他引:1
Schistosomiasis is a parasitic disease mostly found in areas along the Changjiang River of China. The disease is spread solely through an intermediary named oncomelania, so its spread of schistosomiasis can be controlled by properly designing water intakes which prevent oncomelania from entering farming land or residential areas. This paper reports a successful design process and a new oncomelania-free intake device. The design of the new intake is based on a sound research program in which extensive experimental studies were carried out to gain knowledge of oncomelania eco-hydraulic behavior and detailed flow field information through CFD simulation. 相似文献
10.
Douglas P. Looze Markus Kasper Stefan Hippler Orhan Beker Robert Weiss 《Experimental Astronomy》2003,15(2):67-88
This paper develops a compensation algorithm based on Linear–Quadratic–Gaussian (LQG) control system design whose parameters
are determined (in part) by a model of the atmosphere. The model for the atmosphere is based on the open-loop statistics of
the atmosphere as observed by the wavefront sensor, and is identified from these using an auto-regressive, moving average
(ARMA) model. The (LQG) control design is compared with an existing compensation algorithm for a simulation developed at ESO
that represents the operation of MACAO adaptive optics system on the 8.2 m telescopes at Paranal, Chile.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献