全文获取类型
收费全文 | 3138篇 |
免费 | 555篇 |
国内免费 | 781篇 |
专业分类
测绘学 | 244篇 |
大气科学 | 995篇 |
地球物理 | 765篇 |
地质学 | 1489篇 |
海洋学 | 509篇 |
天文学 | 22篇 |
综合类 | 149篇 |
自然地理 | 301篇 |
出版年
2024年 | 12篇 |
2023年 | 16篇 |
2022年 | 79篇 |
2021年 | 80篇 |
2020年 | 94篇 |
2019年 | 130篇 |
2018年 | 86篇 |
2017年 | 131篇 |
2016年 | 123篇 |
2015年 | 132篇 |
2014年 | 152篇 |
2013年 | 181篇 |
2012年 | 138篇 |
2011年 | 149篇 |
2010年 | 143篇 |
2009年 | 215篇 |
2008年 | 221篇 |
2007年 | 247篇 |
2006年 | 209篇 |
2005年 | 221篇 |
2004年 | 199篇 |
2003年 | 156篇 |
2002年 | 164篇 |
2001年 | 141篇 |
2000年 | 158篇 |
1999年 | 135篇 |
1998年 | 107篇 |
1997年 | 111篇 |
1996年 | 114篇 |
1995年 | 79篇 |
1994年 | 91篇 |
1993年 | 62篇 |
1992年 | 55篇 |
1991年 | 42篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 16篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
排序方式: 共有4474条查询结果,搜索用时 343 毫秒
991.
Jonathan D. Phillips 《水文研究》2008,22(14):2424-2437
Instream flow science and management requires identification of characteristic hydrological, ecological, and geomorphological attributes of stream reaches. This study approaches this problem by identifying geomorphic transition zones along the lower Sabine River, Texas and Louisiana. Boundaries were delineated along the lower Sabine River valley based on surficial geology, valley width, valley confinement, network characteristics (divergent versus convergent), sinuousity, slope, paleomeanders, and point bars. The coincidence of multiple boundaries reveals five key transition zones separating six reaches of distinct hydrological and geomorphological characteristics. Geologic controls and gross valley morphology play a major role as geomorphic controls, as does an upstream‐to‐downstream gradient in the importance of pulsed dam releases, and a down‐to‐upstream gradient in coastal backwater effects. Geomorphic history, both in the sense of the legacy of Quaternary sea level changes, and the effects of specific events such as avulsions and captures, are also critical. The transition zones delineate reaches with distinct hydrological characteristics in terms of the relative importance of dam releases and coastal backwater effects, single versus multi‐channel flow patterns, frequency of overbank flow, and channel‐floodplain connectivity. The transitional areas also represent sensitive zones which can be expected to be bellwethers in terms of responses to future environmental changes. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
992.
In this paper, a three‐dimensional smooth particle hydrodynamics (SPH) simulator for modeling grain scale fluid flow in porous media is presented. The versatility of the SPH method has driven its use in increasingly complex areas of flow analysis, including the characterization of flow through permeable rock for both groundwater and petroleum reservoir research. SPH provides the means to model complex multi‐phase flows through such media; however, acceptance of the methodology has been hampered by the apparent lack of actual verification within the literature, particulary in the three‐dimensional case. In this paper, the accuracy of SPH is addressed via a comparison to the previously recognized benchmarks of authors such as Sangani and Acrivos (Int. J. Multiphase Flow 1982; 8 (4): 343–360), Zick and Homsy (J. Fluid Mech. 1982; 115 :13–26) and Larson and Higdon (Phys. Fluids A 1989; 1 (1):38–46) for the well‐defined classical problems of flow through idealized two‐ and three‐dimensional porous media. The accuracy of results for such low Reynolds number flows is highly dependent on the implementation of no‐slip boundary conditions. A new, robust and numerically efficient, method for implementing such boundaries in SPH is presented. Simulation results for friction coefficient and permeability are shown to agree well with the available benchmarks. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
993.
The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent observations and theoretical considerations revealed that the formation of the cusp cannot be treated separately from the processes along the whole dayside magnetopause and that the plasma in regions like cleft or low-latitude boundary layer is of the same origin. Our review of statistical results as well as numerous case studies identified the anti-parallel merging at the magnetopause as the principal source of the magnetosheath plasma in all altitudes. Since effective merging requires a low plasma speed at the reconnection spot, we have found that the magnetopause shape and especially its indentation at the outer cusp is a very important part of the whole process. The plasma is slowed down in this indentation and arising multiscale turbulent processes enhance the reconnection rate. 相似文献
994.
为满足合肥市大房郢水库建设及土地管理部门对用地资井的要求,利用航空遥感片对用地现状进行了调查,采用航测解析法配合其它先进测量手段(GPS全球定位系统、TMS全站仪测量系统)确定用地的各类界线。本文介绍了这次工作的技术路线,土地利用现状调查,用地界线的确定,数字影像线划地籍图的编辑及数据汇总等方面的情况。 相似文献
995.
黔南独山泥盆纪露头层序地层及海平面变化研究 总被引:6,自引:0,他引:6
通过独山地区10余条剖面的追踪调查,分析了独地区泥盆系的层序地层。除河湖相沉积的丹林组外,可识别出31个层序,其平均形成时限约1.29Ma,组成4个层序组;在出露较完整的晚泥盆世中可识别出195个副层序,其平均形成时限为102.6ka。层序相当于Ⅲ级T-R旋回,受沉积作用、冰川作用和构造作用等的影响,更明显地受全球性海平面变化的控制,具有全球性对比的意义,副层序,相当于Ⅳ级T-R旋回,与米兰科维奇效应相关。通过生物组合、地质事件和层序地层的对比,下、中统界线应置于海平面明显下降,并且有古风化层的大河口组之上;中、上统界线应置于望地统组第一次最大海泛面,其艾菲尔阶-吉维特阶的界线应置于鸡泡组之下的风化壳,受地壳运动影响、发生于P.varcus带早中期的独山抬升可进行全球对比;泥盆-石炭系界线应置于Cystophrentis-Psudouralina间隔带之中的汤粑沟组第一次最大海侵面,其弗拉斯阶-法门阶界线应置于生物绝灭与复苏的尧梭组四方坡段第一个层序界面。还探讨了该有关矿产矿的形成与海平面变化的联系。 相似文献
996.
997.
998.
Glen Sparrow 《GeoJournal》2001,54(1):73-83
The cities of San Diego and Tijuana have faced each other across the international boundary for over 100 years. The question
raised by this article is whether they comprise a binational city or region. After a brief comparison of their histories,
economies and political systems, a review of some indications of cross border attitudes and a discussion of binational regionalism
as it is impacted by continentalism, it is concluded that the relationship between these cities is driven by factors of economics
not friendship or trust.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
999.
This paper presents laboratory experiments of aerodynamically fully rough, neutral flow over a series of sinusoidal hills. Two sets of hills, with maximum gradients (slopes) of 0.2 (10°) and 0.4 (20°), were considered.The flow remained attached in the former case while separation occurredin the latter. Characteristics of the mean flow and turbulence statistics are discussed and compared with profiles over a flat surface covered with the same roughness as the hills. Comparisons are made with linear theory predictions for the flow in the inner region and aloft. Accurate measurements of the surface pressure were also made, enabling the comparison between the measured pressure drag and predictions from theoretical and computational work with different turbulent closure schemes. Organised secondary flow in the spanwise direction, observed previously in both experimental and computational studies, was also observed here over the small hills. 相似文献
1000.
广西对越边贸口岸的空间分析 总被引:1,自引:0,他引:1
广西是我国对越边贸的主要省区,拥有全方位开发的口岸,由于区位,交通条件的差异性,口岸群的空间结构具有明显的层次,并在边贸业迹上有所表现,为了优化广西边贸口岸的空间结构,文中提出了突出重点把凭祥、东兴建成现代化边贸口岸、培育特色口岸经济,加大边贸的开放力度、提高口岸群的整体水平,以及建立新型的边境贸易空间组织-边境贸易开发等对策建议。 相似文献