全文获取类型
收费全文 | 1647篇 |
免费 | 394篇 |
国内免费 | 356篇 |
专业分类
测绘学 | 191篇 |
大气科学 | 183篇 |
地球物理 | 445篇 |
地质学 | 977篇 |
海洋学 | 240篇 |
天文学 | 31篇 |
综合类 | 142篇 |
自然地理 | 188篇 |
出版年
2024年 | 4篇 |
2023年 | 38篇 |
2022年 | 54篇 |
2021年 | 57篇 |
2020年 | 76篇 |
2019年 | 76篇 |
2018年 | 68篇 |
2017年 | 85篇 |
2016年 | 95篇 |
2015年 | 97篇 |
2014年 | 110篇 |
2013年 | 126篇 |
2012年 | 134篇 |
2011年 | 140篇 |
2010年 | 84篇 |
2009年 | 124篇 |
2008年 | 94篇 |
2007年 | 116篇 |
2006年 | 113篇 |
2005年 | 92篇 |
2004年 | 87篇 |
2003年 | 67篇 |
2002年 | 64篇 |
2001年 | 45篇 |
2000年 | 47篇 |
1999年 | 42篇 |
1998年 | 34篇 |
1997年 | 37篇 |
1996年 | 34篇 |
1995年 | 28篇 |
1994年 | 17篇 |
1993年 | 16篇 |
1992年 | 23篇 |
1991年 | 9篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2397条查询结果,搜索用时 15 毫秒
991.
992.
Snow cover and runoff modelling in a high mountain catchment with scarce data: effects of temperature and precipitation parameters 下载免费PDF全文
Fan Zhang Hongbo Zhang Scott C. Hagen Ming Ye Dingbao Wang Dongwei Gui Chen Zeng Lide Tian Jingshi Liu 《水文研究》2015,29(1):52-65
Snowmelt is an important source of runoff in high mountain catchments. Snowmelt modelling for alpine regions remains challenging with scarce gauges. This study simulates the snowmelt in the Karuxung River catchment in the south Tibetan Plateau using an altitude zone based temperature‐index model, calibrates the snow cover area and runoff simulation during 2003–2005 and validates the model performance via snow cover area and runoff simulation in 2006. In the snowmelt and runoff modelling, temperature and precipitation are the two most important inputs. Relevant parameters, such as critical snow fall temperature, temperature lapse rate and precipitation gradient, determine the form and amount of precipitation and distribution of temperature and precipitation in hydrological modelling of the sparsely gauged catchment. Sensitivity analyses show that accurate estimation of these parameters would greatly help in improving the snowmelt simulation accuracy, better describing the snow‐hydrological behaviours and dealing with the data scarcity at higher elevations. Specifically, correlation between the critical snow fall temperature and relative humidity and seasonal patterns of both the temperature lapse rate and the precipitation gradient should be considered in the modelling studies when precipitation form is not logged and meteorological observations are only available at low elevation. More accurate simulation of runoff involving snowmelt, glacier melt and rainfall runoff will improve our understanding of hydrological processes and help assess runoff impacts from a changing climate in high mountain catchments. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
993.
A GPS IF signal computer simulation method is proposed in this article. The carrier Doppler frequency and the total propagation
and delay time can be modeled or calculated with the input GPS satellite ephemeris file. The simulated GPS IF signal outputs
to a text file for post-processing and analysis. The simulation signal spectrum is compared with the received real GPS IF
signal spectrum, and the correctness of the simulation result is verified.
Supported by the Research Fund (2008–2009) of State Key Laboratory of Information Engineering in Surveying, Mapping and Remote
Sensing, the National 863 Program of China (No.2008AA12Z308). 相似文献
994.
Thermal oil recovery processes involve high pressures and temperatures, leading to large volume changes and induced stresses. These cannot be handled by traditional reservoir simulation because it does not consider coupled geomechanics effects. In this paper we present a fully coupled, thermal half‐space model using a hybrid DDFEM method. A finite element method (FEM) solution is adopted for the reservoir and the surrounding thermally affected zone, and a displacement discontinuity method is used for the surrounding elastic, non‐thermal zone. This approach analyzes stress, pressure, temperature and volume change in the reservoir; it also provides stresses and displacements around the reservoir (including transient ground surface movements) in a natural manner without introducing extra spatial discretization outside the FEM zone. To overcome spurious spatial temperature oscillations in the convection‐dominated thermal advection–diffusion problem, we place the transient problem into an advection–diffusion–reaction problem framework, which is then efficiently addressed by a stabilized finite element approach, the subgrid‐scale/gradient subgrid‐scale method. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
995.
基于新参考系的极移改正 总被引:1,自引:0,他引:1
本文利用IAU2000决议中关于CIP的定义阐述了极移坐标系和极移坐标的定义,在总结微分旋转矩阵的性质的基础上给出了极移旋转矩阵的详细推导,结合极移产生的主要原因,详细介绍了目前国际上关于极移模型化工作的最新进展,最后给出了获取极移坐标和TIO位置的方法和途径。 相似文献
996.
IRIS数据流的接收及在福建地震台网中的应用 总被引:2,自引:0,他引:2
为实现网络资源共享及提高闽台地区地震定位精度,研究实现了在SeedLink协议和NetSeis/IP协议间进行实时数据传输的方法,使得可以从IRIS上获得准实时数据用于地震速报任务。该方法的应用提高了区域地震台网实时速报系统对中国台湾及台湾海峡地区地震的定位精度。 相似文献
997.
Adrián Pedrozo-Acuña Alec Torres-Freyermuth Qingping Zou Tian-Jian Hsu Dominic E. Reeve 《Coastal Engineering》2010
This paper presents an integrated investigation of physical processes generating impulsive pressures under the action of plunging breakers impinging on gravel beaches. This work is an extension of a recent investigation which suggested that wave impacts from plunging breakers acting on gravel beaches may be a key mechanism to enhance sediment mobilisation. In particular, comparisons of full scale laboratory measurements against model results from a well-validated phase/depth-resolving numerical model based on the Reynolds–Averaged Navier–Stokes (RANS) equations are presented. This represents the first attempt at comparison with such a tool against observed hydrodynamics on steep (slope~1/8) gravel beaches at prototype scale. In order to understand how impulsive pressures are generated under plunging waves, the numerical model is used to carry out a detailed investigation to examine the role of each of the acceleration terms in the momentum balance. Consistent with prior studies, numerical results show that under plunging breakers the local acceleration (∂u/∂t) alone cannot be used as a proxy for pressure gradients. However, the contribution of the third term (w∂u/∂z) of total acceleration is recognized for the first time and indicates that this term has an important role in both the induced pressure gradient and sediment mobilisation as induced by this particular type of breaking. Furthermore, results suggest that a parameterisation of the pressure gradient in terms of ∂u/∂t+u∂u/∂x, may not suffice when dealing with plunging breakers and hence there is a lack of a suitable parameterisation of this process in the present literature. Thus, for different types of breaking it may be necessary to consider a different characterisation of the pressure gradient toward the parameterisation of sediment transport inside the surf zone. 相似文献
998.
在海底地形陡变、垂向密度分层明显的水域,三维σ坐标模式中会出现一种"伪"水平斜压梯度力,并会引起"伪"密度流,以至于影响模拟的精度。垂向上引入双σ坐标变换,建立河口海岸水域三维斜压水流数值模型。数值试验结果表明,在海底地形陡变水域,双σ坐标模式可以减小水平斜压梯度力处理引起的误差。 相似文献
999.
针对影像中梯度较小的模糊边缘,提出了一种自适应梯度的概念和边缘检测算法。基于新的梯度概念,可以根据刀刃曲线的实际形状自适应地计算影像中每一点的梯度,比较理想地消除边缘的渐变效应(模糊)对边缘检测的不利影响。试验表明,这种梯度计算方法对模糊边缘的检测具有较好的效果。 相似文献
1000.
基于梯度幅度直方图和类内方差的边缘提取方法 总被引:1,自引:0,他引:1
针对复杂背景中目标边缘提取的问题,提出一种基于梯度幅度直方图和类内方差进行边缘提取的新方法———CAGH(cluster algorithm based on gradient histogram)算法。该算法先分析经“非最大梯度抑制”后的梯度幅度直方图的特征,确定边缘集中区域,再通过类内方差确定梯度阈值,并利用该阈值确定边缘。在车牌识别中运用该方法提取复杂背景中的车牌边缘,并与Sobel、Canny等算法进行了比较。结果表明,CAGH算法适应性强、提取效率高,提取的是连通性、独立性好的单像素边缘,有利于后续的特征提取和模式识别。 相似文献