全文获取类型
收费全文 | 1536篇 |
免费 | 127篇 |
国内免费 | 117篇 |
专业分类
测绘学 | 265篇 |
大气科学 | 106篇 |
地球物理 | 346篇 |
地质学 | 361篇 |
海洋学 | 293篇 |
天文学 | 34篇 |
综合类 | 71篇 |
自然地理 | 304篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 37篇 |
2021年 | 51篇 |
2020年 | 55篇 |
2019年 | 74篇 |
2018年 | 56篇 |
2017年 | 78篇 |
2016年 | 50篇 |
2015年 | 39篇 |
2014年 | 51篇 |
2013年 | 145篇 |
2012年 | 56篇 |
2011年 | 60篇 |
2010年 | 55篇 |
2009年 | 94篇 |
2008年 | 105篇 |
2007年 | 103篇 |
2006年 | 84篇 |
2005年 | 83篇 |
2004年 | 81篇 |
2003年 | 64篇 |
2002年 | 50篇 |
2001年 | 55篇 |
2000年 | 34篇 |
1999年 | 32篇 |
1998年 | 26篇 |
1997年 | 21篇 |
1996年 | 21篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 13篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1976年 | 2篇 |
排序方式: 共有1780条查询结果,搜索用时 15 毫秒
821.
Assessing the impacts of microtopography on soil erosion under simulated rainfall,using a multifractal approach 下载免费PDF全文
This study aimed to investigate the changing characteristics of microrelief of purple soil and its erosional response during successive stages of water erosion, including splash erosion, sheet erosion, and rill erosion. Methods employed included a rainfall simulator and the use of a laser scanner to generate a digital elevation model. Three artificial tillage practices, including conventional tillage (CT), artificial digging (AD), and ridge tillage (RT), were used to simulate different microrelief patterns. Eighteen artificial rainfall experiments were conducted using three 2 × 1 m boxes with a rainfall intensity of 1.5 mm min?1 on a 15° slope. The results showed that the soil roughness (SR) index values for the tillage slopes were RT > AD > CT. The combined effects of detachment by raindrop impact and transport by run‐off decreased the SR index, whereas rill erosion increased the SR index during rainfall event. Microtopography and drainage networks have strong multifractal behaviours. The multifractal parameters of microtopography reflect the overall characteristics as well as the characteristics of the local soil surface. Within a certain range of threshold values, higher microrelief causes less soil erosion. However, when the parameters of spatial heterogeneity of microtopography exceed the threshold values, a higher degree of microrelief can increase soil erosion. These results help clarify the effect of microtopography on soil erosion and provide a theoretical foundation to guide future tillage practices on sloping farmland of purple soil. 相似文献
822.
???????????????????????Jacobian???????????????????????????????С?????????????????????????С?????????????????????????в?????????С??ж?????????????????????2??????????????????仮??????????????????С??????????н???????????÷???????????????????????????????????????????????????????????????????????????????????????????????????????????С??????????????С?????? 相似文献
823.
824.
台资跨境生产网络的空间重组——电脑企业从珠三角到长三角的转移 总被引:6,自引:1,他引:5
基于从全球生产网络(GPNs) 层面分析区域发展战略性耦合(strategic coupling)的视角,本文探讨自21 世纪初以来,台资跨境生产网络在中国大陆的空间重组,特别是个人电脑 (计算机) 企业从珠三角向长三角地区再布局的动力机制与格局。通过对相关企业的深度访谈与个案研究,指出台资个人电脑企业在珠三角地区与长三角地区的空间再布局是在各区域地方制度下,GPNs 架构中全球领先企业(global lead firms) 与分布在2 个三角洲区域处于 GPNs 较低层级供应企业(supplier firms) 之间进行不同的战略性耦合之结果。研究阐明在经济全球化背景下‘新区域主义’理论对区域发展仅分析区域内部因素之局限性,因应跨地域 (trans-local) 动力的出现,全球生产网络视角下区域内部与外部因素的战略耦合,已成为理解全球经济化下区域发展的重要理论框架。 相似文献
825.
In order to explore the mineral resources buried in sea mud,it is necessary to use seabed resistivity measuring equipment,which works closer to the sediments than ordinary ship-based geophysical measuring equipment. Because of the harsh environment of seafloor,high pressure and highly conductive seawater,marine magnetotelluric method developed slowly. The sea floor environment is similar to the environment of logging, According to the design of dual lateral logging equipment,a new equipment for seafloor electrical resistivity measurement is designed. Four 3D FEM models that contain resistivity abnormal targets are built to test the ability of this equipment to locate different shape of shallow buried resistivity abnormal targets in sea mud. The authors propose the method to correct the response curve while the bottom surface of this equipment is suspended or not parallel to the seafloor. The resistivity of targets can be calculated accurately. 相似文献
826.
827.
Several studies revealed that peak discharges (Q) observed in a nested drainage network following a runoff-generating rainfall event exhibit power law scaling with respect to drainage area (A) as Q(A) = αAθ. However, multiple aspects of how rainfall-runoff process controls the value of the intercept (α) and the scaling exponent (θ) are not fully understood. We use the rainfall-runoff model CUENCAS and apply it to three different river basins in Iowa to investigate how the interplay among rainfall intensity, duration, hillslope overland flow velocity, channel flow velocity, and the drainage network structure affects these parameters. We show that, for a given catchment: (1) rainfall duration and hillslope overland flow velocity play a dominant role in controlling θ, followed by channel flow velocity and rainfall intensity; (2) α is systematically controlled by the interplay among rainfall intensity, duration, hillslope overland flow velocity, and channel flow velocity, which highlights that it is the combined effect of these factors that controls the exact values of α and θ; and (3) a scale break occurs when runoff generated on hillslopes runs off into the drainage network very rapidly and the scale at which the break happens is determined by the interplay among rainfall duration, hillslope overland flow velocity, and channel flow velocity. 相似文献
828.
Evolution,accessibility and dynamics of road networks in China from 1600 BC to 1900 AD 总被引:1,自引:1,他引:0
Before the emergence of modern modes of transport, the traditional road infrastructure was the major historical means of carrying out nationwide socio-economic exchange. However, the history of transport infrastructure has received little attention from researchers. Given this background, the work reported here examined the long-term development of transport networks in China. The national road network was selected for study and the 3500 years from 1600 BC to 1900 AD was chosen as the study period. Indicators were designed for the maturity level of road networks and an accessibility model was developed for the paths of the shortest distance. The evolution of the road network in China since the Shang Dynasty (1600 BC) was described and its major features were summarized to reveal long-term regularities. The maturity level of the road network and its accessibility was assessed and regions with good and poor networks were identified. The relationship between China’s natural, social, and economic systems and the road network were discussed. Our analysis shows that the road network in China has a number of long-term regularities. The continuously expanding road network follows a path of inland expansion especially towards the border areas. However, its coverage and accessibility are characterized by a core-peripheral configuration, which has close relationships with, not only the natural conditions, but also national defense and warfare. The centralization of national power, national land governance, postal transport, the transport of specialized cargos, and international trade are also related to the development of the road network. This research draws attention to the evolving regularities of transport networks. 相似文献
829.
Reconstructing pre-erosion topography using spatial interpolation techniques:A validation-based approach 总被引:1,自引:1,他引:0
Understanding the topographic context preceding the development of erosive landforms is of major relevance in geomorphic research, as topography is an important factor on both water and mass movement-related erosion, and knowledge of the original surface is a condition for quantifying the volume of eroded material. Although any reconstruction implies assuming that the resulting surface reflects the original topography, past works have been dominated by linear interpolation methods, incapable of generating curved surfaces in areas with no data or values outside the range of variation of inputs. In spite of these limitations, impossibility of validation has led to the assumption of surface representativity never being challenged. In this paper, a validation-based method is applied in order to define the optimal interpolation technique for reconstructing pre-erosion topography in a given study area. In spite of the absence of the original surface, different techniques can be nonetheless evaluated by quantifying their capacity to reproduce known topography in unincised locations within the same geomorphic contexts of existing erosive landforms. A linear method (Triangulated Irregular Network, TIN) and 23 parameterizations of three distinct Spline interpolation techniques were compared using 50 test areas in a context of research on large gully dynamics in the South of Portugal. Results show that almost all Spline methods produced smaller errors than the TIN, and that the latter produced a mean absolute error 61.4% higher than the best Spline method, clearly establishing both the better adjustment of Splines to the geomorphic context considered and the limitations of linear approaches. The proposed method can easily be applied to different interpolation techniques and topographic contexts, enabling better calculations of eroded volumes and denudation rates as well as the investigation of controls by antecedent topographic form over erosive processes. 相似文献
830.
Evaluation of digital channel network derivation methods in a glaciated subalpine catchment 下载免费PDF全文
The digital elevation model (DEM) has become an essential tool for an increasing array of mountain runoff analyses, particularly the derivation and mapping of stream channel networks. This study examines how well commonly applied DEM‐based channel derivation methods at different spatial resolutions can represent the channel network for a glaciated Rocky Mountain headwater catchment. The specific objectives are to (1) examine how differences in gridded DEM resolution affect spatially distributed values of local slope, specific contributing area, and topographic wetness index derived from both eight and infinite directional flow algorithms, (2) map the actual stream channel network to examine the influence of surface variables on channel initiation, and (3) assess accuracy of DEM‐derived networks compared with the field surveyed network. Results show that for the same contributing area threshold, increasing grid cell size leads to increased channelization of modeled networks. A plot of local slope versus contributing area reveals a negative relationship similar to that of prior studies in un‐glaciated areas but with breaks in slope at contributing areas that are too small to represent thresholds for channelization. Field survey results and evaluation of DEM‐derived channel networks suggest that channel network formation is not clearly related to surface topographic variables at Loch Vale. Digitally derived channel networks do not accurately predict low order channel locations, but approximations of the channel network with drainage density and headward extent of channelization similar to the observed network can be derived with both a 1 m and 10 m DEM using a contributing area threshold of approximately 4x104 m2. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献