全文获取类型
收费全文 | 24527篇 |
免费 | 181篇 |
国内免费 | 923篇 |
专业分类
测绘学 | 1418篇 |
大气科学 | 1980篇 |
地球物理 | 4537篇 |
地质学 | 11655篇 |
海洋学 | 1012篇 |
天文学 | 1638篇 |
综合类 | 2163篇 |
自然地理 | 1228篇 |
出版年
2022年 | 5篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 4篇 |
2018年 | 4766篇 |
2017年 | 4042篇 |
2016年 | 2590篇 |
2015年 | 243篇 |
2014年 | 99篇 |
2013年 | 39篇 |
2012年 | 1002篇 |
2011年 | 2733篇 |
2010年 | 2017篇 |
2009年 | 2314篇 |
2008年 | 1892篇 |
2007年 | 2361篇 |
2006年 | 56篇 |
2005年 | 198篇 |
2004年 | 407篇 |
2003年 | 410篇 |
2002年 | 250篇 |
2001年 | 47篇 |
2000年 | 51篇 |
1999年 | 13篇 |
1998年 | 22篇 |
1997年 | 2篇 |
1995年 | 1篇 |
1989年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 22篇 |
1980年 | 19篇 |
1979年 | 2篇 |
1976年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
591.
Mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradation 总被引:2,自引:0,他引:2
The mass-front velocities of granular flows results from the joint action of particle size gradations and the underlying surfaces.However,because of the complexity of friction during flow movement,details such as the slope-toe impedance effects and momentum-transfer mechanisms have not been completely explained by theoretical analyses,numerical simulations,or field investigations.To study the mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradations we conducted model experiments that recorded the motion of rapid and long-runout rockslides or avalanches.Flume tests were conducted using slope angles of 25°,35°,45°,and 55° and three particle size gradations.The resulting mass-front motions consisted of three stages:acceleration,velocity maintenance,and deceleration.The existing methods of velocity prediction could not explain the slowing effect of the slope toe or the momentum-transfer steady velocity stage.When the slope angle increased from 25° to 55°,the mass-front velocities dropped significantly to between 44.4% and59.6% of the peak velocities and energy lossesincreased from 69.1% to 83.7% of the initial,respectively.The velocity maintenance stages occurred after the slope-toe and mass-front velocity fluctuations.During this stage,travel distances increased as the angles increased,but the average velocity was greatest at 45°.At a slope angle of 45°,as the median particle size increased,energy loss around the slope toe decreased,the efficiency of momentum transfer increased,and the distance of the velocity maintenance stage increased.We presented an improved average velocity formula for granular flow and a geometrical model of the energy along the flow line. 相似文献
592.
Ken Goto Takahiro Itoh Takahiko Nagayama Rei Utsunomiya Daizo Tsutsumi Takahisa Mizuyama 《山地科学学报》2016,13(2):369-376
Bedload governs riverbed channel variations and morphology, it is necessary to determine bedload discharge through an arbitrary cross section in a mountain river. A new system with submerged load cells has been developed to directly measure bedload discharge. The system consists of: (1) an iron box which is 1 m long, 0.5 m wide and 0.1 m in depth, (2) two submerged load cells 0.7 m apart, (3) a pressure sensor and, (4) an electromagnetic velocity meter. This system has been designed to exclude the effect of the hydraulic pressure of water on direct measurements of bedload particle weight. Initial tests in a laboratory were conducted to examine the accuracy of measurements with the system under aerial conditions. The system has been installed in the supercritical flume in Ashi-arai-dani River of the Hodaka Sedimentation Observatory of the Disaster Prevention Research Institute (DPRI) of Kyoto University to obtain bedload discharge under natural conditions. Flume tests were conducted in this channel by artificial supply of uniform sediment particles of several grain sizes. The average velocity of the sediment particles near the bed was estimated using cross-correlation functions for weight waves obtained by the two load cells. Bedload discharge calculations were based on time integration of the product of sediment velocity and sediment weight obtained by the two load cells. This study clarifies the reasons why bedload measurements are difficult, and provides some solutions using the monitoring systems with submerged load cells through the field measurements. Additionally, the applicability of bedload measurement with the submerged load cells is explained based on experimental artificial sediment supply data. 相似文献
593.
Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide,central Japan.On 19 June 2012,our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits(nine surges) and talus slopes(eight surges).The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow,movement of deposits as a mass,and upward development of the fluid area.The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits.The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits;a continuous water supply from the upper stream by the surface flow might have induced this saturation.The third type was associated with changes in the downstream topography caused by erosion.The flow velocity of most surges was less than 3 m s~(-1) and they usually stopped within 100 m from the initiation point.Surges with abundant pore fluid had a higher flow velocity(about 3- 5 m s~(-1)) and could travel for alonger duration.Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes. 相似文献
594.
Dynamic process simulation with a Savage-Hutter type model for the intrusion of landslide into river 总被引:1,自引:0,他引:1
Natural damming of rivers by mass movements is a very common and potentially dangerous phenomena which has been documented all over the world. In this paper, a two-layer model of Savage-Hutter type is presented to simulate the dynamic procedure for the intrusion of landslide into rivers. The two-layer shallow water system is derived by depth averaging the incompressible Navier-Stokes equations with the hydrostatic assumption. A high order accuracy scheme based on the finite volume method is proposed to solve the presented model equations. Several numerical tests are performed to verify the realiability and feasibility of the proposed model. The numerical results indicate that the proposed method can be competent for simulating the dynamic process of landslide intrusion into the river. The interaction effect between both layers has a significant impact on the landslide movement, water fluctuation and wave propagation. 相似文献
595.
Sami Ullah Muhammad Farooq Muhammad Shafique Muhammad Afra Siyab Fazli Kareem Matthias Dees 《山地科学学报》2016,13(7):1229-1237
Anthropogenic activities and natural processes are continuously altering the mountainous environment through deforestation, forest degradation and other land-use changes. It is highly important to assess, monitor and forecast forest cover and other land-use changes for the protection and conservation of mountainous environment. The present study deals with the assessment of forest cover and other land-use changes in the mountain ranges of Dir Kohistan in northern Pakistan, using high resolution multi-temporal SPOT-5 satellite images. The SPOT-5 satellite images of years 2004, 2007, 2010 and 2013 were acquired and classified into land-cover units. In addition, forest cover and land-use change detection map was developed using the classified maps of 2004 and 2013. The classified maps were verified through random field samples and Google Earth imagery (Quick birds and SPOT-5). The results showed that during the period 2004 to 2013 the area of forest land decreased by 6.4%, however, area of range land and agriculture land have increased by 22.1% and 2.9%, respectively. Similarly, barren land increased by 1.1%, whereas, area of snow cover/glacier is significantly decreased by 21.3%. The findings from the study will be useful for forestry and landscape planning and can be utilized by the local, provincial and national forest departments; and REDD+ policy makers in Pakistan. 相似文献
596.
Experimental study of motion characteristics of rock slopes with weak intercalation under seismic excitation 总被引:1,自引:0,他引:1
In order to investigate the effect of a weak intercalation on slope stability, a large-scale shaking table model test was conducted to study the dynamic response of rock slope models with weak intercalation. The dynamic response of the prototype slopes were studied in laboratory with the consideration of law of similitude. The initiation failure was observed in the rock slope model with a counter-tilt thin-weak intercalation firstly, not in the slope model with a horizontal thin-weak intercalation. Furthermore, it was interesting that the fracture site is shifted from crest top to the slope surface near the weak intercalation, which is different with the location of failure position in a normal layered slope. We also discussed the effect of the dip angle and the thickness of weak intercalation on the failure mechanism and instability mode of the layered rock slope. From the experimental result, it was noted that the stability of the slope with a counter-tilt weak intercalation could be worse than that of the other slopes under seismic excitation. The findings showed the difference of failure in slopes with a horizontal and counter weak intercalation, and implicated the further evaluation of failure of layered slopes caused by seismic loads. 相似文献
597.
Characteristics of viscous debris flow in a drainage channel with an energy dissipation structure 总被引:1,自引:1,他引:0
A new type of drainage channel with an energy dissipation structure has been proposed based on previous engineering experiences and practical requirements for hazard mitigation in earthquakeaffected areas. Experimental studies were performed to determine the characteristics of viscous debris flow in a drainage channel of this type with a slope of 15%. The velocity and depth of the viscous debris flow were measured, processed, and subsequently used to characterize the viscous debris flow in the drainage channel. Observations of this experiment showed that the surface of the viscous debris flow in a smooth drainage channel was smoother than that of a similar debris flow passing through the energy dissipation section in a channel of the new type studied here. However, the flow patterns in the two types of channels were similar at other points. These experimental results show that the depth of the viscous debris flow downstream of the energy dissipation structure increased gradually with the length of the energy dissipation structure. In addition, in the smooth channel, the viscous debris-flow velocity downstream of the energy dissipation structure decreased gradually with the length of the energy dissipation structure. Furthermore, the viscous debris-flow depth and velocity were slightly affected by variations in the width of the energy dissipation structure when the channel slope was 15%. Finally, the energy dissipation ratio increased gradually as the length and width of the energy dissipation structure increased; the maximum energy dissipation ratio observed was 62.9% (where B = 0.6 m and L/w = 6.0). 相似文献
598.
Hydro-hypsometric analysis of tropical river basins,Southwest Coast of India using geospatial technology 总被引:1,自引:0,他引:1
The key aspect in planning and management of water resources is to analyze the runoff potential and erosion status of the river basin. For the detailed investigation of hydrological response, freely available Cartosat-1 (IRS-P5) data was used for the preparation of digital elevation model (DEM). The runoff potential and type of erosive process of 22 river basins originating in the global biodiversity hotspot of Western Ghats, was inferred through hypsometric analysis. Several parameters like Hypsometric integral (HI), maximum concavity (Eh), coordinates of slope inflection point (I) given by a* and h* and normalized height of hypsometric curve (h) were extracted from the hypsometric curves and used for understanding the hydrological responses. From the hypsometric curves, the landform evolution processes were inferred. Contribution of diffusive and fluvial processes in slope degradation of the river basins was understood. Basins with lesser area (<100 km2) were found to have a positive correlation between hypsometric integral and basin area, whereas for large basins no such correlation exists. Based on the study, river basins can be prioritized for the appropriate conservation measures. 相似文献
599.
As an effective livelihood approach to alleviate poverty without rural population migration, ethnic tourism has become the primary choice of economic development in ethnic areas worldwide in addition to traditional livelihood approaches. This article applies the theories of livelihood to study the community evolution driven by tourism livelihood and examine three mountainous tourism communities in different stages of tourist area life cycle. Drawing on the methods of GIS spatial analysis, semi-structured interviews and questionnaires, this article proposes a sustainable livelihood framework for ethnic tourism to explore the evolution of ethnic tourism communities by identifying changes in livelihood assets (natural, financial, social, cultural and human capitals) in the process of tourism development. The results show that the development of ethnic tourism has led to changes in the increase of building land, and the diversification of land use functions with a trend of shifting from meeting local villagers' living needs to satisfying tourists, income composition and uneven distribution of tourism income spatially. Ethnic tourism also led to the deterioration of traditional social management structure, collapse of neighboring relationship, the over- commercialization and staged authenticity of ethnic culture, as well as the gradual vanish of agricultural knowledge with a trend of increasing modern business knowledge and higher education. In addition, these changes, involving livelihood assets from natural, economic, human, social and cultural aspects are interrelated and interactive, which form new evolution characters of ethnic community. This study reveals the conflicts over livelihood approaches which have formed new vulnerabilities to impact on sustainable evolution of ethnic communities. This research provides implications for achieving the sustainable development of ethnic communities with the driving force of tourism livelihood. 相似文献
600.
Elena Polyakova Mikhail Gofarov Yuriy Kutinov Vladimir Beljaev Zinaida Chistova Nikolay Neverov Vadim Staritsyn Alexandr Mineev Sergey Durynin 《山地科学学报》2016,13(4):569-580
This paper considers the problems of the potential development of erosion processes in the natural landscapes of northern taiga in the Russian plain. It is considered that in forest ecosystems, erosion processes are slow and are weakly reflected in the terrain. However, the situation changes radically if the vegetation cover integrity is violated, which is inevitable with the modern methods of developing northern territories. Furthermore, global changes in average annual temperatures and the occurrence of karst processes may be the reason behind the development of erosion processes. The authors suggest a method for determining territories with a varying occurrence probability of erosional processes, based on digital elevation modelling. The territory of the Pinezhsky Nature Reserve (Arkhangelsk region) was chosen as the test plot. Direct field studies were previously used to detect exogenous geological processes in this territory. The authors were the first to suggest digital elevation modelling as a method that allows determining the potential danger of erosion in karst landscapes of the northern taiga. The geomorphometric studies resulted in the determination of areas with the greatest and lowest occurrence probability of erosion processes in the Pinezhsky Nature Reserve. It was established that the most significant erosion type was linear erosion, represented by incised river valleys and karst ravines. Sheet erosion is less significant and occurs as sinkholes, local declines, and chasms over the valleys of subterranean rivers. 相似文献