全文获取类型
收费全文 | 2097篇 |
免费 | 387篇 |
国内免费 | 493篇 |
专业分类
测绘学 | 71篇 |
大气科学 | 115篇 |
地球物理 | 847篇 |
地质学 | 1139篇 |
海洋学 | 316篇 |
天文学 | 47篇 |
综合类 | 77篇 |
自然地理 | 365篇 |
出版年
2024年 | 7篇 |
2023年 | 30篇 |
2022年 | 61篇 |
2021年 | 114篇 |
2020年 | 99篇 |
2019年 | 125篇 |
2018年 | 93篇 |
2017年 | 102篇 |
2016年 | 111篇 |
2015年 | 120篇 |
2014年 | 144篇 |
2013年 | 197篇 |
2012年 | 104篇 |
2011年 | 148篇 |
2010年 | 109篇 |
2009年 | 121篇 |
2008年 | 151篇 |
2007年 | 136篇 |
2006年 | 139篇 |
2005年 | 116篇 |
2004年 | 100篇 |
2003年 | 77篇 |
2002年 | 81篇 |
2001年 | 79篇 |
2000年 | 53篇 |
1999年 | 50篇 |
1998年 | 51篇 |
1997年 | 37篇 |
1996年 | 43篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 20篇 |
1992年 | 12篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有2977条查询结果,搜索用时 31 毫秒
171.
Analysis of the contributions of topographic,soil, and vegetation features on the spatial distributions of surface soil moisture in a steep natural forested headwater catchment
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Surface soil moisture has been extensively studied for various land uses and landforms. Although many studies have reported potential factors that control surface soil moisture over space or time, the findings have not always been consistent, indicating a need for identification of the main factors. This study focused on the static controls of topographic, soil, and vegetation features on surface soil moisture in a steep natural forested headwater catchment consisting of three hillslope units of a gully area, side slope, and valley‐head slope. Using a simple correlation analysis to investigate the effects of the static factors on surface soil moisture at depths of 0–20 cm at 470 points in 13 surveys, we addressed the characteristics of surface soil moisture and its main controlling factors. The results indicated that the mean of surface soil moisture was in the decreasing order of gully area > valley‐head slope > side slope. The relationship between the mean and standard deviation of surface soil moisture showed a convex‐upward shape in the headwater catchment, a negative curvilinear shape in the gully area, and positive curvilinear shapes at the side and valley‐head slopes. At the headwater catchment and valley‐head slope, positive contributions of soil porosity and negative contributions of slope gradient and saturated hydraulic conductivity were the main controlling factors of surface soil moisture under wetter conditions, whereas positive contributions of topographic wetness index and negative contributions of vegetation density were the main controlling factors of surface soil moisture under drier conditions. At the side slope underlain by fractured bedrocks, only saturated hydraulic conductivity and vegetation density were observed to be the controlling factors. Surface soil moisture in the gully area was mainly affected by runoff rather than were static features. Thus, using hillslope units is effective for approximately estimating the hydrological behaviours of surface moisture on a larger scale, whereas dependency between the main static factors and moisture conditions is helpful for estimating the spatial distributions of surface moisture on a smaller scale. 相似文献
172.
Jan Nyssen Wim Clymans Katrien Descheemaeker Jean Poesen Ine Vandecasteele Matthias Vanmaercke Amanuel Zenebe Marc Van Camp Mitiku Haile Nigussie Haregeweyn Jan Moeyersons Kristine Martens Tesfamichael Gebreyohannes Jozef Deckers Kristine Walraevens 《水文研究》2010,24(13):1880-1895
Impact studies of catchment management in the developing world rarely include detailed hydrological components. Here, changes in the hydrological response of a 200‐ha catchment in north Ethiopia are investigated. The management included various soil and water conservation measures such as the construction of dry masonry stone bunds and check dams, the abandonment of post‐harvest grazing, and the establishment of woody vegetation. Measurements at the catchment outlet indicated a runoff depth of 5 mm or a runoff coefficient (RC) of 1·6% in the rainy season of 2006. Combined with runoff measurements at plot scale, this allowed calculating the runoff curve number (CN) for various land uses and land management techniques. The pre‐implementation runoff depth was then predicted using the CN values and a ponding adjustment factor, representing the abstraction of runoff induced by the 242 check dams in gullies. Using the 2006 rainfall depths, the runoff depth for the 2000 land management situation was predicted to be 26·5 mm (RC = 8%), in line with current RCs of nearby catchments. Monitoring of the ground water level indicated a rise after catchment management. The yearly rise in water table after the onset of the rains (ΔT) relative to the water surplus (WS) over the same period increased between 2002–2003 (ΔT/WS = 3·4) and 2006 (ΔT/WS >11·1). Emerging wells and irrigation are other indicators for improved water supply in the managed catchment. Cropped fields in the gullies indicate that farmers are less frightened for the destructive effects of flash floods. Due to increased soil water content, the crop growing period is prolonged. It can be concluded that this catchment management has resulted in a higher infiltration rate and a reduction of direct runoff volume by 81% which has had a positive influence on the catchment water balance. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
173.
Abstract Knowledge of the hydrochemical dynamics of the trace metal manganese (Mn) in upland catchments is required for water quality management. Stream water Mn and other solutes and flow were monitored in two upland catchments in northern England with different soils: one dominated by peat (HS7), the other by mineral soils (HS4). Maximum Mn concentrations occurred at different times in the two catchments: in summer baseflow at HS4 and during late summer storm events at HS7. A two-component chemical mixing model was used to identify the hydrological processes controlling Mn concentrations in stream water. This approach was more successful for HS4 than HS7, probably because of different processes of Mn release in the two catchments and also difficulties in selecting conservative solutes. Factor analysis of the stream water chemistry data set for each catchment was more useful in identifying the controls on Mn release into runoff. The factors indicate that the main source of Mn at HS4 is the hydrological pathway supplying summer baseflow, whereas at HS7 Mn is released during the rewetting of dried peat soils. Manganese concentrations in stream water in upland catchments appear to depend on soil type and antecedent moisture conditions. This has implications for the design of sampling strategies in upland catchments and also for managing the quality of water supplies from such areas. 相似文献
174.
175.
五矩二流太阳风等离子体特性的数值研究 总被引:4,自引:0,他引:4
本文数值求解了各向同性二流太阳风的五矩方程组,得出了1 Rs-2AU区域内太阳风密度、速度、电子和质子温度、它们的热流通量密度q以及非麦克斯韦分布尾部粒子过剩量ξ随日心距离的变化关系.文中比较了二流太阳风五矩模型、四矩模型(ξ=0)和低阶矩模型(不包括q和ξ二个矩方程)的等离子体特性,着重讨论了量ξ对质子温度及其热流通量的影响.结果表明,包括言的五矩方程可改善Te/Tp和qe/qp的计算值与观测值的符合程度. 相似文献
176.
本文旨在研究多孔岩石声衰减机制及衰减和频率的关系,用共振法测量了人工多孔材料10~5Hz以下的弯曲振动能量损耗,发展了前人测量弹性模量的方法.实验结果表明:干燥试样的内耗基本上与频率无关:水饱和试样的内耗比干燥试样的大得多,并在24.0kHz有显著的弛豫峰;饱和试样的内耗主要产生于固体骨架与孔隙流体之间的相对运动.实验结果与Riot理论和局部流体流动机制一致,但未看到Dunn预言的“边界效应”. 相似文献
177.
Robert A. Langel 《Journal of Earth System Science》1990,99(4):581-618
Among the first measurements made from near-Earth orbiting satellites were measurements of the magnetic field. The sources
of that field lie both within the Earth, in its core and crust, and in the surrounding ionosphere and magnetosphere. This
article summarizes some of the methodology and results for studies of the Earth’s mantle and crust. Mantle conductivity studies
can be made either by studying signals impressed on the Earth from outside, e.g., the ionosphere or magnetosphere, or by studying
signals originating in the core and transmitted through the mantle. Crustal field studies begin with a careful selection of
the data and subsequent removal of core and external fields by some sort of filtering. Average maps from different local times
sometimes differ, presumably due to the remaining presence of fields of external origin. Several techniques for further filtering
are discussed. Where large-area aeromagnetic maps are available, crustal maps derived from satellite data can be compared
with upward continued data. In general, the comparisons show agreement, with some differences, particularly in and near the
auroral belts. The satellite data are further reduced by various methods of inverse and forward modelling, sometimes including
reduction to the pole (RTP). These techniques are generally unstable at the equator. Common methods of stabilizing the inversions
include principle components analysis and ridge regression. Because of the presence of the core field, the entire crustal
contribution from the field is not known. Also, there is a basic nonuniqueness to the inverse solutions. Nevertheless, magnetizations
that are interpretable can be derived. 相似文献
178.
Conclusion A simple method of estimating fracture porosity and permeability based on empirical relations between fracture aperture andJRC andJCS can be developed. This shows very close correlation with existing data, using simple discontinuity models. There is quite a good correlation between fracture porosity and permeability and depth for larger initial apertures. 相似文献
179.
提出和介绍了利用GMS-4红外云图资料定量测量淮河流域汛期降水的CT-TRC(cloudtoptemperaturerainfullcurve,云顶温度-降水量曲线)方法及实现步骤,并结合个例进行了分析。 相似文献
180.
From results of two large-eddy simulation of stratus-topped boundary layer,the structure,thermodynamic prop-erties and role of thermals are investigated by using conditional sampling methods,which divided the thermals into thewarm/moist,cool/dry,warm/dry and cool/moist events.The results show that the main turbulent circulation in thestratus-topped boundary layer is composed of the warm/moist updraft and cool/dry downdraft.Below entrainmentregion,the warm/moist updrafts and cool/dry downdrafts are,respectively,positively and negatively buoyant andcontribute most to total fluxes and variances.Evaporative cooling has important effect on the structure and thermodynamic properties of thermals instratus-topped boundary layer. 相似文献