全文获取类型
| 收费全文 | 3758篇 |
| 免费 | 487篇 |
| 国内免费 | 352篇 |
专业分类
| 测绘学 | 90篇 |
| 大气科学 | 104篇 |
| 地球物理 | 1652篇 |
| 地质学 | 905篇 |
| 海洋学 | 488篇 |
| 天文学 | 326篇 |
| 综合类 | 102篇 |
| 自然地理 | 930篇 |
出版年
| 2024年 | 14篇 |
| 2023年 | 24篇 |
| 2022年 | 59篇 |
| 2021年 | 127篇 |
| 2020年 | 158篇 |
| 2019年 | 157篇 |
| 2018年 | 128篇 |
| 2017年 | 139篇 |
| 2016年 | 124篇 |
| 2015年 | 152篇 |
| 2014年 | 155篇 |
| 2013年 | 246篇 |
| 2012年 | 120篇 |
| 2011年 | 174篇 |
| 2010年 | 132篇 |
| 2009年 | 220篇 |
| 2008年 | 223篇 |
| 2007年 | 223篇 |
| 2006年 | 249篇 |
| 2005年 | 222篇 |
| 2004年 | 190篇 |
| 2003年 | 174篇 |
| 2002年 | 144篇 |
| 2001年 | 159篇 |
| 2000年 | 150篇 |
| 1999年 | 153篇 |
| 1998年 | 142篇 |
| 1997年 | 88篇 |
| 1996年 | 56篇 |
| 1995年 | 42篇 |
| 1994年 | 54篇 |
| 1993年 | 27篇 |
| 1992年 | 24篇 |
| 1991年 | 22篇 |
| 1990年 | 14篇 |
| 1989年 | 18篇 |
| 1988年 | 11篇 |
| 1987年 | 19篇 |
| 1986年 | 10篇 |
| 1985年 | 7篇 |
| 1984年 | 11篇 |
| 1983年 | 7篇 |
| 1982年 | 9篇 |
| 1981年 | 7篇 |
| 1980年 | 6篇 |
| 1979年 | 3篇 |
| 1978年 | 1篇 |
| 1977年 | 3篇 |
排序方式： 共有4597条查询结果，搜索用时 31 毫秒
61.
Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes 总被引：6，自引：0，他引：6
Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion. 相似文献
62.
MANUELA PELFINI MAURIZIO SANTILLI 《Geografiska Annaler: Series A, Physical Geography》2006,88(3):223-236
ABSTRACT. Mountain landscapes show rapid evolution, especially at high altitudes, in response to current climate changes. In addition, the greater number of routes and trails made available and the increase in tourism have caused some impacts on mountain areas. Gradual soil erosion has been observed along some hiking trails, with the latter tending to deepen, and with stronger erosion where the trail cuts the slope. Where trails cross forested areas, trees can suffer root damage from foot trampling as roots become exposed. A dendrogeomorphological study was conducted on trees along two hiking trails in upper Valtellina (Italian Alps). On the first trail, in Valle dei Forni, European larches and stone pines, mostly with exposed roots, were sampled. A mean erosion rate of 2.7 mm/a, related to deepening of the footpath, was obtained, and various degrees of root growth disturbance along the trail were observed. In particular, the growth of many sampled roots shows an increase in ring width corresponding with the moment when root exposure occurred, followed, after 3–5 years, by rapid growth suppression. The exposure of many roots has taken place since the 1980s. No significant variations in stem growth were observed, even when there were exposed and damaged roots. Along the second trail, in Valle Alpisella, exposed roots of mountain pines were analysed. A mean erosion rate of 3.2–3.7 mm/a, related to the escarpments bordering the footpath, was obtained, while no significant changes were detected in root growth. 相似文献
63.
64.
根据实测河道地形资料,分析了水库下游河床演变中的河岸侵蚀现象,包括不同横向边界约束条件、不同床沙组成和不同河型河段的河岸侵蚀的特征及规律。结果表明:①水库下游均会有河岸侵蚀发生,但初期一般以下切为主,后期以展宽为主。②对于横向边界约束较强的河段,河岸侵蚀相对较弱;相反,则河岸坍塌严重,河岸侵蚀强烈;③对不同的河床组成和边滩组成的河段:河床较粗特别是形成抗冲粗化层,且边滩组成较细的河段,河岸侵蚀现象非常剧烈;河床组成较细的河段,如果边滩抗冲性较强,则河岸侵蚀现象相对较弱,如果滩地组成较细、容易坍塌,河岸侵蚀也会较强;④不同的河型有着不同的河岸侵蚀现象。分汊河段以主汊为主;游荡段的游荡特性在初期受到抑制;弯曲河段的撇弯切滩现象较普遍。 相似文献
65.
G. L. Geernaert 《Boundary-Layer Meteorology》2002,105(3):433-450
In order to apply Monin–Obukhov similarity theory to estimatethe profiles of mean quantities and surface fluxes from bulk meteorological parameters, the assumptions of homogeneityand stationarity must be valid. Unfortunately, in coastal zones as well as many other regions of interest, theseassumptions are often violated. In this paper, an extension to the theory is presented that considers systematically varyingstate variables. Along-wind variations of windspeed, atmospheric stratification, and roughness are examined with respectto their relative importance to momentum flux divergence, and the drag coefficient is shown to be systematicallylower in coastal zones. For profiles of scalars, an analysis of the set of quasi-homogeneous terms is only speculated,and the relative importance for the terms will strongly depend on which scalar is of interest. 相似文献
66.
67.
The main landforms within the glacially scoured Precambrian rocks of the Swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes, probably during the Mesozoic and with a possible second period of etching during the Neogene. To explore the effect of multiple glacial erosion on the rock surfaces, an island with two different lithologies and with striae from different directions was selected for a detailed study, focusing on the shape of roches moutonnées. Air‐photo interpretation of bedrock lineaments and roches moutonnées combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape. The study reveals a significant difference in shape between roches moutonnées in augen‐granite and orthogneiss. Low elongated and streamlined roches moutonnées occur in the gneiss area, striated by a Late Weichselian ice flow from the NE. This ice flow is subparallel with both the local dominant trend of topographically well‐expressed joints and the schistosity of the gneiss. Frequently, there are no signs of quarrying on the lee‐sides of the gneiss roches moutonnées and hence they resemble the shape of whalebacks, or ruwares, as typically associated with the exposed basal weathering surface found in tropical areas. The granite roches moutonnées were formed by an older ice flow from the ESE, which closely followed the etched WNW–ESE joint system of the granite. Late Weichselian ice flow from the NE caused only minor changes of the landforms. On the contrary, marks of the early ESE ice flow are poorly preserved in the gneiss area, where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and, accordingly, also to the pre‐glacial relief. The study demonstrates that coincidence between ice flow direction and pre‐glacially etched structures is most likely to determine the effects of glacial erosion. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
68.
Channel curvature produces secondary currents and a transverse sloping channel bed, along which the depth increases towards the outer bank. As a result deep pools tend to form adjacent to the outer bank, promoting bank collapse. The interaction of sediment grains with the primary and secondary flow and the transverse sloping bed also causes meanders to move different grain sizes in different proportions and directions, resulting in a consistent sorting pattern. Several models have been developed to describe this process, but they all have the potential to over‐predict pool depth because they cannot account for the influence of erodible banks. In reality, bank collapse might lead to the development of a wider, shallower cross‐section and any resulting flow depth discrepancy can bias associated predictions of flow, sediment transport, and grain‐size sorting. While bed topography, sediment transport and grain sorting in bends will partly be controlled by the sedimentary characteristics of the bank materials, the magnitude of this effect has not previously been explored. This paper reports the development of a model of flow, sediment transport, grain‐size sorting, and bed topography for river bends with erodible banks. The model is tested via intercomparison of predicted and observed bed topography in one low‐energy (5·3 W m?2 specific stream power) and one high‐energy (43·4 W m?2) study reach, namely the River South Esk in Scotland and Goodwin Creek in Mississippi, respectively. Model predictions of bed topography are found to be satisfactory, at least close to the apices of bends. Finally, the model is used in sensitivity analyses that provide insight into the influence of bank erodibility on equilibrium meander morphology and associated patterns of grain‐size sorting. The sensitivity of meander response to bank cohesion is found to increase as a function of the available stream power within the two study bends. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
69.
70.