全文获取类型
收费全文 | 954篇 |
免费 | 212篇 |
国内免费 | 73篇 |
专业分类
测绘学 | 9篇 |
地球物理 | 342篇 |
地质学 | 526篇 |
海洋学 | 175篇 |
天文学 | 8篇 |
综合类 | 30篇 |
自然地理 | 149篇 |
出版年
2024年 | 12篇 |
2023年 | 13篇 |
2022年 | 25篇 |
2021年 | 46篇 |
2020年 | 53篇 |
2019年 | 45篇 |
2018年 | 51篇 |
2017年 | 63篇 |
2016年 | 58篇 |
2015年 | 44篇 |
2014年 | 58篇 |
2013年 | 89篇 |
2012年 | 53篇 |
2011年 | 58篇 |
2010年 | 42篇 |
2009年 | 59篇 |
2008年 | 62篇 |
2007年 | 37篇 |
2006年 | 40篇 |
2005年 | 37篇 |
2004年 | 48篇 |
2003年 | 21篇 |
2002年 | 26篇 |
2001年 | 32篇 |
2000年 | 36篇 |
1999年 | 22篇 |
1998年 | 15篇 |
1997年 | 23篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有1239条查询结果,搜索用时 15 毫秒
81.
82.
Pristine river corridors were characterized by island and floodplain development driven by the inter‐play of flows, sediments and woody vegetation. Here we explore these relationships within topographically controlled settings within the upper, middle and lower reaches of a large, semi‐natural alpine to mediterranean river. These reaches have expanding or contracting valley floors within which we show that there are more or less predictable patterns of stream power and rates of vegetation growth, reflecting water availability during dry periods and also the availability of sand and finer sediment. We relate these to the pattern of island distribution that is repeated within the three reaches and is indicative of the engineering role of riparian trees. Islands are shown to develop within thresholds defined by stream power, rates of woody vegetation growth and rates of sedimentation, and to develop most quickly where riparian species include those capable of sprouting from driftwood. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
83.
L. Allan James 《地球表面变化过程与地形》2006,31(13):1692-1706
An extensive literature about fluvial sediment waves, slugs or pulses has emerged in the past 20 years. The concept has been useful in many respects, but has been applied to diverse phenomena using a variety of definitions. Moreover, inferred linkages between channel‐bed changes and sediment loads are often not justifiable. This paper reviews concepts of large fluvial sediment waves at scales extending to several tens of kilometres. It points out constraints on the inferences that can be made about sediment loads based on changes in channel‐bed elevation at this scale where channel sediment interacts with storage in floodplain and terrace deposits. The type area of G. K. Gilbert's initial sediment‐wave concept is re‐examined to show that neither wave translation nor dispersion occurred in the simple manner commonly assumed. Channel aggradation and return to graded conditions provide an alternative theory explaining Gilbert's observed bed‐elevation changes. Recognizing the evidence and implications of the former passage of a large‐scale bed wave is essential to the accurate diagnosis of catchment conditions and the adoption of appropriate river restoration goals or methods. Sediment loads, water quality, channel morphologic stability and aquatic ecosystems often reflect changes in sediment storage long after the channel bed has returned to grade. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
84.
Fred Worrall Tim P. Burt Gregory R. Hancock Nicholas J.K. Howden John Wainwright 《地球表面变化过程与地形》2020,45(15):3869-3878
This study has hypothesized that for many rivers the trade-off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low-order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that:
- Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit.
- Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events.
- The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle.
- The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.
85.
研究胀缩土的成因,为地表建筑物防灾减灾,有一定的现实意义。根据胀缩土的工程地质特征,平面和垂向分布规律,右江高阶地胀缩土为冲洪积成因。 相似文献
86.
R. A. Henderson 《Australian Journal of Earth Sciences》2013,60(8):1041-1059
Campaspe Formation, a surficial, fluviatile, sand-dominated unit, is extensively developed in the Charters Towers region of north Queensland where it covers an area of 11 000 km2, overlying Paleozoic basement and erosional remnants of Paleogene Southern Cross Formation. In the northern part of its distribution, it is interlayered with, and overlain by, flows of Nulla Basalt. It is dated as mid-Pliocene, based on the 3.48 Ma age of the Myrrilumbing Flow interlayered with its upper part, an antiquity consistent with thin ferricrete locally developed at its surface. The formation defines the Campaspe Surface at an elevation of 200–440 m in the present landscape, falling gently from upland to the west towards the current channel of the Burdekin River. This surface is a relict alluvial plain little incised by easterly flowing tributaries of the Burdekin River and has survived, essentially intact, from the early Pliocene. Geometry of the formation is established from some 2000 mineral exploration drill holes and seismic profiling. It has an average thickness of some 60 m, reaching a maximum of over 200 m. It blankets a pre-existing, low relief landscape in which a basement ridge divides it into two sub-basins that mirror the present drainage. The formation consists of poorly sorted sandstone with minor conglomerate and siltstone, and contains paleosol horizons with associated calcrete. Matrix supported sandstone in the succession, indicates deposition in part from hyperconcentrated flows. Sandstones generally show poorly defined planar layering as typical of ephemeral overbank and terminal splay sediment bodies but beds with cross-lamination, indicating fluvial channel bed forms, are also present. They are characterised by pore-filling silt and mud, largely emplaced by post-depositional infiltration, such that the unit produces essentially no groundwater. Facies attributes are consistent with fluvial deposition in ephemeral, dry climate, distributary system, with inefficient cross-drainage discharge that induced aggradation, resulting in a substantial sediment body perched in the landscape. Framework grain compositions show the formation to be mineralogically mature, representing erosional debris derived from intense weathering in an earlier climatic regime recorded, at least in part, by duricrust developed in the fluviatile–lacustrine Southern Cross Formation of Paleogene age. Such duricrust intervals are now upstanding in the landscape, representing erosional remnants from inverted relief developed in a mid-Cenozoic, pluvial, landscape cycle. Paleoclimatic signature of the Campaspe Formation extends the record of Pliocene aridity, widely recognised elsewhere in Australia, to northeast Queensland. In large part the landscape of the Charters Towers district is relict from the early Pliocene and is in the process of readjusting to more pluvial climatic regimes. By implication, Pliocene aridity has, on a small scale, exerted a strong influence on the present physiography of Australian landscapes. 相似文献
87.
Field stratigraphy, sedimentology and optically stimulated luminescence (OSL) dating have been used to reconstruct the southwest monsoon variability in the semi‐arid region of southern India during the late Holocene. Facies architecture and OSL dating of the water‐lain sediment suggest prevalence of a weak hydrological regime around 3 ka. Following this, a progressive strengthening of monsoon occurred till 2 ka. After 2 ka and until 1 ka fluvial activity was nearly dormant, indicating weakening of the monsoon. Presence of high‐magnitude flood deposits, overbank sedimentation and pedogenesis during 1–0.6 ka indicate intensification of the southwest monsoon in the basin. The onset of aridity was associated with episodic storm surge events that are manifested in the pond sedimentation and localised aeolian accretion. This phase is bracketed between 0.5 ka and 0.2 ka. A renewed phase of monsoonal activity was observed in the form of floodplain aggradation between 180 and 90 years ago. In the past 70 years no significant change in the monsoon performance has been observed. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
88.
《水文科学杂志》2013,58(5)
Abstract Modelling of the rainfall–runoff transformation process and routing of river flows in the Kilombero River basin and its five sub-catchments within the Rufiji River basin in Tanzania was undertaken using three system (black-box) models—a simple linear model, a linear perturbation model and a linear varying gain factor model—in their linear transfer function forms. A lumped conceptual model—the soil moisture accounting and routing model—was also applied to the sub-catchments and the basin. The HEC-HMS model, which is a distributed model, was applied only to the entire Kilombero River basin. River discharge, rainfall and potential evaporation data were used as inputs to the appropriate models and it was observed that sometimes the system models performed better than complex hydrological models, especially in large catchments, illustrating the usefulness of using simple black-box models in datascarce situations. 相似文献
89.
Current high‐resolution palaeoenvironmental records reveal short‐term Holocene coolings. One of these major Holocene rapid climate changes occurred between 3.2 and 2.7 cal. ka BP. The sensitivity of river systems vis‐à‐vis slight and short‐term Holocene climatic variations is a subject of controversy in the scientific community. In this paper, we present a 4.0 to 1.4 cal ka BP palaeoflood record from the Lower Moulouya River (northeastern Morocco) to demonstrate the high sensitivity of semiarid rivers in the southwestern Mediterranean towards Holocene environmental changes. The Lower Moulouya flood deposits are characterised by thick, well‐stratified, predominantly clayey to silty overbank fine sediments. These cohesive sediments show evidence of excellent preservation conditions against fluvial erosion and contain a continuous record of mid to late Holocene flood sequences. The Moulouya palaeoflood record can be interpreted in the context of regional and global high‐resolution proxy data, revealing a strong coupling with Holocene rapid climate changes. The centennial‐scale Moulouya palaeohydrological history will be discussed with palaeoenvironmental data from the same record (palaeomagnetics, sedimentary charcoal record, anthracological analyses, snail analyses) to generate new ideas about the mid to late Holocene hydrological cycle in the southwestern Mediterranean. The deduced features of pronounced Lower Moulouya flooding and the decreased fire recurrence during Holocene cooling remain somewhat inconsistent with the interpretation of other palaeohydrological and paleaoecological records from the southwestern Mediterranean. However, enhanced Lower Moulouya flood frequencies between 3.2 and 2.7 cal. ka BP agree with increased floodplain aggradation in other major river systems of Mediterranean North Africa. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
90.
An experimental investigation was conducted on a flexible riser with and without various strake arrangements. The aim of the present work was to further improve the understanding of the response performance of the vortex-induced vibration (VIV) of a riser with helical strakes. Two current profiles, including uniform and linearly sheared flows, were simulated. The uniform current was simulated by towing the riser model in one direction using the towing carriage, and the linearly sheared current was simulated by fixing one end of the riser and using a driven cantilever to traverse a circular arc. Based on the modal superposition method, the displacement responses were obtained from the measured strain. Strakes with different heights and pitches were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the bare model test show that the lock-in phenomenon displays multi-order characteristics, and the VIV displacement decreases with an increased order of the lock-in regime. The results of the straked model test indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the response performance depends closely on the geometry of the strake configuration. 相似文献