全文获取类型
收费全文 | 2164篇 |
免费 | 134篇 |
国内免费 | 122篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 1篇 |
地球物理 | 1153篇 |
地质学 | 489篇 |
海洋学 | 354篇 |
天文学 | 10篇 |
综合类 | 21篇 |
自然地理 | 389篇 |
出版年
2024年 | 2篇 |
2023年 | 25篇 |
2022年 | 38篇 |
2021年 | 28篇 |
2020年 | 39篇 |
2019年 | 50篇 |
2018年 | 44篇 |
2017年 | 48篇 |
2016年 | 54篇 |
2015年 | 34篇 |
2014年 | 96篇 |
2013年 | 81篇 |
2012年 | 44篇 |
2011年 | 127篇 |
2010年 | 87篇 |
2009年 | 150篇 |
2008年 | 178篇 |
2007年 | 185篇 |
2006年 | 110篇 |
2005年 | 101篇 |
2004年 | 87篇 |
2003年 | 102篇 |
2002年 | 83篇 |
2001年 | 72篇 |
2000年 | 66篇 |
1999年 | 80篇 |
1998年 | 44篇 |
1997年 | 56篇 |
1996年 | 31篇 |
1995年 | 32篇 |
1994年 | 41篇 |
1993年 | 29篇 |
1992年 | 37篇 |
1991年 | 28篇 |
1990年 | 22篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2420条查询结果,搜索用时 31 毫秒
101.
Prediction of threshold conditions and incipient motion is the essential issue for the study of sediment transport. This work compares existing empirical threshold curves proposed for Shields diagram, a method based on the concept of probability of sediment movement, and an empirical method based on movability number. These methods are used to predict the incipient motion conditions for experimental runs taken from various studies. Most of the experimental data, used in this work, have not been used before in derivation of alternative formulations for Shields diagram and other methods. The empirical threshold curves based on the Shields entrainment function was the least successful at predicting the measured incipient motion conditions, while the use of the movability number gives good predictions of critical shear velocity compared with experimental data. 相似文献
102.
Pieter J.C. Honkoop Eilke M. Berghuis Sander Holthuijsen Marc S.S. Lavaleye Theunis Piersma 《Journal of Sea Research》2008,60(4):255-263
The Banc d'Arguin, a non-estuarine area of shallows and intertidal flats off the tropical Saharan coast of Mauritania, is characterised by extensive intertidal and subtidal seagrass beds. We examined the characteristics of intertidal seagrass (Zostera noltii) meadows and bare areas in terms of the presence and abundance of molluscs (gastropods and bivalves). To explain observed differences between molluscan assemblages in seagrass and bare patches, some aspects of the feeding habitat (top-5 mm of the sediment) and of food (organic materials) of molluscs were examined. The novelty of this study is that phytopigments were measured and identified to assess source and level of decay (freshness) of organic material in the sediment and to study their importance as an explanatory variable for the distribution of molluscs. Over an area of 36 km2 of intertidal flats, at 12 sites, paired comparisons were made between seagrass-covered and nearby bare patches. Within seagrass meadows, dry mass of living seagrass was large and amounted to 180 ±10 g AFDM m− 2 (range 75–240). Containing twice the amount of silt per unit dry sediment mass, seagrass sediments were muddier than bare areas; the relative amount of organic material was also larger. The total number of species of bivalves and gastropods amounted to 27, 14 of which were found only in seagrass areas, 4 only in bare and 9 in both types of habitat. Among the three numerically most abundant species, the bivalves Anadara senilis, Dosinia hepatica and Loripes lacteus, the first was numerically most abundant in bare and the other two in seagrass-covered areas. Bare intertidal areas had greater mean total biomass of molluscs (80.5 g AFDM m− 2) than seagrass meadows (30.0 g AFDM m− 2). In both habitats, the bulk of the biomass was made up by A. senilis. Excluding this species, bare mudflats contained on average only 3.1 g AFDM m− 2 and seagrass meadows 6.9 g AFDM m− 2. As compared to previous surveys in 1980–1986, the biomass of A. senilis had increased almost 10-fold and D. hepatica, previously found in very small numbers, had become the most numerous species. However, the total biomass excluding that of A. senilis was similar. Concentrations of phytopigments were similar to those observed at temperate mudflats, indicating that the Banc d'Arguin might not be as oligotrophic as previously thought. Per unit of dry sediment mass, smaller amounts of phytopigments were found in bare than in seagrass areas. Per unit of dry organic material, bare sediments contained most (fresh) phytopigments. This suggests that in seagrass-covered meadows the organic material is more degraded than in bare sediments. Overall, the composition of phytopigments, quite surprisingly, indicated a benthic-diatom-dominated trophic system. Multivariate statistics revealed that patterns of zoobenthic assemblages were correlated with patterns of a combination of four environmental parameters: grain size of the sediment, amount of fresh phytopigments and amounts of leaves and roots of seagrass. 相似文献
103.
104.
105.
Integrated ebb-aligned drainage systems are a feature of tide-dominated marshes, and are generally regarded as major conduits for material exchange. In north Norfolk, highly unsteady creek flows exhibit well-developed velocity and stress transients which result from the discontinuous nature of the tidal prism and the interaction of shallow water tidal inputs with hydraulically rough vegetated surfaces. Marsh morphological development is governed by a form-process feedback, in the sense that the marsh surface acts as a topographic threshold separating the depositional regime of below-marsh tides from the erosional (ebb-dominated) regime of over-marsh tides. Vertical marsh growth results in increasing intermittency of creek sediment transport. Furthermore, velocity transients are associated with large discharges which must be allowed for in material flux computations. Creek flux measurements are not in themselves sufficient to estimate total material budgets, since a large proportion of tidal exchange may take place via the marsh edge. Such studies should focus instead on direct measurement of marsh surface processes. These findings have relevance beyond this back-barrier setting to marshes of different geometry, occupying a broad range of the tidal energy spectrum. 相似文献
106.
Liancong LUO Boqiang QIN Guang GAO Guangwei ZHU Jinshan ZHANG 《国际泥沙研究》2006,21(4):312-320
1 INTRODUCTION Large amounts of nutrients and contaminants such as trace metals are transported into large lakes frominflow rivers and then absorbed onto or associated with fine-grained sediment particles (Sheng and Lick,1979). The nutrients and contamina… 相似文献
107.
108.
109.
110.