全文获取类型
收费全文 | 33648篇 |
免费 | 542篇 |
国内免费 | 392篇 |
专业分类
测绘学 | 1360篇 |
大气科学 | 2552篇 |
地球物理 | 6613篇 |
地质学 | 11832篇 |
海洋学 | 2530篇 |
天文学 | 8048篇 |
综合类 | 160篇 |
自然地理 | 1487篇 |
出版年
2021年 | 299篇 |
2020年 | 302篇 |
2019年 | 350篇 |
2018年 | 906篇 |
2017年 | 853篇 |
2016年 | 1121篇 |
2015年 | 635篇 |
2014年 | 1047篇 |
2013年 | 1786篇 |
2012年 | 1105篇 |
2011年 | 1346篇 |
2010年 | 1134篇 |
2009年 | 1451篇 |
2008年 | 1257篇 |
2007年 | 1200篇 |
2006年 | 1213篇 |
2005年 | 1007篇 |
2004年 | 890篇 |
2003年 | 898篇 |
2002年 | 895篇 |
2001年 | 793篇 |
2000年 | 765篇 |
1999年 | 699篇 |
1998年 | 645篇 |
1997年 | 653篇 |
1996年 | 587篇 |
1995年 | 551篇 |
1994年 | 527篇 |
1993年 | 449篇 |
1992年 | 406篇 |
1991年 | 438篇 |
1990年 | 428篇 |
1989年 | 411篇 |
1988年 | 385篇 |
1987年 | 473篇 |
1986年 | 384篇 |
1985年 | 470篇 |
1984年 | 514篇 |
1983年 | 500篇 |
1982年 | 467篇 |
1981年 | 367篇 |
1980年 | 381篇 |
1979年 | 321篇 |
1978年 | 316篇 |
1977年 | 300篇 |
1976年 | 270篇 |
1975年 | 256篇 |
1974年 | 291篇 |
1973年 | 315篇 |
1972年 | 208篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
933.
934.
935.
Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo 总被引:1,自引:0,他引:1
The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced. 相似文献
936.
A field experiment was conducted to study the effect of different crop sequences on water use, growth and yield of green gram (Vigna radiata (L) Wilezek) during 1992-93 and 1993-94 under rainfed upland condition in Indo-Gangetic plains of West Bengal. Green gram sown in the month of March as pre-rainy (summer) season crop, as and when the winter crops vacated the land, produced highest dry matter of 372 gm-2 which was significantly highest in black gram-yellow sarson sequence. The results of the field experimentation revealed that green gram gave highest grain yield to the extent of 10.80 q/ha when sown after black gram (rainy season) followed by yellow sarson (winter season) while the crop produced 10.63 q/ha under sesame-yellow sarson sequence. Highest water use of 267 mm was achieved in green gram under black gram-yellow sarson sequence and the crop gave water use efficiency of 4.07 kg ha-1mm-1 under black gram-yellow sarson sequences. 相似文献
937.
The distribution of detrital mineral cooling ages in river sediment provides a proxy record for the erosional history of mountain ranges. We have developed a numerical model that predicts detrital mineral age distributions for individual catchments in which particle paths move vertically toward the surface. Despite a restrictive set of assumptions, the model permits theoretical exploration of the effects of thermal structure, erosion rate, and topography on cooling ages. Hypsometry of the source‐area catchment is shown to exert a fundamental control on the frequency distribution of bedrock and detrital ages. We illustrate this approach by generating synthetic 40Ar/39Ar muscovite age distributions for two catchments with contrasting erosion rates in central Nepal and then by comparing actual measured cooling‐age distributions with the synthetic ones. Monte Carlo sampling is used to assess the mismatch between observed and synthetic age distributions and to explore the dependence of that mismatch on the complexity of the synthetic age signal and on the number of grains analysed. Observed detrital cooling ages are well matched by predicted ages for a more slowly eroding Himalayan catchment. A poorer match for a rapidly eroding catchment may result from some combination of large analytical uncertainties in the detrital ages and inhomogeneous erosion rates within the basin. Such mismatches emphasize the need for more accurate thermal and kinematic models and for sampling strategies that are adapted to catchment‐specific geologic and geomorphic conditions. 相似文献
938.
939.
940.
Pradeep Srivastava Indra Bir Singh Shikha Sharma Uma Kant Shukla Ashok Kumar Singhvi 《Geomorphology》2003,54(3-4):279-292
Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to 8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds. 相似文献