全文获取类型
收费全文 | 114357篇 |
免费 | 6961篇 |
国内免费 | 8855篇 |
专业分类
测绘学 | 4735篇 |
大气科学 | 12787篇 |
地球物理 | 23780篇 |
地质学 | 44579篇 |
海洋学 | 11401篇 |
天文学 | 20311篇 |
综合类 | 3876篇 |
自然地理 | 8704篇 |
出版年
2023年 | 686篇 |
2022年 | 1903篇 |
2021年 | 2516篇 |
2020年 | 2232篇 |
2019年 | 2556篇 |
2018年 | 3779篇 |
2017年 | 3526篇 |
2016年 | 4404篇 |
2015年 | 3090篇 |
2014年 | 4361篇 |
2013年 | 6226篇 |
2012年 | 4441篇 |
2011年 | 5349篇 |
2010年 | 4979篇 |
2009年 | 5766篇 |
2008年 | 5178篇 |
2007年 | 5089篇 |
2006年 | 4481篇 |
2005年 | 3789篇 |
2004年 | 3399篇 |
2003年 | 3320篇 |
2002年 | 3181篇 |
2001年 | 2931篇 |
2000年 | 2992篇 |
1999年 | 3153篇 |
1998年 | 2826篇 |
1997年 | 2713篇 |
1996年 | 2416篇 |
1995年 | 2193篇 |
1994年 | 1908篇 |
1993年 | 1738篇 |
1992年 | 1579篇 |
1991年 | 1429篇 |
1990年 | 1325篇 |
1989年 | 1154篇 |
1988年 | 1118篇 |
1987年 | 1049篇 |
1986年 | 987篇 |
1985年 | 1144篇 |
1984年 | 1262篇 |
1983年 | 1123篇 |
1982年 | 1069篇 |
1981年 | 954篇 |
1980年 | 885篇 |
1979年 | 852篇 |
1978年 | 820篇 |
1977年 | 700篇 |
1976年 | 638篇 |
1975年 | 647篇 |
1973年 | 640篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers 总被引:2,自引:1,他引:2
W.H. Blake P.J. Wallbrink S.N. Wilkinson G.S. Humphreys S.H. Doerr R.A. Shakesby K.M. Tomkins 《Geomorphology》2009,104(3-4):105-116
Information on post-fire sediment and nutrient redistribution is required to underpin post-fire catchment management decisions. Fallout radionuclide budgets (210Pbxs, 137Cs and 7Be) were derived to quantify soil redistribution and sediment yield in forested terrain following a moderately severe wildfire in a small (89 ha) water supply catchment in SE Australia. Application of these techniques in burnt terrain requires careful consideration of the partitioning of radionuclides between organic and mineral soil components. Beryllium-7 and 210Pbxs were shown to be closely associated with ash, litter and soil organic matter whereas 137Cs was more closely associated with subsurface coarse mineral soil. Comparison of the three tracer budgets indicated that the dominant sediment source areas were ridgetops and steep valley sideslopes, from which burnt surface material was conveyed to the stream network via pre-existing gullies. Erosion was predominantly driven by sheetwash, enhanced by soil water repellency, and modified by bioturbation which both supplies subsurface sediment and provides sinks for erosive overland flow. Footslope and riparian zones were not important sediment source areas. The estimated event-based (wildfire and subsequent rainfall) sediment yield is 58 ± 25 t km− 2, based on fallout 7Be measurements. The upper estimate of total particulate phosphorus yield (0.70 kg ha− 1) is more than 10 times that at equivalent unburnt sites. This illustrates that, soon after fire, burnt eucalypt forest can produce nutrient loads similar to those of agricultural catchments. The tracer budgets indicate that wildfire is an important control on sediment and phosphorus inputs to the stream network over the decadal timeframe and the pulsed nature of this release is an important concern for water quality management. 相似文献
932.
933.
After describing briefly some of the outstanding features of the uncommonly cold and wet year 1695, one of the coldest years
of the Little Ice Age, an annotated translation is presented of a contemporary review of the unusual weather events of the
year in Europe. The original was published in 1702 in Frankfurt am Main, Germany, in vol. XIV of the seriesTheatrum Europaeum. The annotation relates to the historical events of the year that were substantially affected by the weather, events to which
rather abstruse references are made in the aforementioned contemporary account. In addition to the fact that the contemporary
review describes the uncommon weather conditions of 1695, it appears to be the first extended weather review in history. 相似文献
934.
A coupling model between the canopy layer (CL) and atmospheric boundary layer (ABL) for the study of dry deposition velocity is developed. The model consists of six parts: chemical species conservation equation including absorptive factor; the species uptake action including detailed vertical variation of absorptive element in CL; momen-tum exchange in CL which is represented by a first-order closure momentum equation with an additional larger-scale diffusive term; momentum exchange in ABL which is described by a complete set of the ABL turbulent statistic parameters; absorptivity (or solubility or reflection) at the surface including effects of the physical and chemi-cal characters of the species, land type, seasonal and diurnal variations of the meteorological variables; and deposition velocity derived by distributions of the species with height in CL. Variational rules of the concentration and deposi-tion velocity with both height and time are simulated with the model for both corn and forest canopies. Results pre-dicted with the bulk deposition velocity derived in the paper consist well with experimental data. 相似文献
935.
I. Grudzinska J. Vassiljev L. Saarse T. Reitalu S. Veski 《Journal of Paleolimnology》2017,57(3):257-271
Diatoms, organic matter and magnetic susceptibility in a 10-m-long sediment sequence from coastal Lake Lilaste, Latvia, were analysed to evaluate Holocene environmental changes related to past sea-water intrusions. Lake Lilaste is located ~1 km from the present sea coast in an area with a low uplift rate and a threshold altitude of 0.5 m a.s.l. It was thus considered to be an appropriate site to study the influence of past sea level fluctuations on the lake and its sediments. Variations in diatom community composition, along with sediment lithostratigraphy, show that a shallow, nutrient-rich freshwater lake existed there during the early Holocene. The first brackish-water diatoms appeared concurrent with a sea level rise ca. 8700 ± 50 cal a BP, but long-term, intermittent inputs of brackish water were observed between 6700 ± 40 and 4200 ± 80 cal a BP. During those time spans, diatoms indicate increased nutrient concentrations and high conductivity, a consequence of occasional mixing of brackish and freshwater that promoted biological productivity. Lilaste was isolated from the sea at 4200 ± 80 cal a BP, after which a stable freshwater environment, dominated by planktonic diatoms such as Aulacoseira ambigua, A. granulata, A. islandica and A. subarctica, was established. At 400 ± 50 cal a BP, planktonic diatoms were gradually replaced by Fragilaria spp., indicating the beginning of anthropogenic impact. The reconstructed relative water-level curve from the lake coincides with the eustatic sea level curve from 6800 ± 40 cal a BP onwards. There was a distinct increase in abundance of brackish-water diatoms when the sea level reached the threshold of Lilaste, which at that time was probably about 3 m lower than the present sea level. According to radiocarbon-dated shifts in the diatom community composition, the Litorina Sea transgression was a long-lasting event (ca. 2200 years) in the southern part of the Gulf of Riga, where the land uplift rate was near zero. It culminated more than 1000 years later than at other sites with higher uplift, in the northern part of the Baltic Sea. 相似文献
936.
G. Nishani Musafer M. Helen Thompson E. Kozan R. C. Wolff 《Natural Resources Research》2017,26(2):223-236
A real-world mining application of pair-copulas is presented to model the spatial distribution of metal grade in an ore body. Inaccurate estimation of metal grade in an ore reserve can lead to failure of a mining project. Conventional kriged models are the most commonly used models for estimating grade and other spatial variables. However, kriged models use the variogram or covariance function, which produces a single average value to represent the spatial dependence for a given distance. Kriged models also assume linear spatial dependence. In the application, spatial pair-copulas are used to appropriately model the non-linear spatial dependence present in the data. The spatial pair-copula model is adopted over other copula-based spatial models since it is better able to capture complex spatial dependence structures. The performance of the pair-copula model is shown to be favorable compared to a conventional lognormal kriged model. 相似文献
937.
在旅游业与经济增长作用关系分析基础上,探究了旅游溢出理论,将其产生机制归纳为知识溢出、市场溢出和竞争效应3个方面。实证分析中,构建了包括入境旅游、物质资本、人力资本、科技创新和对外开放的分析框架,运用中国大陆31个省域2000~2014年面板数据对入境旅游的经济增长效应进行实证检验。全局分析得出中国入境旅游在邻近省市以集聚与合作为主,邻近省域入境旅游发展对本地经济增长具有辐射带动作用,且这种空间溢出效应大于直接效应;入境旅游业发达的地区,入境旅游的经济增长效应更大。局部分析发现,随着入境旅游的发展,入境旅游的经济增长效应不断强化,中部部分地区提升幅度最大;入境旅游的经济增长效应存在空间差异性,东部沿海地区贡献最大,西北地区相对最小。 相似文献
938.
939.
940.
Changes in global gravitational energy induced by earthquakes 总被引:1,自引:0,他引:1
Benjamin Fong Chao Richard S. Gross Da-Nan Dong 《Geophysical Journal International》1995,122(3):784-789