全文获取类型
收费全文 | 3663篇 |
免费 | 561篇 |
国内免费 | 419篇 |
专业分类
测绘学 | 106篇 |
大气科学 | 206篇 |
地球物理 | 1616篇 |
地质学 | 1053篇 |
海洋学 | 499篇 |
天文学 | 22篇 |
综合类 | 138篇 |
自然地理 | 1003篇 |
出版年
2024年 | 23篇 |
2023年 | 38篇 |
2022年 | 74篇 |
2021年 | 141篇 |
2020年 | 166篇 |
2019年 | 158篇 |
2018年 | 148篇 |
2017年 | 160篇 |
2016年 | 143篇 |
2015年 | 155篇 |
2014年 | 180篇 |
2013年 | 262篇 |
2012年 | 175篇 |
2011年 | 189篇 |
2010年 | 156篇 |
2009年 | 210篇 |
2008年 | 217篇 |
2007年 | 212篇 |
2006年 | 248篇 |
2005年 | 194篇 |
2004年 | 169篇 |
2003年 | 163篇 |
2002年 | 149篇 |
2001年 | 144篇 |
2000年 | 109篇 |
1999年 | 108篇 |
1998年 | 88篇 |
1997年 | 88篇 |
1996年 | 57篇 |
1995年 | 43篇 |
1994年 | 60篇 |
1993年 | 36篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 12篇 |
1987年 | 19篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 3篇 |
排序方式: 共有4643条查询结果,搜索用时 15 毫秒
71.
Detailed soil erosion studies bene?t from the ability to quantify the magnitude of erosion over time scales appropriate to the process. An inventory balance for 7Be was used to calculate sediment erosion in a 30·73 m2 plot during a series of runoff‐producing thunderstorms occurring over three days at the Deep Loess Research Station in Treynor, Iowa, USA. The inventory balance included determination of the pre‐ and post‐storm 7Be inventories in the soil, the atmospheric in?ux of 7Be during the event, and pro?les of the 7Be activity in the soil following the atmospheric deposition. The erosion calculated in the plot using the 7Be inventory balance was 0·058 g cm?2, which is 23 per cent of the annual average erosion determined using 137Cs inventories. The calculated erosion from the mass balance is similar to the 0·059 g cm?2 of erosion estimated from the amount of sediment collected at the outlet of the 6 ha ?eld during the study period and the delivery ratio (0·64). The inventory balance of 7Be provides a new means for evaluating soil erosion over the time period most relevant to quantifying the prediction of erosion from runoff. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
72.
Flexure of the Indian plate and intraplate earthquakes 总被引:2,自引:0,他引:2
The flexural bulge in central India resulting from India's collision with Tibet has a wavelength of approximately 670 km.
It is manifest topographically and in the free-air gravity anomaly and the geoid. Calculations of the stress distribution
within a flexed Indian plate reveal spatial variations throughout the depth of the plate and also a function of distance from
the Himalaya. The wavelength (and therefore local gradient) of stress variation is a function of the effective elastic thickness
of the plate, estimates of which have been proposed to lie in the range 40–120 km. The imposition of this stress field on
the northward moving Indian plate appears fundamental to explaining the current distribution of intraplate earthquakes and
their mechanisms. The current study highlights an outer trough south of the flexural bulge in central India where surface
stresses are double the contiguous compressional stresses to the north and south. The Bhuj, Latur and Koyna earthquakes and
numerous other recent reverse faulting events occurred in this compressional setting. The N/S spatial gradient of stress exceeds
2 bars/km near the flexural bulge. The overall flexural stress distribution provides a physical basis for earthquake hazard
mapping and suggests that areas of central India where no historic earthquakes are recorded may yet be the locus of future
damaging events. 相似文献
73.
根据南海季风试验(SCSMEX)期间南海内区的三个ATLAS(Autonomous Temperature Line Acquisition System)锚碇浮标资料(1998年4月~1999年4月),采用谐波分析方法对南海上层海洋水温年循环、半年循环加以分离,发现无论在年循环还是在半年循环尺度上,18°N附近SCS1站与13°N附近SCS3站的水温变化次表层与表层呈反位相;15°20′N附近SCS2站水温变化基本上次表层与表层同位相.这说明不同区域上层海洋热变化受不同的正压与斜压模态控制.其次,SCS2、SCS3两点水温年循环振幅均在次表层达到极值;而SCS1在表层达到极大值,在100 m深度达到次极大值.3个站位水温半年循环振幅极值均出现在次表层内,这说明该层内的水温半年循环在温度变化趋势中所占的权重比在表层的权重大. 相似文献
74.
We present a numerical simulation of drag partition over rough surfaces. A computational fluid dynamics model is applied with high resolution to simulatingturbulent flows over arrays of roughness elements positioned on asmooth surface. The skin drag on the surface and the pressure drag on the roughnesselements are computed. The simulated drag partition compares well with wind-tunnelmeasurements and theoretical estimates for similar rough surfaces. This confirms that the computational approach offers an alternative to wind-tunnel and field experiments in studying drag and drag partition. The model is then applied to studying drag partition over rough surfaces with various roughness configurations. It is shown that drag partition depends not only on the magnitude of the roughness frontal area but also on the sizes and arrangement of roughness elements, because (1) the pressure drag coefficient is sensitive to roughness-element dimensions and (2) the arragement of roughness elements lead to different interferences of turbulent wakes. The impact ofthe latter factor is not insignificant. 相似文献
75.
76.
陈伯舫 《地震地磁观测与研究》1997,18(2):40-44
用通日月均值减去夜均值计算的月均值资料和Sompi谱分析法分析了14个台站的北向分量1年周期变化的振幅向相位,时段为1984-1988年。分析结果显示用P^0n(dnd=1,…,6)复合模式的估算的C值较合理。各台的C值比较一致。 相似文献
77.
Zheng Feili Tang Keli Zhang Keli Cha Xuan Bai Hongying Northwest Institute of Soil Water Conservation CAS Ministry of Water Resources Yangling Shaanxi People''s Republic of China 《地理学报(英文版)》1997,(2)
Forlongtime,theproblemaboutnaturalerosionandartificiallyacceleratederosionisindispute.HistoricalgeographersconsiderthathumaninducedecoenvironmentaldestructionisaccountablefortheseveresoilerosionandecoenvironmentdeteriorationonLoessPlateau[7,8].Somegeolog… 相似文献
78.
Zhang Jie Li Shengfeng Zhou Yinkang Du Jinkang Department of Geography Nanjing University Nanjing People''s Republic of China 《地理学报(英文版)》1997,(3)
1.IntroductionResearchonbiokarststretchfromnineteenthcentury,yetonlyrecently,moreefficientandsystematicresearchhasbeenputforwardwiththeappearanceofrelatedspecialterminologyanddetailedreviews[3,11,30,3638],anditseemsthatthereisstilllackofbiokarstinalpineenviro… 相似文献
79.
Tan Ming Qin Xiaoguang Liu Tungsheng Laboratory of Speleothem Paleoclimatology Institute of Geology CAS Beijing People’s Republic of China 《地理学报(英文版)》1997,(3)
I.IntroductionStalagmiteshavebeenappliedtouseashighresolutionpaleoclimaticindicatorssincetheannualgrowthbandingwasrecognizedinstalagmites[1,2].Annualgrowthbandingcanbemanifestedinseveralmannersdefinedby1)alternatingfluorescentandnon-fluorescentcalcite[3,4];2)… 相似文献
80.
A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district 总被引:1,自引:0,他引:1
K. D. van der Post F. Oldfield E. Y. Haworth P. R. J. Crooks P. G. Appleby 《Journal of Paleolimnology》1997,18(2):103-120
Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing. 相似文献