全文获取类型
收费全文 | 6129篇 |
免费 | 1102篇 |
国内免费 | 834篇 |
专业分类
测绘学 | 860篇 |
大气科学 | 836篇 |
地球物理 | 1993篇 |
地质学 | 1513篇 |
海洋学 | 906篇 |
天文学 | 437篇 |
综合类 | 540篇 |
自然地理 | 980篇 |
出版年
2024年 | 18篇 |
2023年 | 59篇 |
2022年 | 181篇 |
2021年 | 203篇 |
2020年 | 231篇 |
2019年 | 292篇 |
2018年 | 206篇 |
2017年 | 270篇 |
2016年 | 254篇 |
2015年 | 260篇 |
2014年 | 319篇 |
2013年 | 354篇 |
2012年 | 402篇 |
2011年 | 376篇 |
2010年 | 310篇 |
2009年 | 382篇 |
2008年 | 341篇 |
2007年 | 422篇 |
2006年 | 407篇 |
2005年 | 344篇 |
2004年 | 293篇 |
2003年 | 281篇 |
2002年 | 220篇 |
2001年 | 218篇 |
2000年 | 183篇 |
1999年 | 182篇 |
1998年 | 196篇 |
1997年 | 112篇 |
1996年 | 116篇 |
1995年 | 115篇 |
1994年 | 114篇 |
1993年 | 106篇 |
1992年 | 60篇 |
1991年 | 49篇 |
1990年 | 29篇 |
1989年 | 51篇 |
1988年 | 28篇 |
1987年 | 25篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有8065条查询结果,搜索用时 15 毫秒
91.
Estimating concentrations or flow rates along a stream network requires specific models. Two classes of models, recently proposed in the literature, are generalized, to the intrinsic case in particular. We present a global construction by ‘streams’, i.e. on the whole set of paths between sources and outlet. Combining stationary or intrinsic one-dimensional random functions leads to stationary or intrinsic models on segments, with discontinuities at the forks. A construction from outlet to sources, leads to stationary or intrinsic models on each stream, without any discontinuity at the forks. The linear variogram is found as a particular case. The extension to the linear model of coregionalization is immediate, allowing a multivariate modelling of concentrations. To cite this article: C. de Fouquet, C. Bernard-Michel, C. R. Geoscience 338 (2006). 相似文献
92.
Philip M. Marren Terence S. McCarthy Stephen Tooth Dion Brandt Glenn G. Stacey Anita Leong Beth Spottiswoode 《Sedimentary Geology》2006,190(1-4):213-226
Along a 28 km reach of the Klip River, eastern Free State, South Africa, mud- and sand-dominated meanders have developed in close proximity within a floodplain wetland up to 1.5 km wide, providing an unusual opportunity to compare their characteristics under similar hydrological conditions. Throughout the reach, the channel bed is grounded on sandstone/shale bedrock although the banks are alluvial, and most river activity occurs during summer high flows. The reach can be divided into three geomorphological zones: Zone 1 (0–11 km), a muddy proximal part with a single meandering channel (w/d < 10) and near-permanent standing water in oxbows and backswamps; Zone 2 (11–17.5 km), a transitional mud-to-sand part with one main channel (w/d 20–30), a number of sinuous palaeochannels and oxbows, and only limited standing water; and Zone 3 (17.5–28 km), a sandy distal part with a single meandering channel (w/d 15–30), scroll bars and oxbows, and little standing water. Each zone also has a distinctive sedimentology: Zone 1 is characterised by an 3–4 m thick succession of basal sand and minor granules overlain by dominantly muddy sediment deposited primarily by oblique accretion in meander bends; Zone 2 is characterised by < 4 m of interbedded sand and mud deposited primarily by lateral point-bar accretion, although a history of avulsions also attests to the importance of abandoned-channel accretion; and Zone 3 is characterised by < 3 m of dominantly sand deposited primarily by lateral point-bar accretion. This unusual downstream sediment coarsening trend, and the associated changes in channel and floodplain character, are independent of sediment inputs from tributaries, and result from a downstream increase in bankfull unit stream power from < 3.5 W m− 2 (Zone 1) to 4–10 W m− 2 (Zone 3). Mud is deposited primarily in low-energy Zone 1 but is conveyed in suspension more effectively through higher energy Zones 2 and 3, only forming drapes over sandy lateral accretion deposits during waning flood stages. The downstream increase in unit stream power is controlled in part by a slight downstream increase in floodplain gradient that may be related to a subtle variation in the erosional resistance of the bedrock underlying the channel bed. These findings add to previous work on meandering rivers by demonstrating that mud-dominated meanders can occur in long-term erosional settings where the channel bed is grounded on bedrock, and that downstream fining trends may be reversed locally. 相似文献
93.
94.
Chironomid remains from the sediment of Lake Vuolep Njakajaure reflect limnological conditions resulting from changing climate and vegetation throughout the Holocene, but do not strictly follow accepted climate trends or the vegetation history based on regional pollen and macrofossil analyses. Chironomid community changes appear to be influenced by organic nutrient input from the surrounding catchment vegetation and lake hydrology, both of which are indirectly responding to some combination of climate change, hypolimnetic oxygen concentration, and changes in basin morphology. The chironomid-based quantitative mean July air-temperature reconstruction differs from other regional quantitative records; this discrepancy is likely related to limnological conditions particular to Lake Vuolep Njakajaure. Comparison of a northern Swedish temperature transfer function and one from western Canada reveals differences in the mean July air-temperature optima of several common taxa, suggesting that the existing conservative estimates of Holocene climate change in northern Sweden may be underestimated due to the limited temperature gradient captured by the Swedish training set. 相似文献
95.
96.
97.
Scott W. White 《Natural Resources Research》2006,15(4):271-281
This paper updates a life-cycle net energy analysis and carbon dioxide emissions analysis of three Midwestern utility-scale
wind systems. Both the Energy Payback Ratio (EPR) and CO2 analysis results provide useful data for policy discussions regarding an efficient and low-carbon energy mix. The EPR is
the amount of electrical energy produced for the lifetime of the power plant divided by the total amount of energy required
to procure and transport the materials, build, operate, and decommission the power plants. The CO2 analysis for each power plant was calculated from the life-cycle energy input data.
A previous study also analyzed coal and nuclear fission power plants. At the time of that study, two of the three wind systems
had less than a full year of generation data to project the life-cycle energy production. This study updates the analysis
of three wind systems with an additional four to eight years of operating data.
The EPR for the utility-scale wind systems ranges from a low of 11 for a two-turbine system in Wisconsin to 28 for a 143-turbine
system in southwestern Minnesota. The EPR is 11 for coal, 25 for fission with gas centrifuge enriched uranium and 7 for gaseous
diffusion enriched uranium. The normalized CO2 emissions, in tonnes of CO2 per GWeh, ranges from 14 to 33 for the wind systems, 974 for coal, and 10 and 34 for nuclear fission using gas centrifuge and gaseous
diffusion enriched uranium, respectively. 相似文献
98.
Paul Augustinus Michael Reid Sofia Andersson Yanbin Deng Mark Horrocks 《Journal of Paleolimnology》2006,35(4):789-805
Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record
of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the
past c. 190 years. A reliable chronology is available from AMS 14C and 210Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of
introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope
geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke
catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent
nuisance algal blooms in 1933, 1934 and 1939 using CuSO4 addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the 210Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant
anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years.
The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually
allowed unambiguous identification of the signatures of change and their causes. 相似文献
99.
The computation of electromagnetic (EM) fields, for 1-D layered earth model, requires evaluation of Hankel Transform (HT)
of the EM kernel function. The digital filtering is the most widely used technique to evaluate HT integrals. However, it has
some obvious shortcomings. We present an alternative scheme, based on an orthonormal exponential approximation of the kernel
function, for evaluating HT integrals. This approximation of the kernel function was chosen because the analytical solution
of HT of an exponential function is readily available in literature. This expansion reduces the integral to a simple algebraic
sum. The implementation of such a scheme requires that the weights and the exponents of the exponential function be estimated.
The exponents were estimated through a guided search algorithm while the weights were obtained using Marquardt matrix inversion
method. The algorithm was tested on analytical HT pairs available in literature. The results are compared with those obtained
using the digital filtering technique with Anderson filters. The field curves for four types (A-, K-, H-and Q-type) of 3-layer
earth models are generated using the present scheme and compared with the corresponding curves obtained using the Anderson
sc heme. It is concluded that the present scheme is more accurate than the Anderson scheme 相似文献
100.
现阶段,地质灾害评估技术人员在评估实践中,往往不注重各评估对象的差异性,采取同一评估模式,结果达不到评估的目的。在对小型水电站进行评估时,野外调查的重点,评估过程中所使用的方法以及危险性现状、预测及综合分区评估等与大型水电站、路线工程、矿山开采、民用建筑等工程有所不同。湄尼多河小型水电站工程位于云南省怒江州福贡县马吉乡境内,地质环境条件复杂,评估级别综合为二级。文章以该电站建设用地地质灾害危险性评估为例,对小型水电站建设用地地质灾害危险性评估的方法与理论进行探讨。 相似文献