全文获取类型
收费全文 | 102078篇 |
免费 | 17624篇 |
国内免费 | 27057篇 |
专业分类
测绘学 | 7158篇 |
大气科学 | 9079篇 |
地球物理 | 16371篇 |
地质学 | 74857篇 |
海洋学 | 10175篇 |
天文学 | 9646篇 |
综合类 | 6725篇 |
自然地理 | 12748篇 |
出版年
2024年 | 403篇 |
2023年 | 1243篇 |
2022年 | 3024篇 |
2021年 | 3605篇 |
2020年 | 3520篇 |
2019年 | 4450篇 |
2018年 | 3457篇 |
2017年 | 3970篇 |
2016年 | 4296篇 |
2015年 | 4633篇 |
2014年 | 6081篇 |
2013年 | 5871篇 |
2012年 | 6441篇 |
2011年 | 6919篇 |
2010年 | 6098篇 |
2009年 | 7130篇 |
2008年 | 6963篇 |
2007年 | 7358篇 |
2006年 | 7213篇 |
2005年 | 6516篇 |
2004年 | 5908篇 |
2003年 | 5486篇 |
2002年 | 4710篇 |
2001年 | 4349篇 |
2000年 | 4138篇 |
1999年 | 3754篇 |
1998年 | 3415篇 |
1997年 | 2776篇 |
1996年 | 2427篇 |
1995年 | 1932篇 |
1994年 | 1882篇 |
1993年 | 1615篇 |
1992年 | 1272篇 |
1991年 | 905篇 |
1990年 | 748篇 |
1989年 | 593篇 |
1988年 | 461篇 |
1987年 | 292篇 |
1986年 | 224篇 |
1985年 | 168篇 |
1984年 | 81篇 |
1983年 | 55篇 |
1982年 | 55篇 |
1981年 | 40篇 |
1980年 | 43篇 |
1979年 | 39篇 |
1978年 | 58篇 |
1977年 | 38篇 |
1976年 | 15篇 |
1954年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The source and hydrochemical makeup of a stream reflects the connectivity between rainfall, groundwater, the stream, and is reflected to water quantity and quality of the catchment. However, in a semi-arid, thick, loess covered catchment, temporal variation of stream source and event associated behaviours are lesser known. Thus, the isotopic and chemical hydrographs in a widely distributed, deep loess, semi-arid catchment of the northern Chinese Loess Plateau were characterized to determine the source and hydrochemical behaviours of the stream during intra-rainfall events. Rainfall and streamflow were sampled during six hydrologic events coupled with measurements of stream baseflow and groundwater. The deuterium isotope (2H), major ions (Cl−, SO42−, NO3−, Ca2+, K+, Mg2+, and Na+) were evaluated in water samples obtained during rainfall events. Temporal variation of 2H and Cl− measured in the groundwater and stream baseflow prior to rainfall was similar; however, the isotope compositions of the streamflow fluctuated significantly and responded quickly to rainfall events, likely due to an infiltration excess, overland dominated surface runoff during torrential rainfall events. Time source separation using 2H demonstrated greater than 72% on average, the stream composition was event water during torrential rainfall events, with the proportion increasing with rainfall intensity. Solutes concentrations in the stream had loglinear relationships with stream discharge, with an outling anomaly with an example of an intra-rainfall event on Oct. 24, 2015. Stream Cl− behaved nonconservative during rainfall events, temporal variation of Cl− indicated a flush and washout at the onset of small rainfall events, a dilution but still high concentration pattern in high discharge and old water dominated in regression flow period. This study indicates rainfall intensity affects runoff responses in a semi-arid catchment, and the stored water in the thick, loess covered areas was less connected with stream runoff. Solute transport may threaten water quality in the area, requiring further analysis of the performance of the eco-restoration project. 相似文献
2.
Sam J. Leuthold Stephanie A. Ewing Robert A. Payn Florence R. Miller Stephan G. Custer 《水文研究》2021,35(2):e14029
In snowmelt-driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high-relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow-derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late-winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid-summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions. 相似文献
3.
A FORTRAN program, consistent with the commercially available finite element (FE) code ABAQUS, is developed based on a three-dimensional (3D) linear elastic brittle damage constitutive model with two damage criteria. To consider the heterogeneity of rock, the developed FORTRAN program is used to set the stiffness and strength properties of each element of the FE model following a Weibull distribution function. The reliability of the program is assessed against available experimental results for granite cylindrical specimens with a throughgoing, flat and inclined fissure. The calibration procedure of the material parameters is explained in detail, and it is shown that the compressive to tensile strength ratio can have a substantial influence on the failure response of the specimens. Numerical simulations are conducted for models with different levels of heterogeneity. The results show a smaller load bearing capacity for models with less homogeneity, representing gradual coalescence of fully damaged elements forming throughout the models during loading. The maximum load bearing capacity is studied for various combinations of inclination angles of two centrally aligned, throughgoing and flat fissures of equal length embedded in cylindrical models under uniaxial and multiaxial loading conditions. The key role of the compressive to tensile strength ratio is highlighted by repeating certain simulations with a lower compressive to tensile strength ratio. It is proven that the peak loads of the rock models with sufficiently small compressive to tensile strength ratios containing two throughgoing fissures of equal length are similar, provided that the minimum inclination angles of the models are the same. The results are presented and discussed with respect to the existing experimental findings in the literature, suggesting that the numerical model applied in this study can provide useful insight into the failure behaviour of rock-like materials. 相似文献
4.
本文回顾了自然和自然贡献情景模型发展的背景、历史和内容,概括总结了自然和自然贡献情景模型的发展进程以及联合国生物多样性与生态系统服务政府间科学—政策平台(IPBES)情景模型的概念框架,讨论了自然和自然贡献情景模型存在的问题和发展方向。为了在全球层面解决现有综合集成模型存在的问题,根据地球表层建模基本定理和生态环境曲面建模基本定理,提出了具有中国原创特点的自然与自然贡献情景模型概念框架。 相似文献
5.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
6.
7.
8.
9.
10.