全文获取类型
收费全文 | 449篇 |
免费 | 67篇 |
国内免费 | 104篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 219篇 |
地球物理 | 142篇 |
地质学 | 71篇 |
海洋学 | 133篇 |
天文学 | 4篇 |
综合类 | 15篇 |
自然地理 | 33篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 15篇 |
2019年 | 9篇 |
2018年 | 13篇 |
2017年 | 5篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 29篇 |
2013年 | 25篇 |
2012年 | 22篇 |
2011年 | 34篇 |
2010年 | 22篇 |
2009年 | 43篇 |
2008年 | 42篇 |
2007年 | 45篇 |
2006年 | 48篇 |
2005年 | 34篇 |
2004年 | 24篇 |
2003年 | 24篇 |
2002年 | 18篇 |
2001年 | 21篇 |
2000年 | 9篇 |
1999年 | 18篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1983年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有620条查询结果,搜索用时 15 毫秒
41.
成都经济区天降水与下渗水元素地球化学特征及土壤元素输入输出通量 总被引:2,自引:0,他引:2
论文研究了成都经济区天降水和下渗水中元素含量、在农田耕层中的输入输出通量及其影响因素.研究表明,研究区雨水中含有大量SO2-4、NO-3等酸性物质,雨水中SO2-4 >NO-3>Cl-.雨水中Ca2 和NH 4含量最高,且NH 4>Ca2 >K >Na >Mg2 .雨水的pH与阴、阳离子摩尔浓度差值具有显著相关性.下渗水中以Ca2 为主要阳离子,且Ca2 >Na >Mg2 >K >NH 4;HCO-3为主要阴离子,且HCO-3>NO-3>SO2-4>Cl->F-,下渗水pH与阳、阴离子摩尔浓度差值具有显著相关性.不同地区雨水中Pb>As>Cd>Se>Hg,下渗水中Pb>As>Se>Cd>Hg,因此,Cd、Pb、Se和Hg等元素累积在耕层中,而As则被下渗水携带迁移出耕层进入地下水.由降雨输入土壤中的Cd通量均大于下渗水输出Cd的通量,局部地区As下渗通量高于雨水输入通量的5.45~13.16倍.土壤中元素的下渗比与土壤质地、pH有关. 相似文献
42.
本文用近年来的大量观测结果,揭示了被长期使用的梯度扩散理论的问题和局限性。讨论了它的成立条件:只有当湍流的涡旋尺度远小于梯度变化的空间尺度时,梯度扩散理论才可以正确使用。对于几种典型的近地层条件,计算了它们的湍流的涡旋尺度,从而说明了梯度扩散理论在这些条件下失败的原因。 相似文献
43.
This paper is a continuation of previous research, which obtained a convenient solution for arbitrary surface fluxes before ponding. By means of Fourier Transformation this has been extended to derive analytical solutions of a linearized Richards' equation for arbitrary input fluxes after surface saturation. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
44.
Farming of wild tuna in coastal areas is a relatively new aquaculture industry and little is known about the magnitude of nutrient discharges to the environment. In this work we present a preliminary model of nitrogen loads from southern bluefin tuna (Thunnus maccoyii) aquaculture in lower Spencer Gulf, South Australia. The model was developed based on feed inputs, estimates of fish metabolism and environmental data. Two pens were monitored over a full grow-out season to determine nitrogen sedimentation fluxes, remineralization at the sediment-water interface and accumulation in the sediments. The model suggests that the high metabolic rates of tuna lead to low retention of nitrogen in fish tissues (7-12% of feed inputs) and high environmental losses (260-502kg Ntonne(-1) growth). Considering Australian annual production of 4380tonnes over initial stocked biomass, total loads can reach 1137tonnes N per year, 86-92% lost as dissolved wastes. The nature of wastes suggests low localized impacts at current stocking densities and holding periods. 相似文献
45.
Water bodies are important storage media for persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs) and this function is increased in coastal regions because their inputs are higher than those to the open sea. The air-water interface is extensively involved with the global cycling of PCBs because it is the place where they accumulate due to depositional processes and where they may be emitted by gaseous exchange. In this work the parallel collection of air, microlayer and sub-superficial water samples was performed in July 2005 at a site in the Venice lagoon to evaluate the summer gaseous flux of PCBs. The total concentration of PCBs (sum of 118 congeners) in air varies from 87 to 273 pg m(-3), whereas in the operationally defined dissolved phase of microlayer and sub-superficial water samples it varies from 159 to 391 pg L(-1). No significant enrichment of dissolved PCB into the microlayer has been observed, although a preferential accumulation of most hydrophobic congeners occurs. Due to this behaviour, we believe that the modified two-layer model was the most suitable approach for the evaluation of the flux at the air-sea interface, because it takes into account the influence of the microlayer. From its application it appears that PCB volatilize from the lagoon waters with a net flux varying from 58 to 195 ng m(-2)d(-1) (uncertainty: +/-50-64%) due to the strong influence of wind speed. This flux is greater than those reported in the literature for the atmospheric deposition and rivers input and reveals that PCB are actively emitted from the Venice lagoon in summer months. 相似文献
46.
Performance of various similarity functions for nondimensional wind and temperature profiles in the surface layer in stable conditions 总被引:3,自引:0,他引:3
The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions. 相似文献
47.
由于海冰覆盖,北极碳汇(Arctic Carbon Sink)在全球碳通量预算中经常被忽略或简单处理。但随着全球变化加剧,北极发生快速变化,北极碳循环及其对全球变化的响应与反馈日趋重要。综合对北极碳汇的研究结果,分析了北极碳汇的来源、变化以及主要调控因子,评估了北极碳汇现状。探讨了在全球变化中,影响北极碳汇变化的因素及其对未来北极碳汇变化趋势的影响。
相似文献
48.
利用架设在珠峰北坡4475m高度处的一套开路涡动协方差测量系统,对曲宗地区的大气状况进行了连续观测.分析了近地层气象要素、辐射平衡各分量和能量平衡各分量在高原雨季前(5月),雨季中(7月)和雨季后(11月)的变化特征.通过分析,发现曲宗地区气温和相对湿度日平均变化均呈单峰单谷型特征,气压则呈双峰双谷型特征,风速日变化为单峰型特征,风速一般在午后突然增大.伴随着雨季的爆发,曲宗地区气温升高,相对湿度增大,气压升高,风速减小.主导风向由东北风转换成西南风.雨季前后,辐射平衡各分量及能量平衡各分量均具有明显的变化趋势. 相似文献
49.
Matthias Mauder Steven P. Oncley Roland Vogt Tamas Weidinger Luis Ribeiro Christian Bernhofer Thomas Foken Wim Kohsiek Henk A. R. De Bruin Heping Liu 《Boundary-Layer Meteorology》2007,123(1):29-54
The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these
flux measurements often do not satisfy a fundamental criterion—closure of the surface energy balance. This study investigates
to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects
of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field
data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat
flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor
separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat
flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor
combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and
also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than
5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers
Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system.
The LI-COR LI-7500 open-path H2O/CO2 gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct
solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor
have since been made.
The National Center for Atmospheric Research is supported by the National Science Foundation. 相似文献
50.
Improved Representation of Land-surface Heterogeneity in a Non-hydrostatic Numerical Weather Prediction Model 总被引:2,自引:1,他引:2
This study focuses on the relevance of accurate surface parameters, in particular soil moisture, and of parameterizations for heterogeneous land surfaces, for the prediction of sensible and latent heat fluxes by a mesoscale weather forecast model with horizontal grid resolution of 7 km. The analysis is based on model integrations for a 30-day period, which are compared both to flux measurements obtained from the LITFASS-2003 field experiment and to high-resolution-model (1-km grid spacing) results. At first, the relevance of improved parameter sets and input data compared to usual operational practice for an accurate prediction of near-surface fluxes is shown and discussed. It is demonstrated that an observation-based land-surface assimilation scheme leads to an improved soil moisture analysis, which is shown to be essential for the realistic simulation of surface fluxes. Secondly, the implementation of two efficient parameterization strategies for subgrid-scale variability of the surface, the mosaic and the tile approach, is presented. Using these methods, the simulations are in better agreement with measurements than simulations with simple aggregation methods that use effective surface parameters. Integrations with the mosaic approach reproduce high resolution simulations very well and more accurately than simulations with the tile method. Finally, the high resolution simulations are analyzed to justify and discuss the approximations underlying both methods. 相似文献