全文获取类型
收费全文 | 611篇 |
免费 | 76篇 |
国内免费 | 73篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 78篇 |
地球物理 | 278篇 |
地质学 | 165篇 |
海洋学 | 101篇 |
天文学 | 34篇 |
综合类 | 18篇 |
自然地理 | 50篇 |
出版年
2023年 | 6篇 |
2022年 | 14篇 |
2021年 | 15篇 |
2020年 | 19篇 |
2019年 | 32篇 |
2018年 | 20篇 |
2017年 | 36篇 |
2016年 | 26篇 |
2015年 | 18篇 |
2014年 | 30篇 |
2013年 | 33篇 |
2012年 | 13篇 |
2011年 | 44篇 |
2010年 | 26篇 |
2009年 | 61篇 |
2008年 | 47篇 |
2007年 | 47篇 |
2006年 | 44篇 |
2005年 | 24篇 |
2004年 | 21篇 |
2003年 | 20篇 |
2002年 | 16篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 19篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有760条查询结果,搜索用时 156 毫秒
521.
The dynamic headspace titration of the volatile constituents of the black truffle(Tuber Melanosporum)isplanned to be carried out with the new gas chromatographic device DCI using Tenax trapping.In thispaper the necessary optimization of the experimental factors which influence the desorption of thevolatiles from the sample and the adsorption on the trap is carried out by experimental design.The studyof the isoresponse curves makes it possible to determine the optimum conditions for a rapid titration ofhighest sensitivity and reproducibility,avoiding water trapping. 相似文献
522.
R.RAJAGOPAL PING-CHI LI Jessup Hall Department of Geography The University of Iow Iowa City IA U.S.A. 《地理学报(英文版)》1991,(3)
It is shown that the presence of 31-35 commonly measured volatile organic compounds(VOCs)inground water can be detected with small error rates by using screening methods which analyze for a subsetof such VOCs.A study of selected data sets indicates that analytical determinations of only from twoto eight VOCs will suffice to detect 95% of all VOC hits.It is also shown that a serially optimal algorithmfor selecting the VOCs for screening is very nearly as accurate as a globally optimal algorithm and mucheasier to implement.These conclusions are supported by empirical evidence from two drinking-water datasets and one hazardous waste site data set.Additional research areas are also outlined. 相似文献
523.
A program for the potentiometric determination of the protonation constants of mononuclear polyproticsubstances is described.A maximum of twelve parameters can be determined simultaneously,includingup to six protonation constants,four electrode calibration parameters,the protolysis constant of thesolvent and the titrant concentration.Optimization is carried out by using the non-simplifiedNewton-Raphson method,which is potentiated by the Marquardt algorithm and a distance speeding-upcoefficient.A direct search method is also used to improve the initial set of values.Variances arecalculated very accurately,since the real Hessian function is used.Statistical weights and ionic strengthcorrections are also considered,The program has been tested by using simulated titration curves ofpolyprotic acids with close constants. 相似文献
524.
An in-channel surficial depositional feature (surficial fine-grained laminae) composed of loosely bound fine sediment deposited during low flow conditions has often been observed in river systems in south-western Ontario. The physical characteristics of this feature have been determined by a direct observation image analysis system. This sediment consists primarily of flocculated fine-grained material. The size distributions of surficial fine-grained laminae and suspended sediment were not significantly different. Each distribution is bimodal in nature and shows a characteristic grain size deficiency in the 4-5 μm size range. This observation suggests that flocculation and not low discharge or low competence is the dominant mechanism for the formation of surficial fine-grained laminae under conditions of low flow in fluvial systems of south-western Ontario. A quantitative assessment of this feature shows its potential importance as a source of fine-grained sediment and associated contaminants for downstream transport. 相似文献
525.
Antoine Thill Stphane Moustier Jean-Marie Garnier Claude Estournel Jean-Jacques Naudin Jean-Yves Bottero 《Continental Shelf Research》2001,21(18-19)
The salt induced flocculation phenomenon is often proposed as a major mechanism influencing the deposition of the suspended matter near the mouth of major rivers. However, depending on the particular mixing conditions between fresh water and marine water and on suspended solids (SS) reactivity, salt induced aggregation may be a minor factor controlling fast deposition of SS. This work combines field studies and laboratory experiments in order to assess the importance of salt induced flocculation in the case of the saltwedge estuary of the Rhône river. Sampling of the mixing zone has been performed in contrasted hydrodynamic conditions from a low water discharge period (500 m3 s−1) to a small flood event (2400 m3 s−1) for particle counting and salinity measurements. Through laboratory experiments, it is shown that the Rhône river particulate matter has a poor average reactivity regarding salt induced flocculation. Considering the hydrodynamics of the estuary, we show that the evolution of the concentration of larger particles (>5 μm) can be explained by settling and dilution. In contrast, the smallest measured fraction (2–5 μm) shows a more complex behavior and settling processes alone cannot explain the observed particle concentrations during the field studies. Four hypotheses are discussed for explaining the 2–5 μm particle concentration evolutions. 相似文献
526.
Transport of suspended solids from a karstic to an alluvial aquifer: the role of the karst/alluvium interface 总被引:1,自引:0,他引:1
N. Massei M. Lacroix H. Q. Wang B. J. Mahler J. P. Dupont 《Journal of Hydrology》2002,260(1-4):88-101
This study focuses on the coupled transport of dissolved constituents and particulates, from their infiltration on a karst plateau to their discharge from a karst spring and their arrival at a well in an alluvial plain. Particulate markers were identified and the transport of solids was characterised in situ in porous and karstic media, based on particle size analyses, SEM, and traces. Transport from the sinkhole to the spring appeared to be dominated by flow through karst: particulate transport was apparently conservative between the two sites, and there was little difference in the overall character of the particle size distribution of the particulates infiltrating the sinkhole and of those discharging from the spring. Qualitatively, the mineralogy of the infiltrating and discharging material was similar, although at the spring an autochthonous contribution from the aquifer was noted (chalk particles eroded from the parent rock by weathering). In contrast, transport between the spring and the well appears to be affected by the overlying alluvium: particles in the water from the well, showed evidence of considerable size-sorting. Additionally, SEM images of the well samples showed the presence of particles originating from the overlying alluvial system; these particles were not found in samples from the sinkhole or the spring. The differences between the particulates discharging from the spring and the well indicate that the water pumped from the alluvial plain is coming from the karst aquifer via the very transmissive, complex geologic interface between the underlying chalk formation and the gravel at the base of the overlying alluvial system. 相似文献
527.
测量平面混合控制网平差与优化设计系统 总被引:1,自引:0,他引:1
在Windows操作系统平台上,将数据库语言Foxpro 2.5 for Windows 和Turbo C语言结合起来,研制出了测量平面混合控制网的整体平差和优化设计系统。本系统操作方便,成果可靠,适用于各种常规工程控制网的平差与优化设计。 相似文献
528.
H. F. P. van den Boogaard M. J. J. Hoogkamer A. W. Heemink 《Stochastic Environmental Research and Risk Assessment (SERRA)》1993,7(2):109-130
For the simulation of the transport of dissolved matter particle models can be used. In this paper a technique is developed for the identification of uncertain parameters in these models. This model calibration is formulated as an optimization problem and is solved with a gradient based algorithm. Here adjoint particle tracks are used for the calculation of the gradient of the cost function. The performance of the calibration method is illustrated by simulations and an application to a river Rhine water quality calamity in November 1986. 相似文献
529.
In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were
deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986
to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were
high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production
and transport, however, differ in both the regions.
In the Arabian Sea, average annual fluxes are over 50gm-2y-1 in the western Arabian Sea and less than 27gm-2 y-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak
flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the
euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central
Bay of Bengal (over
50gm-2y-1) and are least in the southern part of the Bay (37gm-2y-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate
and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns
in the surface waters as a result of increased freshwater and nutrient input from rivers.
Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal
even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual
variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer
from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents. 相似文献
530.
The Siret River has the largest drainage basin (42 274 km2) in Romania. It gathers all the rivers from the eastern part of the Eastern Carpathians, a fact that causes marked asymmetry of the basin. This study is principally concerned with changes in the form of the longitudinal profile and the grain size variability introduced by the Carpathian tributaries. Channel sediment analyses considered the petrography, granulometry, and morphometry of the pebbles, relating these to the river bed and floodplain geometry and to some properties of the drainage basin. The following conclusions arise. The Siret River undergoes an intense regrading of its longitudinal profile, with marked aggradation between transects 24 and 26 (see Figures 1 and 2). This reflects selective accumulation of coarse material due to the massive contribution of the Carpathian tributaries. This phenomenon has been continuous throughout the Holocene, resulting in the gravel sheet formation of the Pericarpathian piedmont. 相似文献