全文获取类型
收费全文 | 1224篇 |
免费 | 78篇 |
国内免费 | 283篇 |
专业分类
测绘学 | 67篇 |
大气科学 | 131篇 |
地球物理 | 475篇 |
地质学 | 563篇 |
海洋学 | 142篇 |
天文学 | 77篇 |
综合类 | 28篇 |
自然地理 | 102篇 |
出版年
2023年 | 13篇 |
2022年 | 13篇 |
2021年 | 11篇 |
2020年 | 8篇 |
2019年 | 14篇 |
2018年 | 16篇 |
2016年 | 23篇 |
2015年 | 15篇 |
2014年 | 23篇 |
2013年 | 23篇 |
2012年 | 50篇 |
2011年 | 64篇 |
2010年 | 25篇 |
2009年 | 35篇 |
2008年 | 56篇 |
2007年 | 76篇 |
2006年 | 65篇 |
2005年 | 70篇 |
2004年 | 62篇 |
2003年 | 54篇 |
2002年 | 47篇 |
2001年 | 45篇 |
2000年 | 96篇 |
1999年 | 20篇 |
1998年 | 20篇 |
1997年 | 30篇 |
1996年 | 25篇 |
1995年 | 44篇 |
1994年 | 29篇 |
1993年 | 58篇 |
1992年 | 35篇 |
1991年 | 47篇 |
1990年 | 37篇 |
1989年 | 30篇 |
1988年 | 27篇 |
1987年 | 39篇 |
1986年 | 27篇 |
1985年 | 30篇 |
1984年 | 44篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1959年 | 5篇 |
1958年 | 7篇 |
1957年 | 4篇 |
1936年 | 4篇 |
1930年 | 3篇 |
1926年 | 4篇 |
1924年 | 4篇 |
排序方式: 共有1585条查询结果,搜索用时 15 毫秒
971.
972.
Remediation of contaminated aquifers demands a reliable characterization of hydraulic connectivity patterns. Hydraulic diffusivity is possibly the best indicator of connectivity. It can be derived using the tidal response method (TRM), which is based on fitting observations to a closed-form solution. Unfortunately, the conventional TRM assumes homogeneity. The objective of this study was to overcome this limitation and use tidal response to identify preferential flowpaths. Additionally, the procedure requires joint inversion with hydraulic test data. These provide further information on connectivity and are needed to resolve diffusivity into transmissivity and storage coefficient. Spatial variability is characterized using the regularized pilot points method. Actual application may be complicated by the need to filter tidal effects from the response to pumping and by the need to deal with different types of data, which we have addressed using maximum likelihood methods. Application to a contaminated artificial coastal fill leads to flowpaths that are consistent with the materials used during construction and to solute transport predictions that compare well with observations. We conclude that tidal response can be used to identify connectivity patterns. As such, it should be useful when designing measures to control sea water intrusion. 相似文献
973.
The EC Water Framework Directive (WFD) suggests using abundance and species composition of intertidal seaweed communities for ecological quality classification of rocky seashores. There are two difficulties with this. According to WFD all sensitive species should be present on a shore. There is no accepted list of sensitive seaweed species and those which may be sensitive in one location may not be so in another. Second, natural successions can result in very large abundance changes of common species, e.g. from almost completely fucoid-dominated shores to almost totally barnacle-dominated shores, without any change in ecological quality. Studies have shown that numerical species richness, not the list of actual species present, is broadly constant in the absence of disturbance. The ephemeral species, possibly the sensitive members of the community, change regularly in such a way as to conserve species richness. It is proposed that species richness on a defined length of shore be used as a criterion of ecological quality. A database of species found on over 300 shores in the British Isles, under strictly controlled sampling conditions, has given ranges of values of species richness to be expected and has allowed for variations in these values due to sub-habitat variability, wave exposure and turbidity to be factored in. A major problem in applying such a tool is the lack of expertise of many workers in critical identification of seaweed species. A reduced species list has been extracted from the database using species commonly present and identifiable with reasonable certainty. A numerical index of ecological quality is proposed based on scores for various aspects of the physical nature of the habitat combined with a score for species richness which may be based on the reduced species list. The scoring system also uses further aspects of community structure, such as ecological status groups and the proportions of rhodophyta, chlorophyta and opportunist species. For this system to be effective there has to be close control of the way in which sampling is carried out to ensure a uniform level of thoroughness. 相似文献
974.
The effect of vertical turbulent mixing on the dynamics of persistent organic pollutants has long been overlooked and its role is still hardly understood. Here we present the first comprehensive analysis of the role of turbulent diffusion on the distribution of those contaminants and its interplay with sinking fluxes. To this end, a 1D dynamic coupled hydrodynamic-contaminant model has been developed and applied to a Mediterranean continental shelf environment. The hydrodynamic sub-model is adapted from COHERENS, the contaminant sub-model is an improvement from the BIODEP model and considers the contaminant in 3 phases: dissolved-colloidal-particulate. The simulation highlights the role of turbulence in determining the POP distribution and variability in the water column. In short, turbulent flux of contaminants strengthens the upward diffusion of sediment entrained contaminants and determines the extent to which inputs from the atmosphere mix into the water column. It acts in parallel with degradation and sinking fluxes, the combined effect yielding a surface enriched - depth depleted - benthic layer enriched region distribution, which presents similarities to reported experimental measures. 相似文献
975.
Ship board testing of a deoxygenation ballast water treatment 总被引:2,自引:1,他引:1
McCollin T Quilez-Badia G Josefsen KD Gill ME Mesbahi E Frid CL 《Marine pollution bulletin》2007,54(8):1170-1178
A ship board trial of a deoxygenation method for treating ballast water was carried out during a voyage from Southampton (United Kingdom) to Manzanillo (Panama). A nutrient solution added to two ballast tanks encouraged bacterial growth, resulting in a gradual change to an anoxic environment. Samples were taken from two treated tanks and two untreated tanks to assess changes in the abundance and viability of zooplankton, phytoplankton and bacteria. The work was carried out before the International Maritime Organization (IMO) standard was agreed so only a broad indication of whether the results achieved the standard was given. For the zooplankton, the standard would have been achieved within 5 or 7 days but the phytoplankton results were inconclusive. The biological efficacy was the result of the combination of several factors, including the treatment, pump damage and an increase in the water temperature during the voyage. 相似文献
976.
Factors affecting marine debris deposition at French Frigate Shoals, Northwestern Hawaiian Islands Marine National Monument, 1990-2006 总被引:1,自引:0,他引:1
Data on the amount and type of small debris items deposited on the beaches of the Hawaiian Islands National Wildlife Refuge Tern Island station, French Frigate Shoals were collected over 16 years. We calculated deposition rates and investigated the relationship among deposition and year, season, El Ni?o and La Ni?a events from 1990 to 2006. In total 52,442 debris items were collected with plastic comprising 71% of all items collected. Annual debris deposition varied significantly (range 1116-5195 items) but was not influenced by season. Debris deposition was significantly greater during El Ni?o events as compared to La Ni?a events. Although often deduced to influence floating marine pollution, this study provides the first quantitative evidence of the influence of El Ni?o/La Ni?a cycles on marine debris deposition. 相似文献
977.
Ballesteros E Torras X Pinedo S García M Mangialajo L de Torres M 《Marine pollution bulletin》2007,55(1-6):172-180
Macroalgae is a biological key element for the assessment of the ecological status in coastal waters in the frame of the European Water Framework Directive (WFD, 2000/60/EC). Here we propose a methodology for monitoring water quality based on the cartography of littoral and upper-sublittoral rocky-shore communities (CARLIT, in short). With the use of spatial databases, GIS, and available information about the value of rocky-shore communities as indicators of water quality, it is possible to obtain an environmental quality index representative of the ecological status of rocky coasts. This index, which completely fulfils the requirements of the WFD, is expressed as a ratio between the observed values in the sector of shore that is being assessed and the expected value in a reference condition zone with the same substrate and coastal morphology (Ecological Quality Ratio, EQR). The application of this index to the coast of Catalonia (North-Western Mediterranean) is presented. 相似文献
978.
Among the various quality elements which the Water Framework Directive requires should be monitored are macroalgae. One aspect of these is the presence, in transitional waters particularly, of large blooms of opportunistic macroalgae, such as Ulva and Enteromorpha. Within the United Kingdom (UK) and Republic of Ireland (RoI) there are currently no set ecological quality objectives or standards for macroalgae. Nor are there standard methods for monitoring macroalgal blooms, although various combinations of aerial photography, remote sensing and measurements on the ground are used. This paper attempts to set a logical framework for the prioritisation of sites for monitoring, the development of a tiered monitoring procedure and the derivation of thresholds for classification. Draft threshold limits for percentage cover and biomass of macroalgae have been derived from the literature. The importance of secondary effects and physico-chemical parameters is discussed. 相似文献
979.
对浙江省安吉县梅溪发电厂粉煤灰贮灰场附近的土壤短剖面进行环境磁学、元素含量及粒度研究.借助模糊聚类分析,分辨出粉煤灰原位堆积、富集、迁移深度及未受污染土壤特征.剖面上部0~14cm为粉煤灰的原位反映,多畴磁铁矿主导了该段样品磁性.其中11~14cm处磁性矿物和金属元素有所富集.14~21cm为粉煤灰在土壤中的迁移深度.迁移段土壤中亚铁磁性矿物及Pb、Zn等金属含量介于底部未污染土壤与上部粉煤灰原位堆积层之间.单畴斜交反铁磁性矿物及来源于上部粉煤灰的多畴磁铁矿共同主导了该段土壤的磁性特征.21cm以下土壤基本未受粉煤灰污染,单畴斜交反铁磁性矿物是其主要磁性矿物.指标聚类分析表明,亚铁磁性矿物主要赋存在粗粉砂中,单畴磁晶粒主要存在于细粉砂中.χ、ARM、SIRM与Cu、Mn、Pb、Zn呈显著相关.结合多元统计分析,磁参数可用于追踪、识别粉煤灰在土壤剖面中的垂向迁移. 相似文献
980.
V.R. BELYAEV E. A. EREMENKO A. V. PANIN Y. R. BELYAEV 《国际泥沙研究》2005,20(3):224-232
Two gullies located in the Protva River basin (central part of European Russia, 100 Ion southwest of Moscow) were studied to investigate the history of their development during Late Holocene. This case study included detailed topographic and geomorphologic surveys, geological cross-sections, and radiocarbon dating of organic matter in gully sediments. For both gullies, incision of the long profile occurred for most of the Holocene, as indicated by the limited presence of infill sediment. However, interruptions of incisions occurred in both gullies during the last -3.5 ka (the Chugunkin gully) or -2.0 ka (the Uzkiy gully), as evident from stratigraphic interpretation of the sediment record. Periods of surface stabilization are represented by buried soil horizons, whereas relatively thick sediment layers suggest periods of predominant aggradation. Similarities and differences in discovered sequences of erosion and accumulation stages for the two gullies point out the importance of distinguishing between local and regional controls. Evidence of the role of internal system behavior and self-development in the gully long-term evolution is clear from asynchronous response of different parts of gullies. In recent times (-500 years), active agricultural development of the study territory resulted in accelerated incisions, followed by deposition of catchment-derived material. The latter was limited to the upper parts of the gullies, while incisions have continued in the middle and lower parts. The impact of human activities (-500 years) has been very limited. This suggests that within the forest zone of the Russian Plain many gullies which were earlier attributed to anthropogenic factors are, in realitv, due to natural phenomena. 相似文献