全文获取类型
收费全文 | 72266篇 |
免费 | 2223篇 |
国内免费 | 651篇 |
专业分类
测绘学 | 1814篇 |
大气科学 | 5273篇 |
地球物理 | 15941篇 |
地质学 | 25491篇 |
海洋学 | 5921篇 |
天文学 | 16519篇 |
综合类 | 158篇 |
自然地理 | 4023篇 |
出版年
2021年 | 547篇 |
2020年 | 654篇 |
2019年 | 804篇 |
2018年 | 1644篇 |
2017年 | 1609篇 |
2016年 | 2023篇 |
2015年 | 1313篇 |
2014年 | 2006篇 |
2013年 | 3705篇 |
2012年 | 2133篇 |
2011年 | 2810篇 |
2010年 | 2474篇 |
2009年 | 3293篇 |
2008年 | 2834篇 |
2007年 | 2695篇 |
2006年 | 2593篇 |
2005年 | 2067篇 |
2004年 | 2039篇 |
2003年 | 1995篇 |
2002年 | 1907篇 |
2001年 | 1706篇 |
2000年 | 1680篇 |
1999年 | 1383篇 |
1998年 | 1355篇 |
1997年 | 1343篇 |
1996年 | 1178篇 |
1995年 | 1153篇 |
1994年 | 1027篇 |
1993年 | 897篇 |
1992年 | 856篇 |
1991年 | 866篇 |
1990年 | 944篇 |
1989年 | 843篇 |
1988年 | 783篇 |
1987年 | 982篇 |
1986年 | 856篇 |
1985年 | 1003篇 |
1984年 | 1180篇 |
1983年 | 1145篇 |
1982年 | 1007篇 |
1981年 | 1040篇 |
1980年 | 898篇 |
1979年 | 848篇 |
1978年 | 883篇 |
1977年 | 792篇 |
1976年 | 730篇 |
1975年 | 721篇 |
1974年 | 743篇 |
1973年 | 763篇 |
1972年 | 483篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
941.
Carbon balance in a salt marsh: Interactions of diffusive export, tidal deposition and rainfall-caused erosion 总被引:1,自引:0,他引:1
Alice G. Chalmers Richard G. Wiegert Paul L. Wolf 《Estuarine, Coastal and Shelf Science》1985,21(6):757-771
Studies of the concentrations of particulate and dissolved organic carbon in the Duplin River, of the tidal exchange of POC and DOC in the marsh, of the standing stock and movement of Spartina alterniflora wrack in the Duplin, and of the removal of carbon from the surface of the marsh by rain were conducted at Sapelo Island, Georgia in order to test three hypotheses about export of carbon from the Duplin River watershed. We found that the gradients in POC and DOC concentrations are such that carbon is being transported down the Duplin River throughout the year, although in smaller quantities than previously believed. In contrast, almost all tidal exchanges within the marsh result in deposition of carbon. Most of this deposited carbon is subsequently eroded as a result of rain falling on the exposed marsh surface, and is washed back into the tidal creeks. This cycle of deposition and erosion is a possible mechanism keeping POC in the thin aerobic surface layer of the marsh, thus increasing its availability to detritivores and aerobic microbes. The standing stock of wrack is only a fraction of the S. alterniflora produced each year, and its export is a negligible term in the carbon balance equation. 相似文献
942.
We consider steady, slowly varying water waves propagating on a steady current over a gently sloping bed, so-called current depth refraction. All expressions are correct to second order in wave amplitude. Formulating the energy equation for the fluctuating motion in terms of wave action (wave energy divided by intrinsic angular frequency) results in an expression, where the dissipative term is strikingly similar to wave action itself. It is simply the ‘extra’ dissipation (per unit area) caused by the fluctuating motion (i.e. total dissipation minus the effect of current acting on total mean bed shear stress) divided by the intrinsic angular frequency. We call it ‘wave action dissipation’. An inconsistency in Phillips' (1977) book is pointed out. A new formula for the calculation of wave amplitudes along rays is set forth. 相似文献
943.
The acute toxicity of bromochlorinated estuarine water (ca. 20%) was determined for several estuarine organisms. The most sensitive species were oysters (Crassostrea virginica, larvae and juveniles) and copepods (Acartia tonsa) with 48-h LC50's of 0·10 to 0·21 mg BrCl/litre. Palaemonetes pugio was most tolerant with a 96-h LC50 of 0·70 mg BrCl/litre. The fish species tested (Menidia menidia, Brevoortia tyrannus and Leiostomus xanthurus) all had a 96-h LC50 of 0·21–0·23 mg BrCl/litre.The BrCl toxicity data are compared with Cl2 toxicity data for the same species. When the LC50's are expressed as equivalents per litre, BrCl is found to be two to four times less toxic than Cl2. The ranking of species in terms of sensitivity is the same for both disinfectants.Some data are provided concerning the decay rates of BrCl and Cl2 in estuarine water. BrCl was found to decay more rapidly than Cl2 at higher ammonia levels (0·25 mg NH4-N/litre). The question of chemical speciation is discussed with particular reference to the differential toxicities. 相似文献
944.
A parametric experimental study is conducted to compare the reflection and transmission characteristics of submerged hemi-cylindrical and rectangular rigid and water-filled flexible breakwater models. The results show that, for the rigid breakwaters, rectangular models are more effective than hemi-cylindrical ones in terms of reduction of transmitted waves. As for the flexible breakwaters, the hemi-cylindrical models may give better wave reflection than rectangular ones. However, the energy loss induced by the rectangular breakwaters is much larger and more significant to result in an overall better efficiency in terms of reduction in wave transmission. The effects of internal pressure show that the lowest pressurized flexible models considered in this work are the most effective in the reduction of the transmitted wave height. Higher wave reflection, lower wave transmission and higher energy loss are obtained consistently at the lower submergence depth ratio. 相似文献
945.
The basic properties of the dynamic model of a turbulent jet formed by a deep-water sewage discharge into the stratified environment of coastal regions are considered. The model developed was used to estimate the parameters of a floating-up jet of deep wastewater discharge from Sand Island into the basin of Mamala Bay (Hawaii) depending on the season and discharge operation mode. The estimates of the float-up depths of the jet and the initial dilution of the jet were estimated on the basis of model calculations using experimental data on the vertical profiles of the water temperature and salinity under the actual conditions of stratification in the study region. It is shown that the further propagation of the wastewater jet depends on tidal events and internal waves generated by tides. The appearance of turbulent jets at the sea surface was recorded. The model estimates of the parameters of the wastewater discharge were compared with the results of experimental measurements. Good agreement was found, which indicates that the physical mechanisms of the propagation of turbulent jets in a stratified medium are adequately described by the model. 相似文献
946.
Free spanning analysis of offshore pipelines 总被引:1,自引:0,他引:1
H. S. Choi 《Ocean Engineering》2001,28(10)
A rigorous procedure was established on the free span analysis of offshore pipelines. The closed form solutions of the beam–column equation, considering tension and compressive force, were derived for the various possible boundary conditions. The solutions can be used to find the natural frequencies of the free spans using the energy balance concept. The results can be applied to improve the current design codes. The improved procedure will yield more realistic calculations of the allowable free span lengths of offshore pipelines. Some calculations are included to present the sensitivity of the axial forces on the allowable free spanning lengths. 相似文献
947.
Baraza J. Ercilla G. Farrán M. Casamor J. L. Sorribas J. Flores J. A. Sierro F. Wersteeg W. 《Marine Geophysical Researches》1997,19(2):115-135
Multibeam bathymetric and ultra high-resolution seismic data reveal that the distal course of the Equatorial Atlantic Mid-Ocean Channel (EAMOC) extends further east and south than was previously known, and is controlled by the presence of morphologic highs related to the Fernando de Noronha Fracture Zone. Distal course of the EAMOC is buried by sediments, and does not have bathymetric expression on the seafloor. The channel fill consists of three seismic sequences, suggesting that the recent geological evolution of the channel is composed of successive phases of decreasing sedimentary activity that finally resulted in its complete burial. Tectonic and volcanic activity related to the Fernando de Noronha Fracture Zone and Ridge, together with the effect of strong pulses of the Antarctic bottom water current during the upper Pliocene are suggested to have contributed to the progressive burial and the final abandonment of the EAMOC. 相似文献
948.
Abstract. Six macrobenthic assemblages of the circalittoral zone found in Strymonikos Gulf, North Aegean Sea, are described and compared with the corresponding ones from other Mediterranean and Atlantic areas. For the delimination of these assemblages and the evaluation of the major environmental gradients governing their distribution, several numerical techniques were used. 相似文献
949.
Major variations in type and rate of tectonic movement in the southwestern margin of the Ulleung Basin coincide in time with
changes in stratal patterns at succession boundaries, suggesting that the effect of tectonism was dominant for the development
of sequence architecture. During the back-arc opening (16–12 Ma), the rise of relative sea level and the high rate of sediment
supply gave rise to sequences with sigmoid progradational patterns. During the back-arc closing (12–6.5 Ma), fall- and rise-dominated
relative sea-level fluctuations resulted in sequences with varying stratal patterns depending upon changes in deposition rate.
The rise-dominated relative sea level has been prevalent during the later stage (6.5 Ma–Present) with low sedimentation rate.
Received: 16 January 1996 / Revision received: 7 February 1997 相似文献
950.
Among the assumptions upon which linear time-invariant models of floating bodies are based is that the body motions are so small that any change in the body’s angular position can be disregarded. However, it is often a major design requirement of a wave energy conversion device that the response amplitude is large, thereby invalidating one of the assumptions of the linear model. In particular, the immersed geometry of a body undergoes considerable variation when it is moved in pitch. With regard to this we investigate the difference in performance between a quasi-linear model in which the change of immersed surface is modelled by time-varying parameters and a basic linear model in which the immersed surface is time-invariant. The time-varying parameter model is realized by interpolation between the appropriate parameter values of a set of linear time-invariant (LTI) models derived for the different immersed surfaces that occur at discrete body displacements. It is shown that the responses predicted using the time-varying parameter model are closer to those measured experimentally than those of a standard frequency-domain model. Particular improvement occurs when the responses are large, such as at or near the resonance frequency. A problem which may limit the general use of the model is also discussed. 相似文献