全文获取类型
收费全文 | 4539篇 |
免费 | 108篇 |
国内免费 | 50篇 |
专业分类
测绘学 | 143篇 |
大气科学 | 383篇 |
地球物理 | 992篇 |
地质学 | 1669篇 |
海洋学 | 404篇 |
天文学 | 811篇 |
综合类 | 11篇 |
自然地理 | 284篇 |
出版年
2023年 | 13篇 |
2022年 | 39篇 |
2021年 | 57篇 |
2020年 | 71篇 |
2019年 | 61篇 |
2018年 | 138篇 |
2017年 | 144篇 |
2016年 | 167篇 |
2015年 | 103篇 |
2014年 | 147篇 |
2013年 | 217篇 |
2012年 | 184篇 |
2011年 | 266篇 |
2010年 | 203篇 |
2009年 | 276篇 |
2008年 | 232篇 |
2007年 | 183篇 |
2006年 | 212篇 |
2005年 | 217篇 |
2004年 | 296篇 |
2003年 | 225篇 |
2002年 | 154篇 |
2001年 | 108篇 |
2000年 | 104篇 |
1999年 | 79篇 |
1998年 | 93篇 |
1997年 | 59篇 |
1996年 | 33篇 |
1995年 | 38篇 |
1994年 | 39篇 |
1993年 | 33篇 |
1992年 | 41篇 |
1991年 | 23篇 |
1990年 | 35篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 16篇 |
1984年 | 19篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 22篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 15篇 |
1971年 | 12篇 |
排序方式: 共有4697条查询结果,搜索用时 0 毫秒
101.
The cross-sectional stability of two tidal inlets connecting the same back-barrier lagoon to the ocean is investigated. The condition for equilibrium is the cross-sectional area tidal prism relationship. In an earlier study [Van de Kreeke, J., 1990. Can multiple inlets be stable? Estuarine, Coastal and Shelf Science 30: 261–273.], using the same equilibrium condition, it was concluded that where two inlets connect the same basin to the ocean ultimately one inlet will close. One of the major assumptions in that study was that the water level in the basin fluctuated uniformly. In hindsight this assumption might be too restrictive. For example, in the Wadden Sea the back barrier lagoon consists of a series of basins, rather than one single basin, separated by topographic highs. These topographic highs limit but do not exclude the exchange of water between the sub-basins. For this reason in the present study, a topographic high in the form of a weir was added, separating the back-barrier lagoon in two sub-basins. The water level in the sub-basins, rather than in the back-barrier as a whole, is assumed to fluctuate uniformly. For this schematization the hydrodynamic equations are solved using a finite difference method. The results, together with the equilibrium condition, yield the equilibrium flow curve for each of the inlets. The intersections of the two equilibrium flow curves represent combinations of cross-sectional areas for which both inlets are in equilibrium. The stability of the equilibriums was investigated using a non-linear stability analysis resulting in a flow diagram. Calculations were carried out for different inlet and weir characteristics. Sinussoidal tides were the same for both inlets. The results show that for relatively large wetted cross-sectional areas over the topographic high, approaching the situation of a single basin, there are no combinations of inlet cross-sectional areas for which both inlets are in a stable equilibrium. This supports the conclusion in the earlier study. For relatively small wetted cross-sectional areas over the topographic high there is one set of stable equilibriums. In that case the two-inlet bay system approaches that of two single-inlet bay systems. 相似文献
102.
M.R.A. van Gent J.S.M. van Thiel de Vries E.M. Coeveld J.H. de Vroeg J. van de Graaff 《Coastal Engineering》2008,55(12):1041-1051
Large-scale physical model tests were performed to quantify the effects of the wave period on dune erosion. Attention was focussed on 2D cross-shore effects in a situation with sandy dunes and extreme water levels and wave conditions. Besides profile measurements, detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near near-shore area. It was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face. Tests with double-peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the Tm − 1,0 spectral mean wave period, better than the peak wave period, Tp. The effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the Dutch coast. More details of the measurements and additional analyses of physical processes are described in an accompanying paper by Van Thiel de Vries et al. [Van Thiel de Vries, J.S.M., van Gent, M.R.A., Reniers, A.J.H.M. and Walstra, D.J.R., submitted for publication. Analysis of dune erosion processes in large scale flume experiments, In this volume of Coastal Engineering.]. 相似文献
103.
Jörg Kröchert Elmar Buchner Luis I. González de Vallejo 《Marine Geophysical Researches》2008,29(3):177-184
Tubular-shaped concretions and concretionary dykes occur in Holocene fossil beach deposits between the township of El Médano
and Punta Roja in southern Tenerife, Canary Islands. These sediment structures have been interpreted either as the result
of (a) the interaction between hot ignimbrites that overflowed wet beaches; (b) fast accumulation of beach sands on hot and
degassing ignimbrites; (c) paleoliquefaction caused by an earthquake (seismites). Based on the interpretation as seismites,
an intense paleoearthquake with a moment magnitude of M = 6.8 was proposed to be responsible for the generation of the paleoliquefaction structures. However, we here reinterpret
the sedimentary structures in question using the general criteria diagnostic for rhizocretions and root tubules with respect
to their orientation, size, branching system, and style of cementation and, thus, consider them, to be of biogenic origin. 相似文献
104.
Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements 总被引:1,自引:0,他引:1
B. van Maanen P.J. de Ruiter Giovanni Coco K.R. Bryan B.G. Ruessink 《Marine Geology》2008,253(3-4):99-106
We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading. 相似文献
105.
Jeremy Hills Margaret Carlisle Martin Le Tissier David Muir Caroline Robinson 《Marine Policy》2009,33(6):887
Management of the coastal zone often focuses on “islands” of high value ecosystems, in terms of economic value or conservation. However, integrated management requires landscape-level analysis of all ecosystem values. The biodiversity portfolio analysis (BPA) method is derived from the logic used in share (equity) portfolio management in terms of balancing within a portfolio the returns with the risks. Optimising the returns from a share portfolio, or a suite of ecosystems in a landscape, is dependent on the relationship between the units in terms of risk and return. Three case studies are then presented to test the applicability of the BPA method at the international (North West Europe), regional (Durham Heritage Coast, UK) and local (part of South Uist, Outer Hebrides, UK) spatial scale. The Biodiversity Portfolio Analysis for NW Europe showed that risk and return were highly correlated in the studied Member States. The ranking of risk and return, with the highest first, was Ireland > UK > France=Netherlands > Belgium. For these Member States the risks to ecosystem service provision were positively correlated with GNI (r=0.97, P<0.01); suggesting that the higher the economic importance of coastal and marine resources in a Member State the more at risk the resources are. The regional and local case studies were more focussed on providing information on which to base Integrated Coastal Zone Management (ICZM) decision making; both case studies used stakeholder participation to determine risks and returns. The conclusions from these two case studies show how the BPA method can be useful in terms of setting ICZM priorities and in addressing local coastal issues. The BPA involves making a number of assumptions, however, it does provide coastal managers with a potential tool to strategically plan due to increased awareness of the interaction between the ecosystems in the portfolio.There is a need for such techniques, which involve stakeholder participation and which create objective outcomes, to support the implementation of ICZM. 相似文献
106.
107.
Enrique Macpherson Bertrand Richer de Forges Kareen Schnabel Sarah Samadi Marie-Catherine Boisselier Antoni Garcia-Rubies 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(2):228-238
We analyzed the distribution patterns of the galatheid squat lobsters (Crustacea, Decapoda, Galatheidae) of the Pacific Ocean. We used the presence/absence data of 402 species along the continental slope and continental rise (200–2000 m) obtained from 54 cruises carried out in areas around the Philippines, Indonesia, Solomon, Vanuatu, New Caledonia, Fiji, Tonga, Wallis and Futuna and French Polynesia. The total number of stations was ca. 3200. We also used published data from other expeditions carried out in the Pacific waters, and from an exhaustive search of ca. 600 papers on the taxonomy and biogeography of Pacific species. We studied the existence of biogeographic provinces using multivariate analyses, and present data on latitudinal and longitudinal patterns of species richness, rate of endemism and the relationship between body sizes with the size of the geographic ranges. Latitudinal species richness along the Western and Eastern Pacific exhibited an increase from higher latitudes towards the Equator. Longitudinal species richness decreased considerably from the Western to the Central Pacific. Size frequency distribution for body size was strongly shifted toward small sizes and endemic species were significantly smaller than non-endemics. This study concludes that a clear separation exists between the moderately poor galatheid fauna of the Eastern Pacific and the rich Western and Central Pacific faunas. Our results also show that the highest numbers of squat lobsters are found in the Coral Sea (Solomon-Vanuatu-New Caledonia islands) and Indo-Malay-Philippines archipelago (IMPA). The distribution of endemism along the Pacific Ocean indicates that there are several major centres of diversity, e.g. Coral Sea, IMPA, New Zealand and French Polynesia. The high proportion of endemism in these areas suggests that they have evolved independently. 相似文献
108.
R. García L. Thomsen H.C. de Stigter E. Epping K. Soetaert E. Koning P.A. de Jesus Mendes 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(8):1012-1026
The role of the Setúbal–Lisbon canyon in accumulation and transport of labile organic matter from the coastal sea and ocean surface water towards the deep sea was assessed by investigating the distribution of organic matter of different quality in sedimentary aggregates and surface sediments of the canyon and adjacent slopes. Total hydrolysable amino acids (THAA) and organic carbon (Corg) were measured from aggregates, and contents of Corg, chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) from sediments. As indices of organic matter (OM) quality THAA:Corg, degradation index (DI), chl a:phaeo, chl a:Corg and C:N ratio were determined. Sediment profiles of chl a and the isotope 210 of lead (210Pb) were used as tracers in a transport model to estimate deposition rates and background levels of the tracers, and sediment mixing rates (Db). Whereas bulk Corg contents of canyon and slope sediments were practically similar at all depths, higher contents of THAA, chl a and CPE, as well as higher THAA:Corg, DI and chl a:Corg, in aggregates and sediments from the upper reaches of the canyon indicate that labile organic matter accumulates in the upper canyon. This is confirmed by higher chl a and 210Pb deposition and Db calculated from the model. Hence, the Setúbal–Lisbon canyon, specially the upper region, acts as a natural trap of organic matter that is transported to the region via lateral transport and vertical settling from primary productivity. Organic matter might be further transported in downward canyon direction via rebound processes. The chl a and 210Pb profiles reveal active sediment mixing by physical processes and/or animal reworking. 相似文献
109.
S. Ponce de León 《Ocean Engineering》2010,37(7):603-610
The sheltering effect of the Balearic Islands in the hindcast wave field was studied for typical Mediterranean wave situations of Llevant, Tramuntana and Mestral and for mild conditions such as the Garbí and Ponent winds. For this purpose, a third generation wave model was applied to the Mediterranean Sea and different patterns of the sheltered areas were found for the various representative situations depending on the wind variability and on the magnitude of the wind speed. From the analysis it was concluded that the sheltered zones created during storms generally persist for short periods of time of the order of 6 h, possibly reaching a maximum of 12 h. In contrast with earlier results obtained for swell dominated ocean areas, it was observed that in this area, due to the short fetches the sea states are mainly local wind seas and thus the wave field behind the islands depends on the local wind. 相似文献
110.
Area usage estimation and spatiotemporal variability in distribution patterns of southern right whales,Eubalaena australis,of southern Brazil 下载免费PDF全文
Eduardo Pires Renault‐Braga Karina Rejane Groch Paulo A. de Carvalho Flores Eduardo R. Secchi Luciano Dalla‐Rosa 《Marine Ecology》2018,39(3)
Southern right whales—Eubalaena australis (Desmoulins, 1822)—migrate seasonally from high‐latitude feeding grounds to coastal breeding and calving grounds at lower latitudes such as the southern coast of Brazil. Understanding how these whales are distributed along the coast is important for monitoring their postwhaling recovery and defining management strategies. In this study, we applied Kernel density estimators to aerial survey data to determine main occurrence and concentration areas of right whales in southern Brazil and investigate inter‐ and intra‐annual distribution patterns between 2003 and 2012. Our results show considerable variation in area usage within and among years, and changes in the general distribution pattern of right whales in the last years of the study. Intra‐annually, higher concentration area tended to expand from July to September and decrease in November. Some areas stood out as high‐density areas for right whales: Ribanceira/Ibiraquera, Itapirubá Sul/Sol, and from Arroio to Gaivota. Some evidences also suggest preferential areas for mother–calf pairs. The higher concentration area of right whales in southern Brazil was estimated at 52,541 km2 and the occurrence area was 682.69 km2, which is the whole study area. As right whale distribution in the region is likely expanding due to this population's current recovery, our study provides essential information for management plan of the Right Whale Environmental Protection Area. 相似文献