全文获取类型
收费全文 | 436篇 |
免费 | 30篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 55篇 |
地球物理 | 93篇 |
地质学 | 133篇 |
海洋学 | 39篇 |
天文学 | 93篇 |
自然地理 | 55篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 14篇 |
2020年 | 10篇 |
2019年 | 15篇 |
2018年 | 16篇 |
2017年 | 16篇 |
2016年 | 20篇 |
2015年 | 22篇 |
2014年 | 20篇 |
2013年 | 24篇 |
2012年 | 17篇 |
2011年 | 30篇 |
2010年 | 25篇 |
2009年 | 32篇 |
2008年 | 15篇 |
2007年 | 32篇 |
2006年 | 18篇 |
2005年 | 19篇 |
2004年 | 12篇 |
2003年 | 16篇 |
2002年 | 10篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 7篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有480条查询结果,搜索用时 15 毫秒
111.
The paper considers opportunities for institutional development within the UK fishing industry that aim to engender a more meaningful incorporation of fishermen's organisations within the policy system. Special attention is given to the location of responsibility within the policy process. This involves an exploration of an alternative approach to consultation and the scope for delegation of management responsibilities to fishermen's organisations. Mechanisms to improve coordination among fishermen's organisations and to strengthen their internal architecture are also considered. The analysis raises important questions to inform the continuing debate over the development and structure of devolved management systems, in particular regarding the assumptions underlying user participation and the representativeness, capabilities and aspirations of fishermen's organisations. 相似文献
112.
John Davison Simon Y.W. Ho Sarah C. Bray Marju Korsten Egle Tammeleht Maris Hindrikson Kjartan Østbye Eivind Østbye Stein-Erik Lauritzen Jeremy Austin Alan Cooper Urmas Saarma 《Quaternary Science Reviews》2011,30(3-4):418-430
This review provides an up-to-date synthesis of the matrilineal phylogeography of a uniquely well-studied Holarctic mammal, the brown bear. We extend current knowledge by presenting a DNA sequence derived from one of the earliest known fossils of a polar bear (dated to 115 000 years before present), a species that shares a paraphyletic mitochondrial association with brown bears. A molecular clock analysis of 140 mitochondrial DNA sequences, including our new polar bear sequence, provides novel insights into the times of origin for different brown bear clades. We propose a number of regional biogeographic scenarios based on genetic data, divergence time estimates and paleontological records. The case of the brown bear provides an example for researchers working with less well-studied taxa: it shows clearly that phylogeographic models based on patterns of modern genetic variation alone can be substantially improved by including data on historical patterns of genetic diversity in the form of ancient DNA sequences derived from accurately dated samples and by using an approach to divergence-time estimation that suits the data under analysis. Using such approaches it has been possible to (i) establish that the processes shaping modern genetic diversity in brown bears acted recently, within the last three glacial cycles; (ii) distinguish among hypotheses concerning species’ responses to climatic oscillations in accordance with the lack of phylogeographic structure that existed in brown bears prior to the last glacial maximum (LGM); (iii) reassess theories linking monophyletic brown bear populations to particular LGM refuge areas; and (iv) identify vicariance events and track analogous patterns of migration by brown bears out of Eurasia to North America and Japan. 相似文献
113.
Jeremy S. Heyl † Brett J. Gladman † 《Monthly notices of the Royal Astronomical Society》2007,377(4):1511-1519
We propose that the presence of additional planets in extrasolar planetary systems can be detected by long-term transit timing studies. If a transiting planet is on an eccentric orbit then the presence of another planet causes a secular advance of the transiting planet's pericentre over and above the effect of general relativity. Although this secular effect is impractical to detect over a small number of orbits, it causes long-term differences when future transits occur, much like the long-term decay observed in pulsars. Measuring this transit-timing delay would thus allow the detection of either one or more additional planets in the system or the first measurements of non-zero oblateness ( J 2 ) of the central stars. 相似文献
114.
Brenton A. Wilder Jeremy T. Lancaster Peter H. Cafferata Drew B. R. Coe Brian J. Swanson Donald N. Lindsay William R. Short Alicia M. Kinoshita 《水文研究》2021,35(1):e13976
Following wildfires, the probability of flooding and debris flows increase, posing risks to human lives, downstream communities, infrastructure, and ecosystems. In southern California (USA), the Rowe, Countryman, and Storey (RCS) 1949 methodology is an empirical method that is used to rapidly estimate post-fire peak streamflow. We re-evaluated the accuracy of RCS for 33 watersheds under current conditions. Pre-fire peak streamflow prediction performance was low, where the average R2 was 0.29 and average RMSE was 1.10 cms/km2 for the 2- and 10-year recurrence interval events, respectively. Post-fire, RCS performance was also low, with an average R2 of 0.26 and RMSE of 15.77 cms/km2 for the 2- and 10-year events. We demonstrated that RCS overgeneralizes watershed processes and does not adequately represent the spatial and temporal variability in systems affected by wildfire and extreme weather events and often underpredicted peak streamflow without sediment bulking factors. A novel application of machine learning was used to identify critical watershed characteristics including local physiography, land cover, geology, slope, aspect, rainfall intensity, and soil burn severity, resulting in two random forest models with 45 and five parameters (RF-45 and RF-5, respectively) to predict post-fire peak streamflow. RF-45 and RF-5 performed better than the RCS method; however, they demonstrated the importance and reliance on data availability. The important parameters identified by the machine learning techniques were used to create a three-dimensional polynomial function to calculate post-fire peak streamflow in small catchments in southern California during the first year after fire (R2 = 0.82; RMSE = 6.59 cms/km2) which can be used as an interim tool by post-fire risk assessment teams. We conclude that a significant increase in data collection of high temporal and spatial resolution rainfall intensity, streamflow, and sediment loading in channels will help to guide future model development to quantify post-fire flood risk. 相似文献
115.
116.
David J. Wilton Grant R. Bigg James D. Scourse Jeremy C. Ely Chris D. Clark 《第四纪科学杂志》2021,36(5):934-945
The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice-rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment-level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000-year time steps during the height of the last BIIS glaciation (31–6 ka bp ). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances. 相似文献
117.
Jisun Park Brent D. Turrin Gregory F. Herzog Fara N. Lindsay Jeremy S. Delaney Carl C. Swisher III Masayuki Uesugi Yuzuru Karouji Toru Yada Masanao Abe Tatsuaki Okada Yukihiro Ishibashi 《Meteoritics & planetary science》2015,50(12):2087-2098
The Hayabusa mission to asteroid 25143, Itokawa, brought back 2000 small particles, which most closely resemble material found in LL4‐6 chondrites. We report an 40Ar/39Ar age of 1.3 ± 0.3 Ga for a sample of Itokawa consisting of three grains with a total mass of ~2 μg. This age is lower than the >4.0 Ga ages measured for 75% of LL chondrites but close to one for Y‐790964 and its pairs. The flat 40Ar/39Ar release spectrum of the sample suggests complete degassing 1.3 Ga ago. Recent solar heating in Itokawa's current orbit does not appear likely to have reset that age. Solar or impact heating 1.3 Ga ago could have done so. If impact heating was responsible, then the 1.3 Ga age sets an upper bound on the time at which the Itokawa rubble pile was assembled and suggests that rubble pile creation was an ongoing process in the inner solar system for at least the first 3 billion years of solar system history. 相似文献
118.
Pichel WG Churnside JH Veenstra TS Foley DG Friedman KS Brainard RE Nicoll JB Zheng Q Clemente-Colón P 《Marine pollution bulletin》2007,54(8):1207-1211
Floating marine debris, particularly derelict fishing gear, is a hazard to fish, marine mammals, turtles, sea birds, coral reefs, and even human activities. To ameliorate the economic and environmental impact of marine debris, we need to efficiently locate and retrieve dangerous debris at sea. Guided by satellite-derived information, we made four flights north of Hawaii in March and April 2005. During these aerial surveys, we observed over 1800 individual pieces of debris, including 122 derelict fishing nets. The largest debris concentrations were found just north of the North Pacific Transition Zone Chlorophyll Front (TZCF) within the North Pacific Subtropical Convergence Zone (STCZ). Debris densities were significantly correlated with sea-surface temperature (SST), chlorophyll-a concentration (Chla), and the gradient of Chla. A Debris Estimated Likelihood Index (DELI) was developed to predict where high concentrations of debris would be most likely in the North Pacific during spring and early summer. 相似文献
119.
Roel Plant Jeremy Walker Scott Rayburg Jacqueline Gothe Teresa Leung 《The Australian geographer》2012,43(1):75-91
Agricultural chemicals are a notoriously intractable source of environmental pollution. Offering enhanced agricultural productivity, they simultaneously risk degrading the ecological basis upon which agriculture depends. This paper considers chemicalisation as a cause of the erosion of aquatic biodiversity and ecosystem resilience, focusing on the Hawkesbury-Nepean River and the small-scale horticulturalists who supply the city's fresh vegetable markets, working under the pressure of urbanisation, retail monopolies, indifferent land-use planning, and often without access to information about pesticide use in the languages they understand. Arguing that standard practices of ‘risk management’ are unable to adequately control chemical contamination, the paper presents findings from interviews with actors within the ‘assemblage’ of institutions with responsibility for agriculture, water quality, and environmental protection, in order to assess the effectiveness of pesticide governance in the Greater Sydney Basin. It appears that pesticide pollution is far from being tamed: it is rarely measured nor monitored, neither is it a priority of any particular agency. Arguing that public health, the long-term viability of local farming and the ecological well-being of the Hawkesbury-Nepean River are mutually consistent goals, we conclude that these vital elements of the common-weal are currently subject to a system of ‘organised irresponsibility’. The paper concludes by proposing several ways forward. 相似文献
120.