全文获取类型
收费全文 | 4195篇 |
免费 | 228篇 |
国内免费 | 20篇 |
专业分类
测绘学 | 127篇 |
大气科学 | 305篇 |
地球物理 | 1706篇 |
地质学 | 1392篇 |
海洋学 | 206篇 |
天文学 | 512篇 |
综合类 | 23篇 |
自然地理 | 172篇 |
出版年
2022年 | 33篇 |
2021年 | 91篇 |
2020年 | 87篇 |
2019年 | 61篇 |
2018年 | 192篇 |
2017年 | 182篇 |
2016年 | 281篇 |
2015年 | 211篇 |
2014年 | 245篇 |
2013年 | 297篇 |
2012年 | 239篇 |
2011年 | 231篇 |
2010年 | 190篇 |
2009年 | 177篇 |
2008年 | 151篇 |
2007年 | 132篇 |
2006年 | 123篇 |
2005年 | 81篇 |
2004年 | 96篇 |
2003年 | 74篇 |
2002年 | 64篇 |
2001年 | 75篇 |
2000年 | 41篇 |
1999年 | 44篇 |
1998年 | 65篇 |
1997年 | 47篇 |
1996年 | 31篇 |
1995年 | 28篇 |
1994年 | 37篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 35篇 |
1990年 | 48篇 |
1989年 | 36篇 |
1987年 | 28篇 |
1986年 | 31篇 |
1985年 | 23篇 |
1984年 | 29篇 |
1983年 | 24篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 29篇 |
1971年 | 36篇 |
排序方式: 共有4443条查询结果,搜索用时 281 毫秒
751.
752.
753.
754.
755.
Cyclogenesis and long-fetched winds along the southeastern coast of South America may lead to floods in populated areas, as the Buenos Aires Province, with important economic and social impacts. A numerical model (SMARA) has already been implemented in the region to forecast storm surges. The propagation time of the surge in such extensive and shallow area allows the detection of anomalies based on observations from several hours up to the order of a day prior to the event. Here, we investigate the impact and potential benefit of storm surge level data assimilation into the SMARA model, with the objective of improving the forecast. In the experiments, the surface wind stress from an ensemble prediction system drives a storm surge model ensemble, based on the operational 2-D depth-averaged SMARA model. A 4-D Local Ensemble Transform Kalman Filter (4D-LETKF) initializes the ensemble in a 6-h cycle, assimilating the very few tide gauge observations available along the northern coast and satellite altimeter data. The sparse coverage of the altimeters is a challenge to data assimilation; however, the 4D-LETKF evolving covariance of the ensemble perturbations provides realistic cross-track analysis increments. Improvements on the forecast ensemble mean show the potential of an effective use of the sparse satellite altimeter and tidal gauges observations in the data assimilation prototype. Furthermore, the effects of the localization scale and of the observational errors of coastal altimetry and tidal gauges in the data assimilation approach are assessed. 相似文献
756.
757.
758.
Caio Vinícius Gabrig Turbay Rangel Marcos Tadeu D’Azeredo Orlando Cláudio De Morisson Valeriano Alexandre de Oliveira Chaves 《International Geology Review》2017,59(15):1966-1973
The Varre-Sai meteorite fell along the border of the states of Espirito Santo and Rio de Janeiro, Brazil; on 19 June 2010 at 5:40 pm. Petrography and X-ray powder diffraction (XRD) indicate that the rock is an L5 S4 chondrite, with blastoporphyritic texture that has not been previously described. Geochemical data based on major and rare-earth elements (REEs) show that Varre-Sai is highly similar to the other L chondrites. In Harker diagrams, Varre-Sai, L, and LL chondrites form a single group, suggesting no significant chemical differences between them and contributing to the long-standing debate of whether LL chondrites form a distinct group or whether they are a subset of the L group. Harker diagrams also define a trend from E to H and L/LL chondrites, similar to the cosmochemical trends suggested by other authors. The behaviour of Fe2O3t and NiO indicates a relationship with Fe-Ni alloys, and their trend in the diagram suggests some chemical differentiation in the ordinary chondrite parental bodies. The REE content in Varre-Sai, normalized to C chondrites, falls in the field of L chondrites and others, but with slight REE enrichment. The chemical differences in chondrites, mainly in REEs, Fe2O3t and NiO could be alternatively interpreted as variations in the inherited agglutinated materials as chondrules, Ca–Al-rich inclusions and Fe–Ni nodules. 相似文献
759.
Taphonomic analysis of small mammal bone remains preyed upon by wildcats (Carnivora: Felidae) from the central Monte Desert (Mendoza,Argentina)
下载免费PDF全文
![点击此处可从《Boreas: An International Journal of Quaternary Research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
José Manuel López María I. Rosi Solana Tabeni Benjamín Bender Horacio Chiavazza 《Boreas: An International Journal of Quaternary Research》2017,46(2):282-293
The present study analysed the taphonomic characteristics of small mammal bone accumulations produced by small felids in an area from the central Monte Desert (Mendoza, Argentina). In order to provide criteria to identify the role that these predators had in the formation of zooarchaeological assemblages, the anatomical representation, bone breakage patterns and degrees of digestive corrosion were evaluated. The main taphonomic results are: low average values for the relative abundance of skeletal elements; greater representation of mandibles, maxillae, isolated incisors, humeri and femora than the remaining elements; preponderance of cranial elements with high proportion of isolated teeth; elevated frequencies of proximal limb bones compared with distal parts; high degree of breakage in all skeletal elements and digestive corrosion on almost all diagnostic bones (mainly moderate and heavy). The values of the studied taphonomic variables indicate that small felids in this area made severe alterations to the bones of their prey (mainly rodents), attributable to the category of extreme modifier, while preserving enough skeletal elements to allow their taphonomic characterization. Tooth marks or grooves on bone surfaces produced by scratching and chewing were not detected. The low relative abundance of skeletal elements, the high degree of breakage and the elevated frequency of elements with digestion traces represent general taphonomic patterns that fall within those reported for other South American small carnivores. 相似文献
760.
The accuracies of three different evolutionary artificial neural network (ANN) approaches, ANN with genetic algorithm (ANN-GA), ANN with particle swarm optimization (ANN-PSO) and ANN with imperialist competitive algorithm (ANN-ICA), were compared in estimating groundwater levels (GWL) based on precipitation, evaporation and previous GWL data. The input combinations determined using auto-, partial auto- and cross-correlation analyses and tried for each model are: (i) GWL t?1 and GWL t?2; (ii) GWL t?1, GWL t?2 and P t ; (iii) GWL t?1, GWL t?2 and E t ; (iv) GWL t?1, GWL t?2, P t and E t ; (v) GWL t?1, GWL t?2 and P t?1 where GWL t , P t and E t indicate the GWL, precipitation and evaporation at time t, individually. The optimal ANN-GA, ANN-PSO and ANN-ICA models were obtained by trying various control parameters. The best accuracies of the ANN-GA, ANN-PSO and ANN-ICA models were obtained from input combination (i). The mean square error accuracies of the ANN-GA and ANN-ICA models were increased by 165 and 124% using ANN-PSO model. The results indicated that the ANN-PSO model performed better than the other models in modeling monthly groundwater levels. 相似文献