全文获取类型
收费全文 | 5764篇 |
免费 | 283篇 |
国内免费 | 44篇 |
专业分类
测绘学 | 155篇 |
大气科学 | 403篇 |
地球物理 | 2150篇 |
地质学 | 1891篇 |
海洋学 | 436篇 |
天文学 | 740篇 |
综合类 | 28篇 |
自然地理 | 288篇 |
出版年
2022年 | 39篇 |
2021年 | 112篇 |
2020年 | 116篇 |
2019年 | 81篇 |
2018年 | 233篇 |
2017年 | 224篇 |
2016年 | 342篇 |
2015年 | 258篇 |
2014年 | 290篇 |
2013年 | 379篇 |
2012年 | 313篇 |
2011年 | 345篇 |
2010年 | 270篇 |
2009年 | 296篇 |
2008年 | 245篇 |
2007年 | 217篇 |
2006年 | 175篇 |
2005年 | 154篇 |
2004年 | 180篇 |
2003年 | 130篇 |
2002年 | 87篇 |
2001年 | 100篇 |
2000年 | 76篇 |
1999年 | 66篇 |
1998年 | 87篇 |
1997年 | 72篇 |
1996年 | 49篇 |
1995年 | 41篇 |
1994年 | 60篇 |
1993年 | 44篇 |
1992年 | 47篇 |
1991年 | 39篇 |
1990年 | 58篇 |
1989年 | 41篇 |
1988年 | 33篇 |
1987年 | 35篇 |
1986年 | 41篇 |
1984年 | 42篇 |
1983年 | 36篇 |
1982年 | 40篇 |
1981年 | 28篇 |
1980年 | 28篇 |
1979年 | 28篇 |
1978年 | 32篇 |
1977年 | 30篇 |
1976年 | 28篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 34篇 |
1971年 | 41篇 |
排序方式: 共有6091条查询结果,搜索用时 546 毫秒
991.
Valentí Rull José A. López-Sáez Teresa Vegas-Vilarrúbia 《Journal of Paleolimnology》2008,40(1):399-411
This paper evaluates the potential usefulness of non-pollen palynomorphs or NPPs (microfossils other than pollen and spores
present in palynological preparations) contained in lake sediments in the paleolimnological reconstruction of high altitude
environments (>4,000 m) from the Venezuelan Andes. A synthetic, quantitative approach is employed, instead of the classical
analytical and mostly qualitative approach commonly used so far for NPPs. The main sources of variation are the Pediastrum–Botryococcus alternation and the relationship between these two algae and animal remains such as Acari legs, postabdomina of Cladocera,
mandibles of other invertebrates, and an unknown type called LVA-1. Other significant microfossils are remains of Rivularia-type and turbellarian oocytes, including Gyratrix. The sequence initiates around 15,000 calibrated years before present (cal BP) with the deglaciation of the lake catchment,
high water levels and still cold climates. A phase of lower lake levels was recorded between about 12,000 and 6,000 cal BP.
Temperatures increased by around 9,000 and 7,000 cal BP, and then decreased until 6,000 cal BP. Since that time, both lake
levels and temperature increased again and stabilized at about 4,000 cal BP, when they reach modern-like values. These results
show a good agreement with previous studies based on pollen, diatom and oxygen isotope analyses, and provide additional paleoecological
information, as for example the possibility of a previously unrecorded Younger Dryas signal. The inclusion of quantitative
NPP analysis in routine paleolimnological studies using synthetic methods is thus recommended. Possible future improvements
are suggested, mainly those related with the development and use of NPP modern analogs. 相似文献
992.
Nikos Nanos Theodoros Grigoratos José Antonio Rodríguez Martín Constantini Samara 《Stochastic Environmental Research and Risk Assessment (SERRA)》2015,29(6):1531-1543
We analyse the concentration of five trace elements (As, Cu, Ni, Pb and Zn) in the topsoil of the Kozani-Ptolemais basin where four coal-fired power plants run to provide almost 47.8 % of electricity requirements in Greece. We assume that if the power plants have altered the spatial (co)variation of the analysed elements through their toxic by-products, their effect would be observable only on a small spatial scale, since deposition of airborne pollutants is more evident if it is near the emission source. We used Factorial Cokriging to estimate the small-scale correlations among soil elements and to compare them to large spatial-scale correlations. Soil samples were collected from 92 sites. Given the low concentrations in soil heavy metals and As, we observed no serious soil contamination risk. We estimated correlations among the analysed elements on two spatial scales. On the larger scale, Ni and As exhibited higher correlation and received higher weights for the first regionalized factor, contrary to Cu, Pb and Zn which weighted more for the second regionalized factor. On the small spatial scale As associated with neither Ni nor other heavy metals. We conclude that soil arsenic has been altered by enrichment caused by some power plants through fly ash deposition and/or disposal. However, enrichment of soil elements was detectable only on the smaller spatial scale because anthropogenic inputs in soil through airborne emissions and subsequent deposition are evident only in the vicinity of the emission source. 相似文献
993.
994.
995.
996.
997.
998.
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
999.
1000.