全文获取类型
收费全文 | 1624篇 |
免费 | 41篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 39篇 |
大气科学 | 177篇 |
地球物理 | 290篇 |
地质学 | 466篇 |
海洋学 | 178篇 |
天文学 | 356篇 |
自然地理 | 166篇 |
出版年
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 15篇 |
2018年 | 19篇 |
2017年 | 17篇 |
2016年 | 39篇 |
2015年 | 38篇 |
2014年 | 26篇 |
2013年 | 115篇 |
2012年 | 50篇 |
2011年 | 65篇 |
2010年 | 29篇 |
2009年 | 67篇 |
2008年 | 76篇 |
2007年 | 72篇 |
2006年 | 76篇 |
2005年 | 55篇 |
2004年 | 60篇 |
2003年 | 59篇 |
2002年 | 65篇 |
2001年 | 51篇 |
2000年 | 35篇 |
1999年 | 45篇 |
1998年 | 31篇 |
1997年 | 27篇 |
1996年 | 22篇 |
1995年 | 26篇 |
1994年 | 18篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 13篇 |
1990年 | 24篇 |
1989年 | 17篇 |
1988年 | 10篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 32篇 |
1984年 | 31篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 22篇 |
1977年 | 22篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 23篇 |
1973年 | 17篇 |
1969年 | 10篇 |
排序方式: 共有1672条查询结果,搜索用时 767 毫秒
941.
942.
943.
Strong contrasts in ambient isotope ratios and in diet suggest stable isotopes in the otoliths of oceanic fish can resolve water masses and geographic areas, promising a powerful multivariate approach for examining population structure and provenance. To test this, whole otoliths were taken from Patagonian toothfish (Dissostichus eleginoides) sampled off the Patagonian Shelf and South Georgia, on either side of a population boundary, and otolith δ18O and δ13C values were measured to see if they could distinguish South American-caught fish from those taken in the Antarctic. Values of otolith δ18O and δ13C predicted capture area with 100% success, validating their use for distinguishing provenance and corroborating the prior evidence of population isolation. Values of δ18O in the otoliths reflected ambient values as well as seawater temperature: low values in Patagonian Shelf fish were consistent with exposure to Antarctic Intermediate Water (AAIW), and high values in South Georgia fish were consistent with exposure to Circumpolar Deep Water (CDW). In contrast, differences in otolith δ13C appeared to reflect diet: relative depletion of otolith δ13C at South Georgia compared to the Patagonian Shelf were most likely linked to differences in sources of metabolic carbon, as well as δ13C in dissolved inorganic carbon (DIC) of seawater. These contrasting properties strongly suggest that stable isotopes can resolve the provenance of toothfish from Antarctic sampling areas that hitherto have been difficult to separate. These results show that, by using the chemistry recorded in otoliths, researchers can exploit biogeochemical variation in fully marine environments to examine the spatial ecology of oceanic fish. 相似文献
944.
Sebastian Wagner Martin Widmann Julie Jones Torsten Haberzettl Andreas Lücke Christoph Mayr Christian Ohlendorf Frank Schäbitz Bernd Zolitschka 《Climate Dynamics》2007,29(4):333-355
This study investigates the atmospheric circulation in transient climate simulations with a coupled atmosphere–ocean general
circulation model (GCM) for the mid-Holocene (MH) period 7–4.5 ka BP driven with combinations of orbital, solar and greenhouse
gas forcings. The focus is on southern South America. Statistical downscaling models are derived from observational data and
applied to the simulations to estimate precipitation in south-eastern Patagonia during the MH. These estimates are compared
with lake level estimates for Laguna Potrok Aike (LPA) from sediments. Relative to pre-industrial conditions (i.e. 1550–1850),
which show extraordinarily high lake levels, the proxy-based reconstructed lake levels during the MH are lower. The downscaled
simulated circulation differences indicate higher LPA precipitation during the MH from March to August, higher annual means,
and reduced precipitation from September to February. Thus the reconstructed lower LPA lake levels can not be explained solely
by the simulated precipitation changes. Possible reasons for this discrepancy are discussed. Based on proxy data from southern
South America hypotheses have also been proposed on the latitudinal position of the southern hemispheric westerlies (SHWs).
In agreement with some of these hypotheses our simulations show an increased seasonal cycle of the latitudinal position of
the SHWs during the MH, which can be explained by the orbital forcing. The simulations also show stronger SHWs over southern
Patagonia during austral summer and weaker SHWs during winter. The downscaling model associates weaker SHWs with increased
precipitation in the LPA region. However, this relationship is only moderate, and therefore the downscaling model does not
support the assumption of a strong link between mean SHWs and precipitation over south-eastern Patagonia, which is the basis
of many proxy-based hypotheses about the SHWs. 相似文献
945.
Erik Kjellström Lars Bärring Daniela Jacob Richard Jones Geert Lenderink Christoph Schär 《Climatic change》2007,81(1):249-265
Probability distributions of daily maximum and minimum temperatures in a suite of ten RCMs are investigated for (1) biases compared to observations in the present day climate and (2) climate change signals compared to the simulated present day climate. The simulated inter-model differences and climate changes are also compared to the observed natural variability as reflected in some very long instrumental records. All models have been forced with driving conditions from the same global model and run for both a control period and a future scenario period following the A2 emission scenario from IPCC. We find that the bias in the fifth percentile of daily minimum temperatures in winter and at the 95th percentile of daily maximum temperature during summer is smaller than 3 (±5°C) when averaged over most (all) European sub-regions. The simulated changes in extreme temperatures both in summer and winter are larger than changes in the median for large areas. Differences between models are larger for the extremes than for mean temperatures. A comparison with historical data shows that the spread in model predicted changes in extreme temperatures is larger than the natural variability during the last centuries. 相似文献
946.
947.
Ismaiel Asad Ismaiel Graham Bird Morag A. McDonald William T. Perkins Timothy G. Jones 《Environmental Earth Sciences》2018,77(2):50
The Great Zab River catchment is a major left-bank tributary of the River Tigris and drains a substantial part of the Kurdistan Region, an autonomous region of Northern Iraq. Within Kurdistan, the water resources of the Great Zab River catchment are under pressure from population increase and are utilized for potable, domestic and agricultural and industrial supply. As with many parts of the world, effective management of water resources within Kurdistan is hindered by a lack of water quality data and established background concentrations. This study therefore represents the first regional survey of river water chemistry for the Great Zab River catchment and presents data on the spatial and temporal trends in concentrations of As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, Li, Mn, Mo, Ni, Pb, Sr, Zn, NO3?, SO42?, F?, Cl? and PO43?, in addition to pH, electrical conductivity, dissolved oxygen and turbidity. As a tool for underpinning the management and monitoring of water quality, background concentrations were defined for the Great Zab catchment using three methods. The influences of geogenic and anthropogenic controls upon spatial and temporal trends in water chemistry are also evaluated. The influence of geogenic loading from underlying bedrock was identifiable within the observed spatial trends, with the most notable differences found between waters sampled from the relatively more volcanic-rich Zagros zone to the north and those sampled from the lower catchment underlain by younger clay-, sand- and siltstones. The greatest anthropogenic influence, identifiable through elements such as Cl? and NO3?, is present in the more highly populated lower catchment. The background concentrations identified in the Great Zab catchment would be those expected as a result of geogenic loading with some anthropogenic influence and represent a more conservative value when compared to those such as the World Health Organization Maximum Admissible Concentration. However, background concentrations represent a powerful tool for identifying potential anthropogenic impacts on water quality and informing management of such occurrences. 相似文献
948.
Brendon?R.?Jones Luke?B.?Brouwers Matthys?A.?DippenaarEmail author 《Hydrogeology Journal》2018,26(3):945-961
Fractures are both rough and irregular but can be expressed by a simple model concept of two smooth parallel plates and the associated cubic law governing discharge through saturated fractures. However, in natural conditions and in the intermediate vadose zone, these assumptions are likely violated. This paper presents a qualitative experimental study investigating the cubic law under variable saturation in initially dry free-draining discrete fractures. The study comprised flow visualisation experiments conducted on transparent replicas of smooth parallel plates with inlet conditions of constant pressure and differing flow rates over both vertical and horizontal inclination. Flow conditions were altered to investigate the influence of intermittent and continuous influx scenarios. Findings from this research proved, for instance, that saturated laminar flow is not likely achieved, especially in nonhorizontal fractures. In vertical fractures, preferential flow occupies the minority of cross-sectional area despite the water supply. Movement of water through the fractured vadose zone therefore becomes a matter of the continuity principle, whereby water should theoretically be transported downward at significantly higher flow rates given the very low degree of water saturation. Current techniques that aim to quantify discrete fracture flow, notably at partial saturation, are questionable. Inspired by the results of this study, it is therefore hypothetically improbable to achieve saturation in vertical fractures under free-draining wetting conditions. It does become possible under extremely excessive water inflows or when not free-draining; however, the converse is not true, as a wet vertical fracture can be drained. 相似文献
949.
950.