全文获取类型
收费全文 | 552篇 |
免费 | 21篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 41篇 |
大气科学 | 55篇 |
地球物理 | 140篇 |
地质学 | 154篇 |
海洋学 | 77篇 |
天文学 | 65篇 |
综合类 | 3篇 |
自然地理 | 46篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 13篇 |
2017年 | 15篇 |
2016年 | 19篇 |
2015年 | 16篇 |
2014年 | 22篇 |
2013年 | 25篇 |
2012年 | 19篇 |
2011年 | 27篇 |
2010年 | 28篇 |
2009年 | 45篇 |
2008年 | 22篇 |
2007年 | 30篇 |
2006年 | 37篇 |
2005年 | 14篇 |
2004年 | 18篇 |
2003年 | 22篇 |
2002年 | 17篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有581条查询结果,搜索用时 15 毫秒
521.
Lars G. Nordn 《水文研究》1990,4(3):197-213
Depletion and recharge dynamics of soil water were studied at two forest sites in Northern and Southern Sweden during three growing seasons. At each site, having a stand of Norway spruce (Picea abies (L.) Karst), soil water potential was measured with 90 gypsum blocks in 30 profiles at depths of 6, 20, and 50 cm from the soil surface. The size of the rectangular plots was 250 m2. The large variation in soil water potential during desiccation periods extended throughout the measuring range of the gypsum blocks. The soil water potentials, at different times and depths, were log-normally distributed during only parts of the desiccation periods. Due to the large variation and skewed distribution of soil water potentials the nonparametric Spearman rank correlation statistics was used to analyse the pattern of desiccation and rewetting of the soil. In the beginning of a dry period the topsoil was more efficiently desiccated than the subsoil at both sites. This difference lasted throughout all droughts at the southern site and was explained by a higher root density in the topsoil. At the northern site, however, there was no difference in the degree of desiccation at different soil depths during an extended desiccation period. While certain microsites tended to be more depleted than others during desiccation periods, this did not prove to be significantly correlated to the rewetting of the soil after the different droughts. There was a vertical relationship in soil water desiccation in which an accentuated water depletion at the 6 cm soil depth was followed by a similar strong depletion 14 cm lower in the same observation profile. Strongly desiccated observation profiles were not necessarily situated close to each other but seemed randomly spread over the site. There was no relationship between extracted water at any of the three soil depths in the different observation profiles and proximity to trees or tree size. 相似文献
522.
The large ice sheets in North America and Europe and the extensive sea-ice cover in the North Atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and, in turn, the location and intensity of the surface winds and jet streams. A general circulation model was used to simulate the January and July patterns of temperature, precipitation, and wind for 18 ka BP. Boundary conditions for the model, consisting of ice-sheet location and height, sea-ice location, and sea-surface temperature were prescribed from CLIMAP (1981). The model results are illustrated and described for the North American/North Atlantic/European sector. The jet stream splits around the North American ice-sheet, and the southern branch strengthens considerably (compared to present) over the southern portion of the United States, the sea-ice margin of the North Atlantic, and the southern edge of the European ice-sheet. Geologic evidence, principally from North America, of wind, temperature and moisture conditions is assessed from sand dune and loess records, estimates of snowline depression, pollen records and lake-level studies. The geologic evidence is generally compatible with the model simulation. 相似文献
523.
Lars Åke Svensson 《Solar physics》1971,18(2):232-243
The level intervals in the ground configurations 3s
23p
k(k = 2, 3, 4) have been studied through the isoelectronic sequences up to Ni. Semiempirical expressions for the parameters F
2(pp) and
p, are derived from observed data and the intervals are recalculated from interpolated or extrapolated values of the parameters. As a result, predicted wavelengths of magnetic-dipole transitions within the configurations are obtained with an estimated uncertainty of one or two Å. 相似文献
524.
The Turkana, Nakuru-Elmenteita, and Naivasha basins in the Rift Valley of East Africa experienced high water levels during the period of 10,000–7000 yr B.P. Analyses of the modern hydrologic and energy budgets for these basins, along with that of Lake Victoria, were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early Holocene. Precipitation must have been at least 150–300 mm/yr (15 to 35%) above the modern average. The precipitation estimates were fairly consistent among the various basins, but no quantitative estimate was made for the additional precipitation required to account for overflow from the Rift Valley lake basins. Discharge from the Lake Victoria basin around 1880 A.D. was considerably above the more recent average, and the increased discharge into the White Nile for that period might have been similar to that of the early Holocene. A sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic-energy budget; changes of albedo, Bowen ratio, and cloudiness were likely to have been of greater importance. 相似文献
525.
Lars Ramberg 《Aquatic Sciences - Research Across Boundaries》1987,49(3):294-302
A dense bloom of a green algae was reduced by 97% within 5 days. This was probably caused by grazing by a colourless flagellate
that increased logarithmic to high number under these days and disappeared as quickly. After this the green algae increased
rapidly and mucilage lumps excreated by the flagellate was colonized by a specialistChlomydomonas.
The flagellate attack was probably enhanced by the high cell density and the physiological weakness of the green algae, due
to a very low growth caused by shortage of inorganic carbon. The grazing seems to have caused an increase of inorganic nutrients
that promoted the following growth of the green algae. 相似文献
526.
Lars Håkanson 《Aquatic Sciences - Research Across Boundaries》1995,57(1):31-53
This study quantifies and ranks variables of significance to predict mean values of Secchi depth in small glacial lakes. The work is based on a new, extensive set of data from 88 Swedish lakes and their catchments. Several empirical models based on catchment and lake morphometric parameters are presented. These empirical models can only be used to predict Secchi depth for lakes of the same type, and the models based on geological map parameters can evidently not be used for time-dependent and site typical predictions of Secchi depth. However, many of the principles behind the results ought to be valid for lakes in general. Various hypotheses concerning the factors regulating the variability in mean Secchi depth among lakes are formulated and tested. The most important variables are: Lake colour (expressing allogenic input of different types of humic materials), total-P and lake temperature (measures of production of autogenic materials). The most important map parameters are: The mean depth (linked to resuspension and lake morphometry) and the ratio between the drainage area and lake area (expressing the linkage between catchment and lake). The predictability of some of the models cannot be markedly improved by accounting for the distribution of the characteristics in the drainage area (using the drainage area zonation technique). The variability in mean Secchi depth from other factors, such as precipitation and anthropogenic load, may then be quantitatively differentiated from the impact of these geological factors, which can statistically explain 68% of the variability in Secchi depth among these lakes. The model based on map parameters can also be used to estimate natural, preindustrial reference values of Secchi depth. 相似文献
527.
We present a method to calculate the radiation pressure force to gravity ratio on meteoroids from their atmospheric flight. Radiation pressure corrections to meteor orbits are negligible for fireballs; of the order of or less than the measurement errors ( 1%) for photographic meteors; of the order of and in some cases substantially larger than the measurement errors ( 10%) for radar meteors. 相似文献
528.
529.
Einar Svendsen Morten Skogen Paul Budgell Geir Huse Jan Erik Stiansen Bjrn dlandsvik Frode Vikeb Lars Asplin Svein Sundby 《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2810
The Norwegian Ecological Model (NORWECOM) biophysical model system implemented with the ROMS ocean circulation model has been run to simulate conditions over the last 25 years for the North Atlantic. Modeled time series of water volume fluxes, primary production, and drift of cod larvae through their modeled ambient temperature fields have been analyzed in conjunction with VPA estimated time series of 3-year-old cod recruits in the Barents Sea. Individual time series account for less than 50% of the recruitment variability; however, a combination of simulated flow of Atlantic water into the Barents Sea and local primary production accounts for 70% of the variability with a 3-year lead. The associated regression predicts increased recruitment between 2007 and 2008 from about 450–700 million individuals with a standard error of nearly 150 million. 相似文献
530.
The effect of boundary layer streaming on the sea bed shear stresses, beneath random waves, is investigated for laminar flow as well as smooth turbulent flow. It is demonstrated how bottom friction formulas for regular waves can be used to obtain the bed shear stresses resulting from steady streaming under random waves. As a result, friction factors for steady streaming under random waves are provided, and the effect of streaming versus the effect of linear waves is discussed. For laminar flow the effect of second order Stokes waves is also included. Examples are included to illustrate the applicability of the present practical method, and results are obtained using data typical for field conditions. 相似文献