全文获取类型
收费全文 | 56579篇 |
免费 | 799篇 |
国内免费 | 612篇 |
专业分类
测绘学 | 1358篇 |
大气科学 | 5137篇 |
地球物理 | 12027篇 |
地质学 | 20834篇 |
海洋学 | 4103篇 |
天文学 | 11055篇 |
综合类 | 106篇 |
自然地理 | 3370篇 |
出版年
2018年 | 738篇 |
2017年 | 695篇 |
2016年 | 1022篇 |
2015年 | 701篇 |
2014年 | 1024篇 |
2013年 | 2481篇 |
2012年 | 1159篇 |
2011年 | 1670篇 |
2010年 | 1434篇 |
2009年 | 2001篇 |
2008年 | 1753篇 |
2007年 | 1624篇 |
2006年 | 1742篇 |
2005年 | 1472篇 |
2004年 | 1518篇 |
2003年 | 1453篇 |
2002年 | 1457篇 |
2001年 | 1234篇 |
2000年 | 1272篇 |
1999年 | 1147篇 |
1998年 | 1097篇 |
1997年 | 1154篇 |
1996年 | 1024篇 |
1995年 | 974篇 |
1994年 | 886篇 |
1993年 | 814篇 |
1992年 | 874篇 |
1991年 | 819篇 |
1990年 | 849篇 |
1989年 | 773篇 |
1988年 | 768篇 |
1987年 | 922篇 |
1986年 | 802篇 |
1985年 | 960篇 |
1984年 | 1113篇 |
1983年 | 1141篇 |
1982年 | 1018篇 |
1981年 | 982篇 |
1980年 | 932篇 |
1979年 | 910篇 |
1978年 | 920篇 |
1977年 | 828篇 |
1976年 | 769篇 |
1975年 | 771篇 |
1974年 | 826篇 |
1973年 | 818篇 |
1972年 | 555篇 |
1971年 | 493篇 |
1970年 | 372篇 |
1969年 | 327篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
James Glimm Shuling Hou Hongjoong Kim Yoon-ha Lee David H. Sharp Kenny Ye Qisu Zou 《Computational Geosciences》2001,5(3):173-197
We consider numerical solutions of the Darcy and Buckley–Leverett equations for flow in porous media. These solutions depend on a realization of a random field that describes the reservoir permeability. The main content of this paper is to formulate and analyze a probability model for the numerical coarse grid solution error. We explore the extent to which the coarse grid oil production rate is sufficient to predict future oil production rates. We find that very early oil production data is sufficient to reduce the prediction error in oil production by about 30%, relative to the prior probability prediction. 相似文献
182.
183.
184.
A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT
e/T
i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT
e 10, ion-acoustic waves would be excited. Ratios ofT
e/T
i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT
e/T
i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A. 相似文献
185.
R. A. SHAKESBY C. J. CHAFER S. H. DOERR W. H. BLAKE P. WALLBRINK G. S. HUMPHREYS 《The Australian geographer》2003,34(2):147-175
Soil water repellency can enhance overland flow and erosion and may be altered by fire. The Christmas 2001 bushfires near Sydney allowed investigation of the relationship between fire severity, water repellency and hydrogeomorphological changes. For two sub-catchments with differences in fire severities in Nattai National Park, south-west of Sydney, this paper considers: (1) the links between fire severity based on SPOT image analysis and ground observation of fire severity and repellency; (2) the textural and organic/minerogenic characteristics of eroded sediment; and (3) erodibility, erosion and deposition of soils in both catchments. Ground surveys show that image analysis reflects well the degree of vegetation consumption by fire, but cannot adequately predict the degree of ground litter consumption, associated soil heating and repellency effects. Fire had varying effects on repellency, leaving it unchanged, destroying it or enhancing it, depending on the soil temperature reached. The main post-fire hydrogeomorphological changes have been widespread erosion and colluvial and alluvial deposition of topsoil in foot-slope locations and river systems, but only localised redistribution of the highly erodible, repellent sandy subsurface layer. The fire did not trigger major geomorphological change in the study area, but fires probably cause important topsoil and nutrient depletion and may also affect water quality. 相似文献
186.
Estimates of the number of undiscovered deposits on a statewide basis offer a different perspective on the nation's undiscovered resources of gold, silver, copper, lead, and zinc. Mean estimates of the number of undiscovered deposits statewide were extracted from the estimates of undiscovered deposits nationwide. More than 50 undiscovered deposits are estimated to occur in Alaska, Arizona, Nevada, and Wisconsin. Estimating the number of undiscovered deposits statewide serves as a measure of a state's total remaining mineral resources in known conventional deposit types. 相似文献
187.
188.
189.
We investigated the incidence of endozoochory in the semi-arid South African Karoo shrubland and Kalahari savanna by dissecting and germinating seed from dung samples from a wide range of wild mammals and domestic livestock. Intact seeds occurred at a mean density of 1575 seeds kg−1in livestock air-dried dung and 3613 seeds kg−1in air-dried dung of indigenous animals. Seedlings emerged from dung at an average density of 153 kg−1air-dry dung. Seeds of Aizoaceae, Mesembryanthemaceae, Chenopodiaceae and Poaceae were abundant in the dung of wild and domestic herbivores as well as such ant- and termite-eating insectivores as aardvark (Orycteropus afer) and bat-eared fox (Otocyon megalotis). Seeds of fleshy-fruited shrubs occurred in dung of browsing herbivores and the bat-eared fox. Five non-indigenous weeds were found in the dung samples. Although the floras of the Kalahari and Karoo are considered to be largely wind and water-dispersed, endozoochory is a primary or secondary dispersal mechanism in many plant families and for many plant life-forms. Animals dispersed seeds of many species characteristic of fertile and disturbed habitats. In Mesembryanthemaceae, a family in which seeds are primarily dispersed very short distances by raindrops, endozoochory enables occasional long-distance dispersal. 相似文献
190.
The ionization quotients of aqueous carbon dioxide (carbonic acid) have been precisely determined in NaCl media to 5 m and from 50° to 300°C using potentiometric apparatus previously developed at Oak Ridge National Laboratory. The pressure coefficient was also determined to 250°C in the same media. These results have been combined with selected information in the literature and modeled in two ways to arrive at the best fits and to derive the thermodynamic parameters for the ionization reaction, including the equilibrium constant, activity coefficient quotients, and pressure coefficients. The variation with temperature of the two fundamental quantities and were examined along the saturation vapor pressure curve and at constant density. The results demonstrated again that for reactions with minimal electrostriction changes the magnitudes and variations of and with temperature are small and, in addition, and are approximately independent of salt concentration.The results have also been applied to an examination of the solubility of calcite as a function of pH (in a given NaCl medium) for the neutral to acidic region both for systems with fixed CO2 pressure and systems where the calcium ion concentration equals the concentration of carbon. The pH of saturated solutions of calcite with PCO2 of 12 bars increases from 5.1 to 5.5 between 100° and 300°C. 相似文献