全文获取类型
收费全文 | 216篇 |
免费 | 4篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 15篇 |
地球物理 | 13篇 |
地质学 | 118篇 |
海洋学 | 7篇 |
天文学 | 7篇 |
综合类 | 2篇 |
自然地理 | 60篇 |
出版年
2018年 | 1篇 |
2017年 | 2篇 |
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 14篇 |
2012年 | 6篇 |
2011年 | 5篇 |
2010年 | 3篇 |
2009年 | 3篇 |
2008年 | 8篇 |
2007年 | 5篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 9篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 19篇 |
1996年 | 13篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1968年 | 3篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1965年 | 2篇 |
1938年 | 1篇 |
排序方式: 共有226条查询结果,搜索用时 15 毫秒
91.
92.
This study was undertaken to test the utility of a geographical information systems (GIS) approach to problems of watershed mass balance. This approach proved most useful in exploring the effects that watershed scale, lithology and land use have on chemical weathering rates, and in assessing whether mass balance calculations could be applied to large multilithological watersheds. Water quality data from 52 stations were retrieved from STORET and a complete GIS database consisting of the watershed divide, lithology and land use was compiled for each station. Water quality data were also obtained from 7 experimental watersheds to develop a methodology to estimate annual fluxes from incomplete data sets. The methodology consists of preparing a composite of daily flux data, calculating a best fit sinusoid and integrating the equation to obtain an annual flux. Comparison with annual fluxes calculated from high resolution data sets suggests that this method predicts fluxes within about 10% of the true annual flux. Annual magnesium fluxes (moles km−2 yr−1) were calculated for all stations and adjusted for fluxes from atmospheric deposition. Magnesium flux was found to be a strong function of the amount of carbonate in the watershed, and silica fluxes were found to increase with the fraction of sandstone present in the watershed. All fluxes were strongly influenced by mining practices, with magnesium fluxes from affected watersheds being 6–10 times higher than fluxes from comparable pristine watersheds. Mining practices enhance chemical weathering by increasing the surface area of unweathered rock to which water has access and by increasing acidity and rate of mineral weathering. Fluxes were also found to increase with watershed size. This scale dependence is most likely caused by the sensitivity of weathering fluxes to even minor quantities of carbonates, which are likely to be found in all lithologies at larger scales. Mass balances were carried out in watersheds where gauged sub-watersheds made up more than 95% of the area. The calculations show large magnesium flux and water balance discrepancies. These errors may be a result of significant groundwater inputs to streams between gauges. The results suggest that improvements in how we measure discharge and estimate fluxes may be required before we can apply mass balance techniques to larger scales. © 1997 John Wiley & Sons, Ltd. 相似文献
93.
94.
In this study, numerical prediction of surges associated with a storm was made through the method of lines (MOL) in coordination with the newly proposed RKARMS (4, 4) method for the meghna estuarine region, along the coast of Bangladesh. For this purpose, the vertically integrated shallow water equations (SWEs) in Cartesian coordinates were firstly transformed into ordinary differential equations (ODEs) of initial valued, which were then soloved using the new RKARMS (4, 4) method. Nested grid technique was employed for resolving the complexities of the region of interest with minimum cost. Fresh water discharge through the lower Meghna River was taken into account along the north east corner of the innermost child scheme. Numerical experiments were performed with the severe cyclone on April 1991 that crossed the coast over the study area. Simulated results by the study were found to be in good agreement with some reported data and were found to compare well with the results obtained by the MOL in addition with the classical 4th order Runge-Kutta (RK (4, 4)) method and the standard finite difference method (FDM). 相似文献
95.
PAUL QUINN 《地球表面变化过程与地形》1996,21(5):487-487
96.
97.
98.
99.
100.
VINCENZO GIANFRANCO DI; ANDRIESSEN PAUL A. M.; GHEZZO CLAUDIO 《Journal of Petrology》1996,37(5):1175-1206
Detailed petrographic and geochemical data and Sr and Nd isotopecompositions of enclaves and host-granite are reported for oneof the largest strongly peraluminous cordierite-bearing intrusionsof the Hercynian Sardinia-Corsica Batholith: the San BasilioGranite. Compared with other peraluminous series, the San BasilioGranite has a non-minimum melt composition andshows variations primarily owing to fractionation of early-crystallizedplagioclase, quartz and biotite. Crystallization age is constrainedat 305 Ma, by Rb-Sr whole-rock age [30523 Ma with (87Sr/86Sr)i= 0.711050.00041], and occurred during late Hercynian tectonicevents. Nd(305Ma) values range from 7.8 to 7.5.The San Basilio Granite contains both magmatic and metamorphicenclaves. Magmatic enclaves, similar to mafic microgranularenclaves common in calc-alkaline granitoids, are tonalitic incomposition and show a variation in silica content from 60.3to 67.7 wt % correlating with a variation in (87Sr/86 Sr) (305Ma)and Nd (305 Ma) from 0.7092 to 0.7109 and from 6.6 to7.4, respectively. Together with petrographic and othergeochemical data, the Sr and Nd isotopic data record differentstages in a complex homogenization process of an unrelated maficmagma with a crustal melt. A process of simple mixing may accountfor the variations of nonalkali elements and, to some extent,of Sr and Nd isotopes, whereas the distribution of alkali elementsrequires diffusioncontrolled mass transfer. Petrographic andmineralogical data on metamorphic enclaves and geochemical modellingfor trace elements in granite indicate melt generation by high-degreepartial melting involving biotite breakdown of a dominantlyquartzo-feldspathic protolith at about T>750800Cand P>6 kbar leaving a granulite facies garnet-bearing residue,followed by emplacement at 3 kbar. Nd(305Ma) values of thegranite fall within the range defined by the pre-existing metamorphicrocks but (87Sr/86Sr) (305Ma) ratios are lower, indicating involvementof at least two distinct components: a dominant crustal componentand a minor well-mixed mafic end-member. These data point toa decoupling between the Sr-Nd isotope systematics and majorand trace element compositions, suggesting that the effect ofthe mafic component was minor on granite major and trace elementconcentrations, but significant on Sr and Nd isotopes. The studyof the magmatic enclaves and the isotopic evidence demonstratethat unrelated mafic magmas, probably derived from the mantle,had a close spatial and temporal association with the productionof on-minimum melt strongly peraluminous granites,and support the proposal that heat from the mafic magma contributedto crustal melting. KEY WORDS: cordierite-bearing granite; enclaves; felsic-mafic interaction; Sardinia-Corsica Batholith; Sr and Nd isotopes
*Corresponding author. 相似文献