全文获取类型
收费全文 | 8593篇 |
免费 | 298篇 |
国内免费 | 97篇 |
专业分类
测绘学 | 202篇 |
大气科学 | 609篇 |
地球物理 | 1966篇 |
地质学 | 3084篇 |
海洋学 | 771篇 |
天文学 | 1312篇 |
综合类 | 19篇 |
自然地理 | 1025篇 |
出版年
2021年 | 90篇 |
2020年 | 116篇 |
2019年 | 118篇 |
2018年 | 176篇 |
2017年 | 168篇 |
2016年 | 233篇 |
2015年 | 206篇 |
2014年 | 202篇 |
2013年 | 502篇 |
2012年 | 255篇 |
2011年 | 321篇 |
2010年 | 285篇 |
2009年 | 349篇 |
2008年 | 327篇 |
2007年 | 289篇 |
2006年 | 328篇 |
2005年 | 243篇 |
2004年 | 309篇 |
2003年 | 275篇 |
2002年 | 272篇 |
2001年 | 188篇 |
2000年 | 176篇 |
1999年 | 144篇 |
1998年 | 148篇 |
1997年 | 108篇 |
1996年 | 122篇 |
1995年 | 118篇 |
1994年 | 133篇 |
1993年 | 120篇 |
1992年 | 117篇 |
1991年 | 111篇 |
1990年 | 103篇 |
1989年 | 90篇 |
1988年 | 93篇 |
1987年 | 124篇 |
1986年 | 104篇 |
1985年 | 169篇 |
1984年 | 193篇 |
1983年 | 149篇 |
1982年 | 134篇 |
1981年 | 137篇 |
1980年 | 113篇 |
1979年 | 129篇 |
1978年 | 126篇 |
1977年 | 110篇 |
1976年 | 98篇 |
1975年 | 96篇 |
1974年 | 71篇 |
1973年 | 87篇 |
1972年 | 54篇 |
排序方式: 共有8988条查询结果,搜索用时 15 毫秒
71.
72.
73.
74.
75.
76.
John Seimenis 《Astrophysics and Space Science》1991,180(2):325-331
We present the generalization of the method of rational Fourier-series approximations for nonlinear ordinary differential equations in galactic dynamics. In previous papers we presented the formulae which describe simple families of periodic orbits with very good accuracy. In this Letter we generalize our formulae to describe periodic orbits of any resonancem:n. 相似文献
77.
78.
Journal of Seismology - 相似文献
79.
80.
Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors influencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26–75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20–47% and 6–28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15–70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3–9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76–82% of the annual TN transport. The remaining dissolved N species, ammonium (NH4-N) and organic N (DON) account for 0·3–1·2% and 13–16% of the annual TN transport, respectively. The TPN and TPP fluxes closely reflect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient fluxes are influenced by point source inputs to the study basins, especially for P, although the TON flux is primarily influenced by diffuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn influenced by land use and basin size. The magnitude of the NH4-N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitrification within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the different nutrient fractions exhibit significant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes. © 1998 John Wiley & Sons, Ltd. 相似文献