收费全文 | 16962篇 |
免费 | 3488篇 |
国内免费 | 4529篇 |
测绘学 | 1634篇 |
大气科学 | 3703篇 |
地球物理 | 4079篇 |
地质学 | 8355篇 |
海洋学 | 2654篇 |
天文学 | 638篇 |
综合类 | 1742篇 |
自然地理 | 2174篇 |
2024年 | 85篇 |
2023年 | 319篇 |
2022年 | 889篇 |
2021年 | 1008篇 |
2020年 | 834篇 |
2019年 | 877篇 |
2018年 | 995篇 |
2017年 | 925篇 |
2016年 | 997篇 |
2015年 | 962篇 |
2014年 | 1087篇 |
2013年 | 1122篇 |
2012年 | 1105篇 |
2011年 | 1148篇 |
2010年 | 1091篇 |
2009年 | 1145篇 |
2008年 | 1032篇 |
2007年 | 913篇 |
2006年 | 798篇 |
2005年 | 713篇 |
2004年 | 471篇 |
2003年 | 496篇 |
2002年 | 560篇 |
2001年 | 514篇 |
2000年 | 469篇 |
1999年 | 602篇 |
1998年 | 526篇 |
1997年 | 480篇 |
1996年 | 455篇 |
1995年 | 366篇 |
1994年 | 353篇 |
1993年 | 315篇 |
1992年 | 270篇 |
1991年 | 225篇 |
1990年 | 175篇 |
1989年 | 145篇 |
1988年 | 126篇 |
1987年 | 64篇 |
1986年 | 74篇 |
1985年 | 44篇 |
1984年 | 39篇 |
1983年 | 23篇 |
1982年 | 41篇 |
1981年 | 15篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1962年 | 5篇 |
1958年 | 16篇 |
As well known, the methods of remote sensing and Bowen Ratio for retrieving surface flux are based on energy balance closure; however, in most cases, surface energy observed in experiment is lack of closure. There are two main causes for this: one is from the errors of the observation devices and the differences of their observational scale; the other lies in the effect of horizontal advection on the surface flux measurement. Therefore, it is very important to estimate the effects of horizontal advection quantitatively. Based on the local advection theory and the surface experiment, a model has been proposed for correcting the effect of horizontal advection on surface flux measurement, in which the relationship between the fetch of the measurement and pixel size for remote sensed data was considered. By means of numerical simulations, the sensitivities of the main parameters in the model and the scaling problems of horizontal advection were analyzed. At last, by using the observational data acquired in agricultural field with relatively homogeneous surface, the model was validated.
相似文献1) The degree of land use sustainability (Dos) is the ration index to organically and systematically integrate the degree of ecological friendliness (DeF), the degree of economic viability (Dev) and the degree of social acceptability (Dsa), able to comprehensively reflect the whole sustainability degree of regional land use
2) Based on the value of Dos, the grading system and standard for the sustainability of land use may be established and totally divided into five grades, namely, the high-degree sustainability, middle-degree sustainability, low-degree sustainability, conditional sustainability and non-sustainability. Meanwhile, the standard for distinguishing sustainability grades has also been confirmed so as to determine the nature of sustainability degrees in different grades. This makes the possibility for the combination of nature determination with ration in research result and provides with the scientific guideline and decision-making gist for better implementation of sustainable land use strategy.
3) The pract 相似文献