全文获取类型
收费全文 | 3481篇 |
免费 | 657篇 |
国内免费 | 787篇 |
专业分类
测绘学 | 525篇 |
大气科学 | 520篇 |
地球物理 | 812篇 |
地质学 | 1480篇 |
海洋学 | 421篇 |
天文学 | 214篇 |
综合类 | 223篇 |
自然地理 | 730篇 |
出版年
2024年 | 28篇 |
2023年 | 72篇 |
2022年 | 121篇 |
2021年 | 149篇 |
2020年 | 192篇 |
2019年 | 183篇 |
2018年 | 159篇 |
2017年 | 184篇 |
2016年 | 213篇 |
2015年 | 212篇 |
2014年 | 236篇 |
2013年 | 265篇 |
2012年 | 271篇 |
2011年 | 219篇 |
2010年 | 193篇 |
2009年 | 205篇 |
2008年 | 210篇 |
2007年 | 216篇 |
2006年 | 208篇 |
2005年 | 144篇 |
2004年 | 173篇 |
2003年 | 147篇 |
2002年 | 125篇 |
2001年 | 121篇 |
2000年 | 127篇 |
1999年 | 112篇 |
1998年 | 88篇 |
1997年 | 75篇 |
1996年 | 50篇 |
1995年 | 52篇 |
1994年 | 47篇 |
1993年 | 36篇 |
1992年 | 18篇 |
1991年 | 24篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有4925条查询结果,搜索用时 31 毫秒
81.
82.
83.
Water and water related matters are important issues in water scarce countries like the Republic of South Africa. This is
especially true for the agricultural sector that relies heavily on the availability of water for irrigation to ensure a secure
food supply. In South Africa, present day irrigation projects rely on government subsidies and are usually large undertakings
beyond the reach of individuals or small communities. The sustainability of such large irrigation projects is still a matter
of debate. A 100 years ago a different set of rules applied. Irrigation could only be achieved by hand-dug furrows that supplied
water in relatively small quantities to small farming communities. This paper investigated such a small community, the Leeukloof
farming community in the Western Cape Province of the Republic of South Africa. For the past 93 years this community has maintained
a furrow that supplies water not only to farms bordering the Perdeberg River, which supplies the furrow with water, but also
to farms not bordering the river. The authors try to reconstruct the changes in agricultural practices brought about by the
furrow, methods of water extraction during each phase, as well as the development and role of the furrow system. The perception
by the community of the necessity of this method of water extraction from the river system is also investigated.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
84.
Weathering: Toward a Fractal Quantifying 总被引:1,自引:0,他引:1
Klaudia Oleschko Jean-Francois Parrot Gerardo Ronquillo Sergey Shoba Georges Stoops Vera Marcelino 《Mathematical Geology》2004,36(5):607-627
Weathering occurs over a wide range of scales. To link features through these scales is a major challenge for interdisciplinary weathering studies. Fractal approach seems to be specially useful for this purpose. We introduce a multistep fractal weathering assessment scheme devoted to extract fractal weathering classifiers from texture analysis of the mineral's image. Our scheme enables to quantitatively estimate the global and local information about the geometry of the weathering pattern. This information is basic to develop geometrical indices of weathering, which can significantly enrich the common qualitative and semiquantitative weathering assessment schemes. To justify the fractal approach, a strong statistical self-similarity has been documented for both the weathering and fresh features of two common silica minerals: quartz and biogenic A-opal (phytolith) over four orders of length scales. The procedure is fast, drastically reduces thresholding bias, promises to be universal, it is valid for genetically different minerals and rock types, scale independent, and specially useful for monitoring the changes in the mineral's roughness during the alteration. Two of the proposed classifiers seem to be potentially useful for direct application in the field and be used by nonspecialist. 相似文献
85.
86.
利用现有重力场模型求定CHAMP卫星加速度计修正参数 总被引:3,自引:0,他引:3
CHAMP卫星加速度计数据的标定是通过确定其尺度因子和偏差参数来完成的.本文基于能量守恒方程,给出利用现有重力场模型标定CHAMP卫星加速度数据的基本原理和数学模型;提出相邻历元间差分算法,大大简化了观测方程,同时避免积分常量的计算.该算法既能同时解算尺度因子和偏差参数,也可任意求解其中之一.基于实测的CHAMP卫星加速度数据,利用EGM96模型和最新公布的EIGEN-2模型进行计算与比较,验证该方法的有效性. 相似文献
87.
88.
中国大陆地区中强地震前加卸载响应比异常时间尺度的统计研究 总被引:4,自引:2,他引:4
加卸载响应比理论是近年来提出的地震预测方法。本文进行了加卸载响应比异常时间尺度的统计研究,其中包括中国大陆地区5.0~8.1级的部分中强地震共30个震例,并得出加卸载响应比异常的时间尺度与未来地震震级之间的拟合函数。结果表明,地震前加卸载响应比异常的时间尺度与未来地震的震级具有正变关系,即震级越高,地震前加卸载响应比异常的时间尺度越长。根据加卸载响应比异常的时间尺度与震级之间的关系可以估计未来地震的发震时间,同时,可以确定加卸载响应比时空扫描过程中时问长度的大小。 相似文献
89.
Storage of large woody debris in the wide, mountain, Czarny Dunajec River, southern Poland, was investigated following two floods of June and July 2001 with a seven‐year frequency. Within a reach, to which wood was delivered only by bank erosion and transport from upstream, wood quantities were estimated for eighty‐nine, 100 m long, channel segments grouped into nine sections of similar morphology. Results from regression analysis indicated the quantity of stored wood to be directly related to the length of eroded, wooded banks and river width, and inversely related to unit stream power at the flood peak. The largest quantities of wood (up to 33 t ha?1) were stored in wide, multi‐thread river sections. Here, the relatively low transporting ability of the river facilitated deposition of transported wood while a considerable length of eroded channel and island banks resulted in a large number of trees delivered from the local riparian forest. In these sections, a few morphological and ecological situations led to the accumulation of especially large quantities of wood within a small river area. Very low amounts of wood were stored in narrow, single‐thread sections of regulated or bedrock channel. High stream power facilitated transport of wood through these sections while the high strength of the banks and low channel sinuosity prevented bank retreat and delivery of trees to the channel. Considerable differences in the character of deposited wood existed between wide, multi‐thread channel sections located at different distances below a narrow, 7 km long, channellized reach of the river. Wood deposited close to the downstream end of the channellized reach was highly disintegrated and structured into jams, whereas further downstream well preserved shrubs and trees prevailed. This apparently reflects differences in the distance of wood transport and shows that in a mountain river wider than the height of trees growing on its banks, wood can be transported long distances along relatively narrow, single‐thread reaches but is preferentially deposited in wide, multi‐thread reaches. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
90.
Photosynthesis-transpiration coupling model at canopy scale in terrestrial ecosystem 总被引:1,自引:0,他引:1
At the hypothesis of big leaf, an ecosystem photosynthesis-transpiration coupling cycle model was established by the scaled SMPT-SB model from single leaf to canopy, and model parameterization methods were discussed. Through simulating the canopy light distribution, canopy internal conductance to CO2 can be scaled from single leaf to canopy by integrating to canopy using the relationship between single internal conductance and photosynthetic photon flux density. Using the data observed by eddy covariance method from the Changbai Mountains site of ChinaFLUX, the application of the model at the canopy scale was examined. Under no water stress, the simulated net ecosystem photosynthesis rate fitted with the observed data very well, the slope and R2 of the line regression equation of the observed and simulated values were 0.7977 and 0.8892, respectively (n = 752), and average absolute error was 3.78 μmol CO2 m-2s-1; the slope, R2 and average absolute error of transpiration rate were 0.7314, 0.4355 and 1.60mmol H2O m-2 s-1, respectively (n = 752). The relationship between canopy photosynthesis,transpiration and external environmental conditions was discussed by treating the canopy as a whole and neglecting the comprehensive feedback mechanism within canopy, and it was noted that the precipitation course affected the transpiration rate simulation badly. Compared to the models based on eco-physiological processes, the SMPT-SB model was simple and easy to be used. And it can be used as a basic carbon and water coupling model of soil-plant-atmosphere continuum. 相似文献