全文获取类型
收费全文 | 50933篇 |
免费 | 12626篇 |
国内免费 | 18503篇 |
专业分类
测绘学 | 9732篇 |
大气科学 | 6989篇 |
地球物理 | 9472篇 |
地质学 | 33255篇 |
海洋学 | 9659篇 |
天文学 | 680篇 |
综合类 | 4407篇 |
自然地理 | 7868篇 |
出版年
2024年 | 435篇 |
2023年 | 1191篇 |
2022年 | 3258篇 |
2021年 | 3959篇 |
2020年 | 3147篇 |
2019年 | 3757篇 |
2018年 | 3245篇 |
2017年 | 3058篇 |
2016年 | 3001篇 |
2015年 | 3699篇 |
2014年 | 3493篇 |
2013年 | 4394篇 |
2012年 | 4765篇 |
2011年 | 4721篇 |
2010年 | 4658篇 |
2009年 | 4280篇 |
2008年 | 4336篇 |
2007年 | 4150篇 |
2006年 | 3927篇 |
2005年 | 3278篇 |
2004年 | 2508篇 |
2003年 | 1787篇 |
2002年 | 1817篇 |
2001年 | 1638篇 |
2000年 | 1359篇 |
1999年 | 647篇 |
1998年 | 264篇 |
1997年 | 228篇 |
1996年 | 156篇 |
1995年 | 126篇 |
1994年 | 92篇 |
1993年 | 94篇 |
1992年 | 84篇 |
1991年 | 50篇 |
1990年 | 63篇 |
1989年 | 29篇 |
1988年 | 22篇 |
1987年 | 33篇 |
1986年 | 49篇 |
1985年 | 39篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 22篇 |
1981年 | 15篇 |
1980年 | 9篇 |
1979年 | 33篇 |
1973年 | 4篇 |
1963年 | 4篇 |
1957年 | 18篇 |
1954年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
831.
沙坡头人工植被固沙区天然降水的入渗和分配研究 总被引:5,自引:3,他引:5
土壤、植被冠层与大气界面间(SVAT)物质传输过程日益成为水文学研究最感兴趣的领域,降水量的迁移与转换是非灌溉区SVAT主要的物质传输过程.干旱半干旱区稀疏灌丛蒸散量占降水量的90%以上,因此对降水入渗与水分在土壤内植物根际区再分配的研究显得十分重要.试验于2001年8月17日至9月30日在中国科学院沙坡头试验站进行,主要观测人工植被荒漠灌木柠条(Caragana korshinskii) 灌丛固定沙丘降水入渗与再分配过程.结果表明:在7次不连续降水过程中,土壤入渗深度与降水强度呈简单线性相关关系,土壤入渗速率约为降水强度的10倍.当次降水过程中降水强度小于0.46 mmh-1时,土壤入渗速率约为0 cm*h-1,此时的降水对沙区土壤基本上没有水分补给作用.受荒漠灌木柠条根系吸水作用的影响,其根系密集剖面深度40~140 cm内降水水分入渗积累不明显.降水入渗速率及入渗深度受土壤剖面初始含水率多寡而变化,干燥土壤剖面有助于提高瞬时入渗速率.降水以后随着时间的推移,人工固沙区微环境内空气温度、湿度等气象条件适宜,柠条生长进入相对旺盛阶段,其根系密集层140 cm深度处土壤含水率在总体上下降的过程中,表现出昼消夜长的趋势,试验期间翌日 8:00时土壤含水率值略高于前一日20:00时水分值 0.1%~0.3%. 相似文献
832.
Yao Chun-xial Li Bao-sheng Jin He-ling David Dian Zhang Yan Man-cun Zhu Yi-zhi Li Hou-xin Zhang Yu-hong Luo Kai-li 《地理学报(英文版)》2002,12(1):65-71
The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley,
which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation
of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian
sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively
active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable
cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since
then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion
and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies
and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess
Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements
is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter
monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial
and interglacial periods since 150 ka BP. 相似文献
833.
The loess mounds are a newly discovered type of landforms on the Laizhou Bay plain south of the Bohai Sea. Research shows that they were formed in the late period of the late Pleistocene when the Bohai Sea was exposed in the period of late Wümu glacial age and changed into plain. As the exposed area is not protected by vegetation, the sediments at the sea bottom are blown and transported southward by strong north winds, and deposit on the coastal plains. As thick loess is accumulated, the loess mound landforms are formed in the direction of down wind. 相似文献
834.
According to archaeological data from about sixty samples the relative stability of physical and human geographical environment in the tropical zone of China is discussed in this paper. Because of the superior natural environment, sufficient food resources and a sparse population resulting in the absence of social requirement to transform the productive forces, the advancement of economy and society was stagnated during prehistorical period in China's tropics. Compared with northern China, the appearance of ground stone tool stagnated about 3,000 years, the beginning of Bronze Age, about 1,000 years, and the agriculture, 2,500-3,000 years. The no ceramics age continued till the early Neolithic Age and the appearance of colour or white ceramics was 2,000 years later than that in northern China. The life form of migration to gather and to hunt continued till the middle Neolithic Age, and the fixed settlement based on agriculture 1,000-2,000 years stagnated. The clan commune just appeared at the end of the Neolithic Age which was 2,000-3,000 years later than that in northern China. 相似文献
835.
836.
837.
838.
839.
840.