首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21300篇
  免费   3907篇
  国内免费   4932篇
测绘学   1112篇
大气科学   4222篇
地球物理   5434篇
地质学   10858篇
海洋学   2494篇
天文学   955篇
综合类   2454篇
自然地理   2610篇
  2024年   101篇
  2023年   415篇
  2022年   927篇
  2021年   1101篇
  2020年   930篇
  2019年   1012篇
  2018年   1205篇
  2017年   1085篇
  2016年   1299篇
  2015年   965篇
  2014年   1288篇
  2013年   1242篇
  2012年   1140篇
  2011年   1216篇
  2010年   1154篇
  2009年   1135篇
  2008年   1012篇
  2007年   989篇
  2006年   772篇
  2005年   826篇
  2004年   588篇
  2003年   622篇
  2002年   622篇
  2001年   574篇
  2000年   672篇
  1999年   1009篇
  1998年   814篇
  1997年   892篇
  1996年   821篇
  1995年   691篇
  1994年   575篇
  1993年   526篇
  1992年   398篇
  1991年   297篇
  1990年   234篇
  1989年   182篇
  1988年   174篇
  1987年   127篇
  1986年   109篇
  1985年   76篇
  1984年   55篇
  1983年   46篇
  1982年   37篇
  1981年   28篇
  1980年   42篇
  1979年   28篇
  1978年   15篇
  1976年   11篇
  1975年   14篇
  1958年   18篇
排序方式: 共有10000条查询结果,搜索用时 60 毫秒
21.
峰局五矿是多年开采的老矿。近些年,随着开采深度的不断增加和开采层次的逐渐下移,来自底部岩溶高承压水的威胁日趋严重,实际生产能力逐年下降。下组煤能否开采是关系到矿井生存与发展的大问题。作者根据多年的研究成果和工作实践,就下组煤带压开采的可行性,谈一些看法和建议。  相似文献   
22.
Ji  Feng  Shi  Yuchuan  Zhou  Huixing  Liu  Haiming  Liao  Yi 《Natural Hazards》2017,87(1):165-184
Farmers along the Amazon River each year face multiple natural hazards that threaten crop production and limit the potential for agricultural development of the expansive floodplain and active channel. In this paper we report the findings of a study of natural hazard-related risk associated with rice production on silt bars in the active channel of the Amazon River near Iquitos, Peru. Data were gathered in four rice producing communities in 2014 using household surveys (n = 83 households), focus group discussions, surveying of land elevations along the Amazon River, and interpretation of remote sensing imagery. The probability, extent, and severity of rice crop shortfalls were estimated for recent production years and the economic losses to farming households were also assessed. Our findings point to a very high risk of crop shortfalls due to natural hazards, suggesting that a good year brings rice farmers bounty and a bad year, near penury. River stage reversals (repiquetes) and edaphic conditions were found to be more problematic than the often cited hazard of high and/or early floods. Also surprisingly, farmers’ perceptions of hazards and risk diverged markedly from actual shortfalls experienced during the production years studied. Our results provide the first quantitative estimates of risk due to the multiple natural hazards along the Amazon River and point to the need to assist lowland farmers with risk mitigation so as to unlock the considerable potential of Amazon floodlands for agricultural production.  相似文献   
23.
Peng  Weilong  Liu  Quanyou  Zhang  Ying  Jia  Huichong  Zhu  Dongya  Meng  Qingqiang  Wu  Xiaoqi  Deng  Shang  Ma  Yongsheng 《中国科学:地球科学(英文版)》2022,65(5):874-881
Science China Earth Sciences - Helium gas is a scarce but important strategic resource, which is usually associated with natural gas. Presently, only one extra-large helium-rich gas field has been...  相似文献   
24.
GEOTAIL卫星于1994年1月15日亚暴期间,在深磁尾(x=96RE)观测到多重等离子体团及与之相对应的高能离子爆,作者以宁静磁尾平衡位形为初态,考虑介质的可压缩性,数值研究亚暴期间磁尾动力学过程.计算结果展现了等离子体团间歇性形成及其运动发展过程.体现了强亚暴事件中储存于碰尾的能量,通过多重等离子体团的排放而逐渐释放的进程.数值结果还表明:持续施加于边界上的晨昏电场及由此引发的驱动重联是导致等离子体团准周期形成的主要因素.此外,作者还考察尾瓣内任一点磁场强度及其分量随时间的演化,它与行进压缩区(TCRs)的观测特征基本相符.  相似文献   
25.
Hemocyte counts and phenoloxidase(PO)activity were examined after hemolymph being incubated in dopamine(DA),noradrenaline(NE)and serotonin(5-HT).Results showed that all the three biogenic amines(BAs)had a significant impact on total hemocyte count(THC),differential hemocyte count(DHC),and intracelluar and extracelluar phenoloxidase(PO)activity.Among these Bas,DA had the strongest effect on the above parameters,whereas 5-HT had the least effect.Preincubation with D1 receptor antagonist SCH23390,D2 receptor antagonist Sulpiride and 1:1 admixture of the two could significantly inhibit the effect of DA on these parameters.SCH23390 showed a stronger inhibitory effect than Sulpiride,and the admixture exhibited the strongest effect.These results suggested that the change of hemocyte count and activation of prophenoloxidase(proPO)system in Litopenaeusvan-namei hemocyte can be regulated by BAs,and DA modulates the two parameters via its receptors.  相似文献   
26.
本文系统地研究了鄂尔多斯盆地南部延长组凝灰岩元素和同位素地球化学特征。结果表明:凝灰岩主量元素具有高K、高Al、Si中等至高、低Na的特征; REE总体特征为轻稀土富集、重稀土亏损,∑REE在4857~40212 μg/g 之间变化,Eu呈现出负异常,在034~082内变化,δCe范围在080~208之间; 微量元素中U含量在312~144 μg/g 之间,Ba、U、Th、Hf和Ce呈明显正异常,Nb、P、Ti和Rb呈现负异常。详细的分析表明:U的富集和一些微量元素如Ta、Dy、Lu等相关,Th与∑REE存在明显的正相关性,这些变化可能和盆地中铀矿的富集有关; 凝灰岩的硫同位素总体呈现深源特征。根据以上的研究对凝灰岩的形成原因及形成环境进行了探讨。鄂尔多斯盆地南部延长组凝灰岩来源于秦岭造山带火山作用,其中的铀元素富集是由于砂岩型铀矿中铀向凝灰岩迁移而被还原所致。本研究对盆地砂岩型铀矿的铀运移和富集机理认识具有一定指导意义。  相似文献   
27.
Runoff coefficients of the source regions of the Huanghe River in 1956–2000 were analyzed in this paper. In the 1990s runoff of Tangnaihai Hydrologic Station of the Huanghe River experienced a serious decrease, which had at- tracted considerable attention. Climate changes have important impact on the water resources availability. From the view of water cycling, runoff coefficients are important indexes of water resources in a particular catchment. Kalinin baseflow separation technique was improved based on the characteristics of precipitation and streamflow. After the separation of runoff coefficient (R/P), baseflow coefficient (Br/P) and direct runoff coefficient (Dr/P) were estimated. Statistic analyses were applied to assessing the impact of precipitation and temperature on runoff coefficients (including Dr/P, Br/P and R/P). The results show that in the source regions of the Huanghe River, mean annual baseflow coefficient was higher than mean annual direct runoff coefficient. Annual runoff coefficients were in direct proportion to annual pre- cipitation and in inverse proportion to annual mean temperature. The decrease of runoff coefficients in the 1990s was closely related to the decrease in precipitation and increase in temperature in the same period. Over different sub-basins of the source regions of the Huanghe River, runoff coefficients responded differently to precipitation and temperature. In the area above Jimai Hydrologic Station where annual mean temperature is –3.9oC, temperature is the main factor in- fluencing the runoff coefficients. Runoff coefficients were in inverse relation to temperature, and precipitation had nearly no impact on runoff coefficients. In subbasin between Jimai and Maqu Hydrologic Station Dr/P was mainly affected by precipitation while R/P and Br/P were both significantly influenced by precipitation and temperature. In the area be-tween Maqu and Tangnaihai hydrologic stations all the three runoff coefficients increased with the rising of annual precipitation, while direct runoff coefficient was inversely proportional to temperature. In the source regions of the Huanghe River with the increase of average annual temperature, the impacts of temperature on runoff coefficients be-come insignificant.  相似文献   
28.
腾格里沙漠地区水化学特征   总被引:6,自引:2,他引:6  
水资源极为匮乏的腾格里沙漠区内所赋存的水主要有两种类型。一种是沙下潜水 ,主要是周边山区由高向低缓慢地向沙漠区运移所致 ,分布较为普遍 ;另一种是沙漠湖水 ,它是在沙漠区内之低洼地段 ,由于沙下潜水的长期溢出汇集于大小不等的长条状湖泊之中 ,以湖表水 (咸水和卤水 )的形式存在。沙下潜水矿化度在 0 .5~ 1 g/L之间 ,为本区的饮用水。湖表水咸苦 ,矿化度较高 ,就同一湖区而言 ,也有咸水和卤水之分 ,咸水矿化度为每升数十克 ,卤水则在 1 0 0~ 440 g/L之间 ,已达到了盐湖类沉积阶段  相似文献   
29.
川西北地区是国内重要的岩金普查靶区。本文对松潘地区的地质特征,泛克立格法原理,及在本区的应用情况作了介绍,对主要计算步骤,给出基本结果,对计算获得的有意义异常区,进行地质解释,并将此方法与趋势面分析进行对比。  相似文献   
30.
定义了对点误差,导出了统计分布密度和分布函数,给出了矩母函数、特征函数和数字特征,以及对点误差及其投影分量和标准化分布。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号