全文获取类型
收费全文 | 1982篇 |
免费 | 472篇 |
国内免费 | 686篇 |
专业分类
测绘学 | 28篇 |
大气科学 | 102篇 |
地球物理 | 550篇 |
地质学 | 1421篇 |
海洋学 | 508篇 |
天文学 | 2篇 |
综合类 | 101篇 |
自然地理 | 428篇 |
出版年
2024年 | 12篇 |
2023年 | 42篇 |
2022年 | 77篇 |
2021年 | 89篇 |
2020年 | 101篇 |
2019年 | 115篇 |
2018年 | 104篇 |
2017年 | 101篇 |
2016年 | 93篇 |
2015年 | 91篇 |
2014年 | 192篇 |
2013年 | 141篇 |
2012年 | 143篇 |
2011年 | 150篇 |
2010年 | 125篇 |
2009年 | 169篇 |
2008年 | 134篇 |
2007年 | 143篇 |
2006年 | 156篇 |
2005年 | 102篇 |
2004年 | 110篇 |
2003年 | 88篇 |
2002年 | 42篇 |
2001年 | 53篇 |
2000年 | 78篇 |
1999年 | 67篇 |
1998年 | 70篇 |
1997年 | 59篇 |
1996年 | 57篇 |
1995年 | 50篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 18篇 |
1991年 | 23篇 |
1990年 | 12篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 16篇 |
1980年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有3140条查询结果,搜索用时 125 毫秒
71.
72.
An experiment on evapotranspiration from citrus trees under irrigation with saline waterwas carried out for 4 months. Two lysimeters planted with a citrus tree in the green house wereused. One lysimeter was irrigated with saline water (NaCl and CaCl2 of 2000 mg/L equivalence,EC = 3.8 dS/m, SAR = 5.9) and the other was irrigated with freshwater using drip irrigation. Theapplied irrigation water was 1.2 times that of the evapotranspiration on the previous day.Evapotranspiration was calculated as the change in lysimeter weight recorded every 30 minutes.The lysimeters were filled with soil with 95.8% sand. The results of the experiment were as follows.(i) The evapotranspiration from citrus tree was reduced after irrigation with saline water. Theevapotranspiration returns to normal after leaching. However it takes months to exhaust the saltfrom the tree. ( ii ) To estimate the impact of irrigation with saline water on the evapotranspirationfrom citrus trees, the reduction coefficient due to salt stress (Ks) was used in this experiment.Evapotranspiration under irrigation with saline water (ETs) can be calculated from evapotranspira-tion under irrigation with freshwater (ET) by the equation ETs = Ks× ET. Ks can be expressed as afunction of ECsw. (iii) The critical soil-water electrical conductivity (ECsw) is 9.5 dS/m, beyondwhich adverse effects on evapotranspiration begin to appear. If ECsw can be controlled at below9.5 dS/m, saline water can be safely used for irrigation. 相似文献
73.
74.
Measures of local relief, regional relief, and slope were calculated from digital elevation models (DEMs) for 50 bedrock units in the Ridge and Valley and Blue Ridge provinces of Tennessee. Each of these measures was normalized and the three were then averaged to produce the erosional resistance index (ERI). Bedrock units with higher ERI values include coarse clastics, intermediate clastics, and metaplutonics. Units with lower values include shales, limestones, limestones plus dolostones, and carbonates plus fine clastics. Dolostones tend to have intermediate values. The calculated ERI values were compared with subjective ratings by a geologist with decades of field experience in east Tennessee. Generally, the agreement between the two ratings was good, the most glaring exception being several shales with improbably high ERI values. These turned out to be thin units cropping out beneath very hard sandstones, allowing them to stand higher and steeper than would otherwise be possible. A systematic method for detecting such erroneously high ERI values is suggested. Inspection of a drainage map superimposed on the geology map shows that in a given area, streams tend to flow on rock units with the lowest ERI values. In addition, statistical analysis shows that bedrock units with the lowest ERI values are, on average, almost three times closer to the nearest stream and six times as likely to have streams flowing on them than are units with highest values. Further, the effect of ERI on stream location is strongest for streams with drainage areas between 1 and 30 km2. Thus, small streams appear to be subject to greater lithologic control than are larger streams. 相似文献
75.
钙贝塔石发现于四川西昌的霓辉石-钠铁闪石脉中,与之共生的矿物是霓辉石,钠铁闪石,钠长石,铈磷灰石,硅钛铈矿,沥青铀矿,重晶石,方解石和彩钼铅矿等。钙贝塔石呈黑色,黑褐色,具八面体晶形,大小为2mm~8mm,条痕为黑色或黄褐色,油脂到沥青光泽,贝壳状断口。摩氏硬度为6.05~6.44(Hv=570.08kg/mm^2~689.06kg/mm^2);无解理,比重4.51(扭力天平法测定),反射率从406nm(13.53%)到659nm(11.87%)。经计算钙贝塔石的化学式为:(Ca,Na,U)2(Nb,Ti)2(0,OH),。钙贝塔石的强X射线:2.975(10.222),2.570(5.400),1.816(9.440),1.549(8.622),1.050(6.844),等轴晶系,α=1.029nm。 相似文献
76.
77.
高压旋喷桩桩周土摩阻力的分析与探讨 总被引:3,自引:0,他引:3
通过对旋喷桩的单桩静载试验,探讨了桩周土摩阻力作用的情况,通过计算和分析,得出了两个结论,为旋喷桩基础的设计提供参考。 相似文献
78.
黄土湿陷性的微结构不平衡吸力成因论 总被引:16,自引:0,他引:16
分析了黄土粒间吸力的基本特性及其湿陷的微观过程,探讨了黄土在应力状态作用下由于浸水增湿而产生的微结构单元体滑移动力和阻力随饱和度、上覆土压力的变化规律,提出了微结构与广义吸力综合效应的湿陷性控制机理---微结构不平衡吸力成因论及其定量模型,从理论上分析了湿陷性与增湿水量、压力及深度的关系,揭示了土湿陷效应的内在规律。文中认为,湿陷过程就是水的楔入导致小孔隙广义吸力的逐渐丧失和大孔隙湿吸力的逐渐增大引起的微结构重建动力与重建阻力间的动态对抗过程。最后,讨论了湿陷的基本条件及影响因素。 相似文献
79.
东濮凹陷是渤海湾盆地的一个次级构造单元,其古近系的两个沉积旋回发育了四套盐岩,分别位于沙河街组第三段的第四亚段(S34)、第三亚段(S33)、第二亚段(S32)和沙河街组的第一段(S1)。主要分布在凹陷北部。每套盐岩又由若干个盐韵律组成,每一单韵律由盐岩和泥岩组成。两套等时对比标志层(深湖沉积)之间盐岩与砂泥岩存在着相变关系。盐岩是在深盆背景下干旱成因的,盐类物质来自地下深处上涌的卤水,分层卤水是盐形成的基本形式,上涌通道是基底大断裂,动力基础是岩浆活动。盐的阻隔作用使其对下部地层的温度、压力均有影响,这种影响有利于加快盐下地层中的有机质成熟与演化。盐又是良好的盖层,且在高温、高压下盐呈塑性状态。勘探实践看出东濮凹陷的岩性油气藏与盐岩的沉积有密切的关系,因此搞清盐岩的分布规律对东濮凹陷下步勘探有着极为重要的意义。 相似文献
80.