全文获取类型
收费全文 | 1004篇 |
免费 | 186篇 |
国内免费 | 157篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 100篇 |
地球物理 | 396篇 |
地质学 | 371篇 |
海洋学 | 108篇 |
天文学 | 80篇 |
综合类 | 59篇 |
自然地理 | 183篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 33篇 |
2021年 | 42篇 |
2020年 | 46篇 |
2019年 | 49篇 |
2018年 | 42篇 |
2017年 | 42篇 |
2016年 | 56篇 |
2015年 | 37篇 |
2014年 | 54篇 |
2013年 | 69篇 |
2012年 | 50篇 |
2011年 | 52篇 |
2010年 | 58篇 |
2009年 | 66篇 |
2008年 | 63篇 |
2007年 | 56篇 |
2006年 | 64篇 |
2005年 | 53篇 |
2004年 | 46篇 |
2003年 | 47篇 |
2002年 | 41篇 |
2001年 | 36篇 |
2000年 | 25篇 |
1999年 | 33篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 20篇 |
1995年 | 25篇 |
1994年 | 18篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1347条查询结果,搜索用时 15 毫秒
991.
992.
Spatial variability of throughfall and stemflow in an exotic pine plantation of subtropical coastal Australia 总被引:3,自引:0,他引:3 下载免费PDF全文
Junliang Fan Kasper T. Oestergaard Adrien Guyot David G. Jensen David A. Lockington 《水文研究》2015,29(5):793-804
Large‐scale exotic pine plantations have been developed for timber production in subtropical Australia. Few studies investigate the spatial variability of both throughfall and stemflow in such managed pine plantations despite their acknowledged effects on the heterogeneity of hydrological and biochemical processes of forested ecosystems. To examine the spatial variability of rainfall under a 12‐year‐old pine plantation in a subtropical coastal area of Australia, we observed gross rainfall, throughfall and stemflow over a 1‐year period. Our results show that the spatial variability of gross rainfall within a 50 m × 50 m plot is minimal. Throughfall is significantly different among three tree zones (midway between rows, west and east side of trunks), particularly for rainfall <50 mm, with the highest throughfall on the east side of the tree trunks (sum = 85% of gross rainfall) and the lowest in the midway between tree rows (sum = 68% of gross rainfall). These spatial patterns persist among 84% of recorded rainfall events. Spatial variability and time stability of throughfall are better explained by canopy interception of the inclined rainfall resulting from the prevailing easterly wind direction throughout the experiment. The annual stemflow is different among individual sample trees, which is mainly ascribed to the difference in tree size (e.g. projected canopy area and stem diameter). The outcomes of this study would help future investigators better design appropriate sampling strategies in these pine plantations under similar climate conditions. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
993.
Rill erosion is a dominant sediment source on sloping lands. However, the amount of soil loss from rills on steep slopes is vastly more than that on gentle slopes because of differences in rill shape and hydraulic patterns. The aims of this paper are to determine the hydrodynamic characteristics of rills and the friction coefficients in steep slope conditions and to propose modifications of some hydraulic parameters used in soil loss prediction models. A series of inflow experiments was conducted on loess slopes. The results show that the geometric and hydraulic properties of rill on the steep loess slopes, which are characterized by the mean width of cross sections, mean velocity and mean depth of flow, are related to discharge and slope gradient in power functions. However, the related exponents to discharge are 0.26, 0.48 and 0.26, respectively, which are different from the exponents derived in previous studies, which were conducted on gentle slopes. The Manning roughness coefficient ranged from 0.035 to 0.071, with an average of 0.0536, and the Darcy–Weisbach friction coefficients varied from 0.4 to 1.9. The roughness coefficients are closely related to the Reynolds numbers and flow volumes; however, the correlations vary with slope gradient. The roughness coefficients are directly proportional to the Reynolds number and the flow volume on steep slopes, in contrast with the roughness coefficients found on gentle slopes, which decrease as the Reynolds number and flow volume increase. This difference is caused by the interactions among the hydraulics of the flow, the shape of the rills and the sediment concentrations on steep slopes. The results indicate that parameters used in models to predict rill erosion have to be modified according to slope gradient. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
994.
995.
新疆不同区域牧业雪灾损失时频变化特征 总被引:2,自引:2,他引:0
选用新疆1952-2013年75个县707条雪灾害资料, 采用灰色关联评估模型以县为单位将新疆分为特重雪灾区、 重雪灾区、 一般雪灾区、 小雪灾区. 用气候趋势系数与五阶函数分析各雪灾区牲畜死亡数额的长期变化趋势, 并用morlet小波分析其振荡周期. 结果表明: 全疆及各雪灾区牲畜死亡数额基本都存在23 a左右的长周期, 14~17 a中周期, 10 a左右的小周期, 另外, 还存在5~7 a的小波动, 各区周期振荡的强弱及时间范围不同. 全疆及特重雪灾区牲畜死亡数额长期以来有减少趋势; 而重雪灾区和一般雪灾区牲畜死亡数额长期以来有微弱的增加趋势. 特重雪灾区牲畜死亡数额长期变化趋势决定了新疆雪灾牲畜死亡数额的长期变化趋势, 全疆平均每年因雪灾牲畜死亡数额以0.4%的速率递减, 特重雪灾区每年按上一年度总额1.4%的速率递减. 由于20世纪80年代开始, 新疆有计划、 大规模推行牧民定居工程, 从而大大提高了新疆牧业抗御自然灾害的能力, 以及冬春气温升高的缘故, 使得全疆及各雪灾区牲畜死亡数额大大减少. 因为各牧业区实施牧民定居工程开始的时间不同, 使全疆各雪灾区牲畜死亡数额高值期(年代际与年际尺度振荡的活跃期)进入低值期(少发时期)的临界时间存在差异. 相似文献
996.
隧道对地下水环境影响科学评价一直是隧道工程界难以回避又备受困扰的技术难题之一,掌握工程因素对地下水资源流失量及水位变化的影响规律是开展地下水环境影响评价的理论基础。以350 km/h高速铁路双线隧道V级围岩“半包”防水复合式衬砌为计算原型,通过隧道瞬态渗流模型分析了地下水资源流失量及水位变化随时间的变化规律。进一步研究了工程因素中水位埋深、施工方法、施工时间、喷射混凝土厚度、喷射混凝土抗渗性能、喷射混凝土开裂渗漏、地层注浆加固方式、地层注浆加固范围、注浆体抗渗性能及隔断墙对地下水资源流失量及水位变化的影响规律。根据工程因素的影响规律,探讨了施工期及运营期地下水环境保护的控制方法和技术措施。 相似文献
997.
Star‐disc coupling is considered in numerical models where the stellar field is not an imposed perfect dipole, but instead a more irregular self‐adjusting dynamo‐generated field. Using axisymmetric simulations of the hydromagnetic mean‐field equations, it is shown that the resulting stellar field configuration is more complex, but significantly better suited for driving a stellar wind. In agreement with recent findings by a number of people, star‐disc coupling is less efficient in braking the star than previously thought. Moreover, stellar wind braking becomes equally important. In contrast to a perfect stellar dipole field, dynamo‐generated stellar fields favor field‐aligned accretion with considerably higher velocity at low latitudes, where the field is weaker and originating in the disc. Accretion is no longer nearly periodic (as it is in the case of a stellar dipole), but it is more irregular and episodic. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
998.
999.
1000.
Transmission losses through the beds of hillslope rills were studied at two sites in the semi‐arid south‐western United States. At one site the rills were sand‐bedded and at the other they were gravel‐bedded. Transmission losses in the sand‐bedded rills are about 66% higher than those in the gravel‐bedded rills. The former show evidence of increasing transmission loss with increasing depth of flow, whereas the latter do not. In both cases, transmission losses in the rills are about an order of magnitude greater than infiltration losses in adjacent interrill areas. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献