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   45篇
  免费   13篇
  国内免费   19篇
大气科学   7篇
地球物理   27篇
地质学   18篇
海洋学   9篇
天文学   1篇
综合类   9篇
自然地理   6篇
  2022年   3篇
  2021年   2篇
  2020年   1篇
  2019年   4篇
  2018年   6篇
  2016年   1篇
  2015年   2篇
  2014年   4篇
  2013年   4篇
  2012年   1篇
  2011年   5篇
  2010年   3篇
  2008年   2篇
  2007年   1篇
  2006年   2篇
  2005年   1篇
  2003年   1篇
  2000年   2篇
  1999年   3篇
  1998年   2篇
  1997年   3篇
  1996年   4篇
  1995年   4篇
  1994年   2篇
  1993年   9篇
  1992年   1篇
  1991年   1篇
  1990年   1篇
  1986年   1篇
  1983年   1篇
排序方式: 共有77条查询结果,搜索用时 15 毫秒
21.
It is important to evaluate the impacts of grasses on soil erosion process so as to use them effectively to control soil and water losses on the Loess Plateau. Laboratory-simulated rainfall experiments were conducted to investigate the runoff and sediment processes on sloped loess surfaces with and without the aboveground parts of grasses and moss (GAM: grass and moss; NGAM: no grass and moss) under slope gradients of 5°, 10°, 15°, 20°, 25° and 30°. The results show that runoff from GAM and NGAM plots increased up to a slope gradient of 10° and decreased thereafter, whereas the runoff coefficients increased with gradient. The average runoff rates and runoff coefficients of NGAM plots were less than those of GAM plots except for the 5° slope. This behaviour may be due to the reduction in water infiltration under moss. The difference between GAM and NGAM plots in average runoff rates varied from 1·4 to 8%. At the same gradients, NGAM plots yielded significantly (α = 0·05) more sediment than GAM plots. Average sediment deliveries for different slopes varied from 0·119 to 3·794 g m−2 min−1 from GAM plots, and from 0·765 to 16·128 g m−2 min−1 from NGAM plots. Sediment yields from GAM plots were reduced by 45 to 85%, compared with those from the NGAM plots. Plots at 30° yielded significantly higher sediments than at the other gradients. Total sediments S increased with slope gradients G in a linear form, i.e. S = 9·25G − 39·6 with R2 = 0·77*, for the GAM plots, and in an exponential model, i.e. S = 40·4 exp(0·1042G) with R2 = 0·93**, for the NGAM plots. In all cases, sediment deliveries decreased with time, and reached a relative steady state at a rainfall duration of 14 min. Compared with NGAM plots, the final percentage reductions in sediment delivery from GAM plots were higher than those at the initial time of rainfall at all slopes. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
22.
This paper deals with the chemical and isotopic compositions of escaped gases from the Rehai geothermal area in Tengchong county of Yunnan Province. Results indicate that there is the mantle-derived magmatic intrusion in shallow crust at this area. Modern mantle-derived magmatic volatiles are being released currently in a steady stream by way of active faults. The escaped gases are mostly composed of CO2, together with subordinate amounts of H2S, N2, H2, CH4, SO2, CO and He. At the studied area, the north-south directed fault is the deepest, and it may be interlinked with the deep-seated thermal reservoir that would be directly recharged by the mantle-derived magmatic volatile. The He, C isotopic evidence reveals that the modern active magma beneath Rehai area may originate from the historical mantle-derived magma which caused the latest eruptive activity of volcanoes in that region.  相似文献   
23.
There are 463 species of woody plant, belonging to 166 genera and 70 families in Shanxi, North China. The floristic composition types of genera and species are abundant, in which the compositions distributed in temperate zones (including North Temperate, Old World Temperate and Temperate Asia), having 62 genera, are dominant and account for 38.27% in total genera (except for Cosmopolitan), and they are major constructive or dominant species of vegetation in Shanxi. The floristic compositions of woody plants of Shanxi, Hebei, Beijing, Shandong, Henan, Jiangxi and Inner Mongolia were studied comparatively by using diversity index, principal component analysis (PCA) and group-average clustering, respectively. The floristic composition of Shanxi was considerably similar to that of Hebei, Beijing and Shandong because they shared a lot of genera, specially genera distributed in temperate regions, such as Rosa, Quercus, Populus, Salix, Acer, Ulmus, Pinus etc. and their eco-environment were considerably similar. However, the flora of Shanxi was comparatively different from that of Henan, Jiangxi and Inner Mongolia by reason that there were distinct differences among their eco-geographic conditions, floristic compositions and constructive or dominant species of dominant vegetation types. Supported by Shanxi Returned Oversea Scholar Foundation, China  相似文献   
24.
There is little information on the performance of vegetative filter strips (VFS) in filtering high‐concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1‐m‐wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min‐1 m‐1) and sediment concentrations of 100–300 kg m‐3 under simulated rainfall and non‐rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D50) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D50 at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D50 of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D50 decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
25.
稻田土壤中甲烷产生率的实验研究   总被引:12,自引:0,他引:12       下载免费PDF全文
本实验旨在研究稻田土壤中甲烷产生率对稻田CH_4排放的影响.观测结果表明:土壤各深度的甲烷产生率有很大的变化范围(1—4639ng·h~(-1)·g~(-1)d.w.).主要的甲烷产生区域是7—17cm深的土壤层,其中以13cm深的土壤层上的生成速率最大.土壤中甲烷产生率与稻田CH_4排放率在水稻生长的某些阶段有较好的相关性,但它的季节变化却不能与排放的季节变化完全耦合.在水稻生长期,土壤中甲烷产生率随时间而增大,并在8月份水稻收割前达到最大,其日平均值在38—767 ng·h~(-1)·g~(-1)d.w.间变动.稻田土壤中甲烷产生率也存在日变化,一般在下午达到最大值,但却没有发现它与土壤温度有明显的相关关系.在不同施肥及水稻品种的稻田土壤中也观测到不同的甲烷产生率.在土壤中产生的甲烷最多只有28.8%被排放到大气中,而其余多于71.2%的则被氧化在土壤中.  相似文献   
26.
We demonstrate a new way of studying interplanetary magnetic field—Ground State Alignment (GSA). Instead of sending thousands of space probes, GSA allows magnetic mapping with any ground telescope facilities equipped with spectropolarimeter. The polarization of spectral lines that are pumped by the anisotropic radiation from the Sun is influenced by the magnetic realignment, which happens for magnetic field (<1 G). As a result, the linear polarization becomes an excellent tracer of the embedded magnetic field. The method is illustrated by our synthetic observations of the Jupiter’s Io and comet Halley. Polarization at each point was constructed according to the local magnetic field detected by spacecrafts. Both spatial and temporal variations of turbulent magnetic field can be traced with this technique as well. The influence of magnetic field on the polarization of scattered light is discussed in detail. For remote regions like the IBEX ribbons discovered at the boundary of interstellar medium, GSA provides a unique diagnostics of magnetic field.  相似文献   
27.
28.
GPS measurement,an effective method for surveying glacier surface topography,has been applied in some glaciers for many years.The Shuiguan River No.4 glacier,a small glacier with its area of 1.84 km2 in 1972,located in the east of the Qilian Mountains,China,was selected to study its ice elevation change using GPS measurement in 2007.This study was conducted on the ablation area with GPS-measured area 0.5 km2.The ice elevation change of the glacier was obtained by comparing the DEM obtained by a 1:50 000 topographic map made in 1972 with the DEM by GPS-measured data acquired in 2007.The differences of the two DEMs showed the thinning condition of the glacier was apparent.The mean thinning was 15±8 m with the mean thinning rate of 0.42±0.22 m a-1 for 1972-2007 in the measurement area,which equaled 0.38±0.20 m yr-1 in water equivalent(w.e.).The prominent thinning occurred on the south part of the glacier,which was the area near the glacier terminus with the maximum thinning of 41±8 m.Assuming the thinning value of 15±8 m for the glacier area below 4640 m a.s.l.,the wasting ice mass was calculated to be 6.4±3.2×10-3 km3 for 1972-2007,corresponding to 5.7±2.8×10-3 km3 w.e.,which meant that the montane runoff released by the glacier was at least 5.7±2.8×106 m3 between 1972-2007.  相似文献   
29.
山西省湿地生物多样性及其保护   总被引:20,自引:2,他引:18  
山西省共有各类湿地2309km^2,包括河口(内陆)湿地、河流湿地、湖泊湿地、水库湿地、沼泽和草甸湿地等,主要分布于各河流流域、湖泊和水库及周围地区。山西湿地有着丰富的生物多样性,其中动物455种、植物1209种,植被类型有70个群系。此外,还分析了山西湿地生物多样性受威胁的现状和原因,提出了山西湿地生物多样性保护对策。  相似文献   
30.
本文对近年来罗布泊深部钾盐成矿规律研究及找矿进展进行了梳理总结。将其大致归纳为"上升卤水补给"及"含水墙"等成钾理论/模式,为罗布泊后备钾资源找矿提供了新的方向和思路,预测了有利成钾靶区。建立以遥感地质解译、高精度重力剖面测量、EH4连续电导率成像以及氡气剖面测量为主的勘查技术体系,实现了盐湖区深部钾盐找矿靶区高效、经济圈定以及钻孔定位。实施深部钾盐科学钻探验证工程,揭示罗布泊深部赋存有丰富、可采的卤水钾盐资源,取得了找矿突破。罗布泊深部钾盐找矿是理论指导找矿取得突破的又一案例,其成矿理论和勘查技术体系可为其他盐湖及古代盐盆地深部卤水钾盐找矿提供借鉴和参考。鉴于罗布泊卤水钾盐的高强度开发态势以及深部找矿的良好前景,有必要进一步加强深部富钾卤水成因机理研究,重点摸清200~500 m层段资源、研发相应开采技术,为罗布泊钾盐基地可持续发展提供后备资源和技术储备。  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