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   78篇
  免费   6篇
  国内免费   2篇
测绘学   1篇
大气科学   6篇
地球物理   18篇
地质学   34篇
海洋学   5篇
天文学   6篇
自然地理   16篇
  2023年   1篇
  2022年   3篇
  2021年   2篇
  2020年   3篇
  2019年   4篇
  2018年   5篇
  2017年   5篇
  2016年   7篇
  2015年   2篇
  2014年   6篇
  2013年   7篇
  2012年   3篇
  2011年   4篇
  2010年   6篇
  2009年   3篇
  2008年   2篇
  2007年   2篇
  2006年   3篇
  2005年   2篇
  2003年   1篇
  2002年   1篇
  2001年   2篇
  1997年   2篇
  1995年   1篇
  1994年   1篇
  1993年   1篇
  1990年   1篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1979年   1篇
  1956年   1篇
排序方式: 共有86条查询结果,搜索用时 491 毫秒
71.
The Aeta Magbukún of Bataan in Luzon, the Philippines remain one of the least known and documented Indigenous tribes. They retain primarily subsistence hunter‐gathering strategies to supply their basic needs. They also strive for recognition of their ancestral connection with, and rights to inhabit their ancestral forest domains, which is threatened by the expansion of agriculture and urban development by non‐Aetas, primarily the majority Tagalog population. The Aeta continue to endure dispossession, poverty and political discrimination through decades of protracted land rights processes. This review explores the potential use of recent genetic evidence in anthropology and human geography to legitimize the Aeta's status as Indigenous people and contradict the perspective of some governments in the Asia Pacific region who question the existence of Indigenous peoples generally, often from fear of land tenure and associated political repercussions. The acquisition and use of genetic research on Indigenous populations is both socially and politically contested through fear of the potential to jeopardize sovereignty claims and lead to biologically‐based discrimination. However, the full implications of the potential use of genetic research to legitimize Indigenous status, as described within this research, has yet to be thoroughly explored. By exploring both the genetic and anthropological evidence using a case study of the Indigenous Aeta, this paper provides a unique approach to building a case for Indigenous rights, occupation, use of ancestral lands, self‐determination, and ultimately, recognition as Indigenous people.  相似文献   
72.

The Gour Oumelalen area exposes Paleoproterozoic (1.9 Ga) marbles and calc-silicate granulites. Some marbles show a specific mineralogy characterized by the presence of a highly aluminous clinopyroxene with Al2O3 content exceeding16 wt%. This clinopyroxene shows a marked zoning with a hedenbergitic core rimmed by fassaite. Phase relations are expressed by spectacular reaction textures in calc-silicate granulites as Opx + Cpx + Pl + H2O  == > Grt + Qtz ± Am and Cpx + Ilm + Pl ± Mt = > Grt + Qtz + Spn. In olivine-bearing marbles, clinopyroxene and dolomite occur around olivine and calcite. According to thermodynamic modeling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCFMASHTO) system, the first stage of metamorphism is located at 800 °C and 6–7 kbar, which is followed by an increase in pressure at 9–10 kbar and 800 °C and an isobaric cooling at 680 °C. The deduced anticlockwise P–T path is consistent with a granulitic metamorphism occurring in an active continental margin context.

  相似文献   
73.
In this study, bentonite (Ben), compost (Com), and biochar (Bio) were used as soil amendments to enhance sandy soil physical properties. A soil column experiment was conducted in a laboratory. Application rates were 3% (weight/weight) of Bio (T1), Ben (T2), and Com (T3). Furthermore, mixtures 1.5% and 1.5% of Bio and Ben (T4), Ben and Com (T5), and Bio and Com (T6), and a mixture 1%, 1%, and 1% of Bio, Ben, and Com (T7) in addition to control treatment were adopted. The mixtures of amendments and sandy soil were concentrated at the top 10 cm of columns. Results revealed that the cumulative evaporation was reduced by 2.3% and 5.7% as a result of using T3 and T5, respectively. However, the remaining treatments enhanced the cumulative evaporation. The application of amendments increased the capacity of the soil to maintain water by 35.4%, 24.4%, 13.3%, and 10.2%, for soils treated with T5, T3, T7, and T4, respectively. The water content at field capacity had the highest increase in the top 10 cm when treatment T3 was used. Although T3 (compost) was the most efficient for enhancing soil physical properties, this study recommends T5 and T7 to improve hydraulic properties of sandy soils. This is due to the fact that biochar and bentonite remain in the soil for a longer period and resist biodegradation while compost overcomes the negative impact of soil chemical properties as a result of biochar and bentonite additions.  相似文献   
74.
The quality of water and sediments of street runoff in Amman,Jordan   总被引:1,自引:0,他引:1  
Metallic content (Cr, Fe, Mn, Cu, Ni, Pb and Cd) of street sediments and street runoff in addition to major inorganic constituents (Ca, Mg, Na, K, HCO3, Cl, NO3 and SO4) of street runoff were determined under semi‐arid conditions. Two sites in the vicinity of Amman during the pluvial year 1998–1999 were chosen for this investigation. A higher quantity of ionic contents was found at site 2 in comparison to site 1 except for iron, which was significantly higher at site 1. This finding was attributed to higher anthropogenic activity and lower rainfall at site 2, which indicates better water quality of street runoff from residential sites than the city centre. The average concentrations of Pb, Cu and Cd in Amman street runoff compared with the highest levels recorded at humid sites of the world as a result the prevailing semi‐arid conditions in the areas investigated. The highest concentrations of all constituents were detected during the first month of sampling, which might be the result of low rainfall, and a long dry period of atmospheric deposition preceding rainfall events. However, high levels of both lead and copper were recorded (below that of iron) which might be attributed to traffic pollution. In contrast, a significant variation between the average concentrations for Cu, Ni and Cr was found in sediments from the two sites. Moreover, a significant difference was detected only for Cu and Mn at each site overtime. The overall pattern of the results suggests that all heavy metal concentrations for street runoff showed a significant variation over time at site 1 whereas only a significant variation was found for Ni at site 2, which can be explained as the result of higher rainfall at site 1 than at site 2. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
75.
Van Driel-Gesztelyi  L.  Wiik  J.E.  Schmieder  B.  Tarbell  T.  Kitai  R.  Funakoshi  Y.  Anwar  B. 《Solar physics》1997,174(1-2):151-162
Observations of the post-flare loops after the X3.9 flare which occurred on 25 June, 1992 at 20:11 UT by the Yohkoh/SXT in X-rays, as well as in H obtained at 5 different observatories, have provided a unique, longest ever, set of data for a study of the relationship between the hot and cool post-flare loops as they evolve. At any given time, the altitude difference between the hot X-ray loops of 6–7× 106 K and the cool H loops of 1.5× 104 K is related to the expansion rate of the loop systems and their cooling time. Therefore, measurements of the expansion rate and relative height of hot and cool loops can provide direct observational values for their cooling times. We measured the altitude of hot and cool loops for 15 and 19 hours, respectively, and found that the cooling time increased as the density of the loops decreased. We found a reasonably good agreement between the observed cooling times and those obtained from model calculations, although the observed values were always somewhat longer than the theoretical ones. Taking into account evolutionary effects, we also found similar shapes and configurations of hot and cool loops during the entire observing period and confirmed that, at any time, hot loops are at higher altitude than cool loops, suggesting that cool loops indeed evolve from hot loops. These results were used to check the validity of the reconnection model.  相似文献   
76.
A field experiment was conducted under rainfed environment of Pothowar region of Pakistan to assess physical properties of soil as influenced by various green manure legumes (sesbania, cluster bean and rice bean) and different P levels (0, 30, 60, 90 kg P2O5/ha). Highest fresh biomass was observed in sesbania (23 t/ha) followed by cluster bean (19 t/ha) and lowest in rice bean (17 t/ha). Maximum crude protein content was found in sesbania (17%) followed by rice bean (10%) and cluster bean (8%). Green manuring crops, on average reduced soil bulk density (5%), enhanced total porosity (8%), and macropores and large mesopores (28%). Maximum reduction (7%) in soil bulk density, and an increase (11%) in total soil porosity and available water (17%) was observed in plots where sesbania was incorporated as green manuring crop. The order of effectiveness in improving bulk density, macro and mesopores was sesbania > cluster bean > rice bean. Sesbania produced greater number of macro and large mesopores about 41% increase over control followed by cluster bean (29%) and rice bean (16%). Phosphorus application showed meager positive impact on various soil physical properties but did not significantly increased porosity or reduced bulk density. Significant differences in volume fraction in pore space suggest that pore space on a volume basis was much higher in sesbania-amended soil than in other treatments.  相似文献   
77.
库车河流域是库车县工矿企业的集中地,是阿克苏地区和库车县今后一个时期开发、治理的重点地区。随着库车河流域开发程度的迅速增大,进一步提高该流域水资源及其循环转化关系的研究程度,及时发现水资源开发利用过程中存在的问题迫在眉睫。文章在广泛收集有关水文、水利现状资料和实际调查数据的基础上,论述了库车河流域的水资源特征,并在分析水资源开发利用中存在问题的基础上,对水资源供需平衡进行了评价。  相似文献   
78.
Electrical responses of the subsurface can be used to identify geologic strata, locate anomalies, detect and delineate contamination, among many other applications. All these applications depend on the spatial variations of electrical properties in the subsurface and the resulting flow pattern of electric current. Due to the heterogeneity of the subsurface and complex boundary conditions, three-dimensional electric current flow problems are not easy to analyze, in particular when the response is frequency- and/or time-dependent. In this paper, a method of electric circuit analogy is proposed to simulate the electrical responses of geomaterials using the circuit simulator SPICE. The technique will allow simulation of more complex electrical conduction behavior of geomaterials without much extra effort. The excellent agreement between simulated results and analytical solutions developed for surface geophysical techniques establishes the viability of the method. Limitations of the approach and potential solutions to relax these limitations, and other potential applications of the technique in geosciences are also discussed.  相似文献   
79.
针对新疆渭干河-库车河三角洲绿洲土壤盐分动态监测中存在的方法问题,首先用灰色关联度模型分析影响形成土壤盐渍化的各因子,并确定其与土壤盐分之间的关联度,然后将人工智能计算技术引入土壤盐分的预测中,经过多次调整网络结构和参数,建立了预测表层土壤盐分的BP神经网络模型和RBF神经网络模型。结果表明:以潜在蒸散量、地下水埋深、地下水矿化度、土壤电导率、总溶解固体、pH值、坡度和土地利用类型8个因素为输入因子,以土壤含盐量为输出因子的BP网络模型和RBF网络模型可有效模拟土壤盐分与其影响因子之间的内在复杂关系,并且有较高的精度。BP网络模型预测误差略低于RBF神经网络。本研究可为分析和预测土壤盐渍化动态规律提供一种有效可行的新途径,是对传统土壤盐分动态研究的补充。  相似文献   
80.
The variation in free amino acids concentration was examined in halophytes under salt stress conditions during spring and autumn in an arid environment, Jordan. The experiment was conducted in the southern region of the Jordan valley, using four different halophytes namely: Atriplex halimus, Atriplex nummularia, Tamarix aphylla L., and Portulaca oleracea L. Free amino acid concentrations showed changeable patterns under salinity stress conditions compared to the non‐stress (control) with respect to plant species and seasonal variation. In general, the amino acid pool during the spring experienced increases in A. halimus with increasing salinity; and decreased in A. nummularia, P. oleracea L., and T. aphylla L., respectively. Whereas, during the autumn, the overall amino acids decreased in A. halimus and A. nummularia, and increased in T. aphylla L. and P. oleracea with increasing salinity. The amino acid pool concentrations of A. halimus L. during the autumn was similar to that in spring under stress condition. Similarly, T. aphylla L. and P. oleracea showed higher rates in the autumn. In contrast, A. nummularia L. exhibited a slight increase in amino acid composition in the autumn under stress conditions. The plantation of salt tolerant plants such as halophytes in saline sodic soils showed an effective decline in soil salinity and can be recommended to be used as a bioreclamation method instead of the traditional leaching method, which requires large amount of fresh water.  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