共查询到20条相似文献，搜索用时 31 毫秒
1.
The structure of liquid water is described by three atom pair distribution functions gOO(r), gOH(r), and gHH(r). These functions have now been derived from neutron diffraction data on four mixtures of light and heavy water. They will provide a crucial and sensitive test for proposed models of liquid water. 相似文献
2.
When insect population density varies within the same cotton field, estimation of abundance is difficult. Multiple population
densities of the same species occur because cotton fields (due to edaphic and environmental effects) are apportioned into
various habitats that are colonized at different rates. These various habitats differ temporally in their spatial distributions,
exhibiting varying patterns of interspersion, shape and size. Therefore, when sampling multiple population densities without
considering the influence of habitat structure, the estimated population mean represents a summary of diverse population distributions
having different means and variances. This single estimate of mean abundance can lead to pest management decisions that are
incorrect because it may over- or under-estimate pest density in different areas of the field. Delineation of habitat classes
is essential in order to make local control decisions. Within large commercial cotton fields, it is too laborious for observers
on the ground to map habitat boundaries, but remote sensing can efficiently create geo-referenced, stratified maps of cotton
field habitats. By employing these maps, a simple random sampling design and larger sample unit sizes, it is possible to estimate
pest abundance in each habitat without large numbers of samples. Estimates of pest abundance by habitat, when supplemented
with ecological precepts and consultant/producer experience, provide the basis for spatial approaches to pest control. Using
small sample sizes, the integrated sampling methodology maps the spatial abundance of a cotton insect pest across several
large cotton fields. 相似文献
3.
以结构化森林经营思想为理论基础,从与水源林涵养水源、保持水土功能密切相关的林分物种组成(树种混交)、种内及种间竞争、空间分布格局、垂直结构4个方面选择混交度、竞争指数、角尺度、林层指数、空间密度指数、开阔比数作为水源林健康经营和林分空间结构优化的目标函数,建立洞庭湖水源林林分多目标空间优化模型,应用改进的群智能粒子群算法求解林分空间结构优化模型,并针对模型输出的目标树空间结构单元制定周密的经营策略。研究结果表明,优化模型能准确定位林分空间关系的薄弱环节,调控措施能显著改善林分空间结构,有利于促进森林生态系统的正向演替,为恢复洞庭湖水源林生态功能和健康经营提供理论依据和技术支撑。应用优化模型进行水源涵养林健康经营突破了传统森林经营模式,为智能信息技术在森林空间经营中的应用提供了新的思路。 相似文献
4.
以历史地图空间解译法为技术手段,从城市水系与园林水源、园林建筑、水系形态与格局等方面,按照唐宋、明清、民国、现代的时间段对东湖水系的演变过程进行梳理,并从城市水系与园林的关联性、水系周边建筑空间及功能变化、水系格局的完善三方面归纳总结水系演变特点:①城市水系与东湖在空间结构上大体经历了完善、破坏、重建和分离4个阶段,而东湖在这4个阶段中,参与到整个城市生态体系的系统中,经历了融合、维持、融入和脱离的生态功能变化;②水系周边建筑空间布局经游赏性空间对列交错、纪念性空间沿岸排布、游赏性空间核心主导和纪念性空间轴线贯穿4个阶段,水系周边建筑功能经纪念性与游赏性共存、纪念性与游赏性增强、纪念性弱化和纪念性恢复的变化,使得东湖从以酬唱游宴为主的衙署园林变为以纪念为主的公共园林;③东湖水系的空间格局由单一水体类型的基本格局变为湖渠溪贯全境的丰富格局.最后从城市与园林水系生态整合、园林水系自身格局保护两个方面给出了启示. 相似文献
5.
利用两期不同年代航空遥感信息源,编制两期土壤侵蚀类型现状图,经过叠合比较,建立动态分类系统,勾绘编制的动态图,能够准确、直观地反映出两个年代,特定区域土壤侵蚀类型空间分布的变化规律、趋势和数量特征,为评价土壤侵蚀,水土保持规划和综合治理决策提供科学依据。结果表明,这种方法合理可行,速度快,精度高。 相似文献
6.
The structure of the first coordination shell in liquid water 总被引：1，自引：0，他引：1
Wernet P Nordlund D Bergmann U Cavalleri M Odelius M Ogasawara H Näslund LA Hirsch TK Ojamäe L Glatzel P Pettersson LG Nilsson A 《Science (New York, N.Y.)》2004,304(5673):995-999
X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen-bonded tetrahedral structure in ice. Upon heating from 25 degrees C to 90 degrees C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen-bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed. 相似文献
7.
The competitive binding of a molecule forming a liquid crystal and a targeted analyte to a common molecular receptor presented at a solid surface possessing nanometer-scale topography is used to trigger an easily visualized surface-driven change in the orientation of a micrometer-thick film of liquid crystal. Diffusion of the targeted analyte from atmosphere to surface-immobilized receptor across the micrometer-thick film of liquid crystal is fast (on the order of seconds), and the competitive interaction of the targeted analyte and liquid crystal with the receptor provides a high level of tolerance to nontargeted species (water, ethanol, acetone, and hexanes). Systems that provide parts-per-billion (by volume) sensitivity to either organoamine or organophosphorus compounds are demonstrated, and their use for imaging of spatial gradients in concentration is reported. This approach does not require complex instrumentation and could provide the basis of wearable personalized sensors for measurement of real-time and cumulative exposure to environmental agents. 相似文献
8.
本研究通过构建农业水资源利用效率指标,采用超效率SBM模型对浙江省2010—2020年的农业用水效率进行了测度,并运用核密度估计法分析了农业用水效率的时空演变规律,最后运用地理探测器探析了农业用水效率的主要影响因素,为解决我国农业水资源短缺和水环境污染问题提供参考。结果表明：浙江省农业用水效率从2010年的1.153上升至2020年的1.661,年均增长3.71%,多年均值为1.535,始终处于有效利用状态,多年稳定性较强,且纯技术效率影响较大;浙北地区农业用水效率高于浙南地区,但地区差异逐步缩小;经济发展水平、政府对农业用水的重视程度、农业种植结构、水资源丰裕程度是影响浙江省农业用水效率空间分异的主要因素,同时农业用水效率的空间分异并不是单一因子作用的结果,而是在多种因子共同作用下非线性增强的结果。据此,浙江省需从提高农民收入水平、加大农业财政投入力度、推进节水灌溉技术、提高农户主体节水意识等方面着手进一步提升农业用水效率。 相似文献
9.
Rutter GM Crain JN Guisinger NP Li T First PN Stroscio JA 《Science (New York, N.Y.)》2007,317(5835):219-222
A single sheet of carbon, graphene, exhibits unexpected electronic properties that arise from quantum state symmetries, which restrict the scattering of its charge carriers. Understanding the role of defects in the transport properties of graphene is central to realizing future electronics based on carbon. Scanning tunneling spectroscopy was used to measure quasiparticle interference patterns in epitaxial graphene grown on SiC(0001). Energy-resolved maps of the local density of states reveal modulations on two different length scales, reflecting both intravalley and intervalley scattering. Although such scattering in graphene can be suppressed because of the symmetries of the Dirac quasiparticles, we show that, when its source is atomic-scale lattice defects, wave functions of different symmetries can mix. 相似文献
10.
When a liquid is cooled below its melting temperature, it usually crystallizes. However, if the quenching rate is fast enough, the system may remain in a disordered state, progressively losing its fluidity upon further cooling. When the time needed for the rearrangement of the local atomic structure reaches approximately 100 seconds, the system becomes "solid" for any practical purpose, and this defines the glass transition temperature Tg. Approaching this transition from the liquid side, different systems show qualitatively different temperature dependencies of the viscosity, and accordingly they have been classified by introducing the concept of "fragility." We report experimental observations that relate the microscopic properties of the glassy phase to the fragility. We find that the vibrational properties of the glass well below Tg are correlated with the fragility value. Consequently, we extend the fragility concept to the glassy state and indicate how to determine the fragility uniquely from glass properties well below Tg. 相似文献
11.
12.
研究合理的植物群落空间比例分配,不仅可为绿量计算提供科学合理的依据,而且还能为绿地系统规划设计提供参考.介绍植物群落生态设计中的关键性问题,包括群落生态关系设计、植物序列选择、垂直结构设计、栖息地设计与保护和生态功能设计.在此基础上,指出合适的空间比例分配主要由当地的气候条件及植物群落的功能来确定,但植物群落空间比例分配还随树种的生态习性、季相和平面结构等的变化而变化.最后,从当地的气候条件及植物群落的功能出发对相关研究成果进行综述,总结出景观群落适宜的乔木、灌木、地被(草坪)、绿地的比例应该在1:2 ～6:20 ～25:25 ～35,而功能群落可依据实际需求在此基础上调整某一层的比例. 相似文献
13.
空间句法理论中的可达性程度已成为衡量城市公园绿地建设的重要指标。为了给南陵县中心城区公园绿地的合理空间布局提供指导与科学依据,以城区公园绿地为研究对象,构建线段模型并进行句法测度与量化分析,结合ArcGIS从全局可达性、局部可达性、感知可达性和心理可达性4个维度指出南陵县在城市道路和公园绿地建设方面存在的不足。结果表明：1）南陵县中心城区公园绿地的全局和局部可达性良好,能在总体层面上满足城市居民的日常游憩要求。但其空间聚集性呈现多核心的圈层分布特征,新老城区公园布局不均衡,部分公园周边道路体系建设不完善;2）中心城区公园绿地整体感知可达性较差,不利于城市居民通过局部路网感知公园的整体空间信息,并且公园整体空间渗透能力不足,心理可达性一般;3）在依据完善城市道路结构、合理增加公园绿地、优化绿地分类分级体系和丰富绿色开放空间等策略优化城市道路和公园绿地空间布局后发现,城市道路和公园绿地的全局与局部整合度值均明显提升,公园空间分布系统性与均衡性方面也得到改善。 相似文献
14.
15.
A PROGRESS REPORT ON THE CONSTRUCTION OF POPULATION AND PHYSIOGRAPHIC MAPS FOR THE STATE OF MISSOURI
Odell CB 《Science (New York, N.Y.)》1943,98(2552):476-477
In addition to the two wall maps and the accompanynig isopleth map of population distribution the project calls for a detailed analysis of the distribution of the population of the state in terms of the physiography. The two maps on the same scale may be published as twin wall maps in order to portray the geographic distribution of population. In a state with such variety in land forms it is hoped that a close relationship between population distribution and physiography can be brought out with great emphasis. It is hoped that this brief report will record the good intentions of the author to provide new maps useful in the field of geographic teaching and research. 相似文献
16.
随着我国城市化进程的不断加快,城市结构日趋复杂,人类活动对生态系统过程和功能的干扰愈发严重,使城市生态系统更加的复杂.城市生态系统是城市居民与其环境相互作用而形成的统一整体,也是人类对自然环境的适应、加工、改造而建设起来的特殊的人工生态系统.城市生态系统的复杂性不但直接体现在其自然、经济和社会三个子系统的结构和过程中,也通过许多市政综合管理要素和对象呈现出来.通过对北京市东城区2009年6月至11月的城市综合监管信息平台立案数据的归类分析,结合东城区地形图和专题图矢量数据,从公用设施类、道路交通类、市容环境类、园林绿化类、房屋土地类、其他设施等因素,分析了东城区城市生态系统及其人类活动的相互关系.把从市政管理信息中提取出的城市管理部件问题与城市生态系统的水、土、气、噪声、视觉污染和固体废弃物等关键要素进行关联分析.通过挖掘东城区生态系统在组成和空间分布上的复杂性和规律性,阐明东城区城市生态系统与人类活动时空关系的复杂性,以期为城市生态系统管理提供借鉴. 相似文献
17.
We used angle-resolved photoemission spectroscopy to measure the electronic dispersion and single-particle spectral function in a liquid metal. A lead monolayer supported on a copper (111) surface was investigated as the temperature was raised through the melting transition of the film. Electron spectra and momentum distribution maps of the liquid film revealed three key features of the electronic structure of liquids: the persistence of a Fermi surface, the filling of band gaps, and the localization of the wave functions upon melting. Distinct coherence lengths for different sheets of the Fermi surface were found, indicating a strong dependence of the localization lengths on the character of the constituent atomic wave functions. 相似文献
18.
19.
为揭示陕西省丹汉江流域典型县土壤侵蚀的空间分布特征,利用地理信息系统技术,通过对石泉县和商南县土壤侵蚀强度分布图和DEM图的叠加,得到土壤侵蚀强度在坡度、坡向、高程、起伏度等地貌因子的空间分布状况。结果表明:研究区水土流失程度较高,土壤侵蚀强度以轻度和中度为主。不同坡度、高程、起伏度下的水土流失面积比例、土壤侵蚀综合指数和土壤侵蚀强度均呈现单峰分布,平地的水土流失面积接近0,不同坡向的土壤侵蚀强度分布差异不显著。不同土壤侵蚀强度的最大值随着坡度升高而增大,但却随着高程的升高而减小。各土壤侵蚀强度的起伏度分布特征均很相似。此结论可为陕西省丹汉江流域中游地区水土保持规划和水土流失治理提供决策依据和理论支持。 相似文献
20.
为改善导致民勤绿洲下游地区土地次生盐化严重、土地退化和生态失衡的不合理土地利用模式,通过历史文献法研究了民勤绿洲农业开发与土地利用空间格局演变史,用遥感方法获取了2008年绿洲土地利用/覆被数据,结合对地貌、地下水和土壤的分布规律的分析,研究表明:1)绿洲农业开垦起源于水源充足、土壤持水性好的中下游区域,并逐渐向外围及上游发展;2)绿洲种植结构不合理,农业用水空间配置不合理,下游灌溉且主要用矿化度高的地下水灌溉导致严重次生盐渍化;3)传统农业模式下绿洲下游已出现生产-生活-生态环境恶化,且农民生计无法保证。为此,本研究提出通过"流"的方式重构土地利用空间布局,即农村居民点向上游迁移,使用膜下滴灌技术发展特色林果,在提高农业经济效益的同时,节约农业用水并利用上游回水为下游地区腾退的农业用地恢复耐盐植被,从而改善绿洲生态系统。 相似文献