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浅水湖泊生态系统对人类干扰的反应会随着干扰力度的改变或增强而出现突然的变化,即发生稳态转换;对其机理和驱动机制的揭示将有助于对湖泊富营养化的控制及恢复。基于"多稳态"理论的稳态转换研究已广泛开展,但对浅水湖泊生态系统稳态转换的驱动机制结论各异,采用的阈值判定方法相差很大,主要有实验观测、模型模拟和统计分析3种。实验观测多关注少数特定指标,指标筛选过程复杂且工作量大;模型模拟虽能从较为全面的尺度上理解生态系统稳态变化的特征和主要机理过程,但在模型误差和不确定性的处理等问题上尚存在不足;统计分析方法基于对长时间序列数据的统计变化规律分析,用以判断或者预警稳态转换现象的发生,是目前最为常用的方法。目前稳态转换领域的研究大都是对已发生的稳态转换进行机制分析或过程反演,对未来预测与预警的问题仍然亟需加强。 相似文献
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Carpenter SR Cole JJ Pace ML Batt R Brock WA Cline T Coloso J Hodgson JR Kitchell JF Seekell DA Smith L Weidel B 《Science (New York, N.Y.)》2011,332(6033):1079-1082
Catastrophic ecological regime shifts may be announced in advance by statistical early warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich, but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early warning signals for an experimentally induced regime shift in an aquatic food web. We gradually added top predators to a lake over 3 years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts. 相似文献
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以油、气、水为试验介质,研究了油气水多相流体在水平管中的流型特征。通过测取水平管内油气水多相流流动的试验段压力降脉动信号,运用扩散分析方法对所测压力降脉动信号进行分析。分析表明,不同流型的信号序列其数值分析的结果具有不同的数值特征,反映了不同多相流流型的流动机帛,因此可用于多相流流型识别。 相似文献
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Neptune receives only 1/900th of the earth's solar energy, but has wind speeds of nearly 600 meters per second. How the near-supersonic winds can be maintained has been a puzzle. A plausible mechanism, based on principles of angular momentum and energy conservation in conjunction with deep convection, leads to a regime of uniform angular momentum at low latitudes. In this model, the rapid retrograde winds observed are a manifestation of deep convection, and the high efficiency of the planet's heat engine is intrinsic from the room allowed at low latitudes for reversible processes, the high temperatures at which heat is added to the atmosphere, and the low temperatures at which heat is extracted. 相似文献
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应用传感器和电子技术提出了一款红外探测型香菇大棚煤气语音报警系统的设计方案.该检测系统采用半导体气敏传感器MQ-7检测环境中的煤气浓度;通过测量电路把煤气的浓度变化转换为相应的电压信号,由信号处理电路进行信号处理及浓度高低显示,一旦超过设定的煤气安全浓度值,当种植人员进入大棚时,红外探测电路控制报警电路发出语音报警信号,提醒种植人员采取安全措施.经实践证明,该系统具有工作可靠、灵敏度高等优点,效果良好. 相似文献
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基于次声波法的管道泄漏检测与定位,是通过检测泄漏流体湍射流作用于管壁产生的次声波而进行泄漏检测与定位。对管道泄漏时检测到的低频声波信号进行分析,泄漏信号在频域的特征表现在10Hz以内,因此选择0.4Hz、3.8Hz和7.2Hz的次声波特征频率作为检测特征量。当发现信号中同时存在两个特征频率功率谱及其能量顺序比率突变时,及时将异常数据及其GPS时间发送给监控主机。监控主机根据接收到的一端基站发送的异常发生时的GPS时间,结合被监控管道的长度和泄漏信号的传播速度,计算出另一端基站捕捉到异常信号的起始时间和数据长度,并向该端基站呼叫对应时间段的数据,然后联合两端数据,依据神经网络模型进行泄漏诊断。根据两端基站检测到异常信号发生的GPS时间的时间差,次声波传播速度和上、下游传感器之间的距离,可以确定泄漏点的具体位置。 相似文献
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Stereoscopic depth discrimination in the visual cortex: neurons ideally suited as disparity detectors 总被引：9，自引：0，他引：9
The possibility has been explored that a subset of physiologically identifiable cells in the visual cortex is especially suited for the processing of stereoscopic depth information. First, characteristics of a disparity detector that would be useful for such processing were outlined. Then, a method was devised by which detailed binocular response data were obtained from cortical cells. In addition, a model of the disparity detector was developed that includes a plausible hierarchical arrangement of cortical cells. Data from the cells compare well with the requirements for the archetypal disparity detector and are in excellent agreement with the predictions of the model. These results demonstrate that a specific type of cortical neuron exhibits the desired characteristics of a disparity detector. 相似文献
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《Science (New York, N.Y.)》1978,200(4338):142-152
The basic features of the U.S. energy supply and utilization system change so slowly that an understanding of the dynamics of major change requires projections that extend several decades. Over such a time most energy-consuming capital stock is replaced, life-styles change, and technology evolves. This study of plausible low energy futures leans heavily on detailed engineering analysis of demand by sector, combined with econometric techniques where appropriate. The results indicate that, given time for the system to respond to prices, regulations, and incentives, U.S. energy demand is very elastic. Consequently, a major slowdown in demand growth can be achieved simultaneously with significant economic growth by substituting technological sophistication for energy consumption. 相似文献
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Color images of faint objects were used to test two hypotheses for the quasi-stellar object (QSO) pair 1146+ 111B,C: gravitational lens or massive string. Blue, red, and near-infrared CCD (charge-coupled device) images of the field of this QSO pair were examined for gravitational lens multiple-image candidates for all four QSO's in the field (B, C, D, and E). No third image of 1146+111B,C was found, down to 4 magnitudes fainter than BC. This result implies a compact lens mass distribution, if B and C are images of the same QSO. C appears to be redder than B in the wavelength region from 700 to 1100 nanometers. This raises the question of whether B and C are images of the same QSO. Three blue stellar objects of unusual color were found at plausible locations for multiple images of the other two QSO's in the field. A very red object was found at a plausible lens position. Under the hypothesis that B and C are lensed images, these color data severely restrict the possible lens models and imaged QSO multiplicities. One possibility is a compact lens mass of 4 x 10(15) solar masses at a redshift of 0.8. Another is an S-shaped massive string. If the spectrum of any of the three anomalous blue objects were available, it would be possible to distinguish between these two models. However, it is difficult to fit the color and intensity data reported here to either simple string or black hole models. Overall, the simplest model consistent with all the data is the no-lens, no-string hypothesis: B and C probably are separate QSO's, but with some spectral similarities. 相似文献
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从多个角度分析了我国农业信息化现状及发展过程中存在的问题,介绍了云计算的优势及用云计算技术构建农业管理信息系统。云计算技术的出现和普及将改变现有农业管理信息系统的工作模式,为农业信息化发展提供源动力。 相似文献
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基于GPRS的大棚智能监控系统的设计与实现 总被引：1，自引：1，他引：1
针对农业对象具有的多样性、多变性以及偏僻分散等特点,提出了一种基于GPRS技术的远程数据采集和控制系统方案。通过GPRS无线通讯技术建立现场监控系统与互联网的连接,将实时采集信息发送到数据服务器,实现大棚现场数据信息的自动获取,远程智能监控农场的执行系统,还可为农业管理部门提供决策依据。 相似文献
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Gaisser TK 《Science (New York, N.Y.)》1990,247(4946):1049-1056
Compact regions in the Milky Way, such as accreting degenerate binary stars, may be sites of acceleration of particles with energies far greater than produced at any man-made accelerator, present or proposed. If so, they would emit characteristic neutral radiation of ultra-high energy, which might be strong enough to be detectable at Earth. The quest for these faint but energetic signals is the focus of more than 50 large, ground-based experiments that are looking for high energy photons or neutrinos from point sources in our galaxy and beyond. Several sources have been claimed, but the signals appear to have unexpected and puzzling features that must be clarified before the field can settle into a routine phase of systematic investigation. In the meantime, the potentially profound implications for particle physics, as well as astrophysics, make this field one of intense activity. 相似文献
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基于移动通信的SMS、Mobile Web技术,开发林产品运输信息平台,可实现检尺员上传检尺短信后,实时采伐监控并加强运输监管。该平台有助于运输公司实现物流通知、空车配货,能提高服务质量,降低物流成本。系统架构成本、运营成本低,具有较强的市场竞争力。 相似文献
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通过盆栽及田间试验 ,探讨了粗有机物质的两种不同施用方式以及不同耕作与覆盖措施对澄城旱地土壤水分状况的影响。结果表明 :在土壤中施用粗有机物质 ,兼有阻止上部土壤水分下渗与保蓄下部土壤水分两种作用 ;土壤水势与温度之间呈极显著的直线关系 ,其直线方程的斜率即为水势的温度效应 ,施用有机物质 ,尤其是集中施用 ,能降低下部土壤的水势温度效应值 ;在当地土壤、气候及技术水平等条件下 ,旋耕是目前比较适宜的蓄水保墒耕作方法 ,而免耕则不宜提倡 ;采用夏闲期并苗期秸秆覆盖 ,能有效提高耕层土壤水分的数量水平和能量水平 ;在某些情况下 ,能量指标比数量指标更适于用来表述土壤的水分状况。 相似文献
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针对遥感图像的选择、解译范围在遥感图像上的定位及生态旅游资源各景观要素信息的RS采集技术进行了概述,研究表明:(1)遥感图像的选择除要考虑具体判读的景观因子外,还要因地而异,选择具体的摄影平台、分辨率、比例尺、拍摄时间、光谱波段等;(2)利用Erdas Imagine软件定位使用方便、操作简单、精度较高,是未来的主要发展方向;(3)在各种景观要素的遥感图像解译中,应综合利用多时相和多源遥感数据,并在GIS支持下,充分发挥人工智能优势,提高解译的精度,为生态旅游资源的科学规划和管理提供可靠的基础资料。 相似文献
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WW Metcalf BM Griffin RM Cicchillo J Gao SC Janga HA Cooke BT Circello BS Evans W Martens-Habbena DA Stahl WA van der Donk 《Science (New York, N.Y.)》2012,337(6098):1104-1107
Relative to the atmosphere, much of the aerobic ocean is supersaturated with methane; however, the source of this important greenhouse gas remains enigmatic. Catabolism of methylphosphonic acid by phosphorus-starved marine microbes, with concomitant release of methane, has been suggested to explain this phenomenon, yet methylphosphonate is not a known natural product, nor has it been detected in natural systems. Further, its synthesis from known natural products would require unknown biochemistry. Here we show that the marine archaeon Nitrosopumilus maritimus encodes a pathway for methylphosphonate biosynthesis and that it produces cell-associated methylphosphonate esters. The abundance of a key gene in this pathway in metagenomic data sets suggests that methylphosphonate biosynthesis is relatively common in marine microbes, providing a plausible explanation for the methane paradox. 相似文献
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针对农村电网谐波源的复杂特性,设计了基于LabVIEW技术的、开放式的农网谐波监测平台,该平台既可以通过高速数据采集卡获取实际电网或标准谐波源中的谐波信号,也可以加载基于LabView或Matlab仿真生成的任意组合的虚拟谐波信号。集成了FFT等多种信号处理方法,用于谐波信号的检测与分析。仿真算例表明,运用基于汉宁窗的FFT方法检测农网常见谐波,可以获得较理想的结果。 相似文献
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对多维OLAP数据分析方法进行了讨论,结合某农产品批发交易市场的特点,以蔬菜农药超标检测主题为核心对蔬菜检测的多维视图进行了设计,并以此为基础实现了蔬菜检测的多维数据分析,从而为加强安全管理,对蔬菜超标追根溯源提供了必要的支持. 相似文献