Analytical investigation of the exact groundwater divide between rivers beyond the Dupuit–Forchheimer approximation     

Ruoyi Li  Xu-Sheng Wang 《水文研究》2021,35(2):e14036

The groundwater divide is a key feature of river basins and significantly influenced by subsurface hydrological processes. For an unconfined aquifer between two parallel rivers or ditches, it has long been defined as the top of the water table based on the Dupuit–Forchheimer approximation. However, the exact groundwater divide is subject to the interface between two local flow systems transporting groundwater to rivers from the infiltration recharge. This study contributes a new analytical model for two-dimensional groundwater flow between rivers of different water levels. The flownet is delineated in the model to identify groundwater flow systems and the exact groundwater divide. Formulas with two dimensionless parameters are derived to determine the distributed hydraulic head, the top of the water table and the groundwater divide. The locations of the groundwater divide and the top of the water table are not the same. The distance between them in horizontal can reach up to 8.9% of the distance between rivers. Numerical verifications indicate that simplifications in the analytical model do not significantly cause misestimates in the location of the groundwater divide. In contrast, the Dupuit–Forchheimer approximation yields an incorrect water table shape. The new analytical model is applied to investigate groundwater divides in the Loess Plateau, China, with a Monte Carlo simulation process taking into account the uncertainties in the parameters.  相似文献   

2007-2017年中国沙尘气溶胶的三维分布特征及输送过程     

贾瑞  刘玉芝  吴楚樵  祝清哲  汪兵 《中国沙漠》2019,39(6):108-117

利用最新发布的CALIPSO产品，构建了2007-2017年中国沙尘气溶胶的三维分布，并结合HYSPLIT-4模式和再分析数据，探讨了沙尘的三维输送过程。结果表明：中国的沙尘排放源区主要是塔克拉玛干沙漠和巴丹吉林沙漠，沙尘气溶胶出现频率分别为60%和35%。塔克拉玛干沙漠排放的沙尘主要（50%~70%）停留在源地0~6 000 m高度，少部分向东输送至甘肃和内蒙古；巴丹吉林沙漠排放的沙尘则主要向东输送。中国沙尘排放量在春季最大，向东输送最强；夏季，东亚夏季风限制了沙尘向东输送；秋季，沙尘排放减弱，输送强度和夏季相当；沙尘排放量在冬季最小，输送最弱。夏季，沙尘在输送过程中可被抬升至高度5 000 m以上，春季次之，秋、冬季的沙尘主要在低层大气输送。沙尘在向东输送的过程中被抬升并和当地人为污染物混合变为污染性沙尘，华北地区污染性沙尘出现频率高达30%；输送到海洋的沙尘也会与洋面上（0~3 000 m高度）的海盐气溶胶混合，出现频率约为10%。  相似文献   

大口径射电望远镜电磁兼容评估技术研究          下载免费PDF全文

刘奇 《天文学报》2021,62(4):46

电磁兼容性是设备或系统的重要性能指标, 也是保障系统的工作效能和提高系统可靠性的重要因素. 大口径射电望远镜运行阶段, 台址周围无线电业务及内部潜在的电磁干扰会降低观测系统灵敏度、影响天文观测的质量. 本论文针对拟建的新疆110 m全向可动射电望远镜(Qi Tai raido Telescope, QTT)开展了系统电磁兼容评估技术及控制方法研究, 具有重要的工程应用价值. 首先, 依据现有电波环境测量方法的不足, 深入分析了仪器设备的关键参数配置方法及测量时间计算方法, 采用Y因子法校准测量数据, 提出一种准实时电波环境测量方法. 面向高重复性宽带频谱, 分析了宽带频谱信号和噪声特征, 结合标准差理论, 提出一种基于邻值比较的信噪分离方法, 并采用邻值统计方法优化关键参数, 提高信噪分离精度. 针对QTT台址, 开发了自动化电波环境监测系统, 该系统6 GHz以下频段系统增益大于40 dB, 系统噪声系数小于2 dB, 测量不确定度小于1.49 dB, 具有极高的系统灵敏度和测量精度; 分析了频谱监测数据流, 设计了基于HDF5 (Hierarchical Data Format version 5)的数据存储格式, 开发了自动化电波环境测量和监控软件及数据处理软件. 依据QTT台址长期监测数据, 评估分析了台址电磁环境、主要干扰源特征及其影响. 其次, 提出大口径射电望远镜馈源口面干扰电平限值量化方法, 建立了基于台址地形的电波传播模型, 分析了现有电波传播模型的优缺点及适应性, 结合QTT台址实际地形及地质特征, 采用Longley-Rice和Two-Ray电波传播模型, 预测分析了QTT台址潜在干扰区域电磁干扰达到射电望远镜的电波路径衰减, 结合大口径射电望远镜天线增益量化方法, 提出设备所在位置干扰电平限值量化方法, 运用该方法对QTT台址潜在干扰区域的干扰电平限值进行量化. 依据设备所在位置干扰电平限值, 调研分析了国内外军用、民用电磁兼容测量标准, 结合电磁干扰对射电天文观测的影响, 提出一种大口径射电望远镜电磁兼容控制方法, 解决了现有电波暗室测量系统无法直接测量评估电子设备电磁兼容的问题, 该电磁兼容控制方法计划应用于QTT建设及运行阶段, 确保系统拥有良好的电磁兼容性. 最后, 依据QTT台址潜在干扰区域干扰电平限值, 结合典型电子设备电磁辐射频谱, 分析了QTT电磁兼容设计需求, 提出电磁兼容设计初步方案. 另外, 针对台址建筑设施内的中低电磁辐射干扰源, 提出一种低成本建筑屏蔽方法, 应用于QTT台址现有建筑.  相似文献   

Solar GLAO and TAO:performance comparisons     

De-Qing Ren  Xi Zhang  Si-Zhen Liu 《天文和天体物理学研究(英文版)》2018,(7)

Multi-conjugate adaptive optics(MCAO),consisting of several deformable mirrors(DMs),can significantly increase the adaptive optics(AO)correction field of view.Current MCAO can be realized by either star-oriented or layer-oriented approaches.For solar AO,ground-layer adaptive optics(GLAO)can be viewed as an extreme case of layer-oriented MCAO in which the DM is conjugated to the ground,while solar tomography adaptive optics(TAO)that we proposed recently can be viewed as star-oriented MCAO with only one DM.Solar GLAO and TAO use the same hardware as conventional solar AO,and therefore it will be important to see which method can deliver better performance.In this article,we compare the performance of solar GLAO and TAO by using end-to-end numerical simulation software.Numerical simulations of TAO and GLAO with different numbers of guide stars are conducted.Our results show that TAO and GLAO produce the same performance if the DM is conjugated to the ground,but TAO can only generate better performance when the DM is conjugated to the best height.This result has important application in existing one-DM solar AO systems.  相似文献   

MicroRNAs identification and bioinformatics analysis in large yellow croaker Larimichthys crocea using an integrated comparative and ab initio approach     

房路京  肖世俊  韩兆方  王志勇 《中国海洋湖沼学报》2018,36(5):1707-1719

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Modelling the effect of changing precipitation inputs on deep soil water utilization          下载免费PDF全文

Ji Qi  Daniel Markewitz  David Radcliffe 《水文研究》2018,32(5):672-686

Forests in the Southeastern United States are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change‐induced alterations could increase drought and lower soil water availability. Drought could alter rooting patterns and increase the importance of deep roots that access subsurface water resources. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall reduction experiment in a loblolly pine plantation of the Southeastern United States to calibrate and validate a hydrological model. The model was accurately calibrated against field measured soil moisture data under ambient rainfall and validated using 30% throughfall reduction data. Using this model, we then tested these scenarios: (a) evenly reduced precipitation; (b) less precipitation in summer, more in winter; (c) same total amount of precipitation with less frequent but heavier storms; and (d) shallower rooting depth under the above 3 scenarios. When less precipitation was received, drainage decreased proportionally much faster than evapotranspiration implying plants will acquire water first to the detriment of drainage. When precipitation was reduced by more than 30%, plants relied on stored soil water to satisfy evapotranspiration suggesting 30% may be a threshold that if sustained over the long term would deplete plant available soil water. Under the third scenario, evapotranspiration and drainage decreased, whereas surface run‐off increased. Changes in root biomass measured before and 4 years after the throughfall reduction experiment were not detected among treatments. Model simulations, however, indicated gains in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios but not when precipitation frequency was adjusted. Deep soil and deep rooting can provide an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand of forests. How this buffering capacity will persist in the face of changing precipitation inputs, however, will depend less on seasonal redistribution than on the magnitude of reductions and changes in rainfall frequency.  相似文献   

Variations in glacier volume and snow cover and their impact on lake storage in the Paiku Co Basin,in the Central Himalayas     

Xingxing Zhang  Rui Wang  Zhijun Yao  Zhaofei Liu 《水文研究》2020,34(8):1920-1933

Glaciers and snow cover are important constituents of the surface of the Tibetan Plateau. The responses of these phenomena to global environmental changes are sensitive, rapid and intensive due to the high altitudes and arid cold climate of the Tibetan Plateau. Based on multisource remote sensing data, including Landsat images, MOD10A2 snow product, ICESat, Cryosat-2 altimetry data and long-term ground climate observations, we analysed the dynamic changes of glaciers, snow melting and lake in the Paiku Co basin using extraction methods for glaciers and lake, the degree-day model and the ice and lake volume method. The interaction among the climate, ice-snow and the hydrological elements in Paiku Co is revealed. From 2000 to 2018, the basin tended to be drier, and rainfall decreased at a rate of −3.07 mm/a. The seasonal temperature difference in the basin increased, the maximum temperature increased at a rate of 0.02°C/a and the minimum temperature decreased at a rate of −0.06°C/a, which accelerated the melting from glaciers and snow at rates of 0.55 × 10⁷ m³/a and 0.29 × 10⁷ m³/a, respectively. The rate of contribution to the lake from rainfall, snow and glacier melted water was 55.6, 27.7 and 16.7%, respectively. In the past 18 years, the warmer and drier climate has caused the lake to shrink. The water level of the lake continued to decline at a rate of −0.02 m/a, and the lake water volume decreased by 4.85 × 10⁸ m³ at a rate of −0.27 × 10⁸ m³/a from 2000 to 2018. This evaluation is important for understanding how the snow and ice melting in the central Himalayas affect the regional water cycle.  相似文献   

Benthic foraminiferal fauna and sediment provenance of Eocene syn-rift sequences in Taiwan: implication for onset of Asian epi-continental marginal seas off China coast     

Huang  Chi-Yue  Shao  Lei  Wang  Ming-Huei  Xue  Wei-Guang  Qiao  Pei-Jun  Cui  Yu-Chi  Hou  Yuan-Li 《Marine Geophysical Researches》2019,40(2):111-127

Marine Geophysical Research - Early Cenozoic rift basins developed commonly on the Mesozoic basement along the SE Asia Continent. However, Eocene–Oligocene sequences were only exposed widely...  相似文献   

A novel coupled FDM-DEM modelling method for flexible membrane boundary in laboratory tests     

Hua-Xiang Zhu  Zhen-Yu Yin  Qiang Zhang 《国际地质力学数值与分析法杂志》2020,44(3):389-404

Difficulties are involved in discrete element method (DEM) modelling of the flexible boundary, that is, the membranes covering the soil sample, which can be commonly found in contemporary laboratory soil tests. In this paper, a novel method is proposed wherein the finite difference method (FDM) and DEM are coupled to simulate the rubber membrane and soil body, respectively. Numerical plane strain and triaxial tests, served by the flexible membrane, are implemented and analysed later. The effect of the membrane modulus on the measurement accuracy is considered, with analytical formulae derived to judge the significance of this effect. Based on an analysis of stress-strain responses and the grain rotation field, the mechanical performances produced by the flexible and rigid lateral boundaries are compared for the plane strain test. The results show that (1) the effect of the membrane on the test result becomes more significant at larger strain level because the membrane applies additional lateral confining pressure to the soil body; (2) the tested models reproduce typical stress and volumetric paths for specimens with shear bands; (3) for the plane strain test, the rigid lateral boundary derives a much higher peak strength and larger bulk dilatation, but a similar residual strength, compared with the flexible boundary. The latter produces a more uniform (or ‘diffuse') rotation field and more mobilised local kinematics than does the former. All simulations show that the proposed FDM-DEM coupling method is able to simulate laboratory tests with a flexible boundary membrane.  相似文献   

Discovery of Four New Clusters in the Cygnus Cloud     

秦松梅  李静  陈力  钟靖 《天文和天体物理学研究(英文版)》2021,(2)

We report the discovery of four new open clusters(named QC 1,QC 2,QC 3 and QC 4)in the direction of Cygnus Cloud and select their members based on five astrometric parameters(l,b,ω,μα*,μδ)of Gaia DR2.We also derive their astrophysical parameters for each new cluster.Structure parameters are generated by fitting the radial density distribution with a King’s profile.Using solar metallicity,we performed isochrone-fitting on their purified color-magnitude diagrams(CMDs)to derive the age of the clusters.The known cluster NGC 7062 in an adjacent area is chosen to verify our identification process.The estimated distance,reddening and age of NGC 7062 are in good agreement with the literature.  相似文献