From micro scale to boundary value problem: using a micromechanically based model

Acta Geotechnica - A 3D multi-scale approach is presented to investigate the mechanical behavior of a macroscopic specimen consisting of a granular assembly, as a boundary value problem. The core...     

Factors influencing the accuracy of the photogrammetry-based deformation measurement method

Triaxial test has been widely used to investigate the stress–strain relationship of unsaturated soils. During triaxial testing, soil volume is an essential parameter to be measured. For an unsaturated soil, due to the presence of air phase, accurate volume/deformation measurement during triaxial testing was a great challenge for researchers. Recently, a photogrammetry-based method has been developed to measure the soil volume/deformation during triaxial testing. Preliminary triaxial test results indicate the new method is simple, accurate, and cost- and time-effective. However, some concerns regarding its measurement accuracy and applicability, which are critical for the dissemination of the photogrammetry-based method, have been raised by other researchers. These concerns were addressed in details in this study. The factors concerning the deformation measurement accuracy were systematically evaluated through a series of triaxial tests on an aluminum cylinder with different confining media and chamber pressures. A sensitivity analysis was carried out to investigate the impact of the system parameters on the volume measurement accuracy of the photogrammetry-based method. In addition, a triaxial test on a saturated sand specimen was conducted to evaluate the influences of mesh density, mesh pattern, and interpolation technique on the volume change measurement accuracy. Finally, some suggestions were provided to improve the accuracy of the photogrammetry-based measurement method.

     

Origin of sulphate in the unsaturated zone and groundwater of a loess aquifer

The use of the sulphate mass balance (SMB) between precipitation and soil water as a supplementary method to estimate the diffuse recharge rate assumes that the sulphate in soil water originated entirely from atmospheric deposition; however, the origin of sulphate in soil and groundwater is often unclear, especially in loess aquifers. This study analysed the sulphur (δ³⁴S-SO₄) and oxygen (δ¹⁸O-SO₄) isotopes of sulphate in precipitation, water-extractable soil water, and shallow groundwater samples and used these data along with hydrochemical data to determine the sources of sulphate in the thick unsaturated zone and groundwater of a loess aquifer. The results suggest that sulphate in groundwater mainly originated from old precipitation. When precipitation percolates through the unsaturated zone to recharge groundwater, sulphates were rarely dissolved due to the formation of CaCO₃ film on the surface of sulphate minerals. The water-extractable sulphate in the deep unsaturated zone (>10 m) was mainly derived from the dissolution of evaporite minerals and there was no oxidation of sulphide minerals during the extraction of soil water by elutriating soil samples with deionized water. The water-extractable concentration of SO₄ was not representative of the actual SO₄ concentration in mobile soil water. Therefore, the recharge rate cannot be estimated by the SMB method using the water-extractable concentration of SO₄ in the loess areas. This study is important for identifying sulphate sources and clarifying the proper method for estimating the recharge rate in loess aquifers.     

Analysis of nonstationarity in low flow in the Loess Plateau of China

An approach for nonstationary low‐flow frequency analysis is developed and demonstrated on a dataset from the rivers on the Loess Plateau of China. Nonstationary low‐flow frequency analysis has drawn significant attention in recent years by establishing relationships between low‐flow series and explanatory variables series, but few studies have tested whether the time‐varying moments of low flow can be fully described by the time‐varying moments of the explanatory variables. In this research, the low‐flow distributions are analytically derived from the 2 basic explanatory variables—the recession duration and the recession coefficient—with the assumption that the recession duration and recession coefficient variables follow exponential and gamma distributions, respectively; the derived low‐flow distributions are applied to test whether the time‐varying moments of explanatory variables can explain the nonstationarities found in the low‐flow variable. The effects of ecosystem construction measures, that is, check dam, terrace, forest, and grassland, on the recession duration and recession coefficient are further discussed. Daily flow series from 11 hydrological stations from the Loess Plateau are used and processed with a moving average technique. Low‐flow data are extracted following the pit under threshold approach. Six of the 11 low‐flow series show significant nonstationarities at the 5% significance level, and the trend curves of the moments of low flow are in close agreement with the curves estimated from the derived distribution with time‐dependent moments of the recession duration and time‐constant moments of the recession coefficient. It is indicated that the nonstationarity in the low‐flow distribution results from the nonstationarity in the recession duration in all 6 cases, and the increase in the recession duration is resulted from large‐scale ecosystem constructions rather than climate change. The large‐scale ecosystem constructions are found to have more influence on the decrease in streamflow than on the increase in watershed storage, thus resulting in the reduction of low flow. A high return period for the initial fixed design value decreases dramatically with an increasing recession duration.     

基于多源数据的新疆人居环境质量评价

针对区域人居环境质量缺乏从自然和人文因子组合综合评价分析的现状,利用遥感影像、站点监测、交通网络等多源数据,获取与人居环境质量密切相关的自然和人文因素共17个影响因子。应用熵权法、GIS空间分析及现代大数据等方法,定量研究新疆人居环境质量综合评价指数的空间分异及其与经济、人口分布的协调性关系。结果表明：① 新疆自然环境本底受水资源及植被覆盖情况影响较大,自然环境本底评价指数高值区域的空间分布特征与绿洲分散分布的特点具有极大的相似性,县域尺度的高质量区聚集分布在天山北坡的伊犁河谷等地,低质量区和一般质量区大面积集中连片地分布在新疆南部和东部。② 人文环境受交通通达程度及基础支撑完善度影响较大,地级市、县级市优于其他县级地区,人文环境质量基本呈现以各地级市、县级市为中心向外辐射递减的空间分异规律。③ 人居环境质量综合评价指数空间分异显著,呈现出以乌鲁木齐、昌吉市等点为中心的局部圈层式递减的空间分异规律。木垒?皮山分界线东南部与西北部人居环境质量的平均水平相差悬殊。④ 整体人居环境质量水平较低,人口分布与人居环境质量综合评价指数空间分布的匹配度不足,一般质量区和低质量区的县域土地面积占比高达84.90%,分布了47.74%的人口,低质量区仍有较大的提升潜力。⑤ GDP水平与人居环境质量保持较高的匹配性,环境经济协调型县市数量较少,环境经济滞后型县市数量最多,经济发展与人居环境相互掣肘,制约着新疆的可持续性发展。     

Characteristics and Origins of Suspended Pyrite in the Mixing Zone of the Yangtze Estuary

For a long time, most studies about pyrite have focused on sediments while only a few have focused on pyrite in water. In this study, a method that combines the scanning electron microscopy(SEM) and the energy dispersive X-ray spectrometry(EDS) was used to compare pyrite particles suspended in water to those in associated bottom sediments, both obtained from the mixing zone of the Yangtze Estuary. It was found that the pyrite particles in the two media have similar morphologies and size distributions. The particle morphology mainly includes two types, single crystal and aggregate, and the particle size mainly ranges from 0.5 to 2 μm. The pyrite particles in water exhibit an increase in relative content towards the sea, and their transport and deposit processes are mainly affected by hydrodynamic conditions. It is concluded that the pyrite particles in the suspended matter mainly derived from the resuspension of sediments, which are products of the early diagenesis. Precursor minerals may appear during the formation of pyrite, but are generally restricted by the diagenetic environment and local microenvironment.     

黑河流域降水统计——动力降尺度问题研究

依据区域气候模式RIEMS2.0输出的3 km高分辨率数据和站点降水记录分析了中国西北黑河流域降水的动力降尺度和统计—动力降尺度问题,检验了多种因子组合下多元线性回归（MLR）和贝叶斯模式平均（BMA）降尺度模型,评估了降尺度降水的均方根误差、相关系数、方差百分率及“负降水”偏差率等方面的统计特征。结果表明,动力降尺度降水相关系数最高,误差也最大,降水方差达到观测值的1.5～2倍;除相关系数外,统计—动力降尺度模型的几个统计特征均最优,纯统计模型次之。检验表明,仅用700 hPa位势高度场、经向风和比湿等构建的统计降尺度模型估计的站点降水相关系数较低,均方根误差也较大。当在统计降尺度模型中引入模式降水因子后站点降水的估计得到明显改善,其中MLR类模型的降水相关系数和方差百分率均明显高于BMA类模型,均方根误差二者相当,但前者“负降水”出现频次明显大于后者,“负降水”偏差主要出现在降水稀少的冬半年及黑河中、下游干旱或极端干旱区,上游出现频率较低,其中MLR类模型“负降水”出现频次明显高于BMA类模型,后者仅出现在黑河中、下游地区。包含模式降水因子的统计—动力降尺度模型能减少“负降水”出现...     

地图符号的认知语义分析与动态生成

面向位置服务背景下地图符号按需和动态表达的需求,提出了一种基于认知语义分析的地图符号结构化描述模型,以及建立在此基础上的地图符号动态生成方法。结构化描述模型立足"现实—认知—符号"的认知语义原理,以地图符号语素为基本单元、以语义结构为描述框架,侧重描写符号图形和语义之间的关联映射机制。动态生成方法的核心是一种规则控制语素组合构造符号的上下文无关文法模型,通过对图形语素、形态结构语素以及符号生成规则系统进行建模,重构了地图符号动态生成的文法编译器。以个性化导航路径符号为例,通过面向不同情境和语义结构的符号动态生成试验,验证了本文方法的可行性。本文研究将有助于进一步理解地图符号的认知和结构原理,并促进相关行业和应用领域地图符号化的按需和动态表达的水平。     

THE CONTRASTS BETWEEN STRONG AND WEAK MJO ACTIVITY OVER THE EQUATORIAL WESTERN PACIFIC IN WINTER

This paper investigates the contrasts between strong and weak Madden-Julian Oscillation (MJO) activity over the equatorial western Pacific during winter using the NCEP reanalysis data. It is shown that the MJO over the equatorial western Pacific in winter shows significant interannual and interdecadal variabilities. During the winters with strong MJO activity, an anomalous cyclonic circulation lies east of the Philippines, strong anomalous easterlies control the equatorial eastern Pacific, and anomalous westerlies extend from the Indian Ocean to the western Pacific in the lower troposphere, which strengthens the convergence and convection over the equatorial western Pacific. The moisture convergence in the lower troposphere is also enhanced over the western Pacific, which is favorable to the activity of MJO. Eastward propagation is a significant feature of the MJO, though there is some westward propagation. The space-time spectral power and center period of the MJO are higher during strong MJO activity winters. The anomalous activity of MJO is closely related to the sea surface temperature (SST) and East Asian winter monsoon (EAWM). During strong MJO activity winters, there are positive/negative anomalies at high/low latitudes in both sea level pressure and 500 hPa geopotential height, and the temperature is lower over the central part of the Chinese mainland, which indicates a strong EAWM. China experiences more rainfall between the Yellow and Yangtze Rivers, but less rainfall south of the Yangtze River. The SSTA is negative near the Taiwan Island due to the impact of strong EAWM and shows a La Ni?a pattern anomaly over the eastern Pacific. During the weak MJO activity winters, the situation is reversed.     

Global ionospheric electron density estimation based on multisource TEC data assimilation

GPS Solutions - We developed a parameterized ionospheric electron density model based on the IRI-2012 model by spherical harmonic expansions in the horizontal and empirical orthogonal functions in...