Guotao Zhang

Heterogeneity of hydraulic conductivity and Darcian flux in the submerged streambed and adjacent exposed stream bank of the Beiluo River, northwest China

Recognizing the heterogeneity of hydraulic conductivity and hyporheic flow is critical for understanding contaminant transfer and biogeochemical and hydrological processes involving streams and aquifers. In this study, the heterogeneity of hydraulic conductivity and Darcian flux in a submerged streambed and its adjacent exposed stream banks were investigated in the Beiluo River, northwest China. In the submerged streambed, Darcian flux was estimated by measurement of vertical hydraulic conductivity (Kv) and vertical head gradient (VHG) using in-situ permeameter tests. On exposed stream banks, both horizontal hydraulic conductivity (Kh) and Kv were measured by on-site permeameter tests. In the submerged streambed, Kv values gradually decreased with depth and the higher values were concentrated in the center and close to the erosional bank. Compared to the exposed stream banks, the Kv values were higher in the streambed. From stream stage to the topmost layer of tested sediment, through increasing elevation, the Kh values increased on the erosional bank, while they decreased on the depositional bank. The values of VHG along the thalweg illustrate that downwelling flux occurred in the deepest area while upwelling flux appeared in the other areas, which might result from the change of streambed elevation. The higher value of the Darcian flux in the submerged streambed existed near the erosional bank.RésuméReconnaître l’hétérogénéité de la conductivité hydraulique et de l’écoulement hyporhéique est essentiel pour comprendre le transfert des polluants et les processus biogéochimiques et hydrologiques concernant les cours d’eau et les aquifères. Dans la présente étude, l’hétérogénéité de la conductivité hydraulique et du flux de Darcy dans le fond noyé du cours d’eau et la berge attenante exposée a été étudiée sur la Rivière Beiluo, Nord-Ouest de la Chine. Dans le fond noyé du cours d’eau, le flux de Darcy a été estimé en mesurant la conductivité hydraulique verticale (Kv) et le gradient de charge hydraulique verticale (GCV) grâce à des tests in situ au perméamètre. Sur les berges exposées du cours d’eau, la conductivité hydraulique horizontale (Kh) et verticale (Kv) ont été mesurées à l’aide de tests sur site au perméamètre. Dans le fond noyé du cours d’eau, les valeurs de Kv décroissent progressivement avec la profondeur et les valeurs les plus élevées sont concentrées dans le centre et proche de la berge soumise à l’érosion. Comparée aux berges exposées du cours d’eau, les valeurs de Kv étaient plus élevées dans le lit du cours d’eau. Entre le niveau du cours d’eau et la couche supérieure du sédiment testé, en raison d’une hauteur croissante, les valeurs de Kh croissent sur la berge soumise à l’érosion, tandis qu’elles décroissent sur la berge avec dépôts. Les valeurs de GCV le long du thalweg illustrent les flux descendants se produisant dans les zones les plus profondes tandis que les flux ascendants se manifestent dans les autres zones, ce qui peut résulter du changement d’altitude du lit du cours d’eau. La valeur plus élevée du flux de Darcy dans le fond noyé du cours d’eau existe près de la berge soumise à l’érosion.ResumenEl reconocimiento de la heterogeneidad de la conductividad hidráulica y del flujo hiporreico es fundamental para la comprensión de la transferencia de contaminantes y procesos biogeoquímicos e hidrológicos que involucran a corrientes de agua y acuíferos. En este estudio, se investigaron la heterogeneidad de la conductividad hidráulica y del flujo Darciano en un lecho sumergido y en las márgenes expuestas de los cursos adyacentes en el río Beiluo, noroeste de China. En el lecho sumergido, se estimó el flujo Darciano por medición de la conductividad hidráulica vertical (Kv) y gradiente de la carga hidráulica vertical (VHG) usando pruebas de permeámetros in situ. En las márgenes expuestas, tanto la conductividad hidráulica horizontal (Kh) y Kv se midieron mediante ensayos con permeámetro in situ. En el lecho sumergido, los valores Kv disminuyeron gradualmente con la profundidad y los valores más altos se concentran en el centro y cerca de la margen erosiva. Los valores Kv fueron mayores en el lecho del río si se los compara con los bancos de la corriente expuestos. A partir del nivel de la capa superior de los sedimentos de la corriente ensayados, al aumentar la elevación, los valores de Kh aumentaron en la margen erosiva, mientras que disminuyeron en la margen de depositación. Los valores de VHG a lo largo del thalweg del cauce ilustran que el flujo descendente se produjo en la zona más profunda, mientras que el flujo de surgencia apareció en las otras áreas, que podrían resultar del cambio de elevación del lecho del río. El valor más alto del flujo Darciano en el lecho sumergido existía cerca de la margen erosiva.摘要正确认识河流沉积物渗透性能的空间异质性在河水-沉积物水交换、溶质运移、生物化学作用等过程研究中十分重要。以中国西北地区北洛河为研究对象,在河床上,采用原位竖管水头下降法对沉积物垂向渗透系数与垂直水力梯度进行测试,并通过达西定律计算渗流量。在与之相邻的河岸带,采用非原位竖管水头下降法对沉积物垂向和水平两种渗透系数进行试验测试,结果表明:河床沉积物垂向渗透系数随深度增加而减小,垂向渗透系数在河道中间及侵蚀岸附近较大;河岸带沉积物较河床沉积物垂向渗透系数值小。同时,河岸带水平渗透系数在侵蚀岸随高程增加而增大,但在沉积岸随高程增高而变小;由于河床高程的变化,沿深泓线,垂直水力梯度在最深的区域为下降流,在其他试验点表现为上升流;此外,河床沉积物渗流量在侵蚀岸附近值较大。ResumoA identificação da heterogeneidade da condutividade hidráulica e o fluxo hiporreico são cruciais para o entendimento do transporte de contaminantes e processos hidrogeológicos e biogeoquímicos envolvendo rios e aquíferos. Nesse estudo, a heterogeneidade da condutividade hidráulica e o fluxo Darciano em um leito submerso e suas margens expostas adjacentes foram investigados no Rio Beiluo, Noroeste da China. No leito submerso, o fluxo Darciano foi estimado pela medição da condutividade hidráulica vertical (Kv) e do Gradiente Vertical da Carga (GVC) utilizando testes in-situ com permeâmetro. Nas margens expostas, a condutividade hidráulica horizontal (Kh) e Kv também foram medidas in-site usando permeâmetro. No leito submerso, os valores de Kv diminuíram gradualmente com a profundidade e os maiores valores estiveram concentrados no centro e próximo da margem em processo de erosão. Comparados com as margens expostas, os valores de Kv foram maiores no leito. Do nível da água até a camada mais alta de sedimentos testada, ao longo do aumento da elevação, os valores de Kh aumentaram na margem em processo de erosão ao passo que diminuíam na margem deposicional. Os valores da GVC ao longo do talvegue ilustram o fluxo de subsidência ocorrido na área mais profunda, enquanto o fluxo de ressurgência apareceu em outras áreas, o que deve resultar na mudança da elevação do leito. O maior valor do fluxo Darciano no leito submerso ocorreu próximo da margem em erosão.

Kirchhoff Inversion In Image Point Coordinates Recast As Source/Receiver Point Processing

Kirchhoff inversion theory tells us that reflection data provides information about the Fourier transform of the reflectivity function at each point in the illuminated subsurface. Thus, inversion formulas expressed as integrals in image point coordinates that closely characterize that Fourier domain are attractive for their relative simplicity. On the other hand, integrals over source/receiver coordinates are more natural to implement on seismic data. We propose a general principle for seismic migration/inversion (MI) processes: think image point coordinates; compute in surface coordinates. This principle allows the natural separation of multiple travel paths of energy from a source to a reflector to a receiver. Furthermore, the Beylkin determinant is particularly simple in this formalism, and transforming to surface coordinates transforms deconvolution-type imaging and inversion operators into convolution-type operators with the promise of better numerical stability.

Variability in the Vertical Hyporheic Water Exchange Effected by Hydraulic Conductivity and River Morphology at a Natural Confluent Meander Bend

River confluences and their associated tributaries are key morphodynamic nodes that play important roles in controlling hydraulic geometry and hyporheic water exchange in fluvial networks. However, the existing knowledge regarding hyporheic water exchange associated with river confluence morphology is relatively scarce. On the 14th and 15th of January, 2016, the general hydraulic and morphological characteristics of the confluent meander bend (CMB) between the Juehe River and the Haohe River in the southern region of Xi’an City, Shaanxi Province, China were investigated. The patterns and magnitudes of vertical hyporheic water exchange (VHWE) were estimated based on a one-dimension heat steady-state model while the sediment vertical hydraulic conductivity (Kv) was calculated via in situ permeameter tests. The results demonstrated that six hydrodynamic zones and their extensions were observed at the CMB during the test period. These zones were likely controlled by the obtuse junction angle and low momentum flux ratio, influencing the sediment grain size distribution of the CMB. The VHWE patterns at the test site during the test period mostly showed upwelling flow dominated by regional groundwater discharging into the river. The occurrence of longitudinal downwelling and upwelling patterns along the meander bend at the CMB was likely subjected to the comprehensive influences of the local sinuosity of the meander bend and regional groundwater discharge, and finally formed regional and local flow paths. Additionally, in dominated upwelling areas, the change in VHWE magnitudes was nearly consistent with that in Kv values, and higher values of both variables generally occurred in erosional zones near the thalweg paths of the CMB, which were mostly made up of sand and gravel. This was potentially caused by the erosional and depositional processes subjected to confluence morphology. Furthermore, lower Kv values observed in downwelling areas at the CMB were attributed to sediment clogging caused by local downwelling flow. The confluence morphology and sediment Kv are thus likely the driving factors which cause local variations in the VHWE of fluvial systems.

Investigating spatial variability of vertical water fluxes through the streambed in distinctive stream morphologies using temperature and head data

Investigating the interaction of groundwater and surface water is key to understanding the hyporheic processes. The vertical water fluxes through a streambed were determined using Darcian flux calculations and vertical sediment temperature profiles to assess the pattern and magnitude of groundwater/surface-water interaction in Beiluo River, China. Field measurements were taken in January 2015 at three different stream morphologies including a meander bend, an anabranching channel and a straight stream channel. Despite the differences of flux direction and magnitude, flux directions based on vertical temperature profiles are in good agreement with results from Darcian flux calculations at the anabranching channel, and the Kruskal-Wallis tests show no significant differences between the estimated upward fluxes based on the two methods at each site. Also, the upward fluxes based on the two methods show similar spatial distributions on the streambed, indicating (1) that higher water fluxes at the meander bend occur from the center of the channel towards the erosional bank, (2) that water fluxes at the anabranching channel are higher near the erosional bank and in the center of the channel, and (3) that in the straight channel, higher water fluxes appear from the center of the channel towards the depositional bank. It is noted that higher fluxes generally occur at certain locations with higher streambed vertical hydraulic conductivity (Kv) or where a higher vertical hydraulic gradient is observed. Moreover, differences of grain size, induced by stream morphology and contrasting erosional and depositional conditions, have significant effects on streambed Kv and water fluxes.RésuméL’étude des relations nappe-rivière est la clé de compréhension des processus de la zone hyporhéique. Les flux verticaux d’eau à travers le lit d’un cours d’eau sont déterminés en utilisant les lois d’écoulement de Darcy et les profils verticaux de température dans les sédiments, pour évaluer les modalités et l’importance des relations nappe-rivière pour la rivière Beiluo, en Chine. Des mesures de terrain ont été réalisées en janvier 2015 pour trois sites avec une morphologie différente comprenant un méandre en coude, un chenal anastomosé, et un chenal rectiligne. Malgré les différences en termes de direction et d’importance des flux, les directions des flux basées sur les profils verticaux de température sont cohérentes avec les résultats des calculs basés sur la loi de Darcy pour le chenal anastomosé, et les tests de Kruskal-Wallis ne montrent pas de différences significatives entre les flux ascendants estimés, basés sur les deux méthodes, pour chaque site. Par ailleurs, les flux ascendants basés sur les deux méthodes montrent des distributions spatiales similaires dans le lit de la rivière, indiquant (1) que des flux d’eau plus importants se produisent pour le méandre en coude, entre le centre du chenal et la rive soumise à l’érosion, (2) que les flux d’eau sont plus importants pour le chenal anastomosé, près de la rive soumise à l’érosion et au centre du chenal, et (3) que pour le chenal rectiligne, des flux d’eau plus importants apparaissent entre le centre du chenal et la rive soumise aux dépôts. Il est noté que les flux les plus importants apparaissent généralement dans certaines zones où la conductivité hydraulique verticale (Kv) du lit de la rivière est plus élevée, ou pour les zones où un gradient hydraulique vertical plus élevé est observé. De plus, des différences de granulométrie, liées à la morphologie du cours d’eau et aux conditions contrastées d’érosion et de dépôt, ont des effets significatifs sur le Kv du lit du cours d’eau et sur les flux d’eau.ResumenInvestigar la interacción del agua subterránea y el agua superficial es clave para comprender los procesos hiporreicos. Los flujos verticales de agua a través del lecho se determinaron usando cálculos de flujos darcianos y perfiles verticales de temperatura de sedimentos para evaluar el patrón y la magnitud de la interacción agua subterránea / agua de superficie en el río Beiluo, China. Las mediciones de campo se realizaron en enero de 2015 en tres morfologías con diferentes flujos, incluyendo una curva de meandro, un canal entrelazado y un canal recto. A pesar de las diferencias de dirección y magnitud del flujo, las direcciones de flujo basadas en perfiles de temperatura verticales están en una buena concordancia con los resultados de los cálculos de flujo darciano en el canal entrelazado y los ensayos de Kruskal-Wallis no muestran diferencias significativas entre la estimación de los flujos ascendentes en base a los dos métodos en cada sitio. Además, los flujos ascendentes basados en los dos métodos muestran distribuciones espaciales similares en el lecho del río, lo que indica (1) que los flujos de agua más altos en la curva del meandro ocurren desde el centro del canal hacia el banco erosivo, (2) que los flujos de agua en el canal entrelazado son más altos cerca del banco erosivo y en el centro del canal y (3) que en el canal recto, los flujos de agua más altos aparecen desde el centro del canal hacia el banco de sedimentación. Se observa que los flujos más altos generalmente ocurren en ciertos lugares con mayor conductividad hidráulica vertical (Kv) del lecho o donde se observa un gradiente hidráulico vertical más alto. Además, las diferencias en el tamaño de los granos, inducidas por la morfología del curso y las condiciones contrastantes de erosión y sedimentación, tienen efectos significativos en el lecho Kv y en los flujos de agua.摘要调查研究地下水和地表水之间的相互作用是深入理解潜流带过程的关键。于2015年1月,以中国北洛河三个不同河流地貌类型(弯曲河道、分叉河道和直河道)为研究对象,在河床上,采用竖管水头下降法对沉积物垂直渗透系数与垂直水力梯度进行原位测试,并通过达西定律计算河床垂直渗流量。同时,在渗透系数测试点位附近,采用不同深度温度同步测定技术,进行潜流带沉积物不同深度的温度原位测定,并基于一维稳态垂直热扩散对流方程计算河床垂直渗流量。通过以上两种方法研究北洛河地下水-地表水相互作用的补给方式和大小。结果表明:两种方法在垂直渗流量方向和大小的测试结果上存在不一致,但在分叉河道渗流量的方向较为一致,而且Kruskal-Wallis非参数检验表明在三种类型河道两种方法计算的上升流没有显著差异,也有相似的空间分布特征:在弯曲河道中间靠近侵蚀岸,渗流量较大;在分叉河道,靠近侵蚀岸和河道中间的渗流量较大;在直河道中间靠近沉积岸,渗流量较大。在每一类型的河道,渗流量较大的测试点位有较大的河床垂直渗透系数或垂直水力梯度。侵蚀沉积过程导致河道的侵蚀岸和沉积岸的沉积物具有不同的颗粒组成,进而影响河床垂直渗透系数和渗流量的变化。ResumoA investigação da interação entre águas subterrâneas e superficiais é chave para entender processos hiporreicos. Os fluxos de água verticais através do leito do rio foram determinados utilizando cálculos de fluxo Darcyano e perfis verticais da temperatura dos sedimentos para avaliar o padrão e magnitude da interação das águas subterrâneas/superficiais no Rio Beiluo, China. Medidas de campo foram obtidas em Janeiro de 2015 em três morfologias de corrente diferentes incluindo a curva do meandro, um canal anastamosado e um canal retilíneo. Apesar das diferenças de direção e magnitude de fluxo, direções de fluxo baseadas nos perfis de temperatura verticais têm boa concordância com os resultados dos cálculos de fluxo Darcyano no canal anastamosado, e os testes de Kruskalo-Wallis não demonstram diferenças significativas entre os fluxos ascendentes estimados nos dois métodos em cada local. Além disso, os fluxos ascendentes baseados nos dois métodos demonstram distribuição espacial similar no leito do rio, indicando (1) que os fluxos de água mais elevados nas curvas dos meandros ocorrem do centro do canal em direção ao banco erosivo, (2) que os fluxos de água no canal anastamosado são maiores próximos ao banco erosivo e no centro do canal, e (3) que no canal retilíneo, fluxos de água mais elevados aparecem no centro do canal em direção ao banco deposicional. É notável que os fluxos mais elevados geralmente ocorrem em certas localidades com condutividade hidráulica (Kv) vertical no leito mais elevada ou onde um gradiente hidráulico mais elevado é observado. Além do mais, diferenças no tamanho da partícula, induzido pela morfologia da corrente e contrastando as condições erosivas e deposicionais, tem efeitos significantes na Kv do leito do rio e nos fluxos de água.

Impacts of climate change on aridity index and its spatiotemporal variation in the Loess Plateau of China, from 1961 to 2014

The Loess Plateau, the transitional zone between humid and arid regions of China, is an important region to examine the regional hydrological cycle and variation in humid and arid regions under global climate change. Aridity index (AI), the ratio of precipitation (P) to potential evapotranspiration (ET0), is an important indicator of regional climate conditions and is also used to classify drylands. In this study, data from 51 national meteorological stations during the period of 1961–2014 were collected to estimate the AI in the Loess Plateau. Results show that a downward trend in annual AI was detected and the boundary of the drylands region based on the AI was expanded across the Loess Plateau over the period of 1961–2014. The spatiotemporal variability of P was the main cause in the AI variations. Furthermore, data analysis suggested the occurrences of the extreme minimum AI values were mostly affected by fluctuations of the two factors (ET0 and P) rather than its corresponding trend during the period. Thus, this study indicated the major driving factor of AI and the relationship between extreme AI values and the global climate anomalies in the Loess Plateau region, and meanwhile, provided an understanding of the impacts of climate change on hydrological cycle in the Loess Plateau of China.

Effects of Hyporheic Water Fluxes and Sediment Grain Size on the Concentration and Diffusive Flux of Heavy Metals in the Streambed

The hyporheic zone regulates physicochemical processes in surface-groundwater systems and can be an important source of heavy metals in fluvial systems. This study assesses the pore water concentrations and diffusive fluxes of heavy metals with respect to the vertical water exchange flux (VWEF) and sediment grain size. Water and sediment samples were collected on August 2016 from upstream Site 1 and downstream Site 2 along the Juehe River in Shaanxi Province, China. Streambed vertical hydraulic conductivity (Kv) and the VWEF were estimated via the standpipe permeameter test method and Darcy’s law. The heavy metal concentrations in the pore water were measured and the diffusive fluxes were calculated using Fick’s first law. The VWEF patterns were dominated by upward flow, and Site 1 featured higher values of Kv and VWEF. Higher Cu and Zn concentrations occurred near the channel centre with coarse sand and gravel and greater upward VWEFs because coarser sediment and greater upward VWEFs cause stronger metal desorption capacity. Additionally, Cu and Zn at the two sites generally diffused from pore water to surface water, potentially due to the upward VWEF. The VWEF and sediment grain size are likely crucial factors influencing the heavy metal concentrations and diffusive fluxes.

True Amplitude One-Way Wave Equations Near Smooth Caustics: An Example

True amplitude one-way wave equations provide full waveform solutions that agree with leading order WKBJ solutions of the full, two-way wave equation away from caustics and other anomalies of the ray field. Both a proof of this fact and numerical examples are available in the literature. Near smooth caustics, ray-theoretic solutions fail for both the one-way and two-way wave equations; the WKBJ amplitude becomes infinite on the caustic although this is not true for the exact solution. For the full wave equation, the asymptotic solution in the neighborhood of smooth caustics is well understood A theory exists for generating asymptotic expansions in terms of Airy functions. These asymptotic solutions having the property that they remain finite on the caustic and accurately represent exact solutions in the neighborhood of the caustic. A corresponding theory for the one-way wave equation has proven to be elusive. Here, however, we present an example in which the one-way wave equation admits the same leading order asymptotic solution in the neighborhood of a smooth caustic as the two-way wave equation. This suggests that the derivation of a ray-theoretic asymptotic solution in the neighborhood of a smooth caustic should ultimately be achieved for the one-way wave equation, as well.