Xunhong Chen

Stream water infiltration, bank storage, and storage zone changes due to stream-stage fluctuations

During a flood period, stream-stage increases induce infiltration of stream water into an aquifer; subsequent declines in stream stage cause a reverse motion of the infiltrated water. This paper presents the results of the water exchange rate between a stream and aquifer, the storage volume of the infiltrated stream water in the surrounding aquifer (bank storage), and the storage zone. The storage zone is the part of aquifer where groundwater is replaced by stream water during the flood. MODFLOW was used to simulate stream–aquifer interactions and to quantify rates of stream infiltration and return flow. MODPATH was used to trace the pathlines of the infiltrated stream water and to determine the size of the storage zone. Simulations were focused on the analyses of the effects of the stream-stage fluctuation, aquifer properties, the hydraulic conductivity of streambed sediments, regional hydraulic gradients, and recharge and evapotranspiration (ET) rates on stream–aquifer interactions. Generally, for a given stream–aquifer system, larger flow rates result from larger stream-stage fluctuations; larger storage volumes and storage zones are produced by larger and longer-lasting fluctuations. For a given stream-stage hydrograph, a lower-permeable streambed, an aquitard, or an anisotropic aquifer of low vertical hydraulic conductivity can significantly reduce the rate of infiltration and limit the size of the storage zone. The bank storage solely caused by the stage fluctuation differs slightly between gaining and losing streams. Short-term rainfall recharge and ET loss in the shallow groundwater slightly influence on the flow rate, but their effects on bank storage in a larger area for a longer period can be considerable.

Analysis of pumping-induced stream - aquifer interactions for gaining streams

Abstract This paper presents analytical solutions that can be used to evaluate stream infiltration and baseflow reduction induced by groundwater pumping in nearby aquifers. Critical time, infiltration reach, and travel times can also be calculated to determine the hydraulic connectivity between the well and the stream. The critical time indicates the earliest time of reversal of hydraulic gradient occurring along the stream–aquifer interface, the infiltration reach is the stream segment where stream water recharges the aquifer, and the shortest travel time for the stream water particle to get into a pumping well is along the meridian line. The transient features of the two stream depletion components, baseflow reduction and stream infiltration, are evaluated separately. The rate of baseflow reduction can be greater than the rate of stream infiltration for a stronger gaining stream. However, for a given distance between the stream and well, a higher pumping rate or a weaker gaining stream results in higher rate of stream infiltration, although the total depletion rate is the same for different pumping rates or varied hydraulic gradient of the baseflow. When a steady-state condition is assumed for a transient flow, the rate and volume of stream infiltration can be overestimated; this overestimation can be very significant in the early stage of pumping.

Determination of the anisotropy of an upper streambed layer in east-central Nebraska, USA

Information on the anisotropy of streambed hydraulic conductivity (K) is a necessity for analyses of water exchange and solute transport in the hyporheic zone. An approach is proposed for the determination of K, developed from existing in-situ permeameter test methods. The approach is based on determination of vertical and horizontal hydraulic conductivity of streambed sediments on-site and eliminates the effects of vertical flow in the hyporheic zone and stream-stage fluctuation, which normally influence in situ permeameter tests. The approach was applied to seven study sites on four tributaries of the Platte River in east-central Nebraska, USA. On-site permeameter tests conducted on about 172 streambed cores for the determination of vertical hydraulic conductivity (Kv) and horizontal hydraulic conductivity (Kh) at the study sites indicate that the study sites have relatively small anisotropic ratios, ranging from 0.74 to 2.40. The ratios of Kh to Kv from individual locations within a study site show greater variation than the anisotropic ratios from the mean or median K at each of the study sites.RésuméL’information sur l’anisotropie de la conductivité hydraulique (K) est nécessaire pour les analyses d’échange d’eau et de transport de soluté dans la zone hyporhéique. Une démarche est proposée pour la détermination de K, dérivée de méthodes existantes de test au perméamètre in situ. La démarche est basée sur la détermination sur site des conductivités hydrauliques verticale et horizontale des sédiments du lit et élimine les effets du flux vertical dans la zone hyporhéique et de la fluctuation du niveau d’eau, qui influencent normalement les tests perméamètre in situ. La démarche a été menée sur sept sites d’étude sur quatre tributaires de la Platte River, Centre-Est Nebraska, USA. Des tests de perméabilité sur site réalisés sur environ 172 carottes pour la détermination des conductivités hydrauliques verticale (Kv) et horizontale (Kh) indiquent des ratios d’anisotropie relativement petits, s’échelonnant de 0.74 à 2.40. Les ratios de Kh sur Kv mesurés sur différents emplacements d’un même site montrent une plus grande variabilité que les ratios d’anisotropie de la moyenne ou de la médiane de K sur chacun de ces sites d’étude.ResumenLa información sobre la anisotropía de la conductividad hidráulica de un cauce (K) es una necesidad para los análisis del intercambio de agua y transporte de solutos en una zona hiporreica. Se propone un método para la determinación de K, desarrollado a partir de métodos de prueba in situ en permeámetros existentes. El método está basado en la determinación de la conductividad hidráulica vertical y horizontal de los sedimentos del cauce en el sitio y elimina los efectos del flujo vertical en la zona hiporreica y las fluctuaciones del estado de la corriente, la cual normalmente influye en las pruebas in situ con los permeámetros. El método fue aplicado en siete sitios de estudio en cuatro tributarios del Platte River en el centro este de Nebraska, EEUU. En los lugares de las pruebas en los permeámetros se llevaron a cabo 172 testigos en el cauce para la determinación de la conductividad hidráulica vertical (Kv) y la conductividad hidráulica horizontal (Kh) en los sitios de estudios indican que los sitios de estudio tienen un cociente de anisotropía relativamente pequeña, oscilando de 0.74 a 2.40. Los cocientes de Kh a Kv a partir de lugares individuales dentro del sitio de estudio muestran mayores variaciones que los cocientes de anisotropía a partir de la K media o mediana en cada uno de los sitios estudiados.摘要河床渗透系数的各向异性比是进行河流潜流带水量交换和溶质运移分析时必不可少的重要信息。本文提出一种测定河床渗透系数的改进渗透方法,该方法是基于原位渗透试验的改进。改进试验中采用非原位操作方法,避免了河水位波动和河流潜流带垂向流对原位试验的影响。本研究采用改进的非原位渗透试验方法分别应用在美国内布拉斯加州中东部普拉特河的四条支流上的七个试验场地。非原位渗透试验共计测定172组河床砂样的垂向渗透系数(Kv)和水平渗透系数(Kh)。结果表明这些试验场地具有较弱的各向异性特征,各向异性比介于0.74和2.40之间。在任意试验场地内,每个试验位置上的Kh/Kv值与使用该场地的Kh与Kv的平均值(或中位数)得到的各向异性比相比,具有更大的变异性。ResumoA informação acerca da anisotropia da condutividade hidráulica (K) do sedimento do leito de rios é uma necessidade para as análises de trocas de água e transporte de solutos na zona hiporreica. Propõe-se uma metodologia para determinar K, desenvolvida a partir de métodos existentes de ensaios de permeâmetro in situ. A metodologia é baseada na determinação, no local, da condutividade hidráulica vertical e horizontal de sedimentos do leito de rios e elimina os efeitos do fluxo vertical na zona hiporreica e da flutuação do nível do rio, que normalmente influenciam os ensaios de permeâmetro in situ. A metodologia foi aplicada a sete casos de estudo em quatro afluentes do rio Platte no Centro-este de Nebraska, nos EUA. Os ensaios de permeâmetro in situ, feitos em cerca de 172 sondagens em sedimentos dos leitos de rios, para a determinação da condutividade hidráulica vertical (Kv) e condutividade hidráulica horizontal (Kh) indicam que as zonas de estudo têm rácios de anisotropia relativamente baixos, variando entre 0.74 e 2.40. Os rácios entre Kh e Kv de locais individuais dentro de uma zona de estudo mostram uma variação maior do que os rácios de anisotropia da K média ou mediana em cada uma das zonas de estudo.

Quantifying time lag of epikarst-spring hydrograph response to rainfall using correlation and spectral analyses

Understanding spring-flow characteristics in karst areas is very important for efficient utilization of water resources. The time lag of a spring-hydrograph response to rainfall is related to karst hydrogeological properties such as thickness, porosity and hydraulic conductivity. The length of the time lag can be determined based on results of the time-series analysis. However, some approaches, with different identifying indicators, give different lengths of the time lag. In this study, the flow-discharge series of two hillslope springs located in a karst area of southwest China were used to compute lengths of the time lag. The thickness and porosity of the epikarst-zone fractures on the two hillslopes were estimated based on a ground-penetrating radar investigation and field measurement. Based on comparison of lengths of the time lag computed by auto- and cross-correlation analyses, the identifying indicators of the time lag were classified into three types for measuring short, intermediate and long-term responses of the spring hydrograph to rainfall. The study also reveals that the time lag of spring-hydrograph response to rainfall in the thick epikarst zone is much longer than that in the thin epikarst zone.RésuméLa compréhension des caractéristiques des écoulements au niveau des sources dans les régions karstiques est essentielle pour une utilisation efficace des ressources en eau. Le décalage temporel de l’hydrogramme à la source en réponse aux précipitations est dû aux propriétés hydrogéologiques telles que l’épaisseur, la porosité et la conductivité hydraulique. La longueur du décalage temporel peut être déterminée à partir des résultats des analyses des séries temporelles. Cependant, certaines approches, avec différents indicateurs, donnent des longueurs différentes pour ce décalage. Dans le cadre de cette étude, les chroniques de débits de deux sources de pente situées dans une région karstique du Sud-Ouest de la Chine ont été analysées pour déterminer la durée de ce décalage temporel. L’épaisseur et la porosité des fractures de la zone épikarstique de deux pentes de collines ont été estimées à partir de résultats d’investigation géophysique au radar et de mesures de terrain. A partir des comparaisons des longueurs de décalage obtenues par analyses corrélatoire simple et croisée, les indicateurs identifiés du décalage ont été classés selon trois types de réponses de l’hydrogramme de la source pour des précipitations (courte, intermédiaire et longue). L’étude révèle également que le décalage de l’hydrogramme de la source en réponse aux précipitations dans un épikarst d’épaisseur importante est beaucoup plus long que pour une zone épikarstique peu épaisse.ResumenEntender las características del flujo de manantiales en áreas kársticas es muy importante para la utilización eficiente de los recursos hídricos. El tiempo de retardo en la respuesta de un hidrograma de un manantial a la precipitación está relacionado con las propiedades hidrogeológicas kársticas, tales como espesor, porosidad y conductividad hidráulica. La longitud del tiempo de retardo puede ser determinada sobre la base de resultados del análisis de series de tiempo. Sin embargo, algunas aproximaciones, con distintos indicadores de identificación, dan distintas longitudes de tiempos de retardo. En este estudio, se usaron las series de flujo de descarga de dos manantiales de ladera situados en un área kársticas del sudoeste de China para computar las longitudes del tiempo de retardo. El espesor y la porosidad de las fracturas de la zona epikárstica en las dos laderas fueron estimados en base a una investigación de un georadar y mediciones de campo. Basado en la comparación de las longitudes del tiempo de retardo computado por análisis de autocorrelacción y cross correlación, los indicadores que identifican el tiempo de retardo fueron clasificados en tres tipos para la medición de respuestas de corto, mediano y largo plazo del hidrograma del manantial a la precipitación. El estudio también revela que la respuesta del tiempo de retardo de un hidrograma de un manantial a la precipitación en la zona del epikarst espeso es mucho mayor que en la zona de epikarst delgado.摘要泉流量特征对喀斯特地区水资源合理利用具有重要的意义。泉流量过程对降雨响应的滞后时间与喀斯特水文地质特征,如厚度、空隙度以及渗透系数等具有密切关系。根据时间序列分析结果可以确定滞后时间。然而,根据不同的判定指标,许多时间序列分析方法可以计算出不同的滞后时间。本研究选取位于中国南方喀斯特地区的两个山坡泉的流量系列计算滞后时间。通过探地雷达勘测和野外调查,对这两个山坡表层岩溶带厚度和裂隙率进行了确定。通过对比自相关和互相关分析计算的滞后时间,将滞后时间判断指标分为三类,分别判断泉流量对降雨的短时、中时和长时响应。同时,本研究结果也显示,较厚表层岩溶带发育的泉流量对降雨响应的滞后时间比较薄表层岩溶带长。ResumoCompreender as caraterísticas do fluxo de nascentes em zonas cársicas é muito importante para a utilização eficiente dos recursos hídricos. O tempo de atraso da resposta à precipitação do hidrograma de uma nascente está relacionado com as propriedades hidrogeológicas do carso, tais como a espessura, a porosidade e a condutividade hidráulica. A dimensão do tempo de atraso pode ser determinada com base nos resultados da análise de séries temporais. No entanto, algumas abordagens, com diferentes indicadores de identificação, dão tempos de atraso diferentes. Neste estudo, foram usadas séries de fluxo de descarga de duas nascentes, localizadas a meia encosta numa área cársica do sudoeste da China, para calcular dimensões de tempos de atraso. A espessura e porosidade das fraturas da zona do epicarso nas duas nascentes foram estimadas com base em investigações de radar de penetração no solo e medições de campo. Com base na comparação dos períodos temporais de atraso determinados por meio de análises de auto correlação e de correlação cruzada, os indicadores que identificam o tempo de atraso foram classificados em três tipos, para medição das respostas do hidrograma da nascente à precipitação: curto, intermédio e longo. O estudo revela ainda que o tempo de atraso do hidrograma de nascente em resposta à precipitação na zona do epicarso com maior espessura é muito maior do que na zona de epicarso pouco desenvolvido.

Influence of bedding orientation on the anisotropy of hydraulic conductivity in a well-sorted fluvial sediment

Abstract The paper describes a permeameter test method for determination of the hydraulic conductivity ( K ) along multi-directions in fluvial sediments with cross beddings. Unlike existing in-situ permeameter methods that determine hydraulic conductivity for submerged streambeds, our method was intended to measure hydraulic conductivity of exposed streambeds or fluvial sediments. The method was applied to the Wei River, Shaanxi Province, Central China for characterization of the anisotropy of K in a well-sorted fluvial sediment. The results illustrated that even in well-sorted sediments, cross-bedding and sediment fabrication (or texture) can lead to varied K values along different measurement directions. The K value was the largest along the dip direction (or the major direction) that is parallel to the orientation of cross bedding and the smallest in the direction perpendicular to the bedding (or the minor direction). The K value in a given direction between the major and minor direction often fell in the range bounded by the K values in the major and minor directions. The anisotropy ratio of K (the ratio of K value between the major and minor directions) in two trenches for this well-sorted fluvial sediment was up to 1.14 to 1.23, respectively. Our results also demonstrated that even for well-sorted sediments, the K values between two sampling points only about 10 cm apart can differ. It is clear that the K distribution strongly correlates to the bedding orientation.

A Geospatial Approach for Assessing Groundwater Vulnerability to Nitrate Contamination in Agricultural Settings

Groundwater is the principal source of drinking water for at least one third of Earth’s human inhabitants. Thus, protection of groundwater is a critical issue in many locales. Nitrates and other contaminants that impact human health are of particular concern. Mapping of aquifer vulnerability to pollution is a critical first step in implementing groundwater management protection programs; however, mapping is often constrained by generalizations inherent in model formulation and availability of data. In this study, a groundwater vulnerability model, which employs data extracted from widely available national and statewide geospatial datasets, is used to evaluate regional groundwater pollution risk in the Elkhorn River Basin, Nebraska, USA. The model, implemented in a geographic information system (GIS), is specifically structured to address risks of nitrate contamination in agricultural landscapes; thus, land use is a key factor. Modeled groundwater vulnerability was found to be positively correlated with nitrate concentrations obtained from sampled wells. The results suggest that the approach documented here could be used effectively to model regional groundwater pollution risk in other areas.

Spatiotemporal patterns of water table fluctuations and evapotranspiration induced by riparian vegetation in a semiarid area

Groundwater evapotranspiration (ETg) links various ecohydrological processes and is an important component in regional water budgets. In this study, an extensive monitoring network was established in a semiarid riparian area to investigate various controls on the spatiotemporal pattern of water table fluctuations (WTFs) and ETg induced by riparian vegetation. Along a vegetation gradient (∼1200 m), diurnal WTFs were observed during a growing season in areas covered by woody species (Populus sect. Aigeiros and Juniperus virginiana) and wet slough vegetation (Panicum virgatum and Bromus inermis) with deeper root systems; whereas, no diurnal WTFs were found in the middle section with shallower-rooted grasses (Poa pratensis and Carex sp.). The occurrence of diurnal WTFs was related to temperature-controlled plant phenology at seasonal scales and to radiation at subdaily scales. Daily ETg in the mid-growing season was calculated using the White method. The results revealed that depth to water table (DTWT) was the dominant control on ETg, followed by potential evapotranspiration (ETp). By combining the effects of DTWT and ETp, it was found that at shallower depths, ETg was more responsive to changes in ETp, due to the closer linkage of land surface processes with shallower groundwater. Finally, exponential relationships between ETg/ETp and DTWT were obtained at the study site, although those relationships varied considerably across the sites. This study demonstrates the complex interactions of WTFs and ETg with surrounding environmental variables and provides further insight into modeling ETg over different time scales and riparian vegetation.

Modeling the groundwater recharge in karst aquifers by using a reservoir model.

The estimation of the groundwater recharge in a karstic system becomes an important challenge due to the great hydrodynamic variability in both time and space. This paper proposes a two reservoir conceptual model to simulate inflow into both the conduit system and the fissure network system based on the analysis of the spring hydrograph. The structure of the model and the governing equations are proposed on the basis of the physical considerations, with the assumption that flow at the outlet of the reservoirs obeys a linear threshold function. The model is applied on the Houzhai karstic underground river basin where it successfully reflects the temporal recharge distribution. The simulated accumulation recharge is 34.29 mm, which is reasonable in relation to the actual rainfall of 92.8 mm. The variations of water volume in two reservoirs represent the storage and transform characteristics of the karst aquifer system. However, this model is particularly well suited to simulate the recharge event after intensive rainfall.

Variation of specific yield with depth in an alluvial aquifer of the Platte River valley, USA

Abstract Estimation of specific yield ( S y ) of an aquifer is of great importance in water resource management. In this study, an experimental drainage method was developed to determine specific yield of an alluvial aquifer of the Platte River valley, Nebraska, USA. Sets of sediment cores with continuous interval depth were collected to plastic tubes using the direct push technique and then taken to the laboratory. During the S y experiment, those sediment cores were re-saturated by placing them vertically in a large water tank. S y was determined by the water drained from the sediments by force of gravity in a bracket. Our results show that the values of S y varied largely with depth at each site and the variability for S y with interval depth between the test sites is also observed. This spatial heterogeneity in S y might result from the variation of grain size, grain shape, sorting and compaction of sediments in different cores with interval depth. The S y for all sediment cores ranged from 0.01 to 0.18 and the mean value was 0.08±0.04. Our drainage method can functionally preserve the sedimentary structures in their original state and it is easier to experiment at a lower expense.

Groundwater evapotranspiration under psammophilous vegetation covers in the Mu Us Sandy Land, northern China

Groundwater is a significant component of the hydrological cycle in arid and semi-arid areas. Its evapotranspiration is an important part of the water budget because many plants are groundwater-dependent. To restore the degraded ecosystems, the need is pressing to further our understanding of the groundwater evapotranspiration (ETg) in arid and semi-arid areas. This study employed the White method to estimate ETg at four sites in the Mu Us Sandy Land in northern China, and the four sites are covered by Salix psammophila (SP site), Artemisia ordosica (AO site), Poplar alba (PA site), and Carex enervis (CE site), respectively. The depth of groundwater table and the duration of drainage were taken into account in calculating the specific yield (Sy) to improve the accuracy of the ETg estimats. Our results showed that from late May to early November 2013 the ETg were 361.87 (SP site), 372.53 (AO site), 597.86 (PA site) and 700.76 mm (CE site), respectively. The estimated ETg rate was also species-dependent and the descending order of the ETg rate for the four vegetation was: C. enervis, P. alba, A. ordosica, and S. psammophila. In addition, the depth of groundwater table has an obvious effect on the ETg rate and the effect varied with the vegetation types. Furthermore, the evapotranspiration for the vegetation solely relying on the water supply from unsaturated layers above the groundwater table was much less than that for the vegetation heavily relying on the water supply from shallow aquifers.