Hydrodynamics of coupled flow above and below a sediment–water interface with triangular bedforms

The hydrodynamics of a system where there is a coupled flow above and below a sediment–water interface (SWI) are not completely understood. We numerically simulate mean two-dimensional, unidirectional, steady, viscous flow in these systems using a sequentially coupled formulation. Simulations were conducted to determine fundamental relationships between bedform geometry, Reynolds number for the water-column flow (Re), interfacial exchange zone depth (d_z) in the sediments, and flux through the SWI (q_int); the latter two parameters play a significant role in biogeochemical and aquatic-life processes across the SWI. d_z and Re are functionally related through an asymptotic growth-curve model while q_int and Re follow a power function. These relationships are dynamically explained by the manner in which pressure gradients along the SWI develop due to current–bedform interactions at different Res and by Darcy’s Law. We found that the coupling between water column and exchange zone flow is controlled by the behavior of the water-column eddy. The eddy detaches at or near the point of minimum pressure along the interface, and reattaches near the point of maximum pressure. These two critical points determine the pressure gradient along the bed surface that controls the exchange zone flow field. Moreover, the reattachment point corresponds to flow divides within the sediments. Lastly, pore-water velocities drop with depth below the SWI, and are larger below the bedform crests than below the troughs.     

Preliminary assessment of crustacean distribution patterns in New Zealand groundwater aquifers

Crustaceans were sampled in 17 groundwater wells at five locations in North and South Islands of New Zealand. Three sampling locations were associated with unconfined aquifers adjacent to gravel‐bed rivers, whereas the other two locations were distant from rivers, but associated with land‐based sewage effluent disposal sites. Crustaceans and basic physico‐chemical parameters were sampled in summer and winter. Total abundance varied from 1 to 1064 individuals/250 litre sample. Copepods dominated the fauna at four of the five locations, and were most abundant immediately downstream of a sewage effluent disposal area. Paraleptamphopus spp. (Amphipoda) was found at all five locations, and abundance was also greatest downstream of sewage effluent disposal areas. Several crustacean taxa were rare or limited in distribution, including the gammarid Paracrangonyx sp. and the anthurid isopod Cruregens sp. Syncarids were widely distributed, but found in low numbers, and no specimens were found immediately below sewage effluent disposal areas. We suggest that groundwater ecology in New Zealand is restricted by lack of knowledge of taxonomy and ecology of key groups, particularly the Copepoda.     

Calculation of head difference at two sides of a cut-off barrier during excavation dewatering

This paper proposed a series of simple equations to calculate the head difference at the two sides of waterproof curtain. The barrier effect of waterproof curtain is considered from two situations with respect to without barrier case: (i) groundwater head difference below the barrier and (ii) groundwater head difference by convergence into the opening. The solution for the first situation can be derived from hydraulic analyses and the second situation can be obtained using a numerical analysis. The final groundwater head difference is the sum of these two situation according to the superposition principal. In the proposed equations, the head difference is expressed as a function of the inserted depth of the barrier into confined aquifer, the ratio of the hydraulic conductivity of the aquifer, the thickness of the aquifer, and hydraulic gradient under normal conditions. Finally, the proposed equation was applied to a field case to verify the validity of the proposed approach. Compared with the field data, the results show that the proposed method is reasonable.     

滨海超大城市多含水层影响地面沉降机制与模型

滨海城市地面高程是宝贵资源,在气候变化、海平面上升的条件下,地面沉降导致滨海城市高程丧失,严重威胁城市地质安全。分析多层含水层对地面沉降的影响规律,研究地面沉降机理,对于保障城市安全高程具有重要意义。本文以上海市采灌深井点位为采样点,采用MAPGIS软件提取各含水层水位和地表沉降值,获取对应点位2011～2019年多层地下水位和地面沉降数据,利用多元回归方法建立了多层分层水位和地面沉降的相关关系。在判定系数R²相对较高区域,地面沉降受深层地下水开采、回灌以及工程降水的影响大;在判定系数R²相对较低区域,地面沉降受工程建设开挖卸荷、高层建筑与密集建筑群长期荷载、地铁振动等影响相对较大。多元回归方程可用于预测未来城市采灌格局下上海市各行政区地面沉降的发展趋势,为上海市地面沉降防控提供参考依据。     

非稳定流单井排放诱发地壳形变的研究──基于Neuman理论的无压含水层完整井

在高精度动态大地测量时,为考虑地下水单井排放对台站坐标的影响,需精细研究不同水流系统和地层模型下相应的定量计算问题．本文将地下水单井排放诱发地表位移的一般公式与Muman理论结合起来,导出了适用于各向异性和滞后反应的无压含水层完整井非稳定流单井排放诱发的地表位移的解析表达式,以20个结点的Hermite求积公式和15个结点的Gauss－laguerre求积公式,编制了相应的计算程序,给出了模拟算例．本文的研究在理论上比基于Boulton理论的相应研究更合理,在实践上此方案是可行的．     

A model to simulate recharge processes of karst massifs

The recharge processes have been evaluated for two karst massifs of southern Italy, the Mt Terminio and Mt Cervialto, characterized by wide endorheic areas. The annual mean recharge has been estimated by Geographic Information System (GIS) tools, from regression of annual mean values of different ground‐elevated rain gauges and thermometers. The recharge has been distinguished for endorheic areas and the other areas of spring catchment, and the ratio between the output spring and input rainfall has been also estimated (recharge coefficient). The annual recharge has been used to calibrate a daily scale model, which allows to estimate the amount of effective rainfall, which is retained as soil moisture; the amount reaching the water table (recharge s.s.); and the amount of rainfall, which develops the runoff and leaves the catchment. All these amounts vary through the hydrological year, in function of soil moisture deficit and daily rainfall intensity. The model allows estimating the recharge conditions through the hydrological year, and it is a useful tool for water management. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrodynamic characteristics of a typical karst spring system based on time series analysis in northern China

In order to study the hydrodynamic characteristics of the karst aquifers in northern China, time series analyses (correlation and spectral analysis in addition with hydrograph recession analysis) are applied on Baotu Spring and Heihu Spring in Jinan karst spring system, a typical karst spring system in northern China. Results show that the auto-correlation coefficient of spring water level reaches the value of 0.2 after 123 days and 117 days for Baotu Spring and Heihu Spring, respectively. The regulation time obtained from the simple spectral density function in the same period is 187 days and 175 days for Baotu Spring and Heihu Spring. The auto-correlation coefficient of spring water level reaches the value of 0.2 in 34–82 days, and regulation time ranges among 40–59 days for every single hydrological year. The delay time between precipitation and spring water level obtained from cross correlation function is around 56 days for the period of 2012–2019, and varies among 30–79 days for every single hydrological year. In addition, the spectral bands in cross amplitude functions and gain functions are small with 0.02, and the values in the coherence functions are small. All these behaviors illustrate that Jinan karst spring system has a strong memory effect, large storage capacity, noticeable regulation effect, and time series analysis is a useful tool for studying the hydrodynamic characteristics of karst spring system in northern China.© 2021 China Geology Editorial Office.     

Aquifer recharge from flash floods in the arid environment: A mass balance approach at the catchment scale

Estimation of the infiltration/natural recharge to groundwater from rainfall is an important issue in hydrology, particularly in arid regions. This paper proposes the application of The Natural Resources Conservation Service (NRCS) mass balance model to develop infiltration (F)–rainfall (P) relationship from flash flood events. Moreover, the NRCS method is compared with the rational and the Ф-index methods to investigate the discrepancies between these methods. The methods have been applied to five gauged basins and their 19 sub-basins (representative basins with detailed measurements) in the southwestern part of Saudi Arabia with 161 storms recorded in 4 years. The F–P relationships developed in this study based on NRCS method are: F = 39% P with R² = 0.932 for the initial abstraction factor, λ = 0.2. However, F = 77% P with R² = 0.986 for λ = 0.01. The model at λ = 0.01 is the best to fit the data, therefore, it is recommended to use the formula at λ = 0.01. The results show that the NRCS model is appropriate for the estimation of the F–P relationships in arid regions when compared with the rational and the Ф index methods. The latter overestimates the infiltration because they do not take λ into account. There is no significant difference between F–P relationships at different time scales. This helps the prediction of infiltration rates for aquifer recharge at ungauged basins from monthly and annual rainfall data with a single formula.     

环境同位素在识别伊犁河谷地下水补给来源中的指示作用

重点分析了研究区潜水、浅层承压水、泉水及地表水δD、δ18O的分布特征, 并对5组水文钻探井地下水样品进行分析. 潜水δD变化范围为-97.32‰~-67.51‰, 平均值为-80.34‰; δ18O为-15.85‰~-10.66‰, 平均值为-12.08‰. 浅层承压水δD为-111.93‰~-68.38‰, 平均值为-84.79‰; δ18O为-16.01‰~-10.52‰, 平均值为-12.30‰. 泉水δD为-102.06‰~-71.63‰, 平均值为-84.10‰; δ18O为-14.21‰~-9.70‰, 平均值为-12.24‰. 地表水δD为-90.53‰~-60.99‰, 平均值为-72.58‰; δ18O在-13.20‰~-9.54‰, 平均值为-11.21‰. 地下水δ13C为-9.4‰~-5.6‰, 平均值为-8.3‰, 极差为3.8‰. 结果表明: 地下水与地表水均起源于当地大气降水. 潜水与浅层承压水水力联系较强, 潜水与浅层承压水属于同一含水系统. 与浅层承压水相比, 深层承压水年龄较大, 在20 ka左右, 属于沉积埋藏水. 深层承压水与浅层承压水的水力联系较弱. 潜水与浅层承压水的δ13C值较为接近, 且接近大气CO2的δ13C值-7‰. 研究区地下水中碳的主要来源为大气CO2.     

Damped least-squares inversion of confined aquifer pumping data based on singular value decomposition

An iterative method for calculating the transmissivity and storage coefficient from pumping test data for a confined aquifer is presented. The method optimizes the fit between the measured and the theoretical data (computed using the Theis equation) in the leastsquare sense. Unlike the existing schemes, this method employs the Levenberg-Marquardt method and the singular value decomposition technique resulting in a stable and rapidly convergent data inversion algorithm. The inverse procedure is initialized by an automatically created starting model derived using a novel technique that operates on the timederivative of the drawdown curve. An important feature of the algorithm is that all the computations are done in logarithmic space which effectively linearizes the pmblem. The proposed method has several advantages over the conventional iterative inversion algorithms because of the linearizing parameterizations at both the forward and inverse stages of the problem. Detailed derivations of the basic equations are provided to guide the potential users as well as applications to field data to demonstrate the usefulness of the proposed algorithm.