Chiara Vassena

Modeling water resources of a highly irrigated alluvial plain (Italy): calibrating soil and groundwater models

Modern and effective water management in large alluvial plains that have intensive agricultural activity requires the integrated modeling of soil and groundwater. The models should be complex enough to properly simulate several, often non-linear, processes, but simple enough to be effectively calibrated with the available data. An operative, practical approach to calibration is proposed, based on three main aspects. First, the coupling of two models built on well-validated algorithms, to simulate (1) the irrigation system and the soil water balance in the unsaturated zone and (2) the groundwater flow. Second, the solution of the inverse problem of groundwater hydrology with the comparison model method to calibrate the model. Third, the use of appropriate criteria and cross-checks (comparison of the calibration results and of the model outputs with hydraulic and hydrogeological data) to choose the final parameter sets that warrant the physical coherence of the model. The approach has been tested by application to a large and intensively irrigated alluvial basin in northern Italy.RésuméLa gestion moderne et efficace de l’eau dans les grandes plaines alluviales siège d’une agriculture intensive requiert la modélisation intégrée du sol et des eaux souterraines. Les modèles doivent être suffisamment complexes pour simuler correctement plusieurs processus souvent non linéaires, mais suffisamment simples pour être calibrés efficacement avec les données disponibles. Une approche de calibration pratique et opérationnelle est proposée, basée sur trois aspects principaux. Premièrement, le couplage de deux modèles construits sur des algorithmes bien validés, pour simuler (1) le dispositif d’irrigation et le bilan en eau du sol au sein de la zone non saturée et (2) l’écoulement d’eau souterraine. Deuxièmement, la résolution du problème hydrogéologique inverse avec la Méthode de Comparaison de Modèle pour caler le modèle. Troisièmement, l’utilisation de critères appropriés et de vérifications croisées (comparaison des résultats du calage et des sorties du modèle avec des données hydrologiques et hydrogéologiques) pour choisir les jeux de paramètres finaux qui garantissent la cohérence physique du modèle. L’approche a été testée par application à un vaste bassin alluvial intensément irrigué du Nord de l’Italie.ResumenEl manejo moderno y efectivo del agua en grandes planicies aluviales que tienen una intensa actividad agrícola requiere el modelado integrado del suelo y del agua subterránea. Los modelos deber ser lo suficientemente complejos como para simular correctamente varios procesos, a menudo no lineales, pero lo suficientemente simple para ser efectivamente calibrado con los datos disponibles. Se propone un enfoque práctico y operativo de la calibración, basada en tres aspectos principales. Primero, el acoplamiento de dos modelos construidos sobre algoritmos bien validados, para simular (1) el sistema de irrigación y el balance de agua en el suelo en la zona no saturada y (2) el flujo de agua subterránea. Segundo, la solución del problema inverso de hidrología de agua subterránea con el Método de comparación de modelos para calibrar el modelo. Tercero, el uso de criterios apropiados y controles cruzados (comparación entre los resultados de la calibración y las salidas del modelo con los datos hidrogeológicos e hidráulicos) para elegir el conjunto de parámetros finales que garanticen la coherencia física del modelo. El enfoque ha sido probado por su aplicación a una gran cuenca aluvial, intensamente irrigada en el norte de Italia.摘要在有密集型农业活动的大型冲积平原进行有效的现代水资源管理需要对土壤和地下水进行综合模拟。模型需要足够复杂,以模拟几种常见的非线性的过程,同时又要足够简单,能够采用现有数据进行校准。本文主要基于以下三个方面,提出了一个具备可操作性且符合实际的校准方法:1. 基于有效算法的两种模型的耦合,以模拟(1)非饱和带灌溉系统和土壤水平衡和(2)地下水流。2. 针对地下水文学的反演问题,采用比较法去校正模型。3.使用合适的准则和交互检验(根据水力和水文地质数据,比较模型的校准结果和模型输出结果)去选择最后的参数设置以确保模型的物理一致性。将该方法已经应用在意大利北部的一个大型密集灌溉冲积平原中进行ResumoUma moderna e eficaz gestão da água nas grandes planícies aluviais com atividade agrícola intensiva requer uma modelação integrada do solo e das águas subterrâneas. Os modelos devem ser suficientemente complexos para simular adequadamente vários processos, muitas vezes de caraterísticas não-lineares, mas suficientemente simples para serem adequadamente calibrados com os dados disponíveis. Neste artigo é proposta uma abordagem para calibração baseada em três aspetos principais. Primeiro, o acoplamento de dois modelos construídos sobre algoritmos bem validados, para simular (1) o sistema de rega e balanço hídrico do solo na zona não saturada e (2) o fluxo de águas subterrâneas. Segundo, uma solução do problema inverso da hidrologia subterrânea, baseada no Método de Comparação do Modelo para calibrar o modelo. Terceiro, o uso de critérios adequados e validações cruzadas (comparação dos resultados de calibração com as saídas do modelo com dados hidráulicos e hidrogeológicos), de modo a selecionar o conjunto final de parâmetros que garantem a coerência física do modelo. A abordagem tem sido testada numa bacia aluvial extensa e intensamente sujeita a rega, localizada no norte da Itália.

Connectivity and single/dual domain transport models: tests on a point-bar/channel aquifer analogue

In porous aquifers, groundwater flow and solute transport strongly depend on the sedimentary facies distribution at fine scale, which determines the heterogeneity of the conductivity field; in particular, connected permeable sediments could form preferential flow paths. Therefore, properly defined statistics, e.g. total and intrinsic facies connectivity, should be correlated with transport features. In order to improve the assessment of the relevance of this relationship, some tests are conducted on two ensembles of equiprobable realizations, obtained with two different geostatistical simulation methods—sequential indicator simulation and multiple point simulation (MPS)—from the same dataset, which refers to an aquifer analogue of sediments deposited in a fluvial point-bar/channel association. The ensembles show different features; simulations with MPS are more structured and characterised by preferential flow paths. This is confirmed by the analysis of transport connectivities and by the interpretation of data from numerical experiments of conservative solute transport with single and dual domain models. The use of two ensembles permits (1) previous results obtained for single realizations to be consolidated on a more firm statistical basis and (2) the application of principal component analysis to assess which quantities are statistically the most relevant for the relationship between connectivity indicators and flow and transport properties.RésuméDans les aquifères poreux, l’écoulement d’eau souterraine et le transport de solutés dépend fortement de la distribution des faciès sédimentaires à l’échelle fine, ce qui détermine l’hétérogénéité du champ de conductivité hydraulique; en particulier, les sédiments perméables connectés peuvent donner lieu à des écoulements préférentiels. Ainsi, la connectivité totale et intrinsèque de faciès définie correctement de manière statistique devrait être corrélée avec les propriétés du transport. Afin d’améliorer l’évaluation de la signification de cette relation, des tests ont été menés sur deux ensembles de réalisations équiprobables, obtenues à partir de deux méthodes différentes de simulation numérique—simulation séquentielle à indicateurs et simulation en points multiples (SPM)—construites à l’aide du même jeu de données se rapportant à un analogue aquifère de sédiments déposés dans un environnement fluvial avec une association de barres et de chenaux ponctuels. Les ensembles montrent différentes caractéristiques ; les simulations avec SPM sont plus structurées et caractérisées par des écoulements préférentiels. Ceci est confirmé par l’analyse des connectivités lors du transport et par l’interprétation des données expérimentales numériques de transport conservatif issues de modèles en domaine simple et dual. L’utilisation des deux ensembles permet (1) de consolider des résultats obtenus au préalable pour des réalisations simples sur une base statistique plus solide et (2) l’application de l’analyse en composante principale pour évaluer quelles quantités sont les plus significatives du point de vue statistique pour la relation entre les indicateurs de connectivité et les propriétés d’écoulement et de transport.ResumenEn acuíferos porosos, el flujo del agua subterránea y el transporte de soluto dependen fuertemente de la distribución de las facies sedimentarias a una escala fina, lo cual determina la heterogeneidad del campo de conductividad; en particular los sedimentos permeables conectados pueden forman trayectorias preferenciales de flujo. Por lo tanto, la estadística correctamente definida, por ejemplo la conectividad total e intrínseca de las facies, debe ser correlacionada con las características del transporte. Con el objeto de mejorar la evaluación de la relevancia de esta relación, se llevaron a cabo algunos ensayos en dos conjuntos de realizaciones equiprobables, obtenidas con dos métodos de simulación geoestadística diferente—simulación de indicador secuencial y simulación de múltiples puntos (MPS)—a partir de un mismo conjunto de datos, los cuales se refieren a una analogía de acuífero sedimentos depositados en una asociación albardón/canal fluvial. Los conjuntos muestran diferentes características; las simulaciones con MPS son más estructuradas y caracterizadas por trayectorias preferenciales de flujo. Esto está confirmado por el análisis de las conectividades del transporte y por la interpretación de datos de experimentos numéricos del transporte conservativo de soluto con modelos de dominios simples y dobles. El uso de los dos conjuntos permite (1) consolidar, sobre una base estadística más firme, los resultados previos obtenidos para realizaciones simples y (2) la aplicación del análisis de la componente principal para evaluar cuales cantidades son estadísticamente más relevantes para la relación entre los indicadores de conectividad y las propiedades de flujo y transporte.摘要在孔隙含水层中地下水流和溶质运移强烈依赖于沉积物中颗粒精细分布的特征,它确定了传导场地的不均匀性,特别是相关联的可渗透沉积层形成优先的水流。因而,适当确定统计资料,例如:总的和固有的连通特质,需要关联其运移特征。以便评价其关系,这些实验产生了两个同等的认识,根据相同的资料以两种不同的地质统计方法—序次指标模拟和多重点模拟(MPS)—获得。其利用了沉积地层在水流的点状阻障/通道中含水层模拟评价。整体效果显示出不同的特征,MPS模拟更突出了优先水流的结构和特点。它根据分析传导连通性和稳定溶质在单/双域传导模型数值实验数据解译确认。应用在两个总体认识上:(1) 原结果在观测单域内的认识是以更牢靠的统计数据为基数,(2) 主成分分析的应用定量统计了在连通指标和流量及传导特性之间的相互关系。ResumoEm aquíferos porosos, o fluxo de água subterrânea e o transporte de solutos dependem fortemente da distribuição das fácies sedimentares à escala fina, que determina a heterogeneidade do campo de condutividades; em particular, os sedimentos permeáveis interconetados que podem formar caminhos de fluxo preferenciais. Por essa razão, é de esperar que certas estatísticas, definidas de forma adequada, tais como a conetividade total e intrínseca das fácies, se correlacionem com as caraterísticas de transporte. A fim de melhorar a avaliação da importância desta correlação, efetuaram-se alguns testes em dois conjuntos de realizações equiprováveis, obtidos com dois métodos geoestatísticos de simulação diferentes—simulação sequencial de indicadores e simulação por múltiplos pontos (MPS)—a partir do mesmo conjunto de dados, referente a um análogo de um aquífero de sedimentos depositados numa sequência fluvial de meandro/canal. Os dois conjuntos apresentam caraterísticas diferentes; as simulações com MPS são mais estruturadas e caraterizam-se por caminhos de fluxo preferenciais. Este facto é confirmado pela análise de conetividades de transporte e pela interpretação de dados de experiências numéricas de transporte de solutos conservativos com modelos de domínio único e duplo. O uso de dois conjuntos permite (1) que os resultados anteriores obtidos por realizações únicas possam ser consolidados numa base estatística mais firme e (2) a aplicação da análise de componentes principais para avaliar quais as quantidades estatisticamente mais relevantes para a relação entre os indicadores de conetividade e as propriedades de fluxo e transporte.

A sensitivity analysis for an evolution model of the Antarctic ice sheet

The evolution of the Antarctic ice sheet for the last 200,000 years is simulated with a finite difference thermomechanical model based on the shallow ice approximation. The model depends on the surface temperature, the ice accumulation rate, the geothermal heat flux and the basal sliding coefficient, which are estimated with large uncertainty. A second-order approximation of the model in a neighborhood of the reference values for these parameters permits the computation of both local and variance-based sensitivity indices. The results show the dominant effect of the surface temperature on the model predictions.

Modeling Groundwater Flow in Heterogeneous Porous Media with YAGMod

Modeling flow and transport in porous media requires the management of complexities related both to physical processes and to subsurface heterogeneity. A thorough approach needs a great number of spatially-distributed phenomenological parameters, which are seldom measured in the field. For instance, modeling a phreatic aquifer under high water extraction rates is very challenging, because it requires the simulation of variably-saturated flow. 3D steady groundwater flow is modeled with YAGMod (yet another groundwater flow model), a model based on a finite-difference conservative scheme and implemented in a computer code developed in Fortran90. YAGMod simulates also the presence of partially-saturated or dry cells. The proposed algorithm and other alternative methods developed to manage dry cells in the case of depleted aquifers are analyzed and compared to a simple test. Different approaches yield different solutions, among which, it is not possible to select the best one on the basis of physical arguments. A possible advantage of YAGMod is that no additional non-physical parameter is needed to overcome the numerical difficulties arising to handle drained cells. YAGMod also includes a module that allows one to identify the conductivity field for a phreatic aquifer by solving an inverse problem with the comparison model method.

dsm.f90: A computer code for the solution of an inverse problem of ground water hydrology by the differential system method

The identification of the transmissivity, T, of a confined aquifer, for which the 2D flow condition holds, can be obtained with the differential system method when several sets of data are available. If we consider stationary flow conditions, the necessary data are at least two couples of piezometric heads and source terms such that the two hydraulic gradients are not parallel at any point, and the value of T at one point only. The method has been implemented in the computer code dsm.f90, developed with the FORTRAN 90 programming language. The computer code allows the user to input several sets of data, an initial estimate of the T field or alternatively several starting points for the integration of the differential system. Moreover, the code performs several checks to identify the T field in a robust way and to avoid unphysical results, e.g. negative or very large values of T.

Identification of Aquifer Transmissivity with Multiple Sets of Data Using the Differential System Method

The mass balance equation for stationary flow in a confined aquifer and the phenomenological Darcy’s law lead to a classical elliptic PDE, whose phenomenological coefficient is transmissivity, T, whereas the unknown function is the piezometric head. The differential system method (DSM) allows the computation of T when two “independent” data sets are available, i.e., a couple of piezometric heads and the related source or sink terms corresponding to different flow situations such that the hydraulic gradients are not parallel at any point. The value of T at only one point of the domain, x0, is required. The T field is obtained at any point by integrating a first order partial differential system in normal form along an arbitrary path starting from x0. In this presentation the advantages of this method with respect to the classical integration along characteristic lines are discussed and the DSM is modified in order to cope with multiple sets of data. Numerical tests show that the proposed procedure is effective and reduces some drawbacks for the application of the DSM.

Exposing high-school students to Geosciences through seminars, laboratory and field demonstrations

To familiarize high school students with the richness of geosciences and their practical applications, Universita degli Studi di Milano has been conducting a number of activities in co-operation with the Liceo G. Bruno (Melzo, MI, Italy). This contribution discusses the number of activities conducted including classroom seminars, laboratory experiments and in-the-field demonstrations.

Numerical modeling of groundwater flow in the coastal aquifer system of the Taranto gulf (southern Italy)

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.