Abstract
The evaluation of the accuracy or reasonableness of numerical models of groundwater flow is a complex task, due to the uncertainties in hydrodynamic properties and boundary conditions and the scarcity of good-quality field data. To assess model reliability, different calibration techniques are joined to evaluate the effects of different kinds of boundary conditions on the groundwater flow in a coastal multi-layered aquifer in southern Italy. In particular, both direct and indirect approaches for inverse modeling were joined through the calibration of one of the most uncertain parameters, namely the hydraulic conductivity of the karst deep hydrostratigraphic unit. The methodology proposed here, and applied to a real case study, confirmed that the selection of boundary conditions is among the most critical and difficult aspects of the characterization of a groundwater system for conceptual analysis or numerical simulation. The practical tests conducted in this study show that incorrect specification of boundary conditions prevents an acceptable match between the model response to the hydraulic stresses and the behavior of the natural system. Such effects have a negative impact on the applicability of numerical modeling to simulate groundwater dynamics in complex hydrogeological situations. This is particularly important for management of the aquifer system investigated in this work, which represents the only available freshwater resource of the study area, and is threatened by overexploitation and saltwater intrusion.
Résumé
L’évaluation de la fiabilité ou du caractère raisonné des modèles numériques d’écoulement d’eaux souterraines est. une tâche complexe, du fait des incertitudes sur les propriétés hydrodynamiques et les conditions aux limites et du manque de données de terrain de bonne qualité. Pour estimer la fiabilité d’un modèle, différentes techniques de calage sont couplées dans le but d’évaluer les effets de différente types de conditions aux limites sur l’écoulement des eaux souterraines dans un aquifère côtier multi-couches dans le Sud de l’Italie. En particulier, les approches tant directes qu’indirectes en vue d’une modélisation inverse sont rapprochées grâce au calage de l’un des paramètres les plus incertains, c’est. à dire la conductivité hydraulique de l’unité hydrostratigraphique profonde karstique. La méthodologie proposée ici, et appliquée à un cas d’étude réel, a confirmé que le choix des conditions aux limites est. l’un des aspects les plus critiques et les plus difficiles de la caractérisation d’un système hydrogéologique en ce qui concerne l’analyze conceptuelle ou la simulation numérique. Les tests pratiques conduits dans cette étude montrent que la caractérisation incorrecte des conditions aux limites empêche une convergence acceptable entre la réponse du modèle aux contraintes hydrauliques et le comportement du système naturel. De tels effets ont un impact négatif sur l’applicabilité du modèle numérique pour simuler la dynamique de l’eau souterraine dans des situations hydrogéologiques complexes. Ceci est. particulièrement important pour la gestion du système aquifère étudié dans ce travail, qui représente la seule ressource en eau douce disponible de l’aire d’étude et qui est. menacé par la surexploitation et l’intrusion d’eau salée.
Resumen
La evaluación de la exactitud o razonabilidad de los modelos numéricos de flujo de agua subterránea es una tarea compleja, debido a las incertidumbres en las propiedades hidrodinámicas y las condiciones de los límites y la escasez de datos de campo de buena calidad. Para evaluar la confiabilidad del modelo se unen diferentes técnicas de calibración para evaluar los efectos de distintos tipos de condiciones de contorno en el flujo de agua subterránea en un acuífero costero multicapa en el sur de Italia. En particular, los enfoques directos e indirectos para el modelado inverso se unieron mediante la calibración de uno de los parámetros más inciertos, a saber, la conductividad hidráulica de la unidad hidroestratotigráfica kárstica profunda. La metodología propuesta aquí, y aplicada a un estudio de caso real, confirmó que la selección de condiciones de borde es uno de los aspectos más críticos y difíciles de la caracterización de un sistema de agua subterránea para el análisis conceptual o la simulación numérica. Las pruebas prácticas realizadas en este estudio muestran que la especificación incorrecta de las condiciones de contorno impide una coincidencia aceptable entre la respuesta del modelo a los esfuerzos hidráulicos y el comportamiento del sistema natural. Tales efectos tienen un impacto negativo en la aplicabilidad del modelado numérico para simular la dinámica del agua subterránea en situaciones hidrogeológicas complejas. Esto es particularmente importante para el manejo del sistema acuífero investigado en este trabajo, que representa el único recurso disponible de agua dulce del área de estudio, y está amenazado por la sobreexplotación y la intrusión de agua salada.
摘要
地下水流数值模型的精确性或合理性评估是一个复杂的工作,这是因为水力特性和边界条件不确定,以及缺乏高质量的野外数据。为了评价模型可靠性,结合不同校正技术评估了不同种类边界条件对意大利南部沿海多层含水层中地下水流的影响。尤其是,通过多数不确定参数的一种参数、即岩溶深层水文地层单元的水力传导率的校准把反演模拟的直接和间接方法结合在一起。这里提出的方法及应用到实际案例的方法证实,边界条件的选择是概念分析或数值模拟中描述地下水系统特征最关键和最难的方面之一。本研究进行的实际试验显示,边界条件的错误确定阻止模型对水力压力的响应与天然系统特性之间的可接受的匹配。这样的效果对模拟复杂水文地质条件下地下水动力学的数值模拟的适用性具有负面影响。这对于本研究中所调查的含水层系统的管理尤为重要,这个含水层系统代表研究区内唯一可利用的淡水资源,受到了超采和海水入侵的威胁。
Riassunto
La valutazione dell’accuratezza e dell’affidabilità di un modello numerico per la simulazione del flusso delle Acque Sotterranee è particolarmente complessa, a causa delle incertezze legate alla definizione delle proprietà idrodinamiche e delle condizioni al contorno e alla scarsità di dati di buona qualità. Per valutare l’affidabilità del modello presentato in questa pubblicazione sono state utilizzate congiuntamente diverse tecniche di calibrazione, per valutare gli effetti di diversi tipi di condizioni al contorno sul flusso idrico simulato in un acquifero costiero multi-strato nell’area di Taranto (Italia meridionale). In particolare, sono stati utilizzati congiuntamente un metodo diretto e un metodo indiretto per la risoluzione di problemi inversi. L’utilizzo congiunto dei due metodi è stato possibile mediante la calibrazione di uno dei parametri maggiormente incerti, cioè la conducibilità idraulica del mezzo fratturato e carsico ospitante l’acquifero profondo. La metodologia proposta, e applicata ad un caso studio reale, ha confermato che la selezione delle condizioni al contorno è tra gli aspetti più critici nella caratterizzazione di un sistema acquifero per la definizione di un modello concettuale o l’implementazione di un modello numerico. Le prove presentate in questa pubblicazione mostrano che una definizione inappropriata delle condizioni al contorno previene una corrispondenza ottimale tra la risposta del modello a determinati stress idraulici e il comportamento del sistema naturale. Ciò ha un effetto negativo sull’applicabilità degli strumenti modellistici per la simulazione del flusso idrico in complessi sistemi idrogeologici. Questo aspetto risulta particolarmente importante per la gestione del sistema acquifero in esame, che rappresenta la sola riserva di acqua dolce nell’area e risulta minacciata da sovrasfruttamento e dall’ingressione di acqua marina.
Resumo
A avaliação da acurácia ou razoabilidade dos modelos numéricos de águas subterrâneas é uma tarefa complexa, devido às incertezas nas propriedades hidrodinâmicas e condições de contorno e à escassez de dados de campo com boa qualidade. Para estimar a confiabilidade, diferentes técnicas de calibração estão unidas com o objetivo de avaliar o efeito de diferentes tipos de condições de contorno no escoamento das águas subterrâneas em um aquífero costeiro de várias camadas no sul da Itália. Em particular, tanto as abordagens direta e indireta para modelagem inversa foram unidas por meio da calibração de um dos parâmetros mais incertos, chamado de condutividade hidráulica de unidades hidroestratigráficas cársticas profundas. A metodologia proposta aqui, e aplicada em um estudo de caso real, confirmou que a escolha das condições de contorno está entre um dos aspectos mais críticos e difíceis da caracterização de um sistema de águas subterrâneas para análise conceitual ou para a simulação numérica. Os testes práticos conduzidos neste estudo mostram que a especificação incorreta das condições de contorno evita uma correspondência aceitável entre a resposta do modelo em relação às tensões hidráulicas e o comportamento do sistema natural. Tais efeitos têm um impacto negativo na aplicabilidade da modelagem numérica para simular a dinâmica das águas subterrâneas em situações hidrogeológicas complexas. Isso é particularmente importante para o gerenciamento do sistema aquífero investigado neste estudo, o qual representa o único recurso de água doce disponível da área de estudo, e está ameaçado pela superexplotação e a intrusão salina.
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Acknowledgements
The activities described in this paper were funded by the Flagship Project RITMARE, La Ricerca Italiana per il Mare, coordinated by the National Research Council and funded by the Ministry for Education, University and Research within the National Research Program 2011–2013. The Hydrogeology Group at the Earth Sciences Department of Technische Universität Darmstadt is kindly acknowledged for scientific contribution to the methodology proposed in this paper, as part of the work was developed while Giovanna De Filippis was hosted there for an internship.
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De Filippis, G., Foglia, L., Giudici, M. et al. Effects of different boundary conditions on the simulation of groundwater flow in a multi-layered coastal aquifer system (Taranto Gulf, southern Italy). Hydrogeol J 25, 2123–2138 (2017). https://doi.org/10.1007/s10040-017-1589-x
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DOI: https://doi.org/10.1007/s10040-017-1589-x