Automatic calibration of CODESA-3D using PEST

We describe here our experience in using the Model Independent Pameter ESTimation (PEST) free software tool [Doherty, 2002] to perform the automatic calibration of the COupled DEnsity-dependent variably SAturated flow and miscible transport (CODESA-3D) groundwater model [Gambolati et al., 1999]. Generally speaking, calibration of a model requires that a suitable method of spatial parameter characterization be defined in order to adjust model parameters until model outputs correspond well to specific laboratory and/or field measurements of the system which is simulated. In particular, for groundwater models the adjustable parameters are usually given by main hydrogeological properties (e.g. hydraulic permeability) and/or system excitations (e.g. abstraction volumes) while control data are represented by piezometric heads and/or salt concentrations measured in the field. Model calibration is a complex task. To perform it for a 3D fully-distributed physically-based hydrological model we need to build up a chain of interdependent software tools and data through the interdisciplinary expertise of GIS experts, modelers and hydrogeologists (Figure 1). The newly generated optimization model is comprised by the two pieces of software CODESA-3D and PEST with the latter wrapping the former up. The optimization model is not restricted in its use solely to the calibration of the groundwater model, through this tool modeler can gain valuable insight into the strengths and weakness of the input dataset allowing future data gathering to be undertaken in an optimal manner. In addition, lessons learned will be applicable also to the estimation of the degree of uncertainty associated with a given calibrated model prediction and to make decisions regarding appropriate levels of model complexity. In the following we discuss in detail the optimization model development and test using synthetic observations generated by the groundwater model itself

Elaborazione di modelli matematico-numerici per la simulazione di flusso e trasporto per problemi dipendenti dalla densità

Il presente rapporto descrive lo stato di avanzamento dell’attività 1 del sottoprogetto al termine del primo anno del progetto MURST/CIPE , ovvero la descrizione della modellazione di fenomeni di flusso e trasporto nel caso di fenomeni dipendenti dalla densità (cioè la presenza stessa dell’inquinante è in grado di influenzare fortemente il campo di moto), con particolare riferimento al caso di dispersione di inquinanti immiscibili in acqua detti NAPL (liquidi organici in fase non acquosa, quali idrocarburi, solventi organici, liquidi oleosi, sostanze bitumose, etc.) possibili di biodegradazione ad opera della flora batterica naturalmente presente , di trasporto di inquinanti chimicamente reattivi nel sottosuolo (NaOH, e altri contaminanti in grado di influenzare il pH dell’ambiente sottostante) e del caso di intrusione dell’acqua marina nel sottosuolo. L’elaborato descrive dunque l’insieme delle equazioni e la formalizzazione degli algoritmi risolutivi dei modelli matematici e delle integrazioni modellistiche e numeriche dei modelli dei seguenti processi: inquinanti immiscibili in acqua; biodegradazione; intrusione di acqua marina; trasporto in non-equilibrio

Implementation and testing of the CODESA-3D model for density-dependent flow and transport problems in porous media

The report describes the implementation and the test phases of the computer code CODESA-3D (Coupled variable Density and Saturation 3-Dimensional model), whose mathematical and numerical models are described in all details in the ten chapter of the book. CODESA-3D is a three-dimensional finite element simulator for flow and solute transport in variably saturated porous media on unstructured domains. The flow and solute transport processes are coupled through the variable density of the filtrating mixture made of water and dissolved matter (salt, pollutants). The flow module simulates the water movement in the porous medium , taking into account different forcing inputs: infiltration/evaporation, recharge/discharge, withdrawal/injection, etc., while the transport module computes the migration of the salty plume due to advection and diffusion processes

Rapporto di ricerca bibliografica (Stato dell'arte dei modelli di flusso e trasporto in mezzi porosi)

In questo primo rapporto vengono presentate le equazioni fondamentali che reggono i fenomeni di flusso e trasporto in mezzi porosi insieme ai metodi numerici che verranno utilizzati per la soluzione di tali equazioni. I modelli matematici in questione sono basati su equazioni differenziali a derivate parziali che impongono il bilancio di massa sia per il fluido che per il soluto (inquinante disciolto in acqua). Queste equazioni vengono scritte in forma generale per un mezzo poroso tridimensionale; in dipendenza dal tipo di applicazione è possibile adottare modelli mono o bidimensionali che portano a semplificazioni notevoli. L'equazione di flusso è sviluppata per il caso di mezzi porosi a saturazione variabile e può essere quindi utilizzata contemporaneamente nella zona insatura (suoli superficiali) e satura (falde freatiche e artesiane). Nell'equazione di trasporto si considerano i processi di dispersione, diffusione e avvezione, insieme ad alcune fenomenologie di interazione chimico-fisica tra il soluto e la matrice porosa. Accanto a queste equazione, si descrive anche un modello, a scala di bacino, di afflussi-deflussi superficiali accoppiato con un modello di infiltrazione. Questo approccio viene tiene conto di fenomeni importanti qualora vi sia una stretta correlazione tra il moto dell'acqua in superficie e il moto dell'acqua nella zona insatura

Modeling and Analysis of Seawater Intrusion in the Coastal Aquifer of Eastern Cap-Bon, Tunisia

A numerical model that treats density-dependent variably saturated flow and miscible salt transport is used to investigate the occurrence of seawater intrusion in the ‘Korba’ aquifer of the eastern coast of Cap-Bon in northern Tunisia. We examine the interplay between pumping regimes and recharge scenarios and its effect on the saline water distribution. More localized simulations are used to examine, in vertical cross sections, the effects of well location and soil type and the role of the vadose zone in possible remediation actions. The exploratory simulations suggest interesting interactions between the unsaturated zone and the saltwater–freshwater interface with possible implications for groundwater exploitation from shallow unconfined coastal aquifers, involving in one case feedback between seawater intrusion and the high pressure head gradients around the pumping-induced drawdown cone and in another case threshold-like interface displacement for tight soils such as clays. The data processing steps undertaken in this GIS and modeling study are described in some detail, and a critical assessment is given of the data availability and of the requirements for successful monitoring and modeling of seawater intrusion risks in heavily exploited coastal aquifers such as those found in the semi-arid regions of the Mediterranean basin. It is shown how, with the aid of GIS, reasonably reliable information can be assembled from maps, surveys, and other sources of geospatial and hydrogeological data, an approach that is necessary in the many regions of the world with acute water resource problems but with limited means for undertaking systematic data acquisition and environmental monitoring actions. Nonetheless the need for more concerted monitoring of relevant parameters and processes and of closer coordination between monitoring and modeling is stressed. An idea of the extent of over-exploitation of the Korba aquifer is obtained by examining the pumping and rainfall/infiltration data, and the simulation results support groundwater pumping as the mechanism for and seawater intrusion as the origin of the salt contamination observed in the soils and subsurface waters of the Korba plain.3-28Pubblicat

Analysis of seawater intrusion phenomena in the Korba coastal aquifer of Tunisia

A geographic information system and a three-dimensional coupled variable density and saturation numerical model are implemented for the Korba coastal aquifer of northeastern Tunisia, and preliminary simulations are performed to investigate seawater intrusion phenomena. The GIS provides an essential platform for data management, enabling the organization and merging of a large volume of data that has been collected in often ad hoc manner from diverse sources over many years. A critical assessment of data quality is provided and the usefulness of GIS and modeling tools is demonstrated, with an aim to encourage more directed and continuous monitoring and characterization of important parameters and processes involved in the contamination phenomena. This approach is currently being extended to two other coastal aquifers, in Sardinia (Italy) and Sahel (Morocco).51-5

Calcolo del bilancio idrico della regione Sardegna con il modello idrologico SWAT

Una diminuzione del 20% delle precipitazioni nell’ultimo trentennio e perdite del 40% nella rete di distribuzione hanno causato un sensibile abbassamento dei livelli idrici medi nei laghi e negli invasi artificiali della Sardegna, con la conseguenza che negli ultimi anni le municipalità, il comparto agricolo e quello industriale hanno subito le impopolari misure di razionamento. Fino ad oggi, gli enti regionali preposti alla gestione delle acque si sono serviti di modelli a scatola chiusa di tipo black box. Tuttavia da un lato appare incerta l’applicabilità di tali modelli in aree prive di misure e dall’altro tale approccio si dimostra carente nel tenere in conto le trasformazione di uso del territorio e le variazioni climatiche. Questo studio presenta l’applicazione del modello idrologico distribuito e fisicamente basato SWAT per la stima del bilancio idrologico della regione Sardegna. La procedura di caratterizzazione del tipo e della copertura del suolo ha seguito un approccio regionale, utilizzando e valorizzando al massimo le frammentarie informazioni disponibili. Per compensare la mancanza di dati di pioggia a passo giornaliero, necessari al modello idrologico, è stato sviluppato un generatore di tempo basato sull’analisi statistica di dati climatici registrati. In base alla distribuzione spaziale delle grandezze statistiche, le stazioni pluviometriche sono state raggruppate in due gruppi. Lo studio ha prodotto una serie di archivi informatici a scala regionale, creati secondo i formati di input del modello SWAT. L’applicazione del modello ai bacini idrologici della Sardegna ha tenuto conto della qualità dell’informazione descrittiva del territorio come, ad esempio, la risoluzione spaziale delle carte di base, la densità delle reti climatiche e la qualità del dato di pioggia, al fine di garantire la massima verosimiglianza del modello concettuale con le singole realtà in studio. Il sistema informatico è stato progettato ed implementato per supportare studi ed analisi mirati alla pianificazione della gestione della risorsa idrica regionale, all’individuazione di aree soggette a stress idrico e allo studio di fenomeni legati al ciclo dell’acqua, quali il ciclo dei sedimenti e dei nutrienti.A 20% rainfall decrease in the last 30 years, prolonged periods of drought and 40% losses in the distribution pipes have substantially lowered available water reserves in Sardinia, with the result of unpopular rationing measures on municipal, industrial and agricultural uses. Up to now black-box models have been the most commonly used methodologies to estimate the regional water budget. Despite their wide use, these models have shown severe limitations in estimating watershed outlet outputs far from the monitoring stations and in covering climate changes properly. In this study, we present the application of the hydrological distributed SWAT model to evaluate the water budget at the regional scale. The model parameterizations with regards to the land cover and soil type are estimated at the regional scale on the basis of available geographic data. In order to generate weather daily inputs for the SWAT model a weather generator has been developed on the basis of a statistical analysis of the Sardinian climatic data. Through a clusterization procedure the climatic gages have been grouped into two homogeneous groups. The regional databases have been formatted in the SWAT fashion. The resulting model input data along with the watershed and HRU spatial discretization criteria have been carefully checked to ensure global consistency at the overall scale. The informatics system has been developed and designed to support decision related to the management of the water resources, to identify areas under water stress, or to evaluate complementary phenomena related to the water cycle as the sediment and the nutrient fate at the catchment’s scale.29-38Pubblicat

On the equivalence of total stress and pressure gradient formulations for predicting land subsidence above compacting gas/oil fields

The solution of the poroelastic equations for predicting land subsidence above productive gas/oil fields may be addressed by the principle of virtual works using either the effective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the finite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate different local forces on the element nodes. The two formulations are briey reviewed and discussed for triangular and tetrahedral finite elements. They are shown to yield different results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modification to both formulations is suggested which allows to correctly solve the problem of a finite reservoir with an infinite pressure gradient, i.e. with a pore pressure discontinuity on its boundary

AQUAGRID: an extensible platform for collaborative problem solving in groundwater protection

AQUAGRID is the subsurface hydrology computational service of the Sardinian GRIDA3 infrastructure, designed to deliver complex environmental applications via a user-friendly Web portal. The service aims to provide to water professionals integrated modeling tools to solve water resources management problems and aid decision making for contaminated soil and groundwater. In this paper, the AQUAGRID application concept and enabling technologies are illustrated. At the heart of the service are the computational models to simulate complex and large groundwater flow and contaminant transport problems and geochemical speciation. AQUAGRID is built on top of compute-Grid technologies by means of the EnginFrame Grid framework. Distributed data management is provided by the Storage Resource Broker data-Grid middleware. The resulting environment allows end-users to perform groundwater simulations and to visualize and interact with their results, using graphs, 3D images and annotated maps. The problem solving capability of the platform is demonstrated using the results of two case studies deployed

Finite element analysis of land subsidence above depleted reservoirs with pore pressure gradient and total stress formulations

SUMMARY The solution of the poroelastic equations for predicting land subsidence above productive gas/oil "elds may be addressed by the principle of virtual works using either the e!ective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the "nite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate di!erent local forces on the element nodes. The two formulations are brie#y reviewed and discussed for triangular and tetrahedral "nite elements. They are shown to yield di!erent results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modi"cation to both formulations is suggested which allows to correctly solve the problem of a "nite reservoir with an in"nite pressure gradient, i.e. with a pore pressure discontinuity on its boundary