Ensemble Kalman filter versus ensemble smoother for assessing hydraulic conductivity via tracer test data assimilation

Abstract. Estimating the spatial variability of hydraulic conductivity K in natural aquifers is important for predicting the transport of dissolved compounds. Especially in the nonreactive case, the plume evolution is mainly controlled by the heterogeneity of K. At the local scale, the spatial distribution of K can be inferred by combining the Lagrangian formulation of the transport with a Kalman-filter-based technique and assimilating a sequence of time-lapse concentration C measurements, which, for example, can be evaluated on site through the application of a geophysical method. The objective of this work is to compare the ensemble Kalman filter (EnKF) and the ensemble smoother (ES) capabilities to retrieve the hydraulic conductivity spatial distribution in a groundwater flow and transport modeling framework. The application refers to a two-dimensional synthetic aquifer in which a tracer test is simulated. Moreover, since Kalman-filter-based methods are optimal only if each of the involved variables fit to a Gaussian probability density function (pdf) and since this condition may not be met by some of the flow and transport state variables, issues related to the non-Gaussianity of the variables are analyzed and different transformation of the pdfs are considered in order to evaluate their influence on the performance of the methods. The results show that the EnKF reproduces with good accuracy the hydraulic conductivity field, outperforming the ES regardless of the pdf of the concentrations

Modeling rainfall-driven transport of Glyphosate in the vadose zone of two experimental sites in North-East Italy

A vertical one-dimensional analysis of infiltration processes and mobility of a tracer (potassium bromide) and a glyphosate-based herbicide, both subjected to hydrological forcing, was performed. Glyphosate is a widespread herbicide whose potential harmfulness and mobility under hydrological forcing have not been fully understood yet. Here, the spatio-temporal evolution of the two compounds was monitored for one year in two experimental sites (Settolo - Valdobbiadene, Colnù - Conegliano), located within the production area of the Prosecco wine (Treviso, Italy). In each experimental site the activities were carried out on two 25 m2 plots located at distances of 50-100 m from each other. The interpretative analyses considered rainwater infiltration as the driving mechanism of the herbicide transport and allowed us to obtain the calibration of a one-dimensional hydrologic model in each monitored plot. Different scenarios of the tracer evolution were simulated considering the pedologic properties of the shallower soil layers, the status of the plant coverage and of the root apparati, leading to a satisfactory reproduction of the observations in both the experimental sites. Modeling the mobility of the herbicide, considering also the degradation to its metabolite AMPA, proved to be more challenging, due to the tendency of glyphosate to be adsorbed to the soil matrix rather than be dissolved in water and transported toward deeper soil layers. Nevertheless, the analysis of model results for tracer and herbicide, compared with in situ observations, suggests that the transport of the glyphosate can take place even when it is adsorbed to the soil, through the movement, triggered by intense precipitation events, of microscopic soil particles within preferential flow paths

Haemodynamics and oxygenation improvement induced by high frequency percussive ventilation in a patient with hypoxia following cardiac surgery: a case report

<p>Abstract</p> <p>Introduction</p> <p>High frequency percussive ventilation is a ventilatory technique that delivers small bursts of high flow respiratory gas into the lungs at high rates. It is classified as a pneumatically powered, pressure-regulated, time-cycled, high-frequency flow interrupter modality of ventilation. High frequency percussive ventilation improves the arterial partial pressure of oxygen with the same positive end expiratory pressure and fractional inspiratory oxygen level as conventional ventilation using a minor mean airway pressure in an open circuit. It reduces the barotraumatic events in a hypoxic patient who has low lung-compliance. To the best of our knowledge, there have been no papers published about this ventilation modality in patients with severe hypoxaemia after cardiac surgery.</p> <p>Case presentation</p> <p>A 75-year-old Caucasian man with an ejection fraction of 27 percent, developed a lung infection with severe hypoxaemia [partial pressure of oxygen/fractional inspiratory oxygen of 90] ten days after cardiac surgery. Conventional ventilation did not improve the gas exchange. He was treated with high frequency percussive ventilation for 12 hours with a low conventional respiratory rate (five per minute). His cardiac output and systemic and pulmonary pressures were monitored.</p> <p>Compared to conventional ventilation, high frequency percussive ventilation gives an improvement of the partial pressure of oxygen from 90 to 190 mmHg with the same fractional inspiratory oxygen and positive end expiratory pressure level. His right ventricular stroke work index was lowered from 19 to seven g-m/m²/beat; his pulmonary vascular resistance index from 267 to 190 dynes•seconds/cm⁵/m²; left ventricular stroke work index from 28 to 16 gm-m/m²/beat; and his pulmonary arterial wedge pressure was lowered from 32 to 24 mmHg with a lower mean airway pressure compared to conventional ventilation. His cardiac index (2.7 L/min/m²) and ejection fraction (27 percent) did not change.</p> <p>Conclusion</p> <p>Although the high frequency percussive ventilation was started ten days after the conventional ventilation, it still improved the gas exchange. The reduction of right ventricular stroke work index, left ventricular stroke work index, pulmonary vascular resistance index and pulmonary arterial wedge pressure is directly related to the lower respiratory mean airway pressure and the consequent afterload reduction.</p

Numerical experiments on dispersion in heterogeneous porous media

The paper discusses numerical simulations of dispersion processes of inert solutes in two-dimensional random log-permeability fields Y(x). A suitable number of realizations is generated preserving a given spatial correlation structure. For each realization a finite element model solves the flow equation and a particle-tracking method solves the transport equation. Ensemble averaging yields then the statistics. The range of σY2 investigated is 0.2÷2.0. The results show that Dagan's linear theory yields acceptable results in an unexpectedly broad range of σY2. Interestingly, a linearization of the flow equation yields larger deviations from the linear theory than the corresponding fully nonlinear model, thereby suggesting that previous conclusions drawn on the limitations of the linear theory might be restrictive

Reply to 'Comment on Nonstationary flow and nonergodic transport in random porous media by N. Suciu and C. Vamos [#2007WR005946]'

The paper reports the replay to comment by Suciu and Vamo\u15f [2007] on a published paper [Darvini and Salandin, 2006] and demonstrates the correctness of the previously obtained results. New insights about the fulfillment of the ergodic hypothesis in numerical experiments are also added

Pianificazione della manutenzione delle reti di distribuzione idropotabile: un approccio basato sull’affidabilità

La nota tratta della pianificazione a lungo termine degli interventi di sostituzione delle condotte nelle reti di distribuzione idropotabile. Il degrado legato all’invecchiamento delle reti riduce infatti le caratteristiche meccaniche e quelle idrauliche, rendendo le condotte soggette a guasti e rotture con frequenza crescente e causando un generale abbassamento del cielo piezometrico nel tempo. Il deterioramento del servizio, sia in termini quantitativi che qualitativi, è la conseguenza inevitabile di tale processo che può essere controllato solo attraverso una accorta programmazione dei processi di riabilitazione e/o sostituzione delle condotte. I costi legati a tali interventi risultano comunque rilevanti e la decisione deve essere presa sulla base di aspetti tecnici e considerazioni economiche che vanno pianificate in un arco temporale adeguato. Nella nota è presentata una tecnica di analisi che permette una previsione degli accadimenti futuri in termini statistici, potendo tenere conto dei processi di deterioramento idraulico e meccanico cui sono sottoposti gli elementi che compongono la rete ed anche di eventuali incrementi della richiesta da parte delle utenze. Tramite il modello proposto è possibile prevedere come le prestazioni del sistema nel suo complesso si modifichino nel tempo e sono identificate le condotte che vengono ad essere causa principale del mancato soddisfacimento dell’utenza in termini di portata e/o prevalenza. Dal confronto di tali informazioni con i costi degli interventi di manutenzione, è possibile individuare su quale singola tratta convenga intervenire per assicurare il massimo beneficio al sistema e quale sia il momento economicamente più conveniente per pianificare le sostituzioni/riabilitazioni delle condotte in un adeguato arco temporale

Examples of subsurface solute spreading driven by inhomogeneous velocity fields

Most of solutions of flow and transport problems available in literature are obtained under assumptions of stationarity of flow field and/or ergodicity of transport, even though in real-world analyses these hypotheses cannot be always verified. For instance, into problems that are met in practical applications, the statistics of flow are often location dependent and the stationarity of flow is violated. The nonstationarity of velocity field may originate from finite domain boundaries, complex flow configurations (pumping and injecting), nonstationarity of medium properties or conditioning of the log conductivity field to measurements of head or conductivity. Moreover the lack of ergodicity related to the finite size of solute sources makes difficult transport analyses that are often carried out by rough schematizations. Some examples of solute dispersion of nonergodic passive solute plume in heterogeneous formations with nonstationary flow conditions are here considered and solved by a new approach. By this method the spatial moments of finite initial volume of solute are obtained from the statistics of velocity field evaluated by first-order perturbation expansion of steady state flow equation in Taylor series combined with a finite element discretization. The approach allows to handle nonstationarity due to several causes and is here applied to some test cases in bounded domains. A comparison of numerical results in terms of particle displacements moments with known solutions available in literature and with Monte Carlo simulations gives a measure of the effect related to the lack of plume ergodicity and of flow spatial stationarity in real-world transport analyses