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

A modeling study of heterogeneity and surface water-groundwater interactions in the Thomas Brook catchment, Annapolis Valley (Nova Scotia, Canada)

A modelling study of the impacts of subsurface\ud heterogeneity on the hydrologic response of a small catchment\ud is reported. The study is focused in particular on the\ud hydraulic connection and interactions between surface water\ud and groundwater. A coupled (1-D surface/3-D subsurface)\ud numerical model is used to investigate, for a range of scenarios,\ud the spatio-temporal patterns of response variables such\ud as return flow, recharge, groundwater levels, surface saturation,\ud and streamflow. Eight scenarios of increasing geological\ud complexity are simulated for an 8 km2 catchment in\ud the Annapolis Valley (eastern Canada), introducing at each\ud step more realistic representations of the geological strata\ud and corresponding hydraulic properties. In a ninth scenario\ud the effects of snow accumulation and snowmelt are also considered.\ud The results show that response variables and significant\ud features of the catchment (e.g. springs) can be adequately\ud reproduced using a representation of the geology and\ud model parameter values that are based on targeted fieldwork\ud and existing databases, and that reflect to a sufficient degree\ud the geological and hydrological complexity of the study area.\ud The hydraulic conductivity values of the thin surficial sediment\ud cover (especially till) and of the basalts in the upstream\ud reaches emerge as key elements of the basin’s heterogeneity\ud for properly capturing the overall catchment response

Intraseasonal Drainage Network Dynamics in a Headwater Catchment of the Italian Alps

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Intraseasonal Drainage Network Dynamics in a Headwater Catchment of the Italian Alps

In the majority of existing studies, streams are conceived as static objects that occupy predefined regions of the landscape. However, empirical observations suggest that stream networks are systematically and ubiquitously featured by significant expansion/retraction dynamics produced by hydrologic and climatic variability. This contribution presents novel empirical data about the active drainage network dynamics of a 5 km2 headwater catchment in the Italian Alps. The stream network has been extensively monitored with a biweekly temporal resolution during a field campaign conducted from July to November 2018. Our results reveal that, in spite of the wet climate typical of the study area, more than 70% of the observed river network is temporary, with a significant presence of disconnected reaches during wet periods. Available observations have been used to develop a set of simple statistical models that were able to properly reconstruct the dynamics of the active stream length as a function of antecedent precipitation. The models suggest that rainfall timing and intensity represent major controls on the stream network length, while evapotranspiration has a minor effect on the observed intraseasonal changes of drainage density. Our results also indicate the presence of multiple network expansion and retraction cycles that simultaneously operate at different time scales, in response to distinct hydrological processes. Furthermore, we found that observed spatial patterns of network dynamics and unchanneled lengths are related to the underlying heterogeneity of geological attributes. The study offers novel insights on the physical mechanisms driving stream network dynamics in low-order alpine catchments

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

Combining Models of Root-Zone Hydrology and Geoelectrical Measurements: Recent Advances and Future Prospects

Recent advances in measuring and modeling root water uptake along with refined electrical petrophysical models may help fill the existing gap in hydrological root model parametrization. In this paper, we discuss the choices to be made to combine root-zone hydrology and geoelectrical data with the aim of characterizing the active root zone. For each model and observation type we discuss sources of uncertainty and how they are commonly addressed in a stochastic inversion framework. We point out different degrees of integration in the existing hydrogeophysical approaches to parametrize models of root-zone hydrology. This paper aims at giving emphasis to stochastic approaches, in particular to Data Assimilation (DA) schemes, that are generally identified as the best way to combine geoelectrical data with Root Water Uptake (RWU) models. In addition, the study points out a more suitable objective function taken from the optimal transport theory that better captures complex geometry of root systems. Another pathway for improvement of geoelectrical data integration into RWU models using DA relies on the use of stem based methods as a leverage to introduce more extensive root knowledge into RWU macroscopic hydrological models

Genetic diversity of Streptococcus suis clinical isolates from pigs and humans in Italy (2003-2007)

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Asymptomatic internal carotid artery stenosis and cerebrovascular risk stratification

Background The purpose of this study was to determine the cerebrovascular risk stratification potential of baseline degree of stenosis, clinical features, and ultrasonic plaque characteristics in patients with asymptomatic internal carotid artery (ICA) stenosis. Methods This was a prospective, multicenter, cohort study of patients undergoing medical intervention for vascular disease. Hazard ratios for ICA stenosis, clinical features, and plaque texture features associated with ipsilateral cerebrovascular or retinal ischemic (CORI) events were calculated using proportional hazards models. Results A total of 1121 patients with 50% to 99% asymptomatic ICA stenosis in relation to the bulb (European Carotid Surgery Trial [ECST] method) were followed-up for 6 to 96 months (mean, 48). A total of 130 ipsilateral CORI events occurred. Severity of stenosis, age, systolic blood pressure, increased serum creatinine, smoking history of more than 10 pack-years, history of contralateral transient ischemic attacks (TIAs) or stroke, low grayscale median (GSM), increased plaque area, plaque types 1, 2, and 3, and the presence of discrete white areas (DWAs) without acoustic shadowing were associated with increased risk. Receiver operating characteristic (ROC) curves were constructed for predicted risk versus observed CORI events as a measure of model validity. The areas under the ROC curves for a model of stenosis alone, a model of stenosis combined with clinical features and a model of stenosis combined with clinical, and plaque features were 0.59 (95% confidence interval [CI] 0.54-0.64), 0.66 (0.62-0.72), and 0.82 (0.78-0.86), respectively. In the last model, stenosis, history of contralateral TIAs or stroke, GSM, plaque area, and DWAs were independent predictors of ipsilateral CORI events. Combinations of these could stratify patients into different levels of risk for ipsilateral CORI and stroke, with predicted risk close to observed risk. Of the 923 patients with <70% stenosis, the predicted cumulative 5-year stroke rate was <5% in 495, 5% to 9.9% in 202, 10% to 19.9% in 142, and <20% in 84 patients. Conclusion Cerebrovascular risk stratification is possible using a combination of clinical and ultrasonic plaque features. These findings need to be validated in additional prospective studies of patients receiving optimal medical intervention alone. Copyright © 2010 by the Society for Vascular Surgery