Simulating surface water and groundwater flow dynamics in tile-drained catchments

Pratique agricole répandue dans les champs sujets à l’accumulation d’eau en surface, le drainage souterrain améliore la productivité des cultures et réduit les risques de stagnation d’eau. La contribution significative du drainage sur les bilans d’eau à l’échelle de bassins versants, et sur les problèmes de contamination dus à l’épandage d’engrais et de fertilisant, a régulièrement été soulignée. Les écoulements d’eau souterraine associés au drainage étant souvent inconnus, leur représentation par modélisation numérique reste un défi majeur. Avant de considérer le transport d’espèces chimiques ou de sédiments, il est essentiel de simuler correctement les écoulements d’eau souterraine en milieu drainé. Dans cette perspective, le modèle HydroGeoSphere a été appliqué à deux bassins versants agricoles drainés du Danemark. Un modèle de référence a été développé à l’échelle d’une parcelle dans le bassin versant de Lillebæk pour tester une série de concepts de drainage dans une zone drainée de 3.5 ha. Le but était de définir une méthode de modélisation adaptée aux réseaux de drainage complexes à grande échelle. Les simulations ont indiqué qu’une simplification du réseau de drainage ou que l’utilisation d’un milieu équivalent sont donc des options appropriées pour éviter les maillages hautement discrétisés. Le calage des modèles reste cependant nécessaire. Afin de simuler les variations saisonnières des écoulements de drainage, un modèle a ensuite été créé à l’échelle du bassin versant de Fensholt, couvrant 6 km2 et comprenant deux réseaux de drainage complexes. Ces derniers ont été simplifiés en gardant les drains collecteurs principaux, comme suggéré par l’étude de Lillebæk. Un calage du modèle par rapport aux débits de drainage a été réalisé : les dynamiques d’écoulement ont été correctement simulées, avec une faible erreur de volumes cumulatifs drainés par rapport aux observations. Le cas de Fensholt a permis de valider les conclusions des tests de Lillebæk, ces résultats ouvrant des perspectives de modélisation du drainage lié à des questions de transport.Tile drainage is a common agricultural management practice in plots prone to ponding issues. Drainage enhances crop productivity and reduces waterlogging risks. Studies over the last few decades have highlighted the significant contribution of subsurface drainage to catchments water balance and contamination issues related to manure or fertilizer application at the soil surface. Groundwater flow patterns associated with drainage are often unknown and their representation in numerical models, although powerful analysis tools, is still a major challenge. Before considering chemical species or sediment transport, an accurate water flow simulation is essential. The integrated fully-coupled hydrological HydroGeoSphere code was applied to two highly tile-drained agricultural catchments of Denmark (Lillebæk and Fensholt) in the present work. A first model was developed at the field scale from the Lillebæk catchment. A reference model was set and various drainage concepts and boundary conditions were tested in a 3.5 ha tile-drained area to find a suitable option in terms of model performance and computing time for larger scale modeling of complex drainage networks. Simulations suggested that a simplification of the geometry of the drainage network or using an equivalent-medium layer are suitable options for avoiding highly discretized meshes, but further model calibration is required. A catchment scale model was subsequently built in Fensholt, covering 6 km2 and including two complex drainage networks. The aim was to perform a year-round simulation accounting for variations in seasonal drainage flow. Both networks were simplified with the main collecting drains kept in the model, as suggested by the Lillebæk study. Calibration against hourly measured drainage discharge data was performed resulting in a good model performance. Drainage flow and flow dynamics were accurately simulated, with low cumulative error in drainage volume. The Fensholt case validated the Lillebæk test conclusions, allowing for further drainage modeling linked with transport issues

A mathematical model for thermal single-phase flow and reactive transport in fractured porous media

In this paper we present a mathematical model and a numerical workflow for the simulation of a thermal single-phase flow with reactive transport in porous media, in the presence of fractures. The latter are thin regions which might behave as high or low permeability channels depending on their physical parameters, and are thus of paramount importance in underground flow problems. Chemical reactions may alter the local properties of the porous media as well as the fracture walls, changing the flow path and possibly occluding some portions of the fractures or zones in the porous media. To solve numerically the coupled problem we propose a temporal splitting scheme so that the equations describing each physical process are solved sequentially. Numerical tests shows the accuracy of the proposed model and the ability to capture complex phenomena, where one or multiple fractures are present

Simulation of reactive solute transport in the critical zone: A Lagrangian model for transient flow and preferential transport

We present a method to simulate fluid flow with reactive solute transport in structured, partially saturated soils using a Lagrangian perspective. In this context, we extend the scope of the Lagrangian Soil Water and Solute Transport Model (LAST) (Sternagel et al., 2019) by implementing vertically variable, non-linear sorption and first-order degradation processes during transport of reactive substances through a partially saturated soil matrix and macropores. For sorption, we develop an explicit mass transfer approach based on Freundlich isotherms because the common method of using a retardation factor is not applicable in the particle-based approach of LAST. The reactive transport method is tested against data of plot- and field-scale irrigation experiments with the herbicides isoproturon and flufenacet at different flow conditions over various periods. Simulations with HYDRUS 1-D serve as an additional benchmark. At the plot scale, both models show equal performance at a matrix-flow-dominated site, but LAST better matches indicators of preferential flow at a macropore-flow-dominated site. Furthermore, LAST successfully simulates the effects of adsorption and degradation on the breakthrough behaviour of flufenacet with preferential leaching and remobilization. The results demonstrate the feasibility of the method to simulate reactive solute transport in a Lagrangian framework and highlight the advantage of the particle-based approach and the structural macropore domain to simulate solute transport as well as to cope with preferential bypassing of topsoil and subsequent re-infiltration into the subsoil matrix

Simulating groundwater and surface water flow and solute transport in tile-drained landscapes

Dans des conditions de climat humide et lorsque les sols sont peu perméables, les systèmes de drainage souterrains sont généralement utilisés pour contrôler le niveau de la nappe phréatique et améliorer la production agricole. Cependant, les drains souterrains modifient à la fois les voies d'écoulement hydrologique et les taux de transport des nutriments des terres cultivées vers les eaux de surface, pouvant détériorer la qualité des eaux souterraines et de surface. De plus, des macropores sont souvent présents dans les sols composés de till argileux, ce qui génère un flux d'eau rapide et riche en nutriments de la surface du sol vers les drains souterrains. Une approche rentable pour réduire le lessivage des nutriments provenant de l'agriculture consiste à imposer des restrictions uniquement dans les zones vulnérables à la contamination de l'eau. Ces zones peuvent être identifiées à l'aide de modèles hydrologiques distribués. Les résultats obtenus sur de petits bassins versants expérimentaux doivent être simplifiés pour être appliqués à des échelles plus grandes, généralement requises pour l'élaboration de politiques. L'objectif de cette étude était d'examiner les avancées et les limitations de l'inclusion des drains souterrains dans les modèles d'écoulement de surface et souterrain. Les objectifs spécifiques étaient de i) démontrer l'utilisation des estimations de conductivité électrique spécifique (CE), pour améliorer les simulations hydrologiques dans un champ drainé, ii) étudier l'efficacité d'un modèle hydrologique et de transport de soluté tridimensionnel pour simuler un test de traçage de bromure (Br) dans un champ drainé et iii) évaluer différents modèles conceptuels de drains souterrains et d'hétérogénéité du sol pour la simulation numérique du drainage dans un bassin versant agricole au Danemark. Les résultats suggèrent que la simulation de la profondeur de la nappe phréatique peut être améliorée par l'inclusion d'hétérogénéités basées sur des estimations de la CE. L'approche des seepage nodes était appropriée pour simuler les débits de drainage, cependant la précision des simulations était meilleure pour les modèles à l'échelle du terrain. À l'échelle du bassin versant, le fait de ne représenter que les drains principaux est approprié pour pouvoir utiliser des maillages plus grossiers et pour simuler le débit des cours d'eau et les faibles profondeurs des eaux de surface dans les zones drainées. Des résultats similaires ont été obtenus lorsque les seepage nodes ont été appliqués sur l'ensemble des zones agricoles, sans tenir compte de l'emplacement spécifique des drains souterrains. Cette dernière approche peut être appliquée lorsque les drains souterrains ne sont pas cartographiés, ce qui est généralement le cas. Une représentation simplifiée de l'hétérogénéité et de la macroporosité peut expliquer les différences entre ls valeurs observées et simulées des charges hydrauliques, débits de drainage et processus de transport de solutés. Les approches de modélisation étudiées dans cette thèse peuvent améliorer la représentation de la dynamique de l'écoulement souterrain et les simulations du transport de substances agrochimiques lessivées des champs cultivés, telles que le nitrate et phosphate.Under humid climate conditions and for low-permeability soils, subsurface tile drains are usually employed to lower the water table and enhance agricultural production. However, tile drains alter both the hydrologic flow pathways in agricultural catchments and the rates of nutrient transport from cropland to surface water bodies, potentially impairing the groundwater and surface water quality. Furthermore, macropores are often present in clayey till soils, generating rapid and nutrient-rich water flow from the ground surface to the tile drains. A cost-effective approach to reduce nutrient leaching from agriculture is to impose restrictions only in vulnerable areas to water contamination, which can be identified using distributed hydrological models. Results on small experimental catchments need to be simplified for application on larger scales, usually required for policy-making purposes. The objective of this study was to investigate the outcomes and limitations of including tile drains in surface and subsurface flow models. Specific objectives were to i) demonstrate the use of electrical conductivity (EC) estimates to improve hydrological simulations in a tile-drained field, ii) investigate the efficiency of a three-dimensional hydrological and solute transport model to simulate a bromide (Br) tracer test in a tile-drained field and iii) assess different conceptual models for tile drains and soil heterogeneity for the numerical simulation of tile drainage in an agricultural catchment in Denmark. The results suggest that the simulation of the water table depth can be improved by the inclusion of heterogeneities based on EC estimates. The seepage nodes approach was suitable to simulate drain discharge, however the accuracy of the simulations was better for the field-scale models. At the catchment scale, representing only the main drains was suitable to reduce the mesh refinement and simulate stream flow and low surface water depths in drained areas. Similar results were obtained when seepage nodes were applied all over the agricultural areas, without considering the specific location of tile drains. The later approach can be applied when tile drains are not mapped, which is usually the case. The misrepresentation of heterogeneity and macroporosity may explain the differences between observed and simulated hydraulic heads, drain discharge and solute transport processes. The modeling approaches investigated in this dissertation can improve subsequent simulations of tile drainage and the transport and fate of leached agrochemicals such as nitrate or phosphate

Two-phase flow in rocks : new insights from multi-scale pore network modeling and fast pore scale visualization

Many geological applications involve the flow of multiple fluids through porous geological materials, e.g. environmental remediation of polluted ground water resources, carbon dioxide storage in geological reservoirs and petroleum recovery. Commonly, to model these applications, the geological materials in question are treated as continuous porous media with effective material properties. Since these properties are a manifestation of what goes on in the pores of the material, we have to study the transport processes at the pore scale to understand why and how they vary over space and time in different rocks and under different conditions. As the high cost of acquiring and testing samples in many of these applications is often a limiting factor, numerical modelling at the pore scale is becoming a key technology to gain new insights in this field. This could be crucial in reducing uncertainties in field scale projects. The work presented in this thesis focuses on the investigation of two-phase flow in sedimentary rocks, and is an integrated numerical and experimental study. It deals primarily with two outstanding issues. First, image-based pore scale simulation methods have difficulties with representing the multiple pore scales in rocks with wide pore size distributions, due to a trade-off in the size and resolution of both modeling and imaging methods. Therefore, performing two-phase flow simulations in a number of important rock types, such as many carbonates and tight, clay-baring sandstones has remained an outstanding challenge. To tackle this problem, a new numerical model was developed to calculate capillary pressure, relative permeability and resistivity index curves during drainage and imbibition processes in such materials. The multi-scale model was based on information obtained from 3D micro-computed tomography images of the internal pore structure, complemented with information on the pores that are unresolved with this technique. In this method, pore network models were first extracted from resolved pores in the images, by using a maximal ball network extraction algorithm. Then, pores which touched regions with unresolved porosity were connected with a special type of network element called micro-links. In the quasi-static simulations that were performed on these network models, the micro-links carried average properties of the unresolved porosity. In contrast to most previous models, the new approach to taking into account unresolved porosity therefore allowed efficient simulations on images of complex rocks, with sizes comparable to single-scale pore network models. It was able to reproduce most of the behaviour of a fully resolved pore network model, for both drainage and imbibition processes, and for different pore scale wettability distributions (water-wet, oil-wet and different mixed-wet distributions). Furthermore, simulations on images of carbonate rocks showed good agreement to experiments. A sensitivity study on carbonate rocks and tight, clay-bearing sandstones produced results that were in qualitative agreement with experiments, and allowed to analyse how the two-phase flow behaviour of these rocks is influenced by their pore scale properties. The second issue which is treated in this thesis is related to the validation of pore scale models. Comparing predicted effective properties to experimentally measured values is useful and necessary, but is complicated by the typical difference in size between the model and the experiment. Furthermore, it does not always give a clear indication of the reasons for an observed mismatch between models and experiments. Comparing two-phase flow models to pore scale experiments in which the evolution of the fluid distributions is visualized is thus extremely useful. However, this requires to image the two-phase flow process while it is taking place in a rock, and it is necessary to do this with time resolutions on the order of tens of seconds and spatial resolutions on the order of micrometers. Previous experimental approaches used synchrotron beam lines to achieve this. In this thesis, we show that such experiments are also possible using laboratory-based micro-computed tomography scanners, which are orders of magnitude cheaper and therefore more accessible than synchrotrons. An experiment in which kerosene was pumped into a water-saturated sandstone is presented, showing that individual Haines jumps (pore filling events) could be visualized during this drainage process. Because the image quality is lower than at synchrotrons, care had to be taken to adapt the image analysis work flow to deal with high image noise levels. The work flow was designed to allow to track the fluid filling state of individual pores. The results indicate that the dynamic effects due to viscous and inertial forces during Haines jumps do not significantly impact the evolution of the fluid distributions during drainage, which may thus be adequately described by quasi-static models