Thomas Kalbacher

OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media

In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open-source initiative for numerical simulation of thermo-hydro-mechanical-chemical processes in porous media. The basic concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multifield problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre- and postprocessing. The OGS idea has been in development since the mid-eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software-engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, for example, for code comparison and validation purposes (DEVOVALEX and CO2 BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however, the parallelization of OO codes still lacks efficiency. High-performance-computing efficiency of OO codes is subject to future research.

Reactive transport codes for subsurface environmental simulation

A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.

Integrated Water Resources Management under different hydrological, climatic and socio-economic conditions

The International Water Research Alliance Saxony (IWAS) project (2009–2014) was dedicated to investigate global challenges concerning integrated water resources management (IWRM) in different model regions in Eastern Europe (region R1), Central and Southeast Asia (R2 and R3), the Middle East (R4) and Latin America (R5). This thematic issue compiles the most important scientific results of the second phase of the IWAS project which was introduced by Kalbus et al. (2012). The main results and lessons learned from the transdisciplinary IWRM project are presented in Seegert et al. (2014). The IWAS project was structured into the above-mentioned model regions (R1–5) and into cross-cutting topics: Q1—Model based scenario analysis, Q2—Technology development, Q3— Governance, and Q4—Capacity development (Fig. 1). The first cross-cutting topic Q1 was dedicated to modelbased scenario analysis of hydrological and climate related processes. Within the first project phase of IWAS a ToolBox for hydrological process simulation was developed and exemplary applied to the IWAS investigation regions (Kalbacher et al. 2012). The toolbox concept has been completed by invoking data integration as well as model visualization methods (Rink et al. 2014; Bilke et al. 2014). Within the second IWAS project phase, the toolbox has been extensively applied to several IWAS model regions: R1 (e.g. Fischer et al. 2014; Koerner et al. 2014; Pavlik et al. 2014), R2 (e.g. Karthe et al. 2014), R4 (e.g. Kloss et al. 2014; Graebe et al. 2013; Subagadis et al. 2014; Walther et al. 2014), R5 (e.g. Borges et al. 2014; Da Anunciacao et al. 2014; Goncalves et al. 2013) to address a variety of questions for surface water and groundwater management. More general works were Barfus and Bernhofer (2014) who applied global climate models (GCM) to different model regions and Pluntke et al. (2014) dealing with uncertainty in hydrological modeling due to data scarcity. The second cross-cutting topic Q2 was focused on technology development, implementation and transfer. Three work packages in Q2 were dealing with the Sewchar concept for sustainable sanitation systems (Fuehner et al. 2012), hydrothermal carbonization for treatment of domestic waste and sewage sludge (Poerschmann et al. 2014), and the development of multisensory systems for the detection of pathogens. The remaining transdisciplinary topics were dealing with aspects of governance (Q3) and capacity development (Q4). Dombrowsky et al. (2014) were discussing socioeconomic questions of water governance in transition countries as well as institutional and legal constraints for transboundary river basin management. Capacity development is an important method for the implementation of IWRM concepts (Leidel et al. 2014). As a result of the IWAS capacity development activities an E-learning platform has been established (Leidel et al. 2013, see http:// www.iwrm-education.de). IWAS Ukraine (R1) was dealing with a variety of IWRM aspects elaborating on socio-economic as well as natural science questions (Dombrowsky et al. 2014; Hagemann et al. 2014) discussing the role of water governance in transition countries as well as institutional and legal constraints on transboundary river basin management. A decision support concept was developed by Leidel et al. J. Seegert O. Kolditz (&) P. Krebs D. Borchardt Technische Universitat Dresden, Dresden, Germany e-mail: ees-editor@ufz.de

The IWAS-ToolBox: Software coupling for an integrated water resources management

Numerical modeling of interacting flow and transport processes between different hydrological compartments, such as the atmosphere/land surface/vegetation/soil/groundwater systems, is essential for understanding the comprehensive processes, especially if quantity and quality of water resources are in acute danger, like e.g. in semi-arid areas and regions with environmental contaminations. The computational models used for system and scenario analysis in the framework of an integrated water resources management are rapidly developing instruments. In particular, advances in computational mathematics have revolutionized the variety and the nature of the problems that can be addressed by environmental scientists and engineers. It is certainly true that for each hydro-compartment, there exists many excellent simulation codes, but traditionally their development has been isolated within the different disciplines. A new generation of coupled tools based on the profound scientific background is needed for integrated modeling of hydrosystems. The objective of the IWAS-ToolBox is to develop innovative methods to combine and extend existing modeling software to address coupled processes in the hydrosphere, especially for the analysis of hydrological systems in sensitive regions. This involves, e.g. the provision of models for the prediction of water availability, water quality and/or the ecological situation under changing natural and socio-economic boundary conditions such as climate change, land use or population growth in the future.

Visual data exploration for hydrological analysis

Hydrological research projects for integrated water resources management such as the IWAS initiative often accumulate large amounts of heterogeneous data from different sources. Given the number of partners taking part in such projects, surveying and accessing the available data sets, as well as searching for a defined subset, becomes increasingly difficult. We propose an integrated approach for a system combining visual data management and numerical simulation which allows to survey and select data sets based on keywords such as a region of interest or given indicators. An adequate 3D visualisation of such subsets helps to convey information and significantly supports the assessment of relations between different types of data. Furthermore, the interface between the visual data management system and finite element codes allows for the straightforward integration of information into the numerical simulation process and the subsequent visualisation of simulation results in a geographical context. We demonstrate typical workflows for integration and processing within the system based on data from the IWAS model region in Saudi Arabia and the TERENO Bode Observatory in the Harz Mountains in Germany. In addition, we present examples for data import and export based on established standard file formats.

A coupled surface/subsurface flow model accounting for air entrapment and air pressure counterflow

This work introduces the soil air system into integrated hydrology by simulating the flow processes and interactions of surface runoff, soil moisture and air in the shallow subsurface. The numerical model is formulated as a coupled system of partial differential equations for hydrostatic (diffusive wave) shallow flow and two-phase flow in a porous medium. The simultaneous mass transfer between the soil, overland, and atmosphere compartments is achieved by upgrading a fully established leakance concept for overland-soil liquid exchange to an air exchange flux between soil and atmosphere. In a new algorithm, leakances operate as a valve for gas pressure in a liquid-covered porous medium facilitating the simulation of air out-break events through the land surface. General criteria are stated to guarantee stability in a sequential iterative coupling algorithm and, in addition, for leakances to control the mass exchange between compartments. A benchmark test, which is based on a classic experimental data set on infiltration excess (Horton) overland flow, identified a feedback mechanism between surface runoff and soil air pressures. Our study suggests that air compression in soils amplifies surface runoff during high precipitation at specific sites, particularly in near-stream areas.

Comments on A mass-conservative switching algorithm for modeling fluid flow in variably saturated porous media, K. Sadegh Zadeh, Journal of Computational Physics, 230 (2011)

Simulating coupled soil-aquifer systems are recently of great scientific interests and need, particularly in terms of getting a correct water exchange between regional climate and terrestrial surface/subsurface water body to predict the future changes in water resources for a sustainable agricultural production and drinking water coverage in areas with increasing aridification. The Richards equation has been widely used in simulating water flow processes in coupled soil-aquifer systems. It has been expressed in three standard forms: i) the pressure head-based form (h-based form) with pressure head as primary variable, ii) the saturation-based form with saturation as primary variable, and iii) the mixed form, in which either pressure or saturation can be chosen as primary variable [1,3,8]. Besides of the saturation-based form, the h-based and the mixed form which use the pressure head as the primary variable can be applied in the unsaturated zone and saturated zone simultaneously [2,6,8]. The mixed form is preferred by several researchers because it conserves mass more precisely [1,2,9], whereas the h-based form often leads to large mass-balance errors for highly non-linear problems such as infiltration into initially dry soil [1,2,9]. In the recent past Sadegh Zadeh [10] stated that the mixed form cannot be applied in the saturated zone: “. . . it is only applicable in unsaturated zones”. Based on this assumption, he proposed a switching algorithm which uses the mixed form of Richards equation in the unsaturated zone and switches to the h-based form in and near the saturated zone based on a threshold value of pressure head (−2.5 cm). The author concluded that this algorithm was designed to produce “rapid convergence for saturated and unsaturated regions for all types of initial and boundary conditions”.

Programm- und Softwareentwicklung für die Grundwassermodellierung

Es ist einsam geworden . . . Als der Erstautor seine Doktorarbeit schrieb (es ist schon eine geraume Zeit her), gab es hierzulande mindestens 10 Forschungseinrichtungen, die sich mit der Methodenund Programmentwicklung fur die Grundwassermodellierung intensiv beschaftigten. Dass es nun weniger geworden sind, hat nicht so sehr mit der deutschen Wiedervereinigung zu tun, sondern ist ein logischer Globalisierungsprozess. Man will immer komplexere Fragestellungen angehen, damit steigt der Aufwand erheblich und so ist die Landschaft der wissenschaftlichen und kommerziellen Software mittlerweile eben fokussierter und recht uberschaubar geworden. Es ist aber auch anders geworden . . . Neben kommerzieller Programmentwicklung und den vielfach unbeachteten, projektspezifischen Einzellosungen gibt es eine dritte Richtung unter Verwendung von soliden kommerziellen, skriptbasierten Tools zur numerischen Losung von Problemen. Mit relativ wenig Programmieraufwand kommt man rasch zu Testbausteinen, die eine Antwort auf eine spezielle wissenschaftliche Fragestellung liefern und kann somit Kernpunkte identifizieren, die es lohnt weiter zu verfolgen, oder man merkt schnell, dass man auf dem Holzweg ist. Als vierte Moglichkeit gibt es noch die wissenschaftliche Softwareentwicklung. Sie ermoglicht die Beantwortung komplexerer Fragestellungen (z. B. Prozesskopplun-