There are a growing number of large-scale, complex hydrologic models that are capable of simulating integrated surface and subsurface flow. Many are coupled to land-surface energy balance models, biogeochemical and ecological process models, and atmospheric models. Although they are being increasingly applied for hydrologic prediction and environmental understanding, very little formal verification and/or benchmarking of these models has been performed. Here we present the results of an intercomparison study of seven coupled surface-subsurface models based on a series of benchmark problems. All the models simultaneously solve adapted forms of the Richards and shallow water equations, based on fully 3-D or mixed (1-D vadose zone and 2-D groundwater) formulations for subsurface flow and 1-D (rill flow) or 2-D (sheet flow) conceptualizations for surface routing. A range of approaches is used for the solution of the coupled equations, including global implicit, sequential iterative, and asynchronous linking, and various strategies are used to enforce flux and pressure continuity at the surface-subsurface interface. The simulation results show good agreement for the simpler test cases, while the more complicated test cases bring out some of the differences in physical process representations and numerical solution approaches between the models. Benchmarks with more traditional runoff generating mechanisms, such as excess infiltration and saturation, demonstrate more agreement between models, while benchmarks with heterogeneity and complex water table dynamics highlight differences in model formulation. In general, all the models demonstrate the same qualitative behavior, thus building confidence in their use for hydrologic applications.

In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open source initiative for numerical simulation of thermo-hydro-mechanical-chemical (THMC) processes in porous media. The basic concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multi-field 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 post-processing. 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, e.g. 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-computin (HPC) efficiency of OO codes is subject to future research.

Abstract. In this paper, we present a stand alone root water uptake model called aRoot, which calculates the sink term for any bulk soil water flow model taking into account water flow within and around a root network. The boundary conditions for the model are the atmospheric water demand and the bulk soil water content. The variable determining the plant regulation for water uptake is the soil water potential at the soil-root interface. In the current version, we present an implementation of aRoot coupled to a 3-D Richards model. The coupled model is applied to investigate the role of root architecture on the spatial distribution of root water uptake. For this, we modeled root water uptake for an ensemble (50 realizations) of root systems generated for the same species (one month old Sorghum). The investigation was divided into two Scenarios for aRoot, one with comparatively high (A) and one with low (B) root radial resistance. We compared the results of both aRoot Scenarios with root water uptake calculated using the traditional Feddes model. The vertical rooting density profiles of the generated root systems were similar. In contrast the vertical water uptake profiles differed considerably between individuals, and more so for Scenario B than A. Also, limitation of water uptake occurred at different bulk soil moisture for different modeled individuals, in particular for Scenario A. Moreover, the aRoot model simulations show a redistribution of water uptake from more densely to less densely rooted layers with time. This behavior is in agreement with observation, but was not reproduced by the Feddes model.

Abstract. In this paper, we present a stand alone root water uptake model called aRoot, which calculates the sink term for any bulk soil water flow model taking into account water flow within and around a root network. The boundary conditions for the model are the atmospheric water demand and the bulk soil water content. The variable determining the plant regulation for water uptake is the soil water potential at the soil-root interface. In the current version, we present an implementation of aRoot coupled to a 3-D Richards model. The coupled model is applied to investigate the role of root architecture on the spatial distribution of root water uptake. For this, we modeled root water uptake for an ensemble (50 realizations) of root systems generated for the same species (one month old Sorghum). The investigation was divided into two Scenarios for aRoot, one with comparatively high (A) and one with low (B) root radial resistance. We compared the results of both aRoot Scenarios with root water uptake calculated using the traditional Feddes model. The vertical rooting density profiles of the generated root systems were similar. In contrast the vertical water uptake profiles differed considerably between individuals, and more so for Scenario B than A. Also, limitation of water uptake occurred at different bulk soil moisture for different modeled individuals, in particular for Scenario A. Moreover, the aRoot model simulations show a redistribution of water uptake from more densely to less densely rooted layers with time. This behavior is in agreement with observation, but was not reproduced by the Feddes model.

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