Exploring environmental variables based on ecotopes derived by remote sensing

LifewatchWB contributes to the European Research Infrastructure Consortium for biodiversity and ecosystem research (Lifewatch) by providing online tools for the visualization of environmental variables. It consists in two Web portals (www.uclouvain.be/lifewatch) providing data about i) weekly anomalies of the dynamic land cover properties (snow cover and vegetation greenness) and ii) high resolution characterization of ecotopes. The topic of this abstract is the ecotope visualization interface. Ecotopes are the smallest ecologically functional units. They can be mapped by intersecting a large number of thematic layers (soil type, land cover, topographic types). However, this creates a lot of polygons with an exponentially increasing number of categorical values combinations. The visualization and analysis of those polygons is therefore difficult. On the other hand, the LifewatchWB ecotopes consist in irregular polygons derived from geographic object-based image analysis (grouping adjacent pixels of similar properties to create a partition of irregular polygons). Each polygon is then characterized using a set of continuous fields: quantitative variables (climate, elevation, slope, artificial light, contextual variables…) are either interpolated or averaged at the level of the ecotopes depending on the resolution of the input layers; categorical variables (soil types, land cover types…) are provided as proportions inside the ecotopes. The ecotope database is an open data layer currently available for the Walloon Region (Belgium). It consists in 1.2 million polygons with more than 80 quantitative fields. Those fields cannot be visualized together, therefore several visualization tools are provided on the interface: 1) each variable can be visualized individually in grey level, 2) any set of three variables can be visualized as a color composite and 3) queries based on potentially all variables can provide binary outputs. In addition, an online tool is available to extract the values of the ecotope characteristics in a set of point locations for further analysis in other softwares. With its polygon-based structure that is homogeneous with respect to the land cover and the topography, ecotopes provides meaningful landscape units with a large set of precomputed variables. Those variables are easy to visualize in a WebGIS and have been combined with species observations to run habitat and biotope models

Discontinuous Galerkin models for coupled surface-subsurface flow

The water cycle is the central component of most environmental systems. A sustainable water management can only be achieved by developing system-wide approaches that encompass the different components of the water cycle. A numerical model is an ideal tool to achieve this goal. In this work, we have developed a suite of numerical models for surface and subsurface water flows. These models solve the 3D Richards equation (RE) and couple it with a 2D surface runoff model based on the non-inertia approximation of the shallow water equations. Unlike classical models of RE, we first consider an explicit time integration scheme to achieve both mass conservation and perfect scaling on High Performance Computers. This saturated/unsaturated subsurface model is then coupled to a surface runoff model, using a coupling method naturally derived from the RE model. In a second step, we consider another formulation that is better suited for water-saturated areas. By slightly degrading the model scalability, we can then handle complex infiltration and evaporation test cases with almost no constraints on the time step and very good numerical efficiency. These models have been thoroughly parameterized and validated on a range of test cases of increasing complexity. They form the building blocks of more complex models that will simulate the water cycle from the raindrop to the ocean.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201

An explicit Discontinuous-Galerkin surface-subsurface water flow model

In this paper, we present a terrestrial hydrological model that explicitly couples the subsurface and surface hydrodynamics. The use of an explicit time integration scheme allows for an optimal par- allel scaling and an efficient coupling between surface and subsurface components. The saturated- unsaturated subsurface module solves Richards equation with a mixed discontinuous-Galerkin (DG) finite element formulation, using both the pressure head h and the water content θ as prognostic vari- ables. θ is used for the unsaturated zone, where it is know to be more efficient, and h is used for the saturated zone, where θ is constant. In the saturated zone, an un-converged false transient method is used in order to replace the elliptic equation by a parabolic one. To allow physical discontinuities between different types of soils, we make use of a modified jump term in the DG formulation for the variable θ. The diffusive wave approximation is applied to the shallow water equation for surface flows including runoff, lakes and small rivers. The surface-subsurface coupling is ensured through a flux continuity constraint The latter is determined by the subsurface model for which a Dirichlet boundary condition is applied, equalizing surface and subsurface pressures. The resulting model is robust and fully conservative

Towards an integrated model of the surface-subsurface water cycle

Surface water and ground water have for a long time been approached as separate components by hydrologists, engineers, and decision makers. In consequence, the relevance of interactions between groundwater and surface water for the aquatic ecosystems has frequently been underestimated. Recent years have experienced a crucial paradigm shift, progressing from defining rivers and aquifers as discrete, separate entities towards an understanding of groundwater and surface water as integral components of a continuum with strong mutual influences between ronoff/river, vadoze zone/aquifer and their interface. We have developed a coupled model of the surface water and ground water. Our model explicitly couples the 2D surface runoff with the 3D saturated-unsaturated subsurface hydrodynamics. We use the false transient method on the saturated zone, where the hydrodynamics is described by an elliptic equation. The surface-subsurface coupling is ensured by setting physical continuity of mass and pressure by appropriated boundary conditions (BC). Th model solves the governing equations with the discontinuous Galerkin finite element method (DGFEM) and an explicit time integration scheme. The DGFEM method allows for unstructured meshes and physical discontinuities between different types of soils, and is locally conservative. The mixed formulation of the Richards equation uses both the pressure head h and the water content w. On the one hand, w is used for the unsaturated zone, where it is know to be more efficient. On the other hand h is used for the saturated zone, where w is constant. The shallow water equation is approximated by the diffusive wave approximation. We will present the overall structure of our model and highlight its advantages. Then we will show its performances for some test cases

Towards a 3D Continuous-Discontinuous Galerkin sub- surface water-flow model

We present a 3D continuous-discontinuous Galerkin subsurface-flow model that will be integrated within the hydrodynamical model SLIM. The model is based on mixed form of Richards equation that takes the retention curve as a constraint on the volumetric water content. A flexible continuous-discontinuous Galerkin discretization is proposed. The model is shown to be exactly mass-conservative and compares well with the state-of-the-art model Hydrus

A fully-explicit discontinuous Galerkin hydrodynamic model for variably-saturated porous media

Groundwater flows play a key role in the recharge of aquifers, the transport of solutes through subsurface systems or the control of surface runoff. Predicting these processes requires the use of groundwater models with their applicability directly linked to their accuracy and computational efficiency. In this paper, we present a new method to model water dynamics in variably- saturated porous media. Our model is based on a fully-explicit discontinuous-Galerkin formulation of the 3D Richards equation, which shows a perfect scaling on parallel architectures. We make use of an adapted jump penalty term for the discontinuous-Galerkin scheme and of a slope limiter algorithm to produce oscillation-free exactly conservative solutions. We show that such an approach is particularly well suited to infiltration fronts. The model results are in good agreement with the reference model Hydrus-1D and seem promising for large scale applications involving a coarse representation of saturated soil

A scalable coupled surface-subsurface flow model.

The coupling of physically-based models for surface and subsurface water flows is a recent concern. The study of their interactions is important both for water resource management and environmental studies. However, despite constant innovation, physically-based simulations of water flows are still time consuming. That is especially problematic for large and/or long-term studies, or to test a large range of parametrizations with an adjoint model. As the current trend in computing sciences is to increase the computational power with additional computational units, new model developments are expected to scale efficiently on parallel infrastructures. This paper describes a coupled surface–subsurface flow model that combines implicit and explicit time discretizations for the surface and subsurface dynamics, respectively. Despite that the surface flow has a faster dynamics than the subsurface flow, we are able to use a unique nearly-optimal time step for each submodel, hence improving the resources use. The surface model is discretized with an implicit control volume finite element method while the subsurface model is solved by means of an explicit discontinuous Galerkin finite element method. The surface and subsurface models are coupled by weakly imposing the continuity of water pressure. By imposing a threshold on the influence coefficients of the control volume finite element method, we can prevent the occurrence of unphysical fluxes in anisotropic elements. The proposed coupling is shown to produce results similar to state-of-the-art models for four different test cases while achieving better strong and weak scalings on up to 192 processors

Lifewatch Wallonia-Brussels : What's up on the land

The Lifewatch-FWB projects aims at providing ready-to-use pan-European geographical information to support biodiversity research. This information is derived by GIS models and remote sensing data analyses. Viewing and downloading services will be freely available to the scientific community