# The iMOD approach - iMOD

**Why iMOD - the iMOD approach**

*One expandable data set covering all possible future areas of interest**Flow model nesting, toggling between grid resolutions and moving to new areas of interest**Fast interactive 2D-analysis of borehole data**Fast 3D-analysis and visualizations**Interactively editing the geometry of the subsurface**Consistency between regional and sub-domain models**Efficient numerical modelling**Leaving the era of building series of individual models behind**Video of iMOD features**Video of Interactive Pathline Simulator tool*

**One expandable data set covering all possible future areas of interest**

The iMOD approach allows gathering the available input data to be stored at its finest available resolution; these data don't have to be clipped to any pre-defined area of interest or pre-processed to any model grid resolution. Resolutions of parameters can differ and the distribution of the resolution of one parameter can also be heterogeneous. In addition, the spatial extents of the input parameters don't have to be the same. iMOD will perform up- and down scaling (Vermeulen, 2006) whenever the resolution of the simulation is lower or higher than that of the available data.

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**Flow model nesting, toggling between grid resolutions and moving to new areas of interest**

This approach allows the modeller to interactively generate models of any sub-domain within the area covered by the data set. When priorities change in time (e.g. due to changing political agenda's) the modeller can simply move to that new area of interest and apply any desired grid resolution. In addition the modeller can edit the existing data set and / or add new data types to the data set. Utilizing the internal up- and down-scaling techniques ensures that sub-domain models remain consistent with the bigger regional model or that the regional model can locally be updated with the details added in the sub-domain model.

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**Fast interactive 2D-analysis and visualizations of boreholes**

Borehole information can be visualized in both 2D and 3D. A special feature is projecting a 3D-borehole data set on a vertical cross-section. Teh position of the cross-section can interactively be moved over a 2D-map overview while simultanously the cross-section of the subsurface including the nearby boreholes is updated.

*: **play specific part of iMOD video** *

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**Fast interactive 2D- and 3D-analysis and visualizations**

iMOD contains very economic zoom-extent-dependent visualization techniques that allow subsets of grids being visualized instantaneously both in 2D and 3D (see Figure).

*: **play specific part of iMOD video** *

The capability of iMOD to rapidly view and edit model inputs is essential to build effective models in reasonable timeframes. The rapid and integrated views of the geologic / hydrostratigraphic models as well as dynamic model output is critical for the public, stakeholders and regulators to understand and trust the model as a valid decision support tool. iMOD is fast even when working from very large data files because it uses a random accessible data format for 2D grids which facilitates instant visualization or editing subsets of such a large grid file.

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**Interactively editing the geometry of the subsurface**

Often geo-modellers are in the process of refining existing geo-models locally and groundwater modellers will generate feedback based on the outcome of the accompanying groundwater model. iMOD facilitates adapting the geometry (see Figure 3), depth and extents of aquifers and aquitards within a pre-defined mask (polygon). Outside of this mask the 3D model of the subsurface remains unchanged, and near the polygon the transition of these adaptations is smooth and manually adaptable. Of course this can also be approached the other way around: once the geo-modellers have finished a local geo-model, the delineation of a polygon can be used to leave the local model unchanged and edit the adjacent area of the local model.

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*Interactively editing the subsurface*

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**Consistency between regional and sub-domain models**

Sometimes the complexity of the geology and uncertainty regarding the regional groundwater flow dynamics necessitated creating a very large regional model in order to consider a sub-domain with hydrogeologically defensible boundary conditions. Vice versa also the modeller wants to integrate local sub-domain models into the regional model to ensure that the sub-domain models do not have incongruent boundary effects, as is common when modelled in isolation of the regional flow dynamics.

*Example of sub-domain model (rectangle) embedded in coarser regional model; both models are based on the same data set, the only difference is the assigned grid-extent and resolution.*

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**Efficient numerical modelling**

Suppose the modeller needs to simulate groundwater flow for the total area covered by the data set, but the theoretical size of the model is far too big to fit in any CPU-memory. iMOD facilitates generating sub models for parts of the whole area of interest with a user-defined resolution depending on how large the available CPU-memory is and how long the modeller permits her/himself to wait for the model calculations to last. To generate a high resolution result for the whole model domain a number of partly overlapping but adjacent sub models are invoked and the result of the non-overlapping parts of the models are assembled to generate the whole picture. The modeller should of course be cautious that the overlap is large enough to avoid edge effects, but this overlap is easily adjustable in iMOD. A big advantage of this approach is that running a number of small models instead of running one large model (if it would fit in memory, which it often will not) takes much less computation time; computation time (T) depends on the number of model cells (n) exponentially: T = f(n^{1,5-2,0}). The approach also allows the utilization of parallel computing, but this is not obligatory. Using this approach means that the modelling workflow is very flexible and not limited anymore by hardware when utilizing iMOD. This enables the modeller to proceed with building groundwater models in any order, region after region or sub regions within larger regions with resolutions that fit the actual needs and still maintain consistency between the different sub models and resolutions.

Another feature is that iMOD generates MODFLOW input direct in memory, skipping the time-consuming production of standard MODFLOW input files (generating standard MODFLOW input files in ASCII format for large transient models may take hours to a full working day); this efficiency is especially useful during the model building phase when checking newly processed or imported data.

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**Leaving the era of building series of individual models behind**

High resolution groundwater flow modelling, necessary to evaluate effects on a local scale, has traditionally been restricted to small regions given the computational limitations of the CPU memory to handle large numerical MODFLOW-grids. Although CPU-memory size doubles every two years (‘Moore's law') the restriction still holds from a hardware point of view. This restriction has traditionally forced a model builder to always choose between (1) building a model for a large area with a coarse grid resolution or (2) building a model for a small area with a fine grid resolution. For some time it appeared that finite element models could fill the gap by refining the grid only where hydrological gradients were anticipated. However, unanticipated stress may also occur in parts of the model area where the grid is not yet refined resulting in a possible undesired underestimation of these effects. Theoretically the modeller could choose to design a finite element network with a high resolution everywhere, but then it becomes more economic to use finite differences. This is why Deltares has based its innovative modelling techniques on MODFLOW considering it is largely seen world-wide as the standard finite difference source code. Still, modellers ideally need an approach that allows: (1) flexibility to generate high resolution model grids everywhere when needed, (2) flexibility to use or start with a coarser model grid, (3) reasonable runtimes / high performance computing and (4) conceptual consistency over time for any part of the area within their administrative boundary. Deltares has invested in understanding all of these requirements and has developed the iMOD software package to advance the methods and approach used by modellers and regulators.

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**Video of iMOD features**

*Examples of iMOD-features.*

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**Video's of Interactive Pathline Simulator tool**

*Demo's of the Interactive Pathline Simulator tool.*

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*References:*

*Vermeulen, P.T.M., 2006. Model-Reduced Inverse Modeling. Ph.D. thesis Delft University of Technology - with ref. - with summary in Dutch. ISBN-10: 90-9020536-5. ISBN-13: 978-90-9020536-6.*