SIMGRO 6 consists of the metaSWAP model for the unsaturated zone and Modflow for the regional groundwater flow. Surface water can be modelled by means of a SIMGRO-based module or SWQN. SIMGRO links these different compartments by means of flux and storativity-exchanges. Its model input contains various hydrological data, like meteorological data, land use, soil types, watercourse trajectories, weirs and so on.

Figure 2 shows an example of how sub-compartments can be schematized. At the bottom layer there are the combined land-use and soil units (which can be obtained from an overlay procedure of land-use and soil maps). Then follows the layer with the cells of the groundwater model. The third layer is formed by the subcatchments of the surface water model. Surface water is modelled as a network of watercourse trajectories that are connected to these subcatchments. In the top layer the schematizations are combined.

Plant/soil-atmosphere interactions
Interactions with the atmosphere are the drivers of the regional water system. Interactions in SIMGRO consist of precipitation, evapotranspiration, vegetation canopy interception/evaporation, ponding water evaporation and bare soil evaporation. Evapotranspiration in SIMGRO is computed using a modified Makkink-method. The natural precipitation can be augmented by sprinkling from surface water or groundwater.

Soil water
Unsaturated flow in the shallow subsoil is modelled using metaSWAP, a dynamic ‘metamodelling’ concept based on steady-state simulations in SWAP. In the soil water submodel the flow is assumed in the vertical direction only. The soil water dynamics are described using a series of three control boxes: one for the root zone, one for the shallow subsoil, and one for the deep subsoil (see figure 3). The flow description of the soil water model ends at the phreatic level, meaning that the boundary between the soil water and groundwater submodels moves up and down, along with the movements of the phreatic level.

Figure 2: Example of how the spatial schematisations of the integrated model can be constructed. The bottom layer involves the units obtained from an overlay of the land use and soil maps. The next layer represents the cells of the groundwater model, followed by the subcatchments of the surface water model in the next layer. The top layer shows how the schematisations have been combined.

Groundwater is simulated using an adjusted version of Modflow made by TNO (no public domain). The coupling with the other domains has been designed according to the Open Modelling Interface philosophy. The latter entails making a mapping table between the subdomains of the top system model and the groundwater model.
Surface water
The SIMGRO-based module for surface water is modelled as a network of watercourse trajectories. Each trajectory is represented by a reservoir which interacts with other linked trajectories. Implementation of SIMGRO involves even the smallest of watercourses. Weirs and pumps can also be simulated as part of the water system.

Water management
The model has a large array of flexible options for simulating the impact of water management, like waste water treatment plants, sprinkling water, weir level control and discharge pumps. The SIMGRO-model has an elaborate set of options for modelling the water management of urban areas, as depicted by the schematic diagram given in figure 3. In dry periods, irrigation of crops is optional and depends on soil moisture and availability of surface water or groundwater. Winter and summer weir levels can be defined, but these can be adjusted according to desired surface water levels or groundwater levels.

Drainage can be simulated by using Modflow or SIMGRO functionalities. The latter entails drainage characteristics for four watercourse types: conventional water courses, field drains (both with entrance and radial resistance, optional horizontal resistance), gulleys (total resistance) and soil surface drainage (resistance < 1 d).