The Grid2MetaSWAP program is incorporated in the iMODFLOW and iMOD framework to be able to couple iMODFLOW with MetaSWAP on a grid level. In essence, the program combines table information, grid data and predefined conditions into information readable for MetaSWAP. The following paragraphs describe the structure of Grid2MetaSWAP, scaling methods used within, and translation of all available data to MetaSWAP input files.
As already mentioned briefly, the Grid2MetaSWAP program couples the MetaSWAP information to an iMODFLOW grid cell. Such a grid cell can be covered by, a selection of, three coverage types; surface water, paved area (urban and roads) and unpaved areas. Usually surface water is not explicitly taken into account by MetaSWAP but is an important parameter for iMODFLOW. In case of the urban (paved) areas some specific characteristics are linked to the grid cell, like soil intrusion, evapotranspiration and runoff to the sewage system. Grid2MetaSWAP reads the listed files and constant values from the CAP block in the iMODFLOW RUNFILE. As shown in Figure A.2, in total 18 parameters can be distinguished of which 4 are split up in a parameter for “urban” and one for “rural” (see also Table 1); ponding depth, runoff resistance, runon resistance, and infiltration capacity. An extensive description of these parameters is given in Chapter 10 of the iMOD manual (“Data set 10: Number of files”, paragraph 10.11). Besides these compulsory parameters that are used to create the needed MetaSWAP input files, additional input files can be added to the RUNFILE. After Grid2MetaSWAP created the coupling files and necessary input files, it makes a copy of the additional input files and places them in the MetaSWAP model folder . An exception on this is made for the files "para_sim.inp" and "mete_grid.inp". These files are copied and rewritten based on certain conditions; this is explained in paragraph "Parameter processing and adjustments".
Table A.1 gives an overview of the compulsory iMODFLOW-MetaSWAP parameters, the used abbreviations in the iMOD manual and examples of file names or default values to be used in the iMODFLOW RUNFILE. The purpose of a value that is given for a certain parameter may differ. A value of "999.0" in case of the artificial recharge capacity means that this variable is turned off during the computation. Similar for ponding depth, a value of 999.0 [m] means that the overland flow will only take place when groundwater level exceeds this; which in The Netherlands will never happen and the ponding depth is technically turned off. A default value for other parameters is set to -9999.0. The soil moisture factor and conductivity factor are set to 1.0 on default. For more information about the specific parameters and what type of values or files to be used, the iMOD manual can be consulted.
Table A.1: Overview of iMODFLOW-MetaSWAP parameters
|abbreviation||iMODFLOW parameter||Example file / default value|
|SFU||Soil physical unit||Bofek2012.idf|
|MET||Meteo station number||Metestat.idf|
|ART||Artificial recharge type||Beregen.idf|
|ARL||Artificial recharge layerBeregen_laag.idf or beregen_locatie.ipf,||default=1.0|
|ARC||Artificial recharge capacity||Default = 999.0|
|PDU + PDR||Ponding depthVXMU_SOPP.idf, VXMU_ROPP.idf, SOF_HYDRO.IDF or||default = 999.0|
|OFU + OFR||Runoff resistance||Default = 1.0|
|ONU + ONR||Runon resistance||Default = 1.0 or -9999 .0|
|QUI + QIR||Infiltration capacity||QUI =0.0 and QIR=0.5|
|PWT||Perched water table||-9999.0|
|SMF||Soil moisture factor||Default = 1.0|
|CFC||Conductivity factor||Default = 1.0|
As briefly mentioned before, seen from the Grid2MetaSWAp code a distinction is made between urban and rural areas. Grid2MetaSWAP processes the parameters based on this division. However, only one of each MetaSWAP table input file (with extension *.inp) is created, containing both data for the urban and for the rural area as shown by Figure A.3. Within the input file set 3 input files are written with specific coupling information; 2 related to the meteorological forcing (precipitation and evapotranspiration grids) and 1 to couple the iMODFLOW cells to the MetaSWAP units.
As already shown in Figure A.3, 11 MetaSWAP input files are created by the Grid2MetaSWAP program. From Table A.2 it can be read which MetaSWAP parameters are included in which input file. In case of the meteorological coupling files ("svat2precgrid.inp" and "svat2etrefgrid.inp") information from the wetted area and urban area parameters are used to fill these coupling files. In the meteorological coupling files, each svat number is coupled to the row and column numbers of the grids in case there is urban or rural area present in the cell. In the iMODFLOW coupling file ("mod2svat.inp") the iMODFLOW cell number is coupled to the svat number and combined with the artificial recharge layer of the specific cell.
In case of the artificial recharge (only an option for cells with rural area) that is stored in the ’scap_svat.inp’ a couple of choices are made in to code before storage. The flow diagram in Figure A.4 shows the possibility of using a grid file (IDF-file) as input or a point file (IPF-file) containing the necessary information. The IPF-file only can contain information about artificial recharge from a groundwater resource.
Each grid file given in the iMODFLOW RUNFILE is scaled towards the right extent using a standardized routine. Different scaling methods are used depending on the type of parameter as can be seen in Table A.3. There are 4 upscaling methods used and 2 downscaling methods.
Upscaling methods (blue coloured in Table A.3):
• Arithmetic mean
• Sum conductance
• Most frequent occurrence
Downscaling methods (orange coloured in Table A.3):
• Block value
• Spatial interpolation
Within the Grid2MetaSWAP program code a number of adjustments are made to be able to create MetaSWAP input files that are readable within the iMODFLOW-MetaSWAP framework. In all cases the following conditions are processed:
• Inactivate constant head boundaries and inactive nodes
• Skip the corners in relation to the anisotropy package
• Change urban areas and detailed land use types into grassland (landuse type = 1)
Land use type 8 (greenhouse horticulture)
Land use type 18 (paved area)
Land use type 19 (dark coniferous forest)
Land use type 20 (heathland vegetation)
Land use type 21 (fruit yards)
Land use type 22 (sport fields)
land use type 23 (unfertilized grassland)
land use type 24 (maize with green)
land use type 25 (potatoes early)
land use type 26 (urban grassland)
• Soil physical unit is equal to 22 or 23 (= sand), it is changed to 9 (=peat)
When looking at cells that are completely our partly covered by urban area, the following specific conditions are taken into account:
• In case there is no ponding depth (VXMU) defined in a specific cell; the total urban area of this cell is set to 0 \(m^2\).
• In case there is no runoff resistance defined in a specific cell; the total urban area of this cell is set to 0 \(m^2\).
• In case there is no runon resistance defined in a specific cell; the total urban area of this cell is set to 0 \(m^2\).
• In case there is no infiltration capacity defined in a specific cell; the total urban area of this cell is set to \(m^2\).
• Minimal urban area is 0 \(m^2\)
A similar set of conditions is in place for the cells containing rural area:
• The surface area dedicated to rural area is calculated by subtracting the total wetted area and the total urban area from the available total cell area:
• In case there is no ponding depth (VXMU) defined in a specific cell; the total area of the cell is set to surface water.
• In case there is no runoff resistance defined in a specific cell; the total area of the cell is set to surface water
• In case there is no runon resistance defined in a specific cell; the total area of the cell is set to surface water
• In case there is no infiltration capacity defined in a specific cell; the total area of the cell is set to surface water
• Minimal root zone depth is 10 cm
• Minimal rural area is 0 \(m^2\)
• Turn off artificial recharge whenever artificial recharge layer equals 0
As given in Figure A.2, the "para_sim.inp" and the "mete_grid.inp" are rewritten. Changes in the "para_sim.inp" include:
• Placing quotes around the "unsa_svat_path" path
• "simgro_opt" settings are not copied in case this option exists
• "idf_per" is always set to "1"; IDF-file output files are always written
• Grid coordinates are newly written
• Number of rows and columns in the grid files are newly written
• The "idf_nodata" value is set to "-9999.00"
Nothing is changed in the "mete_grid.inp". Related to the settings in de "para_sim.inp" and "mete_grid.inp", you need to keep in mind that the following settings are not touched:
• Start year and start day in the "para_sim.inp" are fixed values
• The path to the soil physical unit data base in the "para_sim.inp" is a fixed path and is not touched by Grid2MetaSWAP.
• The paths to the meteorological forcing grids (evapotranspiration and precipitation ascii-files) in "mete_grid.inp" are fixed paths and are not changed by Grid2MetaSWAP.
Finally, some tips for running you model with the iMODFLOW-MetaSWAP framework:
• Do not change the extent of the meteo-grids in between; all the files need to have the same extent as the files given in the first line of the "mete_grid.inp".