MODELING TASK DESCRIPTION
FEFLOW is a groundwater modeling software with groundwater recharge as an input parameter. MIKE SHE, on the other hand, is a software package suitable to calculate spatially and temporally distributed recharge values based on precipitation, evapotranspiration and diverse soil parameters. The coupling of both packages integrates groundwater recharge calculations from MIKE SHE and groundwater flow simulations in FEFLOW.
MIKE SHE stores data in a dfs2 format, which is a proprietary format of the MIKE Software package. The information stored in the dfs2 files, however, can be linked directly to any FEFLOW parameter. The following workflow describes the necessary steps to properly import DFS2 data and to assign recharge distributions in FEFLOW.
Preliminary Note: For the following workflow a complete installation of both software packages, FEFLOW and MIKE SHE on your machine is required !
SOLUTION
Step #01: Settings in MIKE SHE/MIKE Zero
Before starting to integrate the dataset in FEFLOW, the user has to perform some preprocessing steps in MIKE SHE/MIKE Zero. Open the dfs2 file in the Grid Editor of MIKE Zero. Open the Properties window and select Local Coordinates for the Map Projection Type. In the following dialogue choose the third option: Keep 'Geographical Coordinates' and derive 'Map Projection Coordinates' as synchronization method - Fig. 1. Once completed, make sure the item is of type 'Precipitation Rate' and set it in [mm/day] as the default unit - Fig. 2. Save the dfs2 file.
Fig. 1 - Setting Map Projection and Coordinate Synchronization in MIKE Zero.
Fig. 2 - Setting Items Type and Units in MIKE Zero.
Step #02: Time-step definition
Choose a reasonable output time-step length depending on the total simulation time. Time-step lengths are by definition constant and cannot increase or decrease.
FEFLOW interpolates recharge rates linearly between two time-steps. MIKE SHE, in contrast, employs the accumulated reverse mean time-step approach, averaging the values from tn-0.5 > tn > tn+0.5 . From that follows that all time-steps need to be shifted by Δt/2 before importing them into FEFLOW. Two approaches are available:
a) in MIKE Zero open the Editor and shift the time-step Δt/2
b) in FEFLOW add Δt/2 to the reference time in the Import Maps Editor window
A second point to consider are the different temporal datums. MIKE SHE uses the calendrical date whereas FEFLOW a numerical format. To align the reference dates the user has to convert the calendrical start date of MIKE SHE into numerical number. The easiest way to do this is to enter the date into an excel sheet and convert the cell to a number format (0 represents the date 30.12.1899 at 23:00 with each increment reaching the following day at 0:00 hours, 1 equals the 01.01.1900 and so forth).
Step #03: Import dfs2 into FEFLOW
Before importing the dfs2 file into FEFLOW, the recharge parameter needs to be set to steady-state. Open the context menu (right mouse-click) of the recharge parameter ('Source/Sink' or 'Inflow top/bottom'), browse to 'Edit Time Dependency' and select Make Stationary. Please be reminded to divide the values through the layer thickness, if applying the 'Inflow top/bottom' instead of 'Source/Sink'.
Now, you can import the dfs2 file as a map. Go to the Maps Panel and 'Add map'. In the import window set the reference time accordingly and add the time-step shift backward or forward if not already done in Step #02 a) to accommodate the different averaging techniques.
Select Cell data as the spatial data representation in order to activate the area-weighted projection. This regionalization method was developed to support recharge data import for example from MIKE SHE. The area-weighted projection is required to adjust for water balance errors caused by the conversion from a regular point grid used by MIKE SHE to a triangulated mesh in FEFLOW. As such, it is the method of choice for any element-based parameters.
Note: If FEFLOW yields an error of a missing coordinate field, simply right-click on dfs2 and set any coordinate field.
Fig. 3 - Importing a DFS2 file and setting reference date and spatial data representation.
Step #04: Parameter assignment in FEFLOW
Right-click the map and open the 'Link to Parameter' dialogue. On the left column select all time-steps holding the <Shift< key. On the parameter side, open the context menu of 'Sink/source' and select Assign material data to time stages. Navigate to the time-step 0 and double-click to create the parameter link. Don't forget to choose the correct unit which is [mm/day]. If not already available you have to add this user-defined unit in the Units window which can be found in the menu /Tools/Units. Use for example the factor 1/1000 to convert from the default unit m/d.
Once the link is created, select all elements and activate the parameter assignment.
Fig. 4 - Creating parameter link in FEFLOW connecting time-step data from the dfs2 file and the corresponding recharge parameter.
Importing and assigning time-variant recharge data can be time-consuming as dfs2 can be large depending on the spatial and temporal resolution. Wait until the interpolation is finished and assess the results as shown below in Fig 5.
Fig. 5 - Import results of recharge data from MIKE SHE into FEFLOW.
CONCLUSION
With this workflow you will be able to transfer the recharge parameter (what is a result from a MIKE SHE model) into a FEFLOW model to further enhance the output quality of your groundwater model.
FURTHER INFORMATION & USEFUL LINKS
Manuals and Guidelines
FEFLOW 10.0 Documentation - Working with Maps