# iMOD User Manual version 4.4 (html)

#### 7.18Waterbalance

WHY?
The Waterbalance Tool can be used to calculate the sum of budget quantities for different components for specified regions within the model extent and/or the entire model extent. Several budget items can be selected, as well as particular mode layers and/or model output periods, see Section 7.18.1. The output can be analysed in the iMOD Water balance Analyser, see Section 7.18.2.

WHAT?
The Waterbalance Tool uses specific results from iMODFLOW that start with BDG*.IDF and/or MetaSwap output file starting with MSW*.IDF. Those files are stored in folders with identical names, such as BGGFLF\BDGFLF*.IDF. All these files store the flow quantity in m${}^{3}$/day (cubic meters a day). As a rule of thumb the flow quantity is negative whenever it leaves the cell and positive whenever it enters the cell. Flow that leaves the cell from the Right Face (BDGFRF), the Front Face (BDGFFF) and/or the Lower Face (BDGFLF) is negative. However, for flow terms that enter the cell from the Left Face, Back Face and/or Top Face of a model cell, the appropriate flow terms from the adjacent cells need to be multiplied with minus 1, to be consequent. For Internal Boundary Conditions (e.g. rivers, drainage systems, recharge, wells) a similar approach is valid: water that enters the cell is positive, withdrawal of water is negative. Water that flow from a specified water balance region to another region is noted netagive as it leave the corresponding water balance region.

Note: The waterbalance assumes that the dimensions for data that is included are in m$^3/$day. Bear in mind that fluxes that are produced by MetaSWAP are in m$^{3}$/m$^{2}$ and represent cumulative values as specified in the TIME_SIM.INP. Those may NOT overlap with the output file from iMODFLOW.

Example of budget terms inside and outside a computational cell for (left) cell-by-cell flow terms and (right) internal boundary conditions:

The Waterbalance Tool will just add all flow terms within (defined) areas and list them within a summary text files (*.TXT and *.CSV) to be opened with e.g. NotePad, TextPad and/or Excell. Alternatively, it is possible to read in a generated *.CSV file into the waterbalance analyser.

HOW?
To compute a water balance, select the option Toolbox from the main menu, choose Water Balance and then choose Compute Waterbalance to open the Waterbalance window. To analyse the results of a water balance computation, select the option Toolbox from the main menu, choose Water Balance and then choose Analyse Waterbalance to open the Analyse Waterbalance window

##### 7.18.1Compute Waterbalance

Start the Compute Water Balance window by selecting the option Toolbox from the main menu, choose Water Balance and then choose Compute Waterbalance.

Compute Waterbalance window, Result Folder tab; left for results from the Models folders and right from a different folder:

Compute Waterbalance window, Apply To tab for (left) applied to selected shapes only or (right) for all non-NoDataValues within a selected IDF-file:

Note: After a computation for a water balance, iMOD will save an IDF-file at the location where you’ve saved the output file via Create CSV; Create TXT; Create IPF. This IDF files contains the location of the raster cells for each zone. Inspection of this file can lead to adjustment of the polygons used, and/or reuse this particular IDF file for different model scenarios. Remember that in case of overlap in polygons, the value of the zone in the IDF-file becomes negative, so zone 1 might appear as -1. This doesn’t affect the computation of the water balance volumes but has a purpose for finding overlap only.

Example of “table” layout of the water balance:

Example of a “time serie” layout of the water balance:

##### 7.18.2Analyse Waterbalance

To analyse the results of a water balance computation, select the option Toolbox from the main menu, choose Water Balance and then choose Analyse Waterbalance to open the Analyse Waterbalance window. The key thought is that a CSV file is read in first and after that several selection can be made by the different tab on the window. Finally an output can be defined, this can be a schematic representation of the water balance, a time series, a table and/or spatial patterns of water balance term saved in IDF files.

Analyse Waterbalance window, CSV-file tab:

Analyse Waterbalance window, Budget Terms tab:

Analyse Waterbalance window, Graphics Output tab: