The finite difference formulation of the groundwater flow equation is based on conservation of mass, such that the mass of water entering the system through various sources should equal the mass of water leaving the system through the various sinks. The Mass Balance Graphs provide graphical representations of the mass of water (represented as volume, for MODFLOW models on the assumption of the constant density of water in the simulation domains) entering and leaving the system through the various sources and sinks.
The Flow Mass Balance Graphs plot the volume and rates of water entering and leaving the system through the flow boundary conditions, and from aquifer storage at the end of each stress period.
To plot the Flow Mass Balance Graphs, select the View Charts workflow step, click the Mass Balance Button on the upper left corner of the Chart view, and a Mass Balance: MODFLOW window will appear as shown below:
There are four different Flow Mass Balance Graphs:
•Percent Discrepancy plots the temporal changes in the Flow Mass Balance (total flow IN minus total flow OUT) expressed as a percentage of the total flow.
•IN - OUT plots the temporal change in the Flow Mass Balance (total flow IN minus the total flow OUT).
•Time Series plots the temporal flows IN and OUT of the system through the individual sources and sinks (flow boundary conditions and storage).
•Time Step plots a bar chart of the flow IN to the system and the flow OUT of the system through the individual sources and sinks (flow boundary conditions and storage) for selected Output Times.
Note: With both the Time Step and the Time Series graphs, it is important to remember that these graphs present the cumulative volume at a point in time, and not the volumetric loading rate at a point in time.
As mentioned above, the Flow Mass Balance Graphs can be used to plot the flow volume or flow rates of water coming IN and going OUT of the system. By default, the Mass Balance: MODFLOW window plots the flow volume mass balance.
Use the View option in the top menu bar to select either Rates or Volume options, or click the Cumulative volume () or Rate () icon in the toolbar.
On the left-hand side of the Mass Balance: MODFLOW window is a list of all potential sources and sinks for the current flow model (boundary conditions not included in the model will not be listed). These sources and sinks are included in both the IN list and the OUT list. The IN list contains a summary of the flow coming into the system from each individual source term, while the OUT list contains a summary of the flow going out of the system through each individual sink term.
These controls can be used to add or remove the individual source and sink terms from either the Time Series graph or the Time step graph.
The Percent Discrepancy graph and the IN-OUT graph consider only the total flow into and out-of the entire system as a whole. As a result, the settings for the individual IN and OUT terms do not influence the results and appearance of these graphs.
Each of the Flow Mass Balance Graphs can be maximized to occupy the entire Mass Balance window by clicking the maximize button in the top right-hand corner of each graph window.
Alternately, the graphs can be tiled in the Mass Balance window by selecting Window/Tile from the top menu bar.