The Function of Two Variables option is available when creating a function that defines a zone, stage, or elevation. Use this option if the lookup table or relationship you need to define varies as a function of two independent variables. For example, stage at a location may be impacted by both the flow and the backwater of a downstream pool. In such a case, you may define the stage as a function of both flow and elevation. This will allow you to build a two-variable data table to represent your function, as shown in the example in "Figure: Rating Curve as a Function of Two Variables". The independent variables are expressed as a Row Variable and a Column Variable. The Row Variable will be displayed in the left-most column of the data table and vary by row. The column variable varies per column of the data table.

Figure: Rating Curve as a Function of Two Variables

When the Function of Two Variables option is selected, the edit panel of the Independent Variable Definition dialog will display two panes: one for defining the Row Variable and one for defining the Column Variable ("Figure: Independent Variable Definition Dialog - Model Variable"). Each of these two panes allow you to select either a Model, External or State Variable, and are otherwise similar to the other Model Variables panes.

Figure: Independent Variable Definition Dialog - Model Variable

Each of the two (row variable and column variable) Two Variables panes includes:

  • A Variable Type selector—a dropdown list for selecting Model Variable, External Variable, or State Variable as the independent variable.
  • An Interpolation Type selector—a dropdown list for selecting the type of interpolation to use between rows or columns of the independent variable. Interpolation types available are Linear, Cubic, and Step.

Once a variable type has been selected, the rest of the pane populates with the options inherent to either Model Variable, External Variable, or State Variable (as described in the previous sections).