Function of: Two Variables

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.

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.

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).