# WMS:Customizing Equations

Any of the equations can be edited (i.e. pick an equation and then modify it slightly to a particular need) or created using the **Modify Equation** or **User Defined** buttons. Both of these buttons bring up the *Modify Equation* dialog shown below.

If entering this dialog using the **Modify Equation** button, the currently selected equation will appear in the *Equation* edit box. If the current equation has more than one equation (some equations have separate equations for lag time/time of concentration and peaking or storage coefficients) then WMS will ask to choose which equation to modify. If using the **User Defined** button to enter this dialog then the *Equation edit* box will be blank. Type a new equation or modify an existing equation from the keyboard or by using the buttons for add, subtract, multiply, divide, etc. Also enter variable names corresponding to the basin data computed by WMS, or enter variables that to define. The variable definition is displayed and edited in the Definition edit box and units associated with the variable can be set using the Units drop-down combo box.

When finished typing the equation, select the **Parse** button. Doing so will result in any variables being identified and displayed in the Variables text window along with their definitions. The rules of precedence are typical for mathematical computations (order of operations) and are as follows:

- Parenthetical statements
- Functions like ln, log, etc.
- The power function (^)
- Multiply and divide
- Add and subtract
- Left to right

## Related Topics

- Travel Times from Basin Data
- Assigning an Equation
- Editing Basin Variables
- Variables Computed by WMS

WMS – Watershed Modeling System | ||
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