WMS:Regression Tc Arc Computations

To produce accurate runoff results, MODRAT must have data to allow routing of flows through basins and reaches. One of the most important parameters needed for these operations is the time of concentration. Differences in the time of concentration can have marked effects on hydrograph peaks and shapes (temporal runoff distribution), especially when hydrographs from various basins are combined.

As a general rule, the time of concentration calculations should be done after all other parameters have been assigned to each sub-basin of your MODRAT model. The reason for this is that the methods used to compute Tc use rainfall depth or zone, soil type, and percent impervious as variables. If these parameters have been assigned to each sub-basin, WMS will automatically plug those numbers into the equation to compute Tc for the sub-basin.

Once the LACDPW TC equation has been assigned to an arc and all variables defined, WMS will compute the Tc for the arc and display it in the Time Computation Arc Attributes dialog. The computation of the time of concentration requires data from the soil file (lasoilx.dat) to be present. If this file is not present in that folder, you will be prompted to designate the location of the soil file.

To assure the Tc is assigned to the MODRAT model:

1. Select a sub-basin and bring up the Edit MODRAT Attributes dialog.
2. Click the Compute Regression Tc button.
3. Ensure that a Tc has been computed in the Travel Time Computation dialog; edit the Tc arc data if necessary.
4. Click OK and the Tc will be assigned to the sub-basin.
5. Repeat for each sub-basin.

Related Topics

• Automated Tc Calculations
• Compute Regression Tc
• Flow Path Arcs
• Assigning the Regression Equation