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Calculating Runoff With the TR-20 Method
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5 From the Antecedent Moisture Condition list, select the antecedent moisture
condition. The Antecedent Moisture Condition (AMC) list displays three types
of antecedent moisture conditions: Type I, Type II, and Type III:
AMC Type Soil moisture (dormant season) Soil moisture (growing season)
I less than 0.5 inches less than 1.4 inches
II between 0.5 and 1.1 inches between 1.4 and 2.1 inches
III greater than 1.1 inches greater than 2.1 inches
6 In the Runoff Curve Number box, type an RCN.
You can also click the Select button next to the Runoff Curve Number field to
display the Select RCN Type dialog box, then click Single RCN or Composite
RCN to select or compute the RCN for each watershed subarea.
7 In the Duration box, type the duration of the storm. The duration must equal 1
hour for standard TR-20 distribution curves.
8 In the Drainage Area box, type the drainage area of the watershed. You can also
click Select to calculate the drainage area by either drawing or selecting a closed
polyline in the drawing.
9 In the Rainfall box, type a rainfall amount based on a storm-event type for the
site location. You can also click Select and choose a value from the Define
Rainfall-Frequency dialog box.
10 In the Time Increment box, type an appropriate value.
NOTE
The TR-20 method is limited to 300 points on the hydrograph. To reduce
the number of points, use a larger time increment.
11 In the Time of Concentration box, enter a Tc value. You can also click the Select
button next to the Time of Concentration field to calculate the areas Tc.
The calculated Peak discharge for the entire watershed and the computed time
of peak discharge are located above the buttons at the bottom of the SCS TR-20
Unit Hydrograph Method dialog box.
12 You can save and output the calculation results, and view the hydrograph:
If you want to} Then click}
save the calculation results Save.
clear the calculator of its contents New.
save the calculation results to a text file Output.
view the hydrograph Hydrograph.
13 Click OK to exit the dialog box.