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Table Of Contents
Calculating Runoff With the TR-20 Method
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6 Click Yes or No:
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Click Yes to replace ALL of the data in the time column of the Dimensionless
Hydrograph Editor dialog box with the values you just entered in the Range
Settings dialog box.
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Click No to leave the existing values in the time column and append the
new time values to the end of the time column.
7 Click OK to exit the dialog box.
TR-20 Distribution Curve Conditions Tables
The rainfall patterns in the United States can be described by four different
types of dimensionless rainfall distribution curves. These distribution curves are
referred to as I, IA, II, and III, and are described on page B-1 of the TR-55
manual. A distribution curve describes what fraction of the total 24-hour
rainfall has occurred at any given time.
The TR-20 Method uses TR-20 dimensionless curve tables that are based on
rainfall distribution curves I, IA, II, and III. Data is included for both 24-hour
and 48-hour events. You can select a table from the Distribution list in the SCS
TR-20 Unit Hydrograph Method dialog box, and you can also click the Edit
button to add values or ranges or to view the distribution curve.
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TR-20t1: The standard SCS 24-hour, type I distribution, cumulative rainfall
table. The values in this table are in 30 minute intervals.
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TR-20t2: The standard SCS 24-hour, type II distribution, cumulative
rainfall table.
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TR-20t3: The standard SCS 24-hour, type IA distribution, cumulative
rainfall table.
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TR-20t4: The standard SCS 48-hour, type I distribution, cumulative
rainfall table.
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TR-20t5: The standard SCS 48-hour, type II distribution, cumulative
rainfall table.
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TR-20t6: The standard SCS dimensionless distribution, emergency spillway
for freeboard hydrographs, cumulative rainfall table.
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TR-20t7: 24-hour, type III distribution, cumulative rainfall table.
NOTE
Type I, IA, II, and III 24-hour rainfall distributions are discussed in Appendix
B of the TR-55 manual. Type IA is the least intense and type II is the most
intense short duration rainfall. Types I and IA represent the Pacific maritime
climate with wet winters and dry summers. Type III represents Gulf of
Mexico and Atlantic coastal areas where tropical storms bring large 24-
hour rainfall amounts. Type II represents the rest of the country. Figure B-1,
SCS 24-hour rainfall distributions in Appendix B of the TR-55 manual
shows the different distribution curves for each of the four types. You can
also view the distribution curves for the types when you select a table from
the Distribution list in the SCS TR-20 Unit Hydrograph Method dialog box,
click the Edit button, and then click the View button.