Runoff - Calculation Methods


Four techniques are commonly used to estimate runoff - Rational Method, Graphical Peak Discharge Method, Tabular Method (TR-55), and Unit Hydrograph Method.  Descriptions of the first three methods can be found in the math topic pages.  The Unit Hydrograph Method is described in the SCS National Engineering Handbook, Section 4, Hydrology.

The primary factors used to decide on a runoff calculation method are the size of the drainage area and the output information required.  The table below lists acceptable calculation methods for different drainage areas and output requirements.  The plan approving authority may require or accept other calculation methods deemed more appropriate for local conditions.


Output Requirements
Drainage Area
Appropriate Calculation 
Methods
Peak Discharge only
up to 200 acres
up to 2000 acres
up to 20 sq. mi.
1,2,3,4
2,3,4
3,4
Peak Discharge and 
Total Runoff Volume
up to 2000 acres
up to 20 sq. mi.
2,3,4
3,4
Runoff Hydrograph
up to 20 sq. mi.
3,4

1.  Rational Method
2.  Peak Discharge Method
3.  Tabular Method (TR-55)
4.  Unit Hydrograph Method