# Computing Excess Rainfall Hyetograph using SCS CN Method

## Introduction

The objective of this step is to learn how to compute excess rainfall hyetograph from a total rainfall hyetograph using the Soil Conservation Service's (SCS) Curve Number (CN) method. This steps assumes that the user already has a total rainfall hyetograph in MS Excel. This total rainfall hyetograph for a single event is then used as input to separate rainfall abstractions or losses by using the SCS CN method. There are two outputs from this step, including the rainfall abstractions and the excess rainfall hyetograph. This step also requires that the CN for the area is known.

## Conceptual Student Learning Outcomes

• Student demonstrates the understanding of rainfall abstractions and excess
• Student demonstrates the knowledge of SCS CN method for computing rainfall abstractions

## Practical Student Learning Outcomes

• Data analysis using MS Excel
• Use of SCS CN method in Excel for computing continuous abstractions and excess rainfall

One hour

## Data Inputs

• Numerical Value(s): Microsoft Excel file containing rainfall hyetograph for one event at Hall Creek in Indiana: 15 Min Precipitation Data (Excel 2007 (.xlsx) 8kB Apr14 16)
• Numerical Value: Curve Number [Double]

## Data Outputs

• Numerical Values: Rainfall abstractions
• Numerical Values: Excess rainfall hyetograph

• MS Excel

## Instructions

### SCS CN Method

According to the SCS method, the total rainfall (P) is divided into three parts: initial abstraction (Ia), continuous abstraction (Fa) and excess rainfall (Pe). Initial abstraction is the amount of rainfall that is used for satisfying surface depressions, and never leaves the watershed. It is calculated as 20% of the total maximum storage or retention (S) of the watershed. Therefore, Ia = 0.2S. After this initial abstraction is satisfied, the remaining amount of rainfall is used to satisfy remaining soil storage as continuous abstraction (Fa). The expression to compute Fa by using the total amount of rainfall (P)and the maximum retention (S) is given by the following equation:

where, P, Fa, S and Ia have the same unit of length [L]. Usually total rainfall (P) is known, and S is computed by using the CN (assumed to be given for this exercise) as shown in the equation below:

Where S is in inches. If the rainfall data is given in some other units, S needs to be converted to those units before using in the SCS equation.

Essentially, the continuous abstraction term Fa gives the amount of rainfall that is lost as infiltration from the total rainfall. A more detailed description and explanation for the SCS CN methodology can be found in documents listed in the Reference Document and Files section.

### Application of SCS CN Method

Open the Excel file containing the rainfall data. Because the SCS CN method is based on computing excess rainfall from total rainfall, the given rainfall hyetograph is first converted into a cumulative rainfall hyetograph or rainfall mass curve so it will represent the total amount of rainfall after each time step since the beginning of the storm. Create a column to store the cumulative rainfall values as shown below:

Next, find S by using the equation above and CN = 80 to get S = 2.5 inch. A value of S = 2.5 inch means the initial abstraction Ia = 0.5 inch ( Ia = 0.2S). There will be no continuous abstraction or excess rainfall until the total cumulative rainfall becomes greater than Ia. If the total rainfall at a given time step is less than Ia, it means that all rainfall will be lost as initial abstraction. In this case, Ia = total rainfall. Once the total rainfall becomes greater than Ia, then Fa will be calculated for each time step by using the equation above. Once Ia and Fa are calculated, the cumulative excess rainfall (Pe) will then be equal to P - Ia - Fa. The table below shows the set-up in Excel to do these calculations.

Finally, the excess rainfall hyetograph can be obtained by subtracting the cumulative excess rainfall at each time step from the preceding time step as shown below:

Typically, an excess rainfall hyetograph is graphically represented by vertical columns. One can generate such a representation in MS Excel by selecting the 'Excess Rainfall Hyetograph (in)' column, and apply the 'Insert' --> 'Charts/Column/2D' option. Right click on the chart area, and go to 'Select Data', and edit the 'horizontal axis labels' by selecting the range of data in 'Time (h)' column.

At the end of this step, you should have an excess rainfall hyetograph and rainfall loss for each time step.