# Baseflow Separation Using Recession Method

## Introduction

The objective of this step is to learn how to separate baseflow from a total streamflow hydrograph by using a simple recession method. This steps assumes that the user already has a total streamflow hydrograph in MS Excel. This total streamflow hydrograph for a single event is then used as input to separate baseflow by using the recession method.There are two outputs from this step, including the baseflow series and the direct runoff hydrograph.

## Conceptual Student Learning Outcomes

- Student demonstrates the understanding of baseflow within the context of streamflow hydrograph

## Practical Student Learning Outcomes

- Data analysis using MS Excel
- Use of exponential equation in MS Excel

## Student Time

- One hour

## Reference Documents and Files

- None

## Data Inputs

- Numerical Value(s): Microsoft Excel file containing streamflow hydrograph for one event at Hall Creek in Indiana: Hall Creek Streamflow Data (Excel 2007 (.xlsx) 23kB Nov10 14)

## Data Outputs

- Numerical Values: Direct Runoff Hydrograph
- Numerical Values: Baseflow time series

## Required Hardware and Software

- MS Excel

## Related Steps

- Downloading USGS Streamflow Data from Internet into a Text File
- Creating a time series plot in Excel
- Derivation of Unit Hydrograph

### What is baseflow?

Baseflow is a portion of streamflow that is not directly generated from the excess rainfall during a storm event. In other words, this is the flow that would exist in the stream without the contribution of direct runoff from the rainfall. Estimation of baseflow and direct runoff is useful to understand the hydrology of a watershed, including interaction of surface and sub-surface water, role of urbanization on runoff generation and the health of aquatic habitat within a stream. The method presented here is applicable for a single peak hydrograph resulting from a single storm event.

Baseflow can be separated from a total streamflow hydrograph by using a recession model based on the following relationship:

Where Qt is the baseflow at any time t, Q0 is the initial baseflow (at the beginning of the storm event, t = 0) and k = exponential decay constant. The exponential constant can be defined in many ways. A reasonable definition (as used in the HEC-HMS model) is that k is the ratio of the baseflow at time t0 to the baseflow one day earlier. The unit for t in the above equation is days.

### Baseflow Separation Procedure using Recession Method

Open the data provided to you in Microsoft Excel. The data that is provided to you has streamflow values in cfs at 15 minutes interval from 09/24/2000 to 09/30/2000. The data is organized as shown below:

The first step in separating baseflow is to find out when the direct runoff started on the total streamflow hydrograph. This is usually the point, where you will see the value on the streamflow hydrograph rising at a significantly faster rate compared to previous values. This point can be found by visually inspecting the data or by plotting the hydrograph as shown below.

You will see that the direct runoff begins sometime between 7:00 – 9:00 AM on 09/25. Lets use 8:00 AM on 09/25 as the start time of the beginning of the direct runoff. All values until 8:00 AM on 09/25 are considered as baseflow. Copy streamflow values from the second column to the third column (name it as Baseflow) until 8:00 AM on 09/25 as shown below.

After 8:00 AM on 09/25, the baseflow will be computed by using the exponential equation above.The flow value (1.9 cfs) at the time when direct runoff begins becomes Q0 in the recession equation given above. The value for k is computed by taking the ratio of 1.9 cfs and the value one day earlier (1.5 cfs at 8:00 AM on 09/24). Therefore the value of K is 1.9/1.5 = 1.27. Because t has a unit of day, the value of t to compute Q at 8:15 AM on 09/25 is 15/(60*24). Similarly, the t for the next time step is (15 + 15)/(60*24) and so on as shown below. (**Note**: Typically a value of less than 1 is used for k to represent recession in baselow. A value greater than 1 in this example shows that the subsurface flow or baseflow is increasing as the hydrograph is progressing.)

Using the recession equation for Qt, find the baseflow after 8:00 AM 09/25 until the baseflow is less than or equal to the values on the streamflow hydrograph. If it is always less than the values on the streamflow hydrograph, then continue to compute the baseflow until the end of the hydrograph. In this example, the baseflow becomes equal to the streamflow at 05:00 on 09/30 as shown below.

After this time step, the baseflow is equal to the values on the streamflow hydrograph so copy the values from the streamflow column to the baseflow column after 05:00 on 09/30 until the end of the hydrograph as shown below.

What you get in the third column is the baseflow. If you separate the baseflow from the total streamflow (streamflow minus baseflow), you will get direct runoff for each time step.

At the end of this step, you should have a baseflow time series and direct runoff hydrograph.