# Baseflow Separation Using Straight Line Method

## Introduction

This step demonstrates the use of straight line method to separate baseflow from a streamflow hydrograph by using data for a single storm event. This step is developed for a storm event at Hall Creek in Indiana.

## Conceptual Outcomes

• Students demonstrate the understanding of baseflow and direct runoff
• Students can apply straight line method to separate baseflow for a single storm event

## Practical Outcomes

Students can perform simple calculations and data analysis using MS Excel

30 Minutes

## Computing/Data Inputs

Numerical Values: 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

• MS Excel

## Instructions

### 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.

Separating baseflow using the straight line method is suitable only for individual storm events. For continuous hydrographs, other techniques must be used. To separate baseflow using the straight line method, the times at which the direct runoff begins and ends must be determined by looking at the streamflow hydrograph. Open the input streamflow hydrograph in Excel and plot the streamflow values on y-axis as shown in the figure below. By looking at the streamflow values, the beginning of runoff is relatively easy to identify because the hydrograph rises steeply on 09/25/2000. Lets assume that the direct runoff begins at 8:15 AM on 09/25/2000 (streamflow = 1.9 cfs). Next, we need to identify when the direct runoff ends. Generally, this point is located somewhere on the falling limb of the streamflow hydrograph. For a flashy hydrograph, it is reasonable to assume that it point is located towards the end of the of falling limb. In this case, assume that the runoff ends on 8:00 AM on 09/28/2000 (streamflow = 12 cfs). As you can imaging this is an inexact science because it is difficult to exactly distinguish between baseflow and direct runoff purely by using a streamflow hydrograph.

Once the beginning and ending of runoff are determined, create a third column in Excel for baseflow and populate it with values equal to streamflow until 8:15 AM on 09/25/2000. This means that all the discharge until this time is baseflow. Similarly, the baseflow is equal to streamflow after 8:00 AM 09/28/2000 until the end of the hydrograph. The values between 1.9 cfs (at 8:15 AM on 09/25/2000) and 12 cfs (at 8:00 AM on 09/28/2000) are then linearly interpolated to get the complete baseflow hydrrograph. Subtracting the baseflow column from the streamflow values will give you the direct runoff hydrograph as shown below.

At the end of this step, you should have a baseflow time series and direct runoff hydrograph. The following figure shows the baseflow (red area) and the direct runoff (blue line) hydrographs on a single plot.

## Additional Activities and Variants

Baseflow Separation Using Recession Method