Base Flow Separation

Base Flow Separation

This interface facilitates separation of base flow from a stream hydrograph. Separation consists of identifying the points at which the direct runoff starts and ends, and removing this component from the total flow to determine base flow. A number of graphical techniques are available for defining the base flow between end and start points, but these techniques are difficult to implement when separations are to be done for a long continuous record of stream flow (rather than just a single or few storm period hydrographs). Numerical algorithms have been introduced as a means for base flow separation in long hydrographs and are utilized in this IGW-NET interface (see details below in 2. Select Method).

  1. Add Data
  2. Select Method
  3. Parameter Calibration & Validation
  4. Plot Options
  5. Save Results

1. Add Data

Load streamflow

Users can upload a streamflow data file for analysis, or a downloaded baseflow file (see 'Save Result' below).

The format for a streamflow data file is as follows:
[Streamflow data title], [data of analysis]
[start year],[start month],[start day],[start hour]
[multiplier to m3/day]
[days from start],[flow rate]
[days from start],[flow rate]
...

For example:
Streamflow data, 09/13/2021
2003,1,1,0
0.0283168
1,1380
2,1530
...

In this example, the flow rate have units of cubic feet per day, so a multiplier of 0.0283168 is used to convert to cubic meter per day.

USGS Sensor Data

USGS stream flow data extracted with the USGS Sensor Data tool can be imported into this interface for base flow separation. To access the USGS Sensor Data tool, navigate to: Analysis Tools > Data Explorer > USGS Sensor Data

Click 'USGS Sensor Data' button to load extracted stream flow data. This requires first drawing a model domain (Conceptual Model Tools > DrawDomain > ...). The model domain can be smaller than the area used for USGS data extration (in this case, the imported data will be a subset of the original data file).

Once the USGS stream data are successfully loaded, orange markers will appear in the map where data stations are available. Click on a marker to view the station Properties (name, number of points, start date, and end date).

By default, 'Field Data' served by USGS are used for plotting and base flow separation. Click the Daily Data button to check if daily data are available for analysis. If data are available, the user can apply a temporal filter in the prompt that appears (or they can use all daily data that are available).

2. Select Method

One Parameter Digital Filter

Lyne and Hollick (1979) proposed a one parameter digital filter for separating "high frequency", quick flow hydrograph dynamics (or runoff) from the total stream flow . Then, the "low frequency" or base flow dynamics are computed as the total stream flow minus the quick flow / runoff. The high-pass filter is expressed as:

where q_f(i) is the quick flow (high-frequency) response at the i^th sampling interval, q_(i) and q_(i-1) are the original streamflow at the i^th and (i-1) sampling interval; and α is the filter parameter that enables the shape of the separation to be altered. Base flow at the i^th sampling interval, q_(i), can then be computed as:

In general, the filter is run multiple times through a hydrograph dataset, e.g., for daily data threes passes are commonly used: forward, backward, and forward once more. Check the box next to 'Three passes' to apply a the filter in this way (forward, backward, forward). Typical values of α are 0.90 to 0.99 (see e.g., Indarto et al., 2017).

Recursive Digital Filter

Eckhardt (2004) introduced a recursive digital filter method that utilizes the concept of baseflow index (ratio of base flow to the total stream flow). The algorithm for base flow separation is then:

where Q_b(i) is the base flow at the current time interval, Q_b(i-1) is the base flow at the previous time interval, Q_(i) is the total stream flow at the current time step, α is the filter parameter, and BFI_max is the maximum base flow index.

Typical values of α are 0.90 to 0.99 (see e.g., Indarto et al., 2017). Typical BFImax values for classes of catchments are suggested as follows:
- 0.80 for perennial streams with porous aquifers
- 0.50 for ephemeral streams with porous aquifers
- 0.25 for perennial streams with hard rock aquifers.

Modified One Parameter Digital Filter (CM)

Chapman (1990; 1991) showed that the 1-parameter digital filter introduced by Lyne and Hollick (1979) implied base flow would be constant when there was no direct runoff, and proposed a reformulation. Chapman and Maxwell (1996) presented a simplified form where base flow is computed as a weighted average of the direct runoff and the base flow at the previous time interval, i.e.,:

where Q_b(i) is the base flow at the current time interval, Q_b(i-1) is the base flow at the previous time interval, Q_d(i) is the direct runoff at the curret time interval, and k is the algorithm parameter or recession constant during periods of no direct runoff. As total stream flow Q is the sum of base flow and direct runoff, the modified 1-parameter recession algorithm can be expressed as:

Typical k values are 0.62 to 0.99 (see e.g., Indarto et al., 2017).

Boughton Digital Filter

Boughton (1993) introduced a new parameter, C, in place of (1-k) to provide more flexibility when implementing the digital filter algorithm. The separation algorithm then becomes:

Typical k values are 0.72 to 0.99 (see e.g., Indarto et al., 2017). Typical C values are 0.01 to 0.36 (see e.g., Indarto et al., 2017).

3. Parameter Calibration and Validation

A range of typical values is suggested above for each of the available methods. The user may use trial and error to determine parameter values that result in a calculated base flow that closely fits to the observed discharge for each dry period.

For long (muti-year) datasets, consider using an earlier time period (e.g., one year) for parameter calibration, and a later time period (a different year) for comparing the calculated base flow based on calibrated parameter(s) to the dry period discharge (parameter validation).

4. Plot Options

Change plot type

By default, all of the imported data are used for plotting and base flow separation ('All' option).

After calculating base flow the first time, the user may select from the different plot type options.

Select the 'Monthly' option to compute and display monthly averages from the imported dataset (stream flow, base flow, direct runoff, and baseflow index, or the ratio of baseflow to total stream flow). Note that flow is indicated by the left y-axis, and baseflow index is indicated by the right y-axis.

Select the 'Annual' option to compute and display annual averages from the imported dataset.

Select the 'Baseflow Index' option to compute and display baseflow index as a function of time.

Select the 'Mean' option to compute and display the long-term mean from the imported dataset.

Flow Unit

Use the drop-down menu next to 'Unit' to change the flow (y-axis) units for the plots.

Embedded plot options

Inside the plot display, there are 'hover buttons' that allow the user to zoom in, pan, reset axes, etc. The user may click on a data series in the plot legend to toggle its display on or off.

5. Save Results

Click the 'Save result' button to download a .txt file with information of stream flow, direct runoff, base flow, and baseflow index for each sampling time/interval.

The downloaded file can be directly uploaded back into the Base Flow Separation interface with the 'Load streamflow' button.