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A Computer Program for Determining an Index to Base Flow

By Tony L. Wahl
Hydraulic Engineer, Bureau of Reclamation, Lakewood, CO
and Kenneth L. Wahl
Hydrologist, U.S. Geological Survey, Lakewood, CO

Update (August 9, 2013): As a result of the move to Windows 7, the 16-bit Windows version is no longer available. Only the DOS version is provided, and you will need a DOS emulator, such as DOSBox to run it. The Excel_BFI_builder.zip file is useful for setting up data input files. USGS is working to incorporate BFI and other base-flow separation methods into their Groundwater Toolbox, slated for release in late FY13 or early FY14. When that is available I will add a link to this site.

Important Note (7-24-2006): The NWIS web data format has recently changed, which prevents the BFI program from operating with data files downloaded directly from the web. At this time, new data files must be reformatted and the old web data file header must be spliced back onto the file. A suggested procedure for doing so is as follows:

An example of an old-format header is given in the comal.web data file on the download page. Also, the Excel_BFI_builder.zip file contains useful hints.

One other important consideration is the fact the new-format files no longer leave out dates with missing data. Instead, they show up as regular lines with a date, etc., but just a blank where the flow should be. BFI interprets those entries as days of zero flow, not missing data. BFI is written to not process years with missing data, but it will process years with zero-flow days; now it can't tell the difference, so your results will be affected. The NWIS web format may change again in the future to address this problem (since it affects other software as well), but for now, you must be careful when using data files that might span periods of incomplete data.

The latest release of the BFI program is version 4.15, made available from this page on 04/13/2007. This version allows the DOS version of the program to operate multiple times on the same input file, using different base flow separation options. This version can be automated using DOS redirected input. The Windows version was not affected by this change.

Please note that it has been found that the Windows version does not reliably read data in ADAPS card (.crd) format.

To join the BFI Notification List, please send a blank e-mail to join-bfi@listserver.usbr.gov


Hydrologists are often asked to separate runoff into the components that result either from direct runoff or from ground-water discharge. However, doing so is necessarily an inexact science. The processes that produce flow are not simple, and many factors influence the source, timing, and magnitude of stream discharges.

The BFI program was developed to make the base-flow separation process less tedious and more objective. The program implements a deterministic procedure proposed in 1980 by the British Institute of Hydrology. The method combines a local minimums approach with a recession slope test. The program estimates the annual base-flow volume of unregulated rivers and streams and computes an annual base-flow index (BFI, the ratio of base flow to total flow volume for a given year) for multiple years of data at one or more gage sites. Although the method may not yield the true base flow as might be determined by a more sophisticated analysis, the index has been found to be consistent and indicative of base flow, and thus may be useful for analysis of long term base-flow trends. Users should be very cautious about using methods such as this for short-term storm events or for locations where streamflow is affected by upstream regulation, such as reservoir releases. In general, the method interprets most regulated releases as base flow. If the program is used for regulated streams, the effects of regulation must be carefully accounted for through manual adjustment of the program output.

The program accepts input from WATSTORE data files, or files generated from EarthInfo CD-ROM's. The self-extracting files available from this site contain source code and compiled executables for MS-DOS PC's operating in a DOS or Windows environment; the source code can also be compiled as-is using the f77 compiler on most Unix workstations. Since its initial development in 1988 the program has been periodically updated to improve its flexibility and meet output needs for a variety of applications.

Hydrograph of Comal River near New Braunfels, Texas, showing base-flow separation results

For example applications of the BFI program, see:

Tony Wahl's USBR Home Page

Last reviewed: 08/09/13