Water Resources Applications Software

Geochemical || Ground Water || Surface Water || Water Quality || General

Summary of SAC

       sac - Slope-Area Computation program

       SAC follows USGS standardized procedure for computing discharge by
       the slope-area method.  The source is written in Fortran 77.

       The program solves the 1-D steady-state energy and continuity
       equations for discharge given upstream and downstream water-surface

       Version 97-01 1997/05/01 - Added new metric conversion to allow
          input data and the output results to be in either meter-second or
          feet-second units. A new record implements this feature.
          Additionally, the previous version did not correct cross-section
          stationing for skew angles entered on the XS record. The program
          did not compute correctly for sections with level water surfaces
          that were described using the method in Open-File Report 94-360
          for nonlevel water surfaces. The program did not select the
          appropriate water-surface elevation for cross sections described
          by enclosing the figure (Fulford, 1994, p. 21) if a vertical bank
          existed on the right side of the cross section. These errors have
          been corrected.

       Version 96-03 1996/04/19 - The program originally used the mean
          cross-sectional velocity to compute the Froude numbers listed in
          the results for each subarea.  The Froude computation has been
          changed to use an estimated subarea velocity for the subarea
          Froude numbers.

       Version 94-11 1995/08/16 - Version as documented in USGS Open-File
          Report 94-360.

       1993 - J.M. Fulford changed input format to a modified WSPRO format,
          output to an 80-column format, and reorganized code.

       1983 - Revised by W. Kirby.

       1970 - Original code was written by O.G. Lara.

       Cross-section geometry and roughness values that describe the
       channel reach geometry along with measurements of high water marks.

       Text output of discharges and other variables used in the

       The program is written in Fortran 77 with the following extensions:
       variable names longer than 6 characters and use of double quotes for
       character strings.  Generally, the program is easily installed on
       most computer systems. The code has been used on UNIX-based
       computers and DOS-based 386 or greater computers having a math
       coprocessor and 2 mb of memory.

       Fulford, J.M., 1994, User's guide to SAC, a computer program for
          computing discharge by the slope-area method:  U.S. Geological
          Survey Open-File Report 94-360, 31 p.

       Dalrymple, T., and Benson, M.A., 1967, Measurement of peak
          discharges by the slope-area method:  U.S. Geological Survey
          Techniques of Water-Resources Investigations, book 3, chap. A2.

       Surface-Water Hydraulic Analysis (SW2009TC), offered annually at the
       USGS National Training Center.

          U.S. Geological Survey
          Office of Surface Water
          Janice M. Fulford
          Building 2101
          Stennis Space Center, MS 39529

          U.S. Geological Survey
          Hydrologic Analysis Software Support Program
          437 National Center
          Reston, VA 20192

       Official versions of U.S. Geological Survey water-resources analysis
       software are available for electronic retrieval via the World Wide
       Web (WWW) at:


       and via anonymous File Transfer Protocol (FTP) from:

         (path: /pub/software).

       The WWW page and anonymous FTP directory from which the SAC software
       can be retrieved are, respectively:

       wspro88(1) - A computer model for Water-Surface PROfile

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