In Reply Refer To:                                         March 19, 2004
Mail Stop 415


Subject: The Graphical Constituent Loading Analysis System (GCLAS) for computing discharges of suspended sediment and other water-quality constituents


GCLAS is a software package for computing daily discharge (load) records of suspended sediment and other water-quality constituents. It was originally announced and released by the Office of Water Information on March 21, 2001. This memorandum (1) documents the change in user and programming support for the Graphical Constituent Loading Analysis System (GCLAS) (McKallipand others, 2001) from the Office of Water Information to the Offices of Surface Water and Water Quality, and (2) provides a brief background and description of GCLAS.


The techniques sanctioned and used by the U.S. Geological Survey to compute records of daily suspended-sediment discharge (Porterfield, 1972) have changed little since the 1940's. Those techniques, when performed manually, require a considerable expenditure of time and effort, and are subject to human errors unassociated with interpreting the data. Examples of potential human errors include missplotting of concentration values, inaccurate selection of concentration values from manually drawn concentration curves, and basic computational errors.

In the early 1990s, the USGS developed a computer program called SEDCALC (Koltun and others, 1994; U.S. Geological Survey, 1995) that facilitated more efficient computation of suspended-sediment records and eliminated most of the human errors associated with the manual method. SEDCALC was also used successfully to calculate records for constituents other than sediment even though it was not designed for that purpose. Hardware and software systems required for use of SEDCALC are no longer supported within the Water Resources Discipline (WRD). A port of SEDCALC to newer hardware systems was considered but development of a new program, which takes better advantage of modern hardware and software, was a preferable solution.

GCLAS was developed through the cooperation of the Offices of Water Information, Surface Water, and Water Quality. It is now the WRD’s sole software tool for computing daily suspended-sediment discharge records, and is also designed to be used for computing records of daily discharges of other water-quality constituents that are measured on a daily or near-daily basis.


GCLAS features an interactive graphical user interface that permits easy entry of estimated constituent-concentration values and provides new tools to aid in making those estimates. It uses a water-discharge time series, and suspended-sediment or other constituent-concentration values to determine daily constituent discharges and associated daily mean constituent concentrations.

GCLAS includes tools to aid in making estimates of constituent concentrations for periods where concentration data are missing or under-sampled. In addition, GCLAS facilitates analysis and application of cross-section coefficients, which are multipliers used to adjust concentration values obtained from samples collected at a fixed point or vertical so that they are more representative of the discharge-weighted mean concentration at the cross-section and time of interest.

GCLAS includes a number of improvements over SEDCALC, most notably the ability to:
1. Directly compute records for periods of zero or reverse flow,
2. Analyze and apply coefficient adjustments to constituent concentrations as a function of time, streamflow, or both,
3. Compute discharges of constituents other than suspended sediment,
4. Easily view data related to the continuous concentration curve at different levels of detail,
5. Readily display covariate time-series data to provide enhanced visual cues for developing the constituent continuous concentration curve, and
6. Store and display a variety of metadata associated with constituent concentrations.

GCLAS is not suitable for computing constituent discharges when significant numbers of censored values are present in the data set.

The GCLAS software and information on minimum system requirements and installation can be found at User and programming support may be requested by electronic mail sent to

Stephen F. Blanchard, Chief /s/
Office of Surface Water

Timothy Miller, Chief /s/
Office of Water Quality

This memorandum supercedes Office of Surface Water Technical Memorandum 95.09


Koltun, G.F., Gray, J.R. and McElhone, T.J., 1994, User's manual for SEDCALC, a computer program for computation of suspended-sediment discharge: U.S. Geological Survey Open-File Report 94-459, 46 p.

McKallip, T.E., Koltun, G.F., Gray, J.R., and Glysson, G.D., 2001, GCLAS – a graphical constituent loading analysis system: Proceedings of the 7th Federal Interagency Sedimentation Conference, Reno, Nevada, March 25-29, Vol. II, VI-49 to VI-52.

Porterfield, G., 1972, Computation of fluvial-sediment discharge: U.S. Geological Survey Techniques of Water-Resources Investigations of the United States Geological Survey, Book 3, Chapter C2, 66 p.

U.S. Geological Survey, 1995, Endorsement of Sediment Records Calculation Program (SEDCALC): Office of Surface Water Technical Memorandum 95.09, 2 p.