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Water Resources of the United States

GCLAS (Graphical Constituent Loading Analysis System)

GCLAS interface


GCLAS is a program designed primarily to compute daily and annual loadings of constituents transported in streamflow. Use of GCLAS is appopriate when concentration data are collected at relatively high frequency (i.e. approximately daily to more frequent samples depending on the temporal variability of streamflow and concentrations) and concentration data are nearly or completely free of censored (e.g. "less than") values.  Concentration data can be at equal or unequal time intervals. Depending on the frequency with which water-quality data are collected, computation of daily constituent loads can be independent of, to highly dependent on human interpretation of the relation between streamflow and constituent transport.

GCLAS provides a visual environment for evaluating the relation between covariate time series and the constituent chemograph. GCLAS has an interactive graphical interface that permits easy entry of estimated values and provides tools to aid in making those estimates. Some of the features of GCLAS include (1) the ability to directly handle periods of zero or reverse flow, (2) the ability to analyze and apply coefficient adjustments to concentrations as a function of time, streamflow, or both, (3) the ability to compute discharges of a variety of constituents, (4) the ability to easily view data related to the chemograph at varying levels of detail, and (5) the ability to readily display covariate time-series data to provide enhanced visual cues for drawing the constituent chemograph.


GCLAS installer packaged for Unix and Windows (current version that creates tab-delimited [rdb] outputs)

gclas_install_2_20.jar (8.8 MB)

GCLAS installer packaged for Unix and Windows (legacy version that creates card-image outputs)

gclas_install_1_06c.jar (4.5 MB)

Note: Some browsers rename jar files with a ".zip" extension. If that happens, change the extension back to ".jar" before installation.

The Java Runtime Environment (JRE) Version 6 or higher is recommended to run GCLAS. The latest open-source version for Linux and Microsoft Windows platforms can be downloaded from


Koltun, G.F., Eberle, M., Gray, J.R., Glysson, G.D., 2006, User's manual for the Graphical Constituent Loading Analysis System (GCLAS), U.S. Geological Survey Techniques and Methods, 4-C1, 51 p. USGS Techniques and Methods 4-C1, Portable Document Format (PDF) (918 KB).

Point of contact for GCLAS:

U.S.Geological Survey
Office of Surface Water
12201 Sunrise Valley Drive
Mail Stop 415
Reston, VA 20192
(electronic mail:

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