Identifying Changes in Background Water-Quality Conditions, Arkansas River and Fountain Creek, near Pueblo, Colorado

What is a flow-adjusted concentration?

The concentrations of many constituents are affected by streamflow. Typically, adjustment of the concentration data to account for varying streamflow is needed before further analysis to evaluate differences in concentration is done. The effects of streamflow can pose a problem when evaluating changes in specific conductance and dissolved solids because of the inherent inverse relation between these variables and streamflow. The use of flow-adjusted concentrations by the residuals method provides a way to remove the source of much of the variance due to the influence of streamflow. Residuals analysis regresses the measured value on some function of streamflow and uses the residuals from the regression (the observed measurement minus the predicted value) as flow-adjusted values. The residual method works well if a functional form of the measured value to streamflow regression produces a reasonable fit. To estimate daily specific-conductance values from streamflow, instantaneous specific-conductance data and continuous daily streamflow data were input into a multi-parameter regression model. Regression equations for the model can be found in Table 3 of the USGS Scientific-Investigations Report 2004-5024. See the report for a more detailed explanation of flow-adjusted concentrations.