Plotting water quality samples on a hydrograph

When working on water quality in rivers and streams, it’s common to have a continuous flow record but only occasional samples of contaminants.  It is important to know where the samples occur on the hydrograph, particularly when estimating loads.

The figure below shows a plot that summarises the timing and concentration of samples and the relationship to flow.  Concentrations are colour coded.

WhiteDogCk

Nitrogen concentration overlayed on a hydrograph

 

To make the plot, combine two sources of data.

  1. A daily flow record (a column of dates and a column of flows)
  2. Information on samples (a column of dates and a column of concentrations)

Use the sample dates to determine the flow when samples were taken.  Plot the hydrograph and overlay the colour coded concentrations.

There is an example using ggplot and R at this gist.

https://gist.github.com/TonyLadson/4f8c20c6c5c2dd6a95b5.js

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