Rating Curves in All the Right Places…

While plenty of rain has come down in Williamsburg over the past month, not enough rain has fallen on our farm to trigger flow in our stream.  As a result, we’re continuing to analyze samples from the last major flow event in April.  This is a good thing because the analysis of the April storm has turned out to be more complex than we initially anticipated.

First, I’ll back up to the September storm to explain the problems with the April storm.  In September we were only able to take water samples from the stream during a relatively small portion of the flow.  One metric that is important to us about these flows is the total flux of nutrients and sediment off the field.  To get the total fluxes, we needed concentration values from the whole storm.  So using the concentrations we had collected we needed to extrapolate to what the concentrations were during the rest of the storm that we were not able to sample.  To do this we used a rating curve.

A rating curve is an equation created with known data to extrapolate unknown data.  Our rating curve put discharge (volume of water/time) as the independent variable and concentration of a given nutrient as the dependent variable.  For the September storm, my partner, Isaac, had good relationships and was able to, with some confidence, estimate the concentrations of nutrients and suspended sediment during the unsampled part of the storm.  With these concentrations he determined total fluxes (mass) for the storm, which we can compare to other storms.

Back to the April storm data, we tried creating rating curves for nutrients and they did not work out initially.  To get reasonable relationships we had to split the storm into portions and create separate equations for each part.  Anywho, I’ll keep you updated on how it goes. Until then, Ciao faithful readers.





  1. lkfunkhouser says:

    Creating the rating curves sounds extremely complicated! Can you do these graphs and extrapolations in excel or does it require a program more intense?