Analysis Techniques

Streamflow Statistics

Streamflow data are a recorded sample from a much larger period of flows, most of which were not "observed.”  We use streamflow statistics to mathematically manipulate discharge data in order to generate ideas about the larger (longer) "population" of streamflow at a site based on the record (sample).

Streamflow statistics quantify the characteristics of the stream.  Streamflow statistics are calculated depending on the type of statistical method chosen. Choice of a particular method should be based on the type of information available and the amount of detail that is desired. This portion of the web site provides a step-by-step procedure for computing the most commonly used streamflow statistics.

Hydrologic analysis methods can be broken down into four major categories:

• Annual analysis
• Monthly analysis
• Flow-Duration analysis
• Flood Frequency analysis

Our discussion of these analysis techniques will focus on four main questions:

• What is it?
• How is it calculated?
• What kind of graphs and charts can be generated?
• What does this particular information tell you about your stream?

In addition to this information, a detailed tutorial and an example for each type of analysis are included. The tutorials and examples are step-by-step demonstrations of how to perform the indicated hydrologic methods.  USGS data for the Alsea basin (Alsea River at Tidewater) were used for all demonstrations.  The difference between the tutorials and the examples is the amount of data used to perform the analysis.  That is, the tutorials use 10 years of gage data, whereas the examples use the entire period of record (61 years of data) for the gage.  With these two formats you can learn to perform the analysis using a small data set, then apply what you learned to a larger set. Comparing the results of the tutorials with those of the examples can show the importance of obtaining a data set with a long period of record.  When viewing the tutorials and examples, please note that shading is used to highlight new information.

Most of this analysis can be done using a spreadsheet software program such as Microsoft Excel.  The examples presented on this site were all generated using Excel.