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Analysis of Runoff/Streamflow Observations due to Storm Matthew


Overview
In this session we will examine the impact of the rainfall storm on the watershed in terms of runoff and flooding response. We will examine observations from different streamflow gauges located at the outlet of the watershed as well as from those within the watershed. The gauge located at the main outlet of the watershed measures the streamflow (discharge) response of the entire watershed. The interior gauges measure the streamflow response of different sub-watersheds within the main watershed.

Your Task
Work with the field sensors layers (See Sensors in Layers section at the bottom left of this page) to answer the following questions. Download and open this WORD file and use it as a template to record your answers.
  1. Activate the layer that shows the locations of the streamflow gauges. Identify the sub-watersheds associated with each interior gauge. Create a table listing each streamflow gauge number (including the gauge at the main outlet) and the corresponding drainage area of the sub-watershed that the gauge represents.   Download a template for this table; you will be adding more information into the table in the next steps.
  2. Click on each gauge and download its streamflow measurements. Save these observations as separate columns in an EXCEL file.
  3. For each sub-watershed (and the main watershed) generate a plot showing the date/time on the x-axis, the streamflow on the primary y-axis, and the rainfall rate (mm/h) on the secondary y-axis. For the secondary y-axis, display the rainfall plot facing downward (in EXCEL, use the option of “Values in reverse order” under Format Axis). A time series plot of streamflow values is usually called “hydrograph” while a time series plot of rainfall rates is usually called “hyetograph”. Click here for a sample of what your plot should look like.
  4. Estimate the maximum runoff peak (discharge in m3/s) associated with each hydrograph. Record your answers in the same table produced in step 1.
  5. Calculate the time-to-peak (estimated as hours since 10/08/2004 hour 0:00) for each hydrograph measured from the beginning of the rainfall event. Record your answers in the same table produced in step 1.
  6. Calculate the total runoff volume associated with each gauge. Total runoff volume is basically an integration of streamflow values over the duration of the storm. Express the total runoff in terms of volume units (e.g., m3). You will need to add the total streamflow values and then multiply by a conversion factor of 15x60 (that is how many seconds are in a 15-minute interval). Record your answers in the same table produced in step 1.
  7. Express the total runoff in terms of depth units (mm). The runoff depth can be obtained by dividing the runoff volume by the watershed (or sub-watershed) drainage area associated with the streamflow gauge. Include your answers in the same table. What do these ratios tell you about the runoff response of the watershed to the storm?
  8. For each sub-watershed (and the main watershed) estimate the total rainfall that fell over it. This can be estimated by averaging total rainfall depth from whatever number of gauges located within the sub-watershed (refer to previous analysis on total rainfall from different gauges). Include your answers in the same table.
  9. For each sub-watershed (and the main watershed) calculate the runoff-to-rainfall ratio. This ratio is defined as the ratio between the runoff depth and the rainfall depth over the sub-watershed. Include your answers in the same table.
  10. Discuss and comment on your results and try to provide justification for the differences that you observe in runoff volumes, peaks, and runoff-to-rainfall ratios between the different sub-watersheds and the main watershed

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