This applet is to be used in connection with the Flood Watch CSC in section 4.11. It implements a hydrograph, consisting of data for a rainfall event and the accompanying data for the discharge from a river into which the rain drains.

The object of the CSC is to obtain the centroids for the rainfall event and the ensuing rainfall discharge, thus to find the lag time between the rainfall event and the rainfall discharge.

Click here for more instructions below.

Three hydrographs are provided—an example, and two hydrographs for the Gorge River that are the subject of the CSC. The object of the CSC is to obtain the centroids for the rainfall event and the ensuing rainfall discharge, thus to find the lag time between the rainfall event and the rainfall discharge. By studying such statistics one can learn how the water shed of the river has changed over time, presumably through development, building of levees, etc.

Several important definitions:

• The discharge data gathered in the field is measured in m^3/second. It is plotted in the second column of the table and in the graph. Note that times displayed in the first column are in hours.
  
  
• The total discharge of the stream consists of two components— the base flow discharge (BFD) that represents the discharge of the stream in the absence of a rainfall event, and the rainfall discharge (RFD) that represents the discharge of the stream over and above the base flow discharge.
  
  
• The applet draws the graph of the raw discharge data and draws a line to represent the base flow of the stream (the base flow is just the minimum of the raw discharge data). The area below the line is the BFD and the area above the line and under the curve is the RFD.
  

The applet computes various statistics including the total rainfall (in inches), the total RFD (in m^3), the centroids of the rainfall and of the RFD, the base flow of the stream (in m^3/sec), and the lag time between the centroids (in hours). You will interpret these in doing the CSC. You will also enter your own rainfall data and compute for it all of the statistics mentioned above. (Note that the applet computes the rainfall statistics for the three examples but hides them when you enter your own data.)

Units: The units on the vertical axis of the graph are cubic meters per second (m^3/second). The units on the green vertical axis are for the rainfall data. These are measured in inches per hour (in/hour). The horizontal axis is in units of hours. It is customary in drawing hydrographs to plot the discharge data and the rainfall data on the same time scale since it is the lag time that is of greatest importance. The rainfall data and discharge data are linked through knowledge of the area of the watershed of the stream. The number of inches of rain in the storm multiplied by the area of the watershed should be approximately equal to the RFD.

Notes:

• The text field below the graph displays the rainfall data. Compare the six numbers in the text field with the definition of the rainfall function given in the standard form for a piecewise defined function. This text field is editable, thus you can enter your own rainfall data following the same convention used in the three examples (6 numbers separated by commas).
  
  
• Clicking the graph will alternatively hide or show the rain fall graph.
  
  
• Scroll to the end of the table to see the sum of the right hand column.
  
  

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