LAR 512 – Fall 2002

 

PROBLEM #16 – ESTIMATING STREAM  LOW FLOW & GROUNDWATER RECHARGE RATES

 

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Let’s continue to use the Falling Creek Watershed near Drewrys Bluff, Virginia.  Go to the USGS website http://waterdata.usgs.gov/va/nwis/nwis and proceed through the following steps:  (1) go to surface water data, (2) go to stream flow data, (3) check the site number box and hit the submit button, (4) put the site number 0238500 in the box and hit the submit button, and (5) put in the date range of 1943-01-01 to 1943-12-31 and hit the submit button.  This will give you a discharge graph for this gauging station for 1943.  Print this graph and repeat step (5) again for each year through 1962 and print each of these graphs.

 

  1. Calculate a 7 Q 10 value for the gauging station by determining for each year the lowest flow that occurs for 7 days or more.  Set up a table of the values for the twenty years of records.  Arrange the discharge rates in a table in descending order, starting with the largest discharge rate.  In the second column, rank the discharge rates from the largest having a rank of 1, to the smallest having a rank of 19.  In the third column, compute a “Recurrence Interval”.  Compute the “Recurrence Interval” using the following formula:

 

T = 100 (m / n + 1) ,  where:

 

T =  Recurrence Interval (years)

 

n = number of values, in this case 19 values

 

n + 1 = number of values +1, in this case 20

 

m =  rank number assigned to the discharge rate

 

  1. Plot the data on a log-log graph with the x-axis being both the “Exceedance Probability” and the “Recurrence Interval (years)” and the y-axis being “Annual Maximum Discharge (cfs)”.  See Figure 10-13 of Dunne and Leopold.  Draw a line that best fits the data points to create a “Low Flow Frequency Curve”.
  2. Reading from the graph, what is the low flow value for a 10-year frequency?  ____________ cfs

 

  1. Since, the low flow discharge rate is a measure of the groundwater seepage, what is the average discharge rate/sq.mi. of the drainage basin?  __________cfs/sq.mi.