"Flow Duration Curve"
Flow duration is defined in "Applied Hydrogeology"
by C.W. Fetter, Jr. as the probability that a
given flow will be equalled or exceeded.
The probabilities are obtained by simply counting
the number of equal or higher measurements for each
measured flow rate.  Dividing by the total number
of measurements gives the flow duration as a
fraction.  The fractions are then sorted by 
discharge value and converted to percent.
The resulting curve always falls monotonically 
with increasing flow rate.