Flow duration curve and its applicability

A flow duration curve (FDC) is a plot of river discharge against the percentage of time that discharge is equalled or exceeded. It is drawn by arranging recorded (daily or monthly) flows in descending order and plotting each flow against its exceedance probability, $P = \dfrac{m}{n+1}\times 100\%$ (m = rank, n = number of values).

 Q (m3/s)
  |\
  | \
  |  \__
  |     \____
  |          \______
  |                 \_______
  +--------------------------> % time exceeded
  0     45  65        95   100

Applicability

1. Design discharge: the turbine flow is chosen at a set exceedance, e.g. $Q_{40}$–$Q_{45}$ for ROR projects in Nepal.
2. Installed capacity and energy: the area under the FDC (limited by the design flow) gives the energy available; it is used to compute annual energy and plant factor.
3. Firm power: flow at 90–95% exceedance gives the firm (dependable) power.
4. Number and size of units: the shape of the curve shows how long the plant will run at part load, so it guides unit sizing.
5. Riparian release and water rights: low-flow end of the curve shows the minimum flow to release downstream.
6. Comparison of sites: a flat curve means steady flow (good for ROR); a steep curve means highly variable flow needing storage.