monthly flow over a period of several years, to determine the plant capac-
ity and estimated output which are dependent on the average flow of the
stream and its distribution during the year; (b) low flows, to assess the
primary, firm, or dependable power.
(a) Streamflow data analysis
A typical streamflow hydrograph, including a dry period from which the
frequency of occurrence of a certain flow during the period can be calcu-
lated (Worked example 12.1), is shown in Fig. 12.6.
The flow duration curve (Fig. 12.7) is a plot of the streamflow in
ascending or descending order (as ordinate) and its frequency of occur-
rence as a percentage of the time covered by the record (as abscissa). Also
shown on this figure is the flow duration curve of a flow regulated by
storage. Losses due to evaporation and leakage from the proposed reser-
voir, and flow rates relating to low-water management downstream of the
reservoir and to any other water demand, have to be taken into account in
arriving at the regulated flow duration curve.
(b) Power duration curve
If the available head and efficiency of the power plant are known, the flow
duration curve in Fig. 12.7 may be converted into a power duration curve
by changing the ordinate to the available power (i.e. gQH).
The power which is available for 95% to 97% of the time on the
reservoir regulated schemes is usually considered to be the primary or firm
power, and the area of the power duration curve under the minimum
amount of flow available for 95% to 97% of the time thus gives the total
amount of the primary power. Primary power is not necessarily produced
continuously. If pondage and interconnection facilities are available, the
STREAMFLOW DATA 503
Fig. 12.6 Streamflow hydrograph