ABSTRACT
For a gaging station, an FDC is a plot of streamflow values from high to low against the percent of time these values are either equaled or exceeded. The plot considers the full range of flows and is constructed over a specified period of time scaled between 0% and 100%. The time interval for constructing an FDC depends on the need, but daily average discharge values are usually used; sometimes weekly, monthly, or seasonally averaged values can also be used. However, averaging over longer time intervals obscures details of the variations in flow and the effect of varying time interval is not the same for all streams. The difference between an FDC based on daily discharge values and that based on monthly values can be as high as 35%, as noted by Foster (1934). For large streams where flow from day to day is almost uniform, weekly FDC may be almost the same as daily FDC, whereas for flashy streams with sudden floods lasting for a few hours in a day, daily and weekly FDCs will be greatly different.
