Summary:

Basing on four 49-year time series of daily flows in the Mała Wisła catchment, two methods of creating flow duration curves (FDCs) were analysed: a traditional method where one FDC is produced (period-of-record FDC) using all flow from N-year period, and mean annual FDC created as the average of N annual FDCs. The latter FDC differs from the former especially in the area of minim flows. In this area, for assumed exceedance durations p = 60, 70, 80 i 90%, a Qp taken from a mean annual FDC is higher from about 10% for p = 60% to even more than 20% for p = 90%. This increase and its amount are of value for selecting threshold flow when defining drought and for water supply design.
The application of the second method implies that instead of a single value of exceedance time, p% or 365p days, and a single value of flow Qp, an N-element set of exceedance times of a fixed flow value, and an N-element set of flows Qp for fixed p are given. These set exhibit the variability of both the exceedance time and the flows Qp for fixed p. Both variabilities were studied and were shown to be considerable.