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 ...
In Polish hydrology and water management the term characteristic flow exists denoting a specific value of flow at the given cross-section of a river calculated as the long-term minimum, mean, median or maximum calculated using the annual minimum, mean, median or maximum flow taken for each year from a series of (usually) daily flows. Some of these characteristic flows are used to define the low-flow (o drought) periods while the another criterion: a percentage flow Qp taken from the long-term flow duration curve is also widely used. In the paper the study on the frequency structure the empirical exceedance probability of a given characteristic flow made for some low and average characteristic flows (SNQ, WNQ, NSQ and SSQ) is presented. The results show that the exceedance probability of a given characteristic flow is variable, and the amount of this variability may be large, as is the case of WNQ and NSQ. So assigning a characteristic flow to a single FDC quantile value Qp (as can be find in the literature) cannot be justified.Correlation analysis made for the pairs ( , characteristic flow), ( , catchment area) and ( , gauging station elevation) revealed some significant correlations. Only ...
