Probabilistic evaluation of flow duration curves in semi-arid basins

The flow duration curve (FDC) is a relationship between any given discharge value and the percentage of time that this discharge is exceeded, or, in other words, the relationship between magnitude and frequency of streamflow discharges. In this work, according to a probabilistic approach to the FDCs, the percentage of time is considered as a fraction of the year and the FDC is derived linking each discharge to the expected value of the fraction of the year during which such discharge is exceeded. In particular, a method for the evaluation of the FDC with a fixed return period T (T-year FDC) is proposed. The FDC is considered as a function bounded above and below. The upper and lower bounds are assumed equal to the corresponding T-year fractiles of, respectively, the annual maximum and minimum probability distribution of the mean daily discharge. The FDC is then modeled as a logarithmic transformed discharge following a Pearson Type 1 distribution (Beta function). In the case of intermittent flow regimes, which characterize the study cases presented (Puglia, Southern Italy), such approach had to be used in order to produce a satisfactory agreement between abserved and modeled FDCs. The methodology proposed seems promising in the framework of the regional evaluation of the T-year FDCs and when dealing with ungauged river catchments.