Probabilistic Flow Duration Curves for Use in Environmental Planning and Management

When dealing with the planning and management of water uses in a river, the knowledge of the probabilistic structure of minima of daily or of multiple-day flows is often required. As a tool for straightforward determination of different levels of flow minima, flow duration curves (FDCs) are particularly suited for planning purposes. In this paper, FDCs referred to annual samples are interpolated with lognormal curves, and their probabilistic structure is obtained through the statistical analysis of the two lognormal parameters. Distribution of these parameters is shown to be normal so that the discharge with a given duration and return period T can be easily evaluated. To build FDCs in ungaged basins, relations between the moments of the parameters and catchment characteristics have been investigated with reference to the data available in the Basilicata region (Italy). For both parameters, most of the variance of the first moment can be explained by the Base Flow Index (BFI), which can be estimated from geology. The second moment can be derived considering that the coefficient of variation is constant over the whole region. Since the curves are considered in dimensionless form, estimation of the mean annual runoff is finally needed to obtain the dimensional probabilistic FDCs in ungaged sites.