Using the stochastic index method for calculation of flow duration curves on ephemeral basins

Availability of tools to simulate discharges regime, in particular low flow regime is a fundamental topic in the planning and management of water resources. The correct representation of a river frequency regime is in fact really useful in different hydrological applications, as analysis of hydro-electrical feasibility of a river, in the management of water resources and in environmental planning. In this field a fundamental problem is the flush of waste water in rivers that has no flow for percentage of time bigger than a fixed value. In this situation it is not useful get an only index, but it is necessary to use flow duration curves (FDC). This curve describes river discharges versus the percentage of time that these are exceeded. Therefore this work combines the stochastic index model, that enables to reproduce the period of record FDC of a river without regard persistency and seasonality of the series, to calculate conditional distribution F(y|Y>0), with the theory of total probability, that allows to evaluate the percentage of time the river is dry. This procedure is applied to a nested catchment in the Lazio region.