The flow duration curve (FDC) is a fundamental signature of the hydrological cycle to support water
management strategies. Despite many studies on this topic, its estimation in ungauged basins is still a relevant
issue as the FDC is controlled by different types of processes at different time-space scales, thus resulting quite
sensitive to the specific case study.
In this work, a regional spatially-smooth procedure to evaluate the annual FDC in ungauged basins is proposed,
based on the estimation of the L-moments (mean, L-CV and L-skewness) through regression models valid for
the whole case study area. In this approach, homogeneous regions are no longer required and the L-moments are
allowed to continuously vary along the river network, thus providing a final FDC smoothly evolving for different
locations on the river. Regressions are based on a set of topographic, climatic, land use and vegetation descriptors
at the basin scale. Moreover, the model ensures that the mean annual runoff is preserved at the river confluences,
i.e. the sum of annual flows of the upstream reaches is equal to the predicted annual downstream flow.
The proposed model is adapted to incorporate different βsub-modelsβ to account for local information within
the regional framework, where man-induced alterations are known, as common in non-pristine catchments. In
particular, we propose a module to consider the impact of existing/designed water withdrawals on the L-moments
of the FDC.
The procedure has been applied to a dataset of daily observation of about 120 gauged basins on the upper Po
river basin in North-Western Italy