Regionalization of flow-duration curves for ungauged catchments in southern peninsular Italy

Study region The study covers southern peninsular Italy, including Campania, Calabria, Basilicata, Puglia, and marginal parts of Lazio and Molise. The analysis uses 58 hydrometric stations reconstructed from historical SIMN daily discharge records across heterogeneous Mediterranean catchments. Study focus This study develops a regional framework for estimating long-term flow-duration curves in ungauged Mediterranean catchments. Daily discharge is represented by a mixed formulation that separates flow occurrence from non-zero discharge magnitude. The non-zero component is mean-normalized and described with a log-normal model. Flow occurrence, distribution parameters, mean non-zero discharge, and base-flow index are regionalized using climatic, physiographic, land-cover, drainage-network, and hydrogeological descriptors. Principal component analysis supports hydro-physiographic interpretation, partial least squares regression derives prediction equations, and jackknife validation assesses transferability. New hydrological insights for the region Separating zero-flow occurrence from positive-flow magnitude provides a transferable and physically interpretable basis for regional flow-duration curve estimation in heterogeneous Mediterranean basins. Jackknife validation showed high stability for mean non-zero discharge (R² = 0.98), and the perennial-river location parameter (R² = 0.81), while intermittent-flow parameters retained acceptable predictive skill (R² ≈ 0.70). Reconstructed quantiles agreed closely with observations across most exceedance probabilities, highlighting associations between regional flow regimes and climate, catchment scale, drainage connectivity, and subsurface storage.