An Open-Source Tool for Generating Hourly Synthetic Streamflow Series in Ungauged Basins Using Regional Flow-Duration Curves

Predicting streamflow in ungauged catchments using regionalization methods has been extensively studied. While numerous free and open-source software (FOSS) tools exist for predicting regional flow-duration curves (FDCs) at ungauged sites, a general FOSS tool specifically designed to generate continuous streamflow series from these FDCs is lacking. This study introduces FDC2Q_t, an R-package developed within a collaboration between the University of Bologna, the Po River Basin Authority, and the Emilia-Romagna Regional Authority. FDC2Qt generates long synthetic hourly streamflow series in ungauged catchments, utilizing a regional dataset of daily streamflow observations from neighboring sites and basin morphoclimatic descriptors. The methodology comprises three key steps: (1) FDC Prediction: a regional period-of-record FDC is predicted for the ungauged site by combining an index-flow approach (Castellarin et al., WRR, 2004) with a region of influence approach (Burn, WRR, 1990); (2) Daily Streamflow Synthesis:  a synthetic daily streamflow series is generated at the ungauged site using a non-linear spatial interpolation method based on FDCs (Smakhtin et al., HSJ, 1997; Smakhtin, J. Hydrol., 2001), referencing one or more observed daily streamflow series from neighboring catchments; (3) Hourly Streamflow Downscaling: the synthetic daily series is downscaled to an hourly time step using a scaling law that primarily considers the morphological features of the ungauged catchment. Validation experiments demonstrate that the synthetic hourly streamflows accurately capture the primary hydrological characteristics of observed streamflow series and exhibit reliability comparable to that of hourly streamflow series simulated by regionalized lumped rainfall-runoff models.