Quantifying representative runoff coefficients for small basins: a practical recipe for data-limited conditions

The runoff coefficient is a key hydrological variable, but its dependence on event severity and high variability across events makes selecting a representative value for a catchment challenging. This study presents a practical approach to estimate it when high-resolution time series of rainfall and streamflow data are unavailable. Based on over 1000 historical events recorded in the Hydrological Yearbooks, we empirically estimate annual runoff coefficients for 60 small- to medium-sized catchments in Italy. We develop a novel procedure to unambiguously match maximum discharges with corresponding rainfall depths, addressing situations where the dates of occurrence of hydrological extremes were not consistently recorded. We also show that pairing rainfall and discharge maxima by frequency provides equally reasonable estimates of runoff coefficients as pairing by temporal coincidence. A consistency analysis and examination of cases with outlying runoff coefficients finally provide useful insights for basin classification to support, for instance, regional flood frequency analyses.