Where can I get this darn number? Estimating 1000-year return periods in ungauged catchment areas using continuous simulation

Estimating extreme flood events with a return period of 1000 years or more is particularly challenging in ungauged catchments. Traditional methods often rely on statistical extrapolation of peak discharges or regionalized design values, both of which are subject to considerable uncertainty. Our study examines whether continuous hydrological simulations could offer a useful second opinion to these traditional methods. The continuous simulations are based on a model chain that starts with a stochastic weather generator, which produces long synthetic time series of precipitation and temperature. These time series then serve as input to a hydrological model. As there are no direct streamflow observations for ungauged catchments, the model parameters must be regionalized to realistically configure the hydrological model. Initial tests, in which gauged catchments were treated as ungauged, were promising, suggesting that continuous simulation using a stochastic weather generation combined with a hydrological model could provide a robust basis for estimating extreme floods in regions with limited data.