Uncertainty analysis for estimating flood frequencies for ungauged catchments using rainfall-runoff models

Continuous simulation of flows for ungauged catchments is a methodology being developed for estimating flood frequencies where no flood records exist. This involves driving a rainfall-runoff model with either simulated or observed rainfalls, using values of the rainfall-runoff model parameters derived using a generalisation procedure based on analysing sets of parameter values for calibrated catchments. This paper examines the uncertainty associated with such generalised parameters, and carries this through to estimate the uncertainty of the generalised flood frequency curves for ungauged catchments. The approach used distinguishes two sources of uncertainty: the uncertainty in the parameters calibrated for individual catchments, and the uncertainty with which parameters for an ungauged catchment can be estimated based on calibrated parameters for other catchments and descriptors of those catchments. The uncertainty associated with estimates for ungauged catchments can then be reduced compared with a more direct approach, firstly by allowing one of the components of uncertainty to be omitted, and secondly by allowing the introduction of weighting schemes which reduce the effect of catchments where calibration uncertainty is high.