| 15 Dec 2025
An Uncertainty Quantification Framework for Simulation-based Flood Frequency Analysis
Abstract. Flood frequency analysis (FFA) is essential for flood risk management and infrastructure design, yet the uncertainty associated with flood quantile estimates is often poorly characterized or disregarded – especially under data-scarce conditions. Existing uncertainty quantification methods are frequently subjective, overly complex, or impractical for routine engineering use. We introduce a simulation-based uncertainty quantification framework – UQ-flood – that integrates a process-based hydrologic model, a stochastic weather generator, and a residual error model (REM). Designed for annual maxima, the REM accounts for model bias and residual variability, enabling the generation of probabilistic streamflow ensembles tailored to extreme event analysis. We apply UQ-flood to three Canadian watersheds with long streamflow records and contrasting hydroclimatic conditions. We compare its performance against traditional statistical FFA using the Generalized Extreme Value distribution and Bayesian inference. UQ-flood yields flood quantile estimates consistent with long-record statistical methods but with substantially narrower uncertainty bounds. Under short-record conditions (e.g., 30 years), UQ-flood maintains statistically consistent estimates, while statistical FFA produces wide, often impractical uncertainty intervals. Additional experiments reveal that omitting the REM introduces systematic bias in flood magnitude estimates. UQ-flood avoids parametric assumptions about flow distributions, circumvents hydrologic model biases, and is adaptable to data-limited conditions. By explicitly propagating uncertainty from hydrologic simulation to flood quantiles, UQ-flood offers a practical alternative for robust flood risk management, including applications in infrastructure design and floodplain mapping. We recommended integrating residual error models into continuous simulation frameworks to improve bias correction and uncertainty quantification in flood risk estimation.
The authors present a novel framework for Uncertainty Quantification of flood-frequency for Annual Maxima estimates that combines model-generated data with a Residual Error Model (REM). Their formalism provides a realistic and easily implementable method for at-a-station Flood Frequency Analysis (FFA). This work is a significant improvement over previous techniques that rely on either statistical assumptions about the properties of the Annual Maxima random variable or purely model-based generation of deterministic values. The careful treatment of model error allows for a smooth combination of these two approaches, provinding norrow and physically-constrained estimates of uncertainty bounds.
Specific Comments:
1. Clarify if the underlying HBV model was calibrated using only 30-years of the data in the experiments that were comparing 30 vs. 108 years estimates.
2. Clarify if the AR model for residual error was deemed sufficient to characterize autocorrelations or if this was a subjective decision
3. Include the lack of "parameter uncertainty" estimation in Section 5.3. It looks to me that a different set of parameters with similar performance had been chosen; the final uncertainty band could have changed. Therefore, a bootstrapping of multiple similarly performing model parameterizations can lead to a widening of the uncertainty band.
4. Further addressing the issue of uncertainty in streamflow estimates due to errors in the rating curves. I suggest citing and comparing the uncertainty bands reported here with those found by Velásquez, N., Krajewski, W.F. Effect of streamflow measurement error on flood frequency estimation. Stoch Environ Res Risk Assess 38, 2903–2910 (2024). https://doi.org/10.1007/s00477-024-02707-1
5. I suggest the authors comment on any findings in their analysis that suggest whether annual maxima produced by different flood mechanisms (e.g., snowmelt vs rainfall generated) can be detected, and if a conditional REM model may be needed to address those differences.
Line 123: superscript for square kilometers
Line 389: replace "providing" with "provides"