A Proposal for a New, Simplified Methodology for Design Flow Modeling in Ungauged Catchments

Abstract The primary objective of this research was to propose a new methodology for calculating design flows in ungauged catchments, addressing uncertainties associated with outdated empirical formulas. Focusing on four catchments in the eastern United States, the methodology comprises two main steps: 1) determination of peak flow series using the Event-Based Approach for Small Ungauged Basins (EBA4SUB) rainfall–runoff model and 2) calculation of design flows based on the generated series from EBA4SUB, utilizing the best-fitted statistical distribution. Linear scaling was applied to reduce the percent bias (PBIAS) error in design flows. The study encompassed trend analysis for hydrometeorological data, estimation of peak flow series using EBA4SUB, and the calculation of design flows with the proposed methodology. Validation involved comparing estimated design flows from observed time series with those calculated via the Graphical Peak Discharge (GPD) method and the Rational Formula. Results indicated no significant ( α = 0.05) trends in peak flow series and identified the Pearson type III distribution as best fitted for design flows. Hydrological modeling underscored the critical role of antecedent-moisture conditions in calculating design flows. The calculated PBIAS values of <10% for model validation suggest the proposed methodology is suitable for use in ungauged catchments. Significance Statement Design flows are pivotal in environmental management for several reasons. First, they aid in planning water infrastructure with minimal environmental impact. Second, they inform flood protection strategies vital for safety and property protection. Understanding design flows is crucial for safeguarding aquatic ecosystems and determining minimum flows for a healthy environment. They are also vital in managing water-related risks such as floods, particularly amid climate change. Currently, estimating design flows in ungauged catchments relies heavily on multivariate correlation analysis, which may be outdated due to climate changes and land-use alterations. Our research proposes a novel methodology using the Event-Based Approach for Small Ungauged Basins (EBA4SUB) rainfall–runoff model and statistical distributions to calculate design flows in ungauged catchments to address these concerns.