Quantifying Future Shifts in Intensity–Duration–Frequency (IDF) in Singapore: A comparison of methods

Intensity-Duration-Frequency (IDF) curves are critical for urban drainage design and flood risk mitigation, particularly in highly urbanized regions like Singapore, where short-duration extreme rainfall events pose significant challenges. This study quantifies future changes in IDF curves and their associated uncertainties under two representative emission scenarios: SSP 2-4.5 and SSP 5-8.5. To construct future IDF curves, we compare two methods. First, we use a stochastic downscaling methodology that makes use of the AWE-GEN weather generator, to downscale precipitation projections from 25 Global Climate Models (GCMs) to the local point scale. The results show that the magnitude of future extreme precipitation quantiles is expected to get higher toward the end of the 21st century under both future scenarios. Higher-emission scenarios lead to substantial intensification of rare precipitation events, accompanied by a large uncertainty. However, internal climate variability is the dominant source of uncertainty, with climate model and emission scenario uncertainties being less relevant. Second, the results are compared with outputs of the TENAX (Temperature dependent Non-Asymptotic statistical model for eXtreme return levels) model, a novel framework that incorporates temperature as a covariate in a physically consistent manner to project rainfall return levels in a warmer climate using fewer inputs. This study compares state-of-the-art methodologies for computing IDF representative of future climates and provides actionable insights for engineers and policymakers to update urban stormwater design guidelines and enhance resilience against future rainfall extremes.