Regionalization of Intensity-Duration-Frequency Coefficients in Uganda

Rainfall intensity‒duration‒frequency (IDF) curves are essential for hydrological design and flood management, yet their development in Uganda is limited by scarce subdaily rainfall data. This research regionalized the IDF coefficients using physical and climatic predictors. IDF coefficients (a, b, c) were derived from observed hourly rainfall and from disaggregated daily rainfall using aligned time distribution (TD) factors. The aligned TD method produced consistent coefficients (b: 0.404 to 0.65; c: 1.01 to 1.079) with near-perfect R² values, while the empirical method based on observed hourly data exhibited greater variability, the coefficient a ranged from 32.37 at Soroti to 678.54 at Gulu. Multiple regression analysis (MRA) using elevation, mean, and standard deviation as predictors, performed strongly, particularly with disaggregated data (R² > 92% for all coefficients). Validation showed predicted coefficients closely matched actual values, especially coefficients b and c (percentage error within ±10%), though coefficient a exhibited larger deviations. The MRA model, based on observed data, also performed well for coefficients b and c (R² = 97.65% and 96.25%, respectively). This research demonstrates that regionalizing IDF coefficients using MRA is feasible and effective, providing practical tools for estimating design storm intensities in gauged and ungauged catchments, while recommending cautious application beyond the calibration domain.