Importance of Copula-Based Bivariate Rainfall Intensity-Duration-Frequency Curves for an Urbanized Catchment Incorporating Climate Change

The intensity-duration-frequency (IDF) curve is a critical input for designing hydraulic infrastructure such as stormwater drainage systems. The last few decades have witnessed drastic changes in rainfall patterns and changes in their extremes, mostly associated with the impact of climate change. Therefore, it is important to study the possible drift in the IDF curve associated with the changing trends in historical rainfall data and future climatic conditions. Based on the understanding of the negative correlation between rainfall intensity and duration, this study used a bivariate copula-based approach for IDF curve development (considering historical rainfall data) and compared it with the conventional empirical models and univariate frequency analysis. The study identified the Frank copula (from 24 candidate copulas) with its parameters estimated by Bayesian inference and a hybrid-evolution Monte Carlo Markov Chain. In addition, the IDF curve was developed for four future climate scenarios corresponding to three different time periods. The proposed methodology was demonstrated for an urban catchment in Northeast India, for which the future climate scenarios were not considered for previous IDF curve development. The rainfall intensity from one of the empirical models and univariate analysis compared well with the corresponding values obtained using the IDF developed using bivariate analysis for short durations (=3 h). Both the time periods and different climate scenarios had a significant influence on the rainfall intensities compared to the historical bivariate IDF data. The recommendation from this study for this area is that to account for the influence of near future climate change on infrastructure with a design life of 50 years and <50 years, the representative concentration pathway (RCP) 6.0 scenario would yield the critical IDF curve. For long-term planning with a design life >50 years, it is desirable to consider the IDF curve based on the RCP 8.5 scenario. For the intermittent period P2 (2048-2074), both RCP 6.0 and RCP 8.5 exhibited a similar trend in terms of rainfall intensity. The results from this study and the literature recommend comprehensive studies on specific urban catchments to incorporate the impact of climate change on the IDF curve for assessing the adequacy of hydraulic structures from a future perspective.