Modeling Nonstationary Extreme Rainfall for Updating Intensity–Duration–Frequency Curves across Indian Cities under Changing Climate

Abstract Nonstationarity in extreme rainfall, driven by an intensifying hydrological cycle under a changing climate, challenges the reliability of conventional stationary intensity–duration–frequency (IDF) curves used in hydrologic design. Analyzing annual and seasonal timeframes, we employed innovative trend analysis to detect trends in maximum rainfall over durations of 1 to 48 h. A set of 10 covariate combinations, resulting in 9,225 models per location and timeframe, was used. Nonstationarity is initially introduced in the location parameter and subsequently in the location and scale parameters of the generalized extreme value distribution. Annual trends indicate an increase in 24 h rainfall in northern and central regions, contrasted by decreases in coastal southern and northern areas. The southwest (SW) monsoon trends indicate increases in the north and southern peninsular regions, with decreases in the north and west. In contrast, the northeast (NE) monsoon trends show increases in the north, east, and south as well as decreases in the southern peninsular areas. Nonstationarity predominantly affects the generalized extreme value location parameter in the western, southern coastal, and southern peninsular regions during the southwest monsoon and in the central and southern peninsular regions during the northeast monsoon. The eastern, southern peninsular, and central areas, traditionally dominated by the NE monsoon, exhibit more extreme conditions during the SW monsoon. In contrast, the south coastal, central, and northern regions, typically driven by the SW monsoon, show increased extremes of the NE monsoon. Annual nonstationary rainfall intensities increase up to 58.4% (Dehradun) and 72.9% (Varanasi); SW monsoon rises reach 37.5%–50.9%, while NE monsoon increases range 20.3%–42.2%. The global temperature anomaly, El Niño–southern oscillation phase changes, and regional temperature change are identified as significant covariates, with temporal nonstationarity limited to northern regions for specific durations. These findings underscore the need for region-specific IDF curves to address the evolving patterns of extreme rainfall under climate change.