Assessment of climate change impacts on runoff and hydrological drought risk using the VIC-3L model and four-variate D-vine copulas in the Upper Bhima basin, India

This study proposed a D-vine copula-based methodology to compute the four-variate joint return period of drought events, incorporating severity, duration, peak, and interarrival time. The VIC-3L hydrological model, calibrated using differential evolution, was employed to simulate gridded runoff in the Upper Bhima basin using climate projections from five CMIP6 GCMs under the SSP2-4.5 and SSP5-8.5 scenarios. Future runoff projections reveal substantial seasonal shifts relative to a historical period. In the future, early monsoon runoff (June–August) is expected to decline, while post-monsoon runoff (September–December) might increase. INM-CM5-0 and MIROC6 models have projected higher runoff in post-monsoon months, whereas IPSL-CM6A-LR consistently simulates lower runoff. To characterize the hydrological droughts, the study employed a 3-month Standardized Runoff Index (SRI) and run theory. Spatiotemporal analysis of extreme hydrological droughts over the Upper Bhima basin indicates a decline in drought duration and severity but an increase in peak and reduced interarrival time under future scenarios. Under the SSP2-4.5 scenario for extreme droughts, the basin was projected to experience critically decreasing return periods, about 17.7% (19%) of the study area in the mid-(late-) century.