On the occurrence of the observed worst flood in Mahanadi River basin under the warming climate

Floods cause enormous damage to infrastructure and result in the loss of human lives. Most river basin-scale floods in India occur during the summer monsoon season. While extreme precipitation events and floods are projected to increase under the warming climate, it remains unclear how the frequency of the worst flood that occurred during the observed period will change in the future. Using the observations and simulations conducted using the Variable Infiltration Capacity (VIC) model, we identified the worst flood during the observed period of 1971–2012 in the Mahanadi River basin. The observed worst flood at Basantpur occurred in August 2003, exceeding 33,000 m³/s and a return period of 132 years. Kantamal station of the Mahanadi River basin experienced the worst flood in 2008 with a magnitude of 20,000 m³/s and a return period of 64 years. Multi-day extreme precipitation upstream of the gauge stations driven by the moisture transport from the Bay of Bengal caused the observed worst floods in the basin. We used bias-corrected projections from the six global climate models (GCMs) that participated in the coupled model intercomparison project 6 (CMIP6) under the SSP1-2.6 and SSP5-8.5 scenarios to estimate the projected changes in the frequency and magnitude of floods. The frequency of the worst observed floods is projected to rise at both stations under the projected future climate. In addition, the magnitude of floods of 10, 20, and 50-year return periods are projected to rise at both stations in the Mahanadi basin. The projected rise in the frequency of the worst floods and extreme precipitation in the basin will pose challenges for agriculture and infrastructure.