Nandamuri Yamini Rama, Poulomi Ganguli, Chandranath Chatterjee

Analyzing of trends in flood magnitude and the timing of the dates of flood occurrences of large river basins across the globe are essential for understanding changes in water availability (high or low flows) and assessing the fidelity of global hydrological models. Our research is motivated by the recent six major consecutive floods in Mahanadi (years: 2001, 2003, 2006, 2008, 2011 and 2013) River Basin (MRB), which is one of the largest peninsular Rivers in India with a catchment area of 14,1589 km2. We examine the altered risk of flooding focusing on changes in the flood regimes and a shift in the timing of floods over the past four decades (1970-2016) using hydrometric observations across the MRB. A framework for identification of flood regime changes is developed using monsoonal maxima peak discharge (MMPD) and peak over threshold (POT) events at 24 stream gauges over the basin. We find a mix of (insignificant) up/downward trends in flood magnitude at Upper MRB (Region I). On the other hand, the middle reaches of the basin (Region II) showed an upward trend in flood magnitude, with a larger number of sites detect significant trends in flood magnitude for the POT events. Further, we find the downward trends in MMPD series at Region I is field significant (at 10% significance level) whereas none of the trends in POT series show field significance. Only a few stations detected abrupt changes in the flood time series, and they are spatially clustered at Region I, whereas Region II showed no evidence of change points. A delayed (or earlier) shift in flood timing is apparent for most of the sites, notwithstanding the mean date of flood occurrence is in August irrespective of the type of flood series. The outcomes of the study contribute to ensuring flood resilience at densely populated large river basins.