Nandamuri Yamini Rama, Poulomi Ganguli, Chandranath Chatterjee

This study contributes to the understanding of the timing of occurrence of floods and role of the catchment wetness in flood processes (i.e., magnitude and the timing of floods) over one of the largest tropical pluvial river basin system, Mahanadi, in India. Being located in the monsoon ‘core’ region (18° - 28° N latitude and 73° - 82° E longitude) and its proximity to Bay of Bengal, Mahanadi River Basin (MRB) system is vulnerable to tropical depression-induced severe storms and extreme precipitation-induced fluvial floods during southwest monsoon. Here we examine the incidence of flooding over MRB in recent decades (2007-2016) using monsoonal maxima peak discharge (MMPD) and peak over threshold (POT) events at 12 stream gauges, spatially distributed over the basin. We find the mean dates of flood occurrences are temporally clustered in the month of August for all gauges irrespective of the type of flood series. Our results reveal, sensitiveness of runoff responses (Flood Magnitude, FM and the Flood Timing, FT) to lagged d-day mean catchment wetness [CW] and corresponding catchment properties. Although we identify moderate to strong positive correlation between CW and flood properties at various lags, for the MMPD events, the nature of association between CW and FM, ranges between negative to modestly positive for the catchments with fine-textured soil, whereas catchments with medium textured soil showed moderately positive correlations. Further, we find FT is more strongly correlated (as manifested by statistically significant correlations) to CW rather than FM. Overall, we observe, the correlation of CW versus FT is negative, where the flood timing is relatively irregular. The outcomes of the study helps to improve predictability of floods, which can in turn enhance existing flood warning techniques.