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 dday 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.

Abstract. Understanding trends in flood severity and the persistence in peak discharge timing along a vast river network is vital for basin-scale flood risk management and reinsurance purposes. While earlier studies have primarily focused on analysis of either trends in floods or its seasonality independently, here for the first time, we assess coincidence of changes in peak discharge and shifts in its timing in one of the largest peninsular rivers (drainage area of 141 589 km2), Mahanadi River Basin (MRB), in India during 1970–2016. Our research is motivated by the recent six major consecutive floods over MRB during the years 2001, 2003, 2006, 2008, 2011, and 2013. We analyze flood properties using peak fluvial discharge indicators, Monsoonal (from June 1–end of September, during the Indian summer monsoon period), Maxima Flood (MMF) and Peak over Threshold Flood (POTF) events. While we find a blend of (insignificant) up/downward trends in flood magnitude at Upper MRB (Region I), 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 floods for the POTF events. Although the average dates of peak discharge in the basin are concentrated in August, notwithstanding the nature of flood samplings, a delayed (or earlier) shift in flood timing is apparent for most of sites. Further, we detect potential hotspots, where up/downward trends in flood magnitude coincide with early (or delayed) dates of flood occurrences. Based on observational evidence, here we show that up to one-third of sites show an up/downward trend in peak discharge with a distinct shift in the flood timing throughout the MRB. The outcomes of the study call for developing efficient adaptation strategies to ensure regional flood resilience since variations in the peak discharge timing should not be confounded with (insignificant) changes in its magnitude.

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