Quantification of Precipitation Extremeness over a Large Indian River Basin using Weather Extremity Indices

Quantifying the extremeness of precipitation events in a spatio-temporally consistent manner, especially over a large river basin with hydro-climatic heterogeneity, poses a key challenge in flood risk assessment. The Mahanadi River basin (MRB) in India is located in the core monsoon region and often experiences tropical cyclone-induced severe rainfall. Extreme precipitation in the upstream sub-basins of MRB, leads to major flooding in the densely populated lower sub-basin (delta) region. Existing studies typically quantify precipitation extremes for the basin as a whole and thereby often overlook spatial heterogeneity, resulting in an underestimation of the variability and distribution of extreme rainfall across sub-basins. This study, therefore, applies the Weather Extremity Index (WEI) and its cross-scale extension (xWEI) at the sub-basin scale to further investigate flood-generating processes.

Both WEI and xWEI quantify extremeness as a function of the spatial extent and rarity (frequency) of precipitation events. However, WEI characterises events by their maximum extremeness at a specific duration of rainfall accumulation, whereas xWEI captures extremeness across the entire spatio-temporal scale. In this study, WEI is applied as an event-centred diagnostic, while xWEI is computed continuously in time for the sub-basins. Both indices are used to examine precipitation extremeness during six major flood events that occurred in the delta region since 2000.

The WEI time series shows systematically high values at medium to long accumulation durations (5–10 days) in the upstream sub-basins prior to flood events in the delta. An exception is observed for the 2006 flood, during which high WEI values occurred directly over the delta on the flood date. The absence of a consistent dominant duration in WEI over the delta suggests that local precipitation alone is not sufficient to explain flood occurrence. A similar pattern is observed in the xWEI time series, with higher values in the upstream catchments during the days preceding delta floods. In addition, xWEI highlights pronounced extremeness at short to medium durations (2–5 days) in the upstream basins. Across all six events, xWEI consistently reveals a clear upstream–downstream evolution of precipitation extremeness in the MRB, with maxima appearing in the upper sub-basin at 2–3 days before and in the middle sub-basin 1–2 days before any major flood event at delta.