Extreme rainfalls and floods have caused severe socio-economic and environmental losses in most parts of the world and are predicted to exacerbate due to the changing climate. Highly saturated soil, extreme rainfall, and heavy snowmelt are the most common flood triggers. However, the relative contributions of extreme, long rainfall, antecedent soil moisture, and snowmelt and how they are vary with time and change from catchment to catchment are not fully understood. This information is critical for a better understanding of flood generation mechanisms and can improve flood risk management plans and strategies. We examined precipitation, streamflow, and estimated anticipated soil moisture (anticipated precipitation index) from more than 146 hydrological stations across the UK. Our main objectives were: creating flood type classes according to hydrometeorological characteristics, identifying the contribution of independent variables (soil moisture, snow, rainfall) and understanding the spatial and temporal variability of mutual information.

A simple empirical relationship between the peak flow and precipitation was used to estimate the anticipated precipitation index, used as a proxy for antecedent soil moisture. The relative importance of each variable and its respective flood-generating processes were identified using multilinear regression and decision tree approaches. Based on catchment average rainfall, gauged streamflow, and estimated anticipated soil moisture data across the UK, we confirm that most of peak flows are strongly associated with both the extreme rainfall and antecedent soil moisture conditions above the 90th percentile. There is a clear difference in flood magnitude and their respective generating mechanisms between regions, and regions with an expected decrease in anticipated soil moisture into the future were highly statistically correlated with a decrease in annual average peak flood magnitude. The role of extreme rainfall is the most dominant factor across the UK; however, seasonal total rainfall is not a strong influencing factor of peak floods in the southern UK. Extreme rainfall and peak floods are positively corrected with catchment drainage area. This linkage between drainage area and the most common flood generation mechanisms is crucial to quantifying the magnitude and level of flood risk in ungauged catchments.