Spatial and temporal variability of flood seasonality in Wales

High-magnitude floods across Europe within the last decade have resulted in the widespread reassessment of flood risk; this coupled with the introduction of the Water Framework Directive (2000) has increased the need for a detailed understanding of seasonal variability in flood magnitude and frequency. Mean day of flood (MDF) and flood seasonality were calculated for Wales using 30 years of gauged river-flow records (1973-2002). Noticeable regional variations in timing and length of flood season are evident, with flooding occurring earlier in small catchments draining higher elevations in north and mid-west Wales. Low-altitude regions in West Wales exposed to westerly winds experience flooding during October-January, while large eastern draining catchments experience later flooding (January-February). In the northeast and mid-east regions December-January months experience the greatest number of floods, while the southeast has a slightly longer flood season (December-February), with a noticeable increase in January floods. Patterns obtained from MDF data demonstrate their effectiveness and use in analysing regional patterns in flood seasonality, but catchment-specific determinants, e. g. catchment wetness, size and precipitation regime are important factors in flood seasonality. Relatively strong correlations between precipitation and flood activity are evident in Wales, with a poorer relationship between flooding and weather types and the North Atlantic Oscillation (NAO). Copyright (C) 2010 John Wiley & Sons, Ltd