Abstract
AbstractUsing data from 520 gauging stations in Britain and gridded rainfall datasets, the seasonality of storm rainfall and flood runoff is compared and mapped. Annual maximum (AMAX) daily rainfall occurs predominantly in summer, but AMAX floods occur most frequently in winter. Seasonal occurrences of annual daily rainfall and flood maxima differ by more than 50% in dry lowland catchments. The differences diminish with increasing catchment wetness, increase with rainfalls shorter than daily duration and are shown to depend primarily on catchment wetness, as illustrated by variations in mean annual rainfall. Over the whole dataset, only 34% of AMAX daily flood events are matched to daily rainfall annual maxima (and only 20% for 6-hour rainfall maxima). The discontinuity between rainfall maxima and flooding is explained by the consideration of coincident soil moisture storage. The results have serious implications for rainfall-runoff methods of flood risk estimation in the UK where estimation is based on a depth–duration–frequency model of rainfall highly biased to summer. It is concluded that inadequate treatment of the seasonality of rainfall and soil moisture seriously reduces the reliability of event-based flood estimation in Britain.
Highlights
Seasonality is an obvious feature of most global climates and is demonstrated in seasonal variations in extreme rainfall and flood occurrence
As a first step in assessing the variability of the seasonality of rainfall and flow, the percentage of Annual maximum (AMAX) daily rainfall and peak flow occurring in summer is illustrated in Figure 1(a) and 1(b)
Over the greater part of eastern England and Scotland more than 60% of AMAX daily rainfall occurs in summer, whereas in the same areas, fewer than 20% of AMAX peak flows occur in summer
Summary
Seasonality is an obvious feature of most global climates and is demonstrated in seasonal variations in extreme rainfall and flood occurrence. In Britain, long-duration frontal rainfall and occasional snow dominate in winter, while convective storms in summer increase the frequency and intensity of short-period rainfall. This seasonality is intensified by higher temperatures and evaporation in summer compared with winter, with resulting seasonal variation in soil moisture status at the onset of storm rainfall.
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